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A Woman’s Metabolism

This excerpt is from the book, Running for Women. It’s published with permission of Human Kinetics

Metabolic Differences

Metabolism refers to all of the energy-requiring chemical reactions occurring inside your body. At any

one time, trillions of reactions are going on inside of you, including the growth of new tissue, muscle contraction, and the breakdown of food for energy. The resting metabolic rate—the amount of energy needed during resting conditions—is lower in females because of their smaller body mass and muscle mass. When you run, your metabolic rate increases dramatically because of the increased demand for energy. The faster your metabolic pathways can use the available fuel to regenerate energy for muscle contraction, the faster you will be able to run any race.

While your nervous system controls your body’s faster functions, like the initiation of reflexes and movement, hormones control the slower functions, like the regulation of growth and metabolism and the development of reproductive organs. Much of metabolism is under the direction of hormones, which act as conductors, initiating signals that lead to the transportation and use of fuel. And the two predominant fuels for running are carbohydrate and fat, which provide energy on a sliding scale. At slower speeds, your muscles rely more on fat and less on carbohydrate, and as you increase your running pace, the energy contribution from fat decreases while the energy contribution from carbohydrate increases.

Carbohydrate Metabolism

The hormone insulin is responsible for carbohydrate metabolism. Consuming carbohydrate elevates your blood glucose concentration and increases insulin concentration. The increase in circulating insulin, which is secreted from your pancreas, stimulates specific proteins to transport the glucose from your blood into your muscles, where it is either used for immediate energy by your cells or stored as muscle glycogen for later use. Males typically have more glycogen stored in their muscles. Longer races like the marathon are limited, in part, by the amount of stored glycogen. Therefore, the lower muscle glycogen in women’s muscles can partly explain why they cannot run marathons as fast as men.

Research has shown that men also are more responsive to carbohydrate loading than women. In other words, women do not increase muscle glycogen as much as men in response to consuming more carbohydrate in their diets. However, some of this research is clouded by the fact that women consume fewer total calories than men, so the lack of glycogen storage may be due to a lower caloric or carbohydrate intake by women rather than an inherent sex difference in the ability to store glycogen. When women increase their total caloric intake as they also increase the amount of carbohydrate in their diets, they increase their muscle glycogen content by a similar amount as men. From a training perspective, while men simply need to increase the percentage of their calories coming from carbohydrate in order to carbo load and store more glycogen, women need to also increase the total number of calories in their diets to get the same effect.

Because carbohydrate is the predominant fuel source during running and the only fuel source at speeds faster than acidosis threshold, research has focused on how the hormonal differences between men and women affect insulin and alter carbohydrate metabolism. Most research has found that women use less carbohydrate than men when exercising at similar intensities.

When you finish a workout that severely lowers your muscle glycogen content, it’s important to replenish the carbohydrates so you can resynthesize more glycogen to be prepared for your next run. In fact, refueling nutrient-depleted muscles is possibly the single most important aspect of optimal recovery from training and racing. Scientists first discovered in the late 1960s that endurance performance is influenced by the amount of stored glycogen in skeletal muscles, and that intense endurance exercise decreases muscle glycogen stores. The faster you can resynthesize muscle glycogen, the faster your recovery. Research has shown that the rate of glycogen synthesis in the first few hours following a workout (the time when you are best able to store glycogen because the cells are most sensitive to insulin) is similar between the sexes. This suggests that recovery rates between males and females are similar, at least the component of recovery affected by the resynthesis of fuel.

Fat Metabolism

As a consequence of not using as much carbohydrate during exercise, women rely more on fat than men. Indeed, it has been estimated that women use about 75 percent more fat than do men while running or cycling at 65 to 70 percent V·O2max. Women get about 39 percent of their energy from fat during exercise at 65 percent V·O2max, while men get about 22 percent of their energy from fat. However, the percentage of energy derived from fat varies significantly from person to person because factors such as training status, muscle fiber type, muscle glycogen content, and mitochondrial density all play a role.

While it is difficult to tease out the exact reasons for the difference between the sexes in the metabolism of carbohydrate and fat, it appears that estrogen is at least partly responsible. Research done on rats has shown that when male rats are given estrogen, they deplete less glycogen during exercise; the concentration of fatty acids in the blood increases, suggesting a greater availability of fat for energy; and they can exercise for longer periods before becoming exhausted. Increasing the amount of fatty acids circulating in the blood favors their use by muscle during exercise, resulting in a decreased reliance on muscle glycogen and blood glucose, thus delaying glycogen depletion and hypoglycemia, or low blood sugar, and postponing fatigue.

This switch in fuel use to a greater reliance on fat at the same running speed also occurs from endurance training. Training enhances fat use by increasing the mitochondria in your muscles, allowing for more aerobic metabolism and the sparing of muscle glycogen. This shift in the energy source for muscular activity is a major advantage in delaying the onset of fatigue in running events that are limited by the availability of muscle glycogen—marathons and ultramarathons. Because humans’ carbohydrate stores are limited, the difference in metabolism between the sexes may give female runners an advantage for very long endurance activities, during which there is a greater need to conserve carbohydrate and a greater use of fat because of the slower pace. In 2002 and 2003, Pam Reed showed that science may be on to something, by winning the 135-mile (217K) Badwater Ultramarathon, beating all of the men. In shorter races, however, when there is a greater demand to generate energy quickly for muscle contraction, relying more on fat will slow the pace because energy is derived much more quickly from carbohydrate than from fat.

