sports nutrition
Carbohydrate intake during exercise
Submitted by admin on Tue, 02/02/2010 - 20:06 "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."
"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
Submitted by admin on Thu, 09/17/2009 - 01:00 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."
"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
Submitted by admin on Mon, 08/17/2009 - 20:22 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."