Protein Metabolism

The third macronutrient, protein, is often neglected in metabolism because it accounts for only 3 to 6 percent of the amount of energy expended while running. Rather, protein is used primarily for other things, such as building, maintaining, and repairing muscle, skin, and blood tissue, as well as aiding in the transportation of materials through the blood. Protein can be thought of as your body’s scaffolding and cargo. However, it can be used for energy if inadequate amounts of fat and carbohydrate are available because the body’s requirement for energy takes priority over tissue building. Although the amount of protein you use for energy may be small, even a small contribution to your daily run may be large if you run a lot and run often.

Exercise increases the use of amino acids from protein breakdown, and the amount of amino acids that your muscles use is inversely related to the amount of glycogen in the muscle. When glycogen is abundant, muscles rely on glycogen, but when glycogen is low, muscles begin to rely more on amino acids. Research has shown that females use less protein during exercise than do males. Because endurance-trained females use less muscle glycogen and rely more on fat than endurance-trained males, protein breakdown seems to be inhibited in females by virtue of the greater muscle glycogen.

How our bodies use protein

This is an excerpt from Nutrient Timing for Peak Performance. It’s published with permission of Human Kinetics.

The body doesn’t have a large storage depot for protein, as it does for carbohydrate and fat. The protein we eat from food has to be handled as we eat it. Like rookies sitting on the bench waiting for their chance to play, the amino acids in the pool are ready and waiting to be utilized. Either the amino acids are used within a limited time to build a body protein, or they are transformed.

If amino acids in the pool aren’t needed to become a protein, the body is equipped to reconfigure them either back to glucose to be used as energy or into fat. To transform an amino acid, the liver strips off the nitrogen, which may then be incorporated into DNA, RNA, or a nonessential amino acid. Excess nitrogen may also be incorporated into urea, or ammonia, both of which are excreted in the urine. In order to eliminate these, water is needed, so a high protein intake can result in excess fluid loss. The remaining part of the stripped-down amino acid may be reconfigured into glucose, and it is burned for energy.

How much protein do we need?

As you can see, our bodies can do a lot of things with the protein we eat. The recommended dietary allowance (RDA) for protein for people over the age of 18 is not a huge amount—.8 gram per kilogram or .36 gram per pound. For younger people who are still growing, ages 4 to 13, it’s .95 gram per kilogram or .43 gram per pound; and for ages 14 to 18, it’s .85 gram per kilogram or .39 gram per pound. These amounts represent the average daily dietary intake level sufficient to meet the nutrient requirements of about 97 percent of people in these age groups. However, athletes need more.

How much protein should be recommended is a hotly debated subject among some because, although the previous recommendations will supposedly maintain health, they do not necessarily represent the optimal intake or cover the needs of an athlete in training. Muscle damage does occur as a result of exercise, and additional protein is needed for repairing tissue. Optimal protein intake for athletes not only maintains health but also supports muscle growth, preserves bone integrity, and in certain instances helps with weight management. The type and intensity of training, duration, frequency, your fitness level, and your weight will all be considered when determining your protein needs (we will discuss this in more detail later).

What happens with excess protein intake?

Athletes do need more protein than their sedentary counterparts, but there still is a limit to how much can be used. Often when people consume excess protein, the ammonia formed as a by-product of protein metabolism cannot be eliminated through urine. So it is lost in sweat. If your sweat has an ammonia odor, your protein intake may be higher than your body needs. In healthy people, the body will employ protective mechanisms so that ammonia doesn’t build up to toxic levels. The rate that food leaves the stomach slows as a way to try to protect the body from ammonia overload. That’s why very high protein intake can sometimes make people feel nauseated.

Additionally, staying hydrated is a challenge for many athletes, and an excessive amount of protein intake requires fluid to break down amino acids and rid the body of nitrogen. If protein is consumed too close to practice, there is an increased demand for oxygen by working muscles and organs that process protein. In research published in the British Journal of Sports Medicine (Wiles, 1991), subjects thought the exercise was harder one hour after having a high-protein meal compared with those having only water; their Rate of Perceived Exertion (RPE) was higher. If your protein intake is very high, it is also likely that you are not taking in adequate carbohydrate, which will negatively affect performance. As is the case with many nutrients, too much of one can displace enough of another and cause imbalances. Power athletes can handle a lower percentage of calories coming from carbohydrate (as low as 42 percent, perhaps, although not necessarily ideal); however, some power athletes eat protein almost exclusively. The main point is that while athletes do need more protein than inactive people, excessive amounts can hurt performance.

Learn the advantages of nutrient timing

This is an excerpt from Nutrient Timing for Peak Performance. It’s published with permission of Human Kinetics.

What Are the Benefits of Nutrient Timing?
There are several benefits of nutrient timing. These involve maximizing your body’s response to exercise and use of nutrients. The Nutrient Timing Principles (NTP) help you do the following:

  • Optimize fuel use so that you remain energized throughout your training

  • Ensure that you repair and strengthen your muscles to the best of your genetic potential
  • Ingest sufficient nutrients to keep you healthy and able to fight off infection, limiting the suppression of the immune system often experienced with intense training
  • Recover from your training so that you are ready for your next practice, event, or training session with well-fueled muscles

Energy
When sports nutritionists talk about energy, we are referring to the potential energy food contains. Calories are potential energy to be used by muscles, tissues, and organs to fuel the task at hand. Much of the food we eat is not burned immediately for energy the minute it’s consumed. Rather, our bodies digest, absorb, and prepare it so that it can give us the kind of energy we need, when we need it. We transform this potential energy differently for different tasks. How we convert potential energy into usable energy is based on what needs to get done and how well prepared our bodies are; how we fuel endurance work is different from how we fuel a short, intense run. It is helpful to understand that you must get the food off your plate and into the right places in your body at the right time.

Clients consistently ask us, “What can I eat to give me energy?” For you, “energy” may have different meanings, depending on what you’re referring to and how you’re feeling. If you’re talking about vitality, liveliness, get-up-and-go, then a number of things effect this: amount of sleep, hydration, medical conditions, medications, attitude, type of foods eaten, conditioning and appropriate rest days, and timing of meals and snacks. Food will help a lack of energy only if the problem is food related. You may think that’s obvious, but it’s not to some. If you’re tired because you haven’t slept enough, for instance, eating isn’t going to give you energy. However, if your lack of energy is because you’ve eaten too little, your foods don’t have “staying power,” you go for too long without eating, or you don’t time your meals and snacks ideally around practice or conditioning, then being strategic with food intake can help you feel more energetic. What, how much, and when you eat will affect your energy.

Nutrient timing combined with appropriate training maximizes the availability of the energy source you need to get the job done, helps ensure that you have fuel ready and available when you need it, and improves your energy-burning systems. You may believe that just eating when you are hungry is enough, and in some cases this may be true. But, many times, demands on time interfere with fueling or refueling, and it takes conscious thought and action to make it happen. Additionally, appetites are thrown off by training, so you may not be hungry right after practice, but by not eating, you are starving while sitting at your desk in class or at work. Many athletes just don’t know when and what to eat to optimize their energy stores.

By creating and following your own Nutrition Blueprint and incorporating the NTP, your energy and hunger will be more manageable and consistent, whether you are training several times a week, daily, participating in two-a-days, or are in the midst of the competitive season.


Recovery

During the minutes and hours after exercise, your muscles are recovering from the work you just performed. The energy used and damage that occurred during exercise needs to be restored and repaired so that you are able to function at a high level at your next workout. Some of this damage is actually necessary to signal repair and growth, and it is this repair and growth that results in gained strength. However, some of the damage is purely negative and needs to be minimized or it will eventually impair health and performance. Providing the right nutrients, in the right amounts, at the right time can minimize this damage and restore energy in time for the next training session or competition.

The enzymes and hormones that help move nutrients into your muscles are most active right after exercise. Providing the appropriate nutrients at this crucial time helps to start the repair process. However, this is only one of the crucial times to help repair. Because of limitations in digestion, some nutrients, such as protein, need to be taken over time rather than only right after training, so ingesting protein throughout the day at regular intervals is a much better strategy for the body than ingesting a lot at one meal. Additionally, stored carbohydrate energy (glycogen and glucose) and lost fluids may take time to replace.

By replacing fuel that was burned and providing nutrients to muscle tissue, you can ensure that your body will repair muscle fibers and restore your energy reserves. If you train hard on a daily basis or train more than once a day, good recovery nutrition is absolutely vital so that your muscles are well stocked with energy. Most people think of recovery as the time right after exercise, which is partially correct, but how much you take in at subsequent intervals over 24 hours will ultimately determine your body’s readiness to train or compete again.

Muscle Breakdown and Muscle Building

Nutrient timing capitalizes on minimizing muscle tissue breakdown that occurs during and after training and maximizing the muscle repair and building process that occurs afterwards. Carbohydrate stored in muscles fuels weight training and protects against excessive tissue breakdown and soreness. Following training, during recovery, carbohydrate helps initiate hormonal changes that assist muscle building. Consuming protein and carbohydrate after training has been shown to help hypertrophy (adding size to your muscle). The proper amount and mix of nutrients taken at specific times enables your body to utilize them most efficiently—that’s one of the Nutrient Timing Principles.

Immunity

Nutrient timing can have a significant impact on immunity for athletes. Strenuous bouts of prolonged exercise have been shown to decrease immune function in athletes. Furthermore, it has been shown that exercising when muscles are depleted or low in carbohydrate stores (glycogen) diminishes the blood levels of many immune cells, allowing for invasion of viruses. In addition, exercising in a carbohydrate-depleted state causes a rise in stress hormones and other inflammatory molecules. The muscles, in need of fuel, also may compete with the immune system for amino acids. When carbohydrate is taken, particularly during longer-duration endurance training (two to three hours), the drop in immune cells is lessened, and the stress hormone and inflammatory markers are suppressed. Carbohydrate intake frees amino acids, allowing their use by the immune system. Carbohydrate intake during endurance training helps preserve immune function and prevent inflammation.

Certain vitamins and minerals also play a role in immunity: iron, zinc, and vitamins A, C, E, B6, and B12. However, excess intake of iron, zinc, and vitamins A, C, and E can have the opposite effect and in some cases impair the body’s adaptation to training. An eating plan incorporating all of these nutrients in reasonable quantities, such as amounts found in food, can help athletes maintain immunity. The quality of the foods selected is very important and needs to be just as much of a priority as the focus on carbohydrate or protein, for example. For instance, eating a bagel for the carbohydrate but also including an orange for the vitamin C is important; drinking a protein shake can be helpful at the right time, but including some lean steak or shellfish for the iron and zinc is also essential.

Injury Prevention
Did you know that dehydration and low blood sugar can actually increase your risk of injury? Avoiding injury due to poor nutrition is absolutely within your control. Inadequate hydration results in fatigue and lack of concentration. Low blood sugar results in inadequate fueling to the brain and central nervous system. This leads to poor reaction time and slowness. Poor coordination as a result can lead to missteps, inattention, and injury.

Additionally, chronic energy drain (taking in fewer calories and nutrients than needed) will increase your risk of overuse injuries over time. Stress fractures are one example; poor tissue integrity can happen when athletes think solely about calories taken in but not the quality of the calories consumed. This is what is behind the phrase “overfed but undernourished.” Eating lots of nutrient-poor foods will not provide your body with the building blocks for healthy tissues and overall repair. Inadequate protein will also hinder the rebuilding of damaged muscles during training. If muscles are not completely repaired, they will not be as strong as they could be and will not function optimally. The damaged muscle fibers can lead to soft-tissue injuries. Both protein and carbohydrate along with certain nutrients are needed to help with this repair. For instance, gummy bears may provide carbohydrate, but they don’t contain any vitamin E, which is helpful in repairing soft-tissue damage that occurs daily during training. Therefore, the goal is both an appropriate quantity and an appropriate quality in food selection.

How our bodies use protein

“The body doesn’t have a large storage depot for protein, as it does for carbohydrate and fat. The protein we eat from food has to be handled as we eat it. Like rookies sitting on the bench waiting for their chance to play, the amino acids in the pool are ready and waiting to be utilized. Either the amino acids are used within a limited time to build a body protein, or they are transformed.

If amino acids in the pool aren’t needed to become a protein, the body is equipped to reconfigure them either back to glucose to be used as energy or into fat. To transform an amino acid, the liver strips off the nitrogen, which may then be incorporated into DNA, RNA, or a nonessential amino acid. Excess nitrogen may also be incorporated into urea, or ammonia, both of which are excreted in the urine. In order to eliminate these, water is needed, so a high protein intake can result in excess fluid loss. The remaining part of the stripped-down amino acid may be reconfigured into glucose, and it is burned for energy.

How much protein do we need?

As you can see, our bodies can do a lot of things with the protein we eat. The recommended dietary allowance (RDA) for protein for people over the age of 18 is not a huge amount—.8 gram per kilogram or .36 gram per pound. For younger people who are still growing, ages 4 to 13, it’s .95 gram per kilogram or .43 gram per pound; and for ages 14 to 18, it’s .85 gram per kilogram or .39 gram per pound. These amounts represent the average daily dietary intake level sufficient to meet the nutrient requirements of about 97 percent of people in these age groups. However, athletes need more.

How much protein should be recommended is a hotly debated subject among some because, although the previous recommendations will supposedly maintain health, they do not necessarily represent the optimal intake or cover the needs of an athlete in training. Muscle damage does occur as a result of exercise, and additional protein is needed for repairing tissue. Optimal protein intake for athletes not only maintains health but also supports muscle growth, preserves bone integrity, and in certain instances helps with weight management. The type and intensity of training, duration, frequency, your fitness level, and your weight will all be considered when determining your protein needs (we will discuss this in more detail later).

What happens with excess protein intake?

Athletes do need more protein than their sedentary counterparts, but there still is a limit to how much can be used. Often when people consume excess protein, the ammonia formed as a by-product of protein metabolism cannot be eliminated through urine. So it is lost in sweat. If your sweat has an ammonia odor, your protein intake may be higher than your body needs. In healthy people, the body will employ protective mechanisms so that ammonia doesn’t build up to toxic levels. The rate that food leaves the stomach slows as a way to try to protect the body from ammonia overload. That’s why very high protein intake can sometimes make people feel nauseated.

Additionally, staying hydrated is a challenge for many athletes, and an excessive amount of protein intake requires fluid to break down amino acids and rid the body of nitrogen. If protein is consumed too close to practice, there is an increased demand for oxygen by working muscles and organs that process protein. In research published in the British Journal of Sports Medicine (Wiles, 1991), subjects thought the exercise was harder one hour after having a high-protein meal compared with those having only water; their Rate of Perceived Exertion (RPE) was higher. If your protein intake is very high, it is also likely that you are not taking in adequate carbohydrate, which will negatively affect performance. As is the case with many nutrients, too much of one can displace enough of another and cause imbalances. Power athletes can handle a lower percentage of calories coming from carbohydrate (as low as 42 percent, perhaps, although not necessarily ideal); however, some power athletes eat protein almost exclusively. The main point is that while athletes do need more protein than inactive people, excessive amounts can hurt performance.”

This is a podcast from an author of Nutrient Timing for Peak Performance. It’s published with permission of Human Kinetics.

Carbohydrate intake during exercise

“Carbohydrate … during exercise of about 45 minutes or longer can improve endurance capacity and performance.” That’s what the authors of Sport Nutrition explain in this excerpt reprinted here with permission of the publisher, Human Kinetics.

“Convincing evidence from numerous studies indicates that carbohydrate feeding during exercise of about 45 minutes or longer (Jeukendrup 2004, 2008; Jeukendrup et al. 1997) can improve endurance capacity and performance. Studies have also addressed questions of which carbohydrates are most effective, what feeding schedule is the most effective, and what amount of carbohydrate to consume is optimal. Other studies have looked at factors that could possibly influence the oxidation of ingested carbohydrate, such as muscle glycogen levels, diet, and exercise intensity. Mechanisms by which carbohydrate feeding during exercise may improve endurance performance include the following.

* Maintaining blood glucose and high levels of carbohydrate oxidation. Coyle et al. (1986) demonstrated that carbohydrate feeding during exercise at 70% of V.O2max prevents the drop in blood glucose that was observed when water (placebo) was ingested. In the placebo trials, the glucose concentration started to drop after 1 hour of exercise and reached extremely low concentrations (2.5 mmol/L) at exhaustion after 3 hours. With carbohydrate feeding, glucose concentrations were maintained above 3 mmol/L, and subjects continued to exercise for 4 hours at the same intensity. Total-carbohydrate oxidation rates followed a similar pattern. A drop in carbohydrate oxidation occurred after about 1.5 hours of exercise with placebo, and high rates of carbohydrate oxidation were maintained with carbohydrate feeding. When subjects ingested only water and exercised to exhaustion, they were able to continue again when glucose was ingested or infused intravenously. These studies showed the importance of plasma glucose as a substrate during exercise.
* Glycogen sparing in the liver and possibly muscle. Carbohydrate feedings during exercise “spare” liver glycogen (Jeukendrup et al. 1999), and Tsintzas and Williams (Tsintzas et al. 1998) discussed a potential muscle glycogen sparing effect. Generally, muscle glycogen sparing is not found during cycling (Jeukendrup et al. 1999), but it may be important during running (Tsintzas et al. 1995).
* Promoting glycogen synthesis during exercise. After intermittent exercise, muscle glycogen concentrations were higher when carbohydrate was ingested than when water was ingested (Yaspelkis et al. 1993). This finding could indicate reduced muscle glycogen breakdown. But the ingested carbohydrate was possibly used to synthesize muscle glycogen during the low-intensity exercise periods (Keizer et al. 1987a).
* Affecting motor skills. Few studies have attempted to study the effect of carbohydrate drinks on motor skills. One such study investigated 13 trained tennis players and observed that when players ingested carbohydrate during a 2-hour training session (Vergauwen et al. 1998), stroke quality improved during the final stages of prolonged play. This effect was most noticeable when the situations required fast running speed, rapid movement, and explosiveness.
* Affecting the central nervous system. Carbohydrate may also have central nervous system effects. Although direct evidence for such an effect is lacking, the brain can sense changes in the composition of the mouth and stomach contents. Evidence, for instance, suggests that taste influences mood and may influence perception of effort. An interesting observation provides support for a central nervous system effect. When a hypoglycemic person bites a candy bar, that person’s symptoms almost immediately decrease, and the person feels better again long before the carbohydrate reaches the systemic circulation and the brain. The central nervous system effect may also explain why some studies report positive effects of carbohydrate during exercise on performance lasting approximately 1 hour (Jeukendrup et al. 1997). During exercise of such short duration, only a small amount of the carbohydrate becomes available as a substrate. Most of the ingested carbohydrate is still in the stomach or intestine. Studies in which athletes rinsed their mouths with carbohydrate (but did not ingest any) during 1-hour time trials showed performance improvements similar to those that occurred when the athletes ingested the carbohydrate (Carter et al. 2004). Others (Pottier et al. 2008) recently confirmed these findings.

Whether the central nervous system effects of glucose feeding are mediated by sensory detection of glucose or perception of sweetness is not known, although studies with placebo solutions with identical taste to glucose solutions suggest that sweetness is not the key factor. Brain imaging studies also show that increased brain activity is specific to carbohydrates.
Feeding Strategies and Exogenous Carbohydrate Oxidation

A greater contribution of exogenous (external) fuel sources (carbohydrate) spares endogenous (internal) sources, and the notion that a greater contribution from exogenous sources increases endurance capacity is enticing. The contribution of exogenous substrates can be measured using stable (or radioactive) isotopic tracers. The principle of this technique is simple: The ingested substrate (e.g., glucose) is labeled, and the label can be measured in expired gas after the substrate has been oxidized. The more the ingested substrate has been oxidized, the more of the label (tracer) will be recovered in the expired gas. Knowing the amount of tracer ingested, the amount of tracer in the expired gas, and the total CO2 production enables us to calculate exogenous substrate oxidation rates.

The typical pattern of exogenous glucose oxidation rates is shown in figure 6.6. The labeled CO2 starts to appear 5 minutes after ingestion of the labeled carbohydrate. During the first 75 to 90 minutes of exercise, exogenous carbohydrate oxidation continues to rise as more and more carbohydrate is emptied from the stomach and absorbed in the intestine. After 75 to 90 minutes a leveling off occurs, and the exogenous carbohydrate oxidation rate reaches its maximum value and does not increase further. Several factors have been suggested to influence exogenous carbohydrate oxidation including feeding schedule, type and amount of carbohydrate ingested, and exercise intensity.”

Connection between diet and muscle cramping

Base training starts soon if it hasn’t already which brings on longer bouts of exercise.  The longer you go, you more you need to know how your body’s chemical reactions are happening. This excerpt from Vegetarian Sports Nutrition is reprinted with permission by Human Kinetics.

“If you look at the information presented in most exercise physiology
and sports nutrition books, you will notice an obvious omission of
discussions of muscle cramps. This is probably because little is known
about muscle cramps. Nonetheless, I am a true believer that imbalances
of fluid or the mineral electrolytes—sodium, potassium, calcium, and
magnesium—in the diet should be ruled out as contributors to all
nocturnal and exercise-associated cramps.

Fluid Imbalances and Dehydration
Whether fluid imbalances and mild dehydration can trigger muscle
cramping is open to debate. Although we know that muscle cramps can and
do occur with severe dehydration and heat injury, there is no
conclusive evidence that consuming adequate fluid with or without
electrolytes will prevent typical nocturnal or exercise-associated
cramping. In fact, studies have found that runners, cyclists, and
triathletes who develop cramps during an endurance event are no more
likely to be dehydrated or to have lost greater amounts of bodily water
than are those who do not develop cramps during the same race. In my
practice, however, I have noted anecdotally that maintaining a proper
fluid balance indeed helps many endurance and team athletes avoid
cramps, particularly those that occur after exercise or when sleeping
at night. In one case, I worked with a male tennis player from
Switzerland who had a history of severe cramping and fatigue after
practice that was relieved by a regular and diligent fluid-consumption
schedule. In her book, well-known sport nutritionist Nancy Clark tells
an amusing story about a runner who eliminated his painful muscle
cramps by following the simple postexercise advice to first drink water
for fluid replacement and then have a beer for social fun.

Sodium
Sodium is one of the main positively charged mineral ions or
electrolytes in body fluid. The body needs it to help maintain normal
body-fluid balance and blood pressure, and in conjunction with several
other electrolytes, it is critical for nerve impulse generation and
muscle contraction. Sodium is distributed widely in nature but is found
in rather small amounts in most unprocessed foods. In most developed
countries, however, a significant amount of sodium is added from the
salt shaker (1 teaspoon [6 g] contains 2,325 milligrams of sodium) or
by food manufacturers in processing (as listed on the food label).
Because sodium intake can vary, the typical Western diet contains 10 to
12 grams of salt (3.9 to 4.7 g of sodium) per day.

Because
sodium plays an important role in regulating blood pressure and fluid
and electrolyte balance, the body has an effective mechanism to help
regulate the levels of sodium in the blood on a variety of sodium
intakes. If the sodium concentration in the blood starts to drop, a
series of complex events leads to the secretion of a hormone called
aldosterone, which signals the kidneys to retain sodium. If sodium
levels are too high, aldosterone secretion is inhibited, which allows
the kidneys to eliminate some sodium through urination. Another
hormone, called antidiuretic hormone (ADH), also helps
maintain normal sodium levels in body fluids by signaling the kidney to
retain water and sodium. Typically, levels of both aldosterone and ADH
increase during exercise, which helps conserve the body’s water and
sodium stores.

Actual sodium-deficient states caused by
inadequate dietary sodium are not common because the body’s regulatory
mechanisms are typically very effective. Humans even have a natural
appetite for salt, which helps assure that they take in enough sodium
to maintain sodium balance. Indeed, I have great memories of eating
salty tortilla chips wet with a little water—so more salt would
stick—after long cycling races in Arizona. Thankfully, these
sodium-conserving mechanisms are activated in athletes who lose
excessive sodium and other electrolytes during prolonged sweating.

Although muscle cramps are reported to occur during the
sodium-deficient state, some researchers believe that alterations in
sodium balance are not involved in exercise-associated cramps. This is
despite the fact that significantly lower postexercise serum sodium
concentrations have been found in endurance athletes who experienced
cramps during a race compared to those who did not develop cramps. One
of the reasons this is downplayed may be because serum sodium
concentrations remain within the normal range, despite being
significantly lower in the athletes with muscle cramps.

Nevertheless, it is important for athletes to consume enough sodium to
replace what is lost through sweat. Despite the regulatory mechanisms
discussed earlier, it is possible for vegetarian athletes to be at risk
for muscle cramps and other problems because of low sodium intake. The
reason is most likely because they ignore their salt craving
cues—eating mostly unprocessed and unsalted foods—while continuing to
lose considerable salt through sweating. The recommendation set by the
USDA’s Dietary Guidelines for Americans to keep sodium intake to 2.3
grams or less per day is not appropriate for most athletes because of
their higher sodium losses. Thus, while it is not likely that low
sodium intake is the cause of cramps in most athletes, it is certainly
possible that a vegetarian athlete prudently following a low-sodium
diet for health reasons might experience muscle cramps that would be
relieved with more liberal use of the salt shaker.

Potassium
Potassium is the major electrolyte found inside all body cells,
including muscle and nerve cells. It works in close association with
sodium and chloride in the generation of electrical impulses in the
nerves and the muscles, including the heart muscle. Potassium is found
in most foods, but is especially abundant in fresh vegetables,
potatoes, certain fruits (melon, bananas, berries, citrus fruit), milk,
meat, and fish.

Potassium balance, like sodium balance, is
regulated by the hormone aldosterone. A high serum potassium level
stimulates the release of the hormone aldosterone, which leads to
increased potassium excretion by the kidneys into the urine. A decrease
in serum potassium concentration elicits a drop in aldosterone
secretion and hence less potassium loss in the urine. As with sodium
and calcium, potassium is typically precisely regulated, and
deficiencies or excessive accumulation are rare. Potassium
deficiencies, however, can occur with conditions such as fasting,
diarrhea, and regular diuretic use. In such cases, low blood–potassium
concentrations, called hypokalemia, can lead to muscle cramps
and weakness, and even cardiac arrest caused by impairment in the
generation of nerve impulses. Similarly, high blood–potassium
concentrations, or hyperkalemia, are also not common but can
occur in people who take potassium supplements far exceeding the
recommended daily allowance. High blood–potassium concentrations can
also disturb electrical impulses and induce cardiac arrhythmia.

Even though little evidence is available to support a link between
potassium intake and muscle cramps, it is quite interesting that most
athletes—and non-athletes alike—think that the banana is the first line
of defense in preventing muscle cramps. If only it were that simple.
Furthermore, athletes following vegetarian diets are not likely to
experience muscle cramping as a result of low potassium intake because
the vegetarian diet provides an abundance of potassium. An athlete who
is recovering from an intestinal illness, restricting calories, or
taking diuretics or laxatives should, nevertheless, make an effort to
consume potassium-rich foods, particularly if he or she is experiencing
muscle cramping. Because of the dangers of hyperkalemia, potassium
supplements are not recommended unless closely monitored by a
physician. The recommended daily intake for potassium is 4,700
milligrams per day for adults.

Calcium
As discussed in chapter 6, the vast majority of calcium found in the
body is found in the skeleton where it lends strength to bone. Calcium,
however, is involved in muscle contractions, including that of the
heart, skeletal muscles, and smooth muscle found in blood vessels and
intestines, as well as the generation of nerve impulses. Blood calcium
is tightly controlled and regulated by several hormones, including
parathyroid hormone and vitamin D.

Although impaired muscle
contraction and muscle cramps are commonly listed as symptoms of
calcium deficiency, many exercise scientists feel that low calcium
intake is not likely to play a role in most muscle cramps. This is
because if dietary calcium intake were low, calcium would be released
from the bones to maintain blood concentrations and theoretically
provide what would be needed for muscle contraction. This thinking,
however, does not completely rule out the possibility that muscle
cramping could be caused by a temporary imbalance of calcium in the
muscle during exercise. Certainly, we know that people with inborn
errors in calcium metabolism in skeletal muscle (which will be
discussed later) are prone to muscle cramping.

Despite so
little being known about low calcium intake and muscle cramps, calcium
is one of the nutritional factors people most associate with relieving
cramps, second only to the potassium-rich banana. Although to my
knowledge studies have not assessed whether dietary or supplemental
calcium affects exercise cramps in athletes, a recent report found that
calcium supplementation was not effective in treating leg cramps
associated with pregnancy. On the other hand, anecdotal reports from
athletes are common. Nancy Clark tells of a hiker who resolved muscle
cramps by taking calcium-rich Tums and of a ballet dancer whose
cramping disappeared after adding milk and yogurt to her diet. Because
calcium intake can be low in the diet of some vegans and vegetarians,
inadequate calcium should also be ruled out in vegetarians experiencing
muscle cramps.

Magnesium
In addition
to its role in bone health, magnesium plays an important role in
stabilizing adenosine triphosphate (ATP), the energy source for muscle
contraction, and also serves as an electrolyte in body fluids. Muscle
weakness, muscle twitching, and muscle cramps are common symptoms of
magnesium deficiency.

Limited data have suggested that
magnesium status is indirectly related to the incidence of muscle
cramps. In these studies of endurance athletes, the athletes who
developed muscle cramps were found to have serum magnesium
concentrations that were different from their competitors who did not
cramp. The research, however, presents a confusing story because serum
magnesium was significantly lower in cyclists who cramped during a
100-mile (160 km) bike ride and significantly higher in runners who
cramped during an ultradistance race. In both studies, serum magnesium
remained within the normal range but was low-normal in the cyclists who
cramped and high-normal in the runners. Interestingly, studies in
pregnant women have found that supplementation with magnesium (taken as
magnesium lactate or magnesium citrate in doses of 5 millimoles in the
morning and 10 millimoles in the evening ) show promise for treating
pregnancy-associated leg cramps. Research, however, has not addressed
whether dietary or supplemental magnesium can prevent or reduce muscle
cramps in athletes.

Vegetarian athletes are not likely to
experience muscle cramping as a result of low magnesium intake because
the typical vegetarian diet is abundant in magnesium. Low magnesium
intake, however, is possible for people restricting calories or eating
a diet high in processed foods. Low magnesium intake should be ruled
out in cramp-prone athletes.

Carbohydrate
Inadequate carbohydrate stores have also been implicated as a potential
cause of muscle cramps. Theoretically, it makes sense that hard-working
muscles might experience cramping in association with the depletion of
its power source—carbohydrate. While all athletes should consider the
recommendations presented earlier to optimize performance, athletes
with a history of cramping during prolonged exercise should ensure that
they consume adequate carbohydrate during exercise and in the days
before and days following an endurance event.”

Nutrition truths for endurance athletes

Some practical wisdom on endurance sports nutrition from the book is “Endurance Sports Nutrition“, reprinted with permission by Human Kinetics

“You are responsible for experimenting in training (before the actual
event or race) to discover and build a repertoire of acceptable foods
and drinks, and any other supplements, that you will use to meet your
fluid, energy, and electrolyte needs during long-distance events and
races. You must figure out the basics—what and how much you need to eat
and drink and when you need to eat and drink it. Don’t neglect to put
your strategies to the test in various weather conditions at your
intended race pace or intensity.

  • The only way that drinking and eating on the move become automatic
    on the day of the event or race is by practicing beforehand. Aim to be
    consistent and stick with what you know. When your favorite or old
    standby is no longer working, however, you must be willing to try
    something new. If you’re contemplating tackling ultralength challenges,
    you first need to establish smart drinking and refueling habits in
    longdistance events and races.
  • Consider how your body processes foods during exercise. Blood flow
    to the gastrointestinal tract falls as your pace or intensity
    increases, making it harder to digest and absorb foods that you take
    in. In addition, your ability to consume and absorb calories when
    running (because of significant jostling of the stomach) is far less
    (by as much as 50 percent) than when cycling. Rely on simple
    carbohydrates during high-intensity efforts or when you need a rapid
    energy boost. Choose electrolyte replacement drinks, energy gels (take
    with water) and sport chews, glucose tablets, and  if tolerated, soda
    or juice. During longer efforts of moderate intensity, add  solid foods
    and high-calorie liquid drinks to boost your calorie intake and your
    spirits.
  • Refuel frequently instead of eating a large quantity at any one
    time, which diverts blood away from your working muscles. In other
    words, spread your hourly energy needs over 15- to 20-minute
    increments. Don’t try to cram it all down on the hour mark. The best
    sports drinks, high-calorie liquid drinks, energy gels, and energy bars
    for you are the ones that go down and stay down.
  • Hitting the wall means that you have essentially depleted your
    muscle glycogen stores. Your legs (and other major muscle groups) have
    gone on strike, even though you may have been consuming adequate fluids
    and calories. Your training, or lack thereof, improper pacing, and
    general fatigue can contribute to this phenomenon. You will often be
    able to continue and finish, albeit not with the desired performance.
  • Bonking, when the body completely shuts down because of a severe
    drop in blood sugar, is a much more serious situation. The glycogen
    stored in muscles and the liver is essentially gone. Muscles and, more
    important, the brain are not receiving sufficient fuel. If left
    untreated, you may become increasingly irritable, confused, and
    disoriented. You could find yourself sitting or lying down and could
    possibly lapse into a coma. Stop whatever activity you were engaged in
    and boost your blood sugar by consuming readily absorbable
    carbohydrates, such as sports drinks, energy gels, soda, fruit juice,
    or glucose tablets, if available. Seek or ask for medical attention if
    necessary.
  • The best way to avoid bonking is to create a calorie buffer. Liquid
    calories in the form of electrolyte replacement drinks and high-energy
    liquid products are favored because they tend to be well tolerated and
    require less effort to get down than solid foods do. Large male
    endurance athletes often have to consciously work to consume enough
    calories (for example, as much as 500 calories per hour of prolonged
    cycling as compared to 300 calories per hour for smaller female
    athletes) to stay in energy balance.
  • Athletes who struggle with sensitive stomachs and other
    gastrointestinal problems are advised to learn beforehand what sports
    drink will be served during races and organized events. They can then
    train with that product or, if they will have access to water, carry
    their own acceptable powdered sports drink in premeasured baggies and
    reconstitute it along the way.
  • The less fit you are, the fewer shortcuts you can take. Knowing
    what you can survive on and still perform well with comes with
    experience. If you are less fit or less efficient (a novice rider or
    trail runner, for example), you need to drink and eat on a regular
    schedule. Set your watch or bike computer and train yourself to drink
    every 15 to 20 minutes and refuel every 30 to 60 minutes to keep pace
    with the energy that you’re expending.”