Category Archives: Nutrition

The Body’s Fuel Sources

Our ability to run, bicycle, ski, swim, and row hinges on the capacity of the body to extract energy from ingested food. As potential fuel sources, the carbohydrate, fat, and protein in the foods that you eat follow different metabolic paths in the body, but they all ultimately yield water, carbon dioxide, and a chemical energy called adenosine triphosphate (ATP). Think of ATP molecules as high-energy compounds or batteries that store energy. Anytime you need energy—to breathe, to tie your shoes, or to cycle 100 miles (160 km)—your body uses ATP molecules. ATP, in fact, is the only molecule able to provide energy to muscle fibers to power muscle contractions. Creatine phosphate (CP), like ATP, is also stored in small amounts within cells. It’s another high-energy compound that can be rapidly mobilized to help fuel short, explosive efforts. To sustain physical activity, however, cells must constantly replenish both CP and ATP.

Our daily food choices resupply the potential energy, or fuel, that the body requires to continue to function normally. This energy takes three forms: carbohydrate, fat, and protein. (See table 2.1, Estimated Energy Stores in Humans.) The body can store some of these fuels in a form that offers muscles an immediate source of energy. Carbohydrates, such as sugar and starch, for example, are readily broken down into glucose, the body’s principal energy source. Glucose can be used immediately as fuel, or can be sent to the liver and muscles and stored as glycogen. During exercise, muscle glycogen is converted back into glucose, which only the muscle fibers can use as fuel. The liver converts its glycogen back into glucose, too; however, it’s released directly into the bloodstream to maintain your blood sugar (blood glucose) level. During exercise, your muscles pick up some of this glucose and use it in addition to their own private glycogen stores. Blood glucose also serves as the most significant source of energy for the brain, both at rest and during exercise. The body constantly uses and replenishes its glycogen stores. The carbohydrate content of your diet and the type and amount of training that you undertake influence the size of your glycogen stores.

The capacity of your body to store muscle and liver glycogen, however, is limited to approximately 1,800 to 2,000 calories worth of energy, or enough fuel for 90 to 120 minutes of continuous, vigorous activity. If you’ve ever hit the wall while exercising, you know what muscle glycogen depletion feels like. As we exercise, our muscle glycogen reserves continually decease, and blood glucose plays an increasingly greater role in meeting the body’s energy demands. To keep up with this greatly elevated demand for glucose, liver glycogen stores become rapidly depleted. When the liver is out of glycogen, you’ll “bonk” as your blood glucose level dips too low, and the resulting hypoglycemia (low blood sugar) will further slow you down. Foods that you eat or drink during exercise that supply carbohydrate can help delay the depletion of muscle glycogen and prevent hypoglycemia.

Fat is the body’s most concentrated source of energy, providing more than twice as much potential energy as carbohydrate or protein (9 calories per gram versus 4 calories each per gram). During exercise, stored fat in the body (in the form of triglycerides in adipose or fat tissue) is broken down into fatty acids. These fatty acids are transported through the blood to muscles for fuel. This process occurs relatively slowly as compared with the mobilization of carbohydrate for fuel. Fat is also stored within muscle fibers, where it can be more easily accessed during exercise. Unlike your glycogen stores, which are limited, body fat is a virtually unlimited source of energy for athletes. Even those who are lean and mean have enough fat stored in muscle fibers and fat cells to supply up to 100,000 calories—enough for over 100 hours of marathon running!

Fat is a more efficient fuel per unit of weight than carbohydrate. Carbohydrate must be stored along with water. Our weight would double if we stored the same amount of energy as glycogen (plus the water that glycogen holds) that we store as body fat. Most of us have sufficient energy stores of fat (adipose tissue or body fat), plus the body readily converts and stores excess calories from any source (fat, carbohydrate, or protein) as body fat. In order for fat to fuel exercise, however, sufficient oxygen must be simultaneously consumed. The second part of this chapter briefly explains how pace or intensity, as well as the length of time that you exercise, affects the body’s ability to use fat as fuel.

As for protein, our bodies don’t maintain official reserves for use as fuel. Rather, protein is used to build, maintain, and repair body tissues, as well as to synthesize important enzymes and hormones. Under ordinary circumstances, protein meets only 5 percent of the body’s energy needs. In some situations, however, such as when we eat too few calories daily or not enough carbohydrate, as well as during latter stages of endurance exercise, when glycogen reserves are depleted, skeletal muscle is broken down and used as fuel. This sacrifice is necessary to access certain amino acids (the building blocks of protein) that can be converted into glucose. Remember, your brain also needs a constant, steady supply of glucose to function optimally.

Fuel Metabolism and Endurance Exercise

Carbohydrate, protein, and fat each play distinct roles in fueling exercise.

Carbohydrate

  • Provides a highly efficient source of fuel—Because the body requires less oxygen to burn carbohydrate as compared to protein or fat, carbohydrate is considered the body’s most efficient fuel source. Carbohydrate is increasingly vital during high-intensity exercise when the body cannot process enough oxygen to meet its needs.
  • Keeps the brain and nervous system functioning—When blood glucose runs low, you become irritable, disoriented, and lethargic, and you may be incapable of concentrating or performing even simple tasks.
  • Aids the metabolism of fat—To burn fat effectively, your body must break down a certain amount of carbohydrate. Because carbohydrate stores are limited compared to the body’s fat reserves, consuming a diet inadequate in carbohydrate essentially limits fat metabolism.
  • Preserves lean protein (muscle) mass—Consuming adequate carbohydrate spares the body from using protein (from muscles, internal organs, or one’s diet) as an energy source. Dietary protein is much better utilized to build, maintain, and repair body tissues, as well as to synthesize hormones, enzymes, and neurotransmitters.

Fat

  • Provides a concentrated source of energy—Fat provides more than twice the potential energy that protein and carbohydrate do (9 calories per gram of fat versus 4 calories per gram of carbohydrate or protein).
  • Helps fuel low- to moderate-intensity activity—At rest and during exercise performed at or below 65 percent of aerobic capacity, fat contributes 50 percent or more of the fuel that muscles need.
  • Aids endurance by sparing glycogen reserves—Generally, as the duration or time spent exercising increases, intensity decreases (and more oxygen is available to cells), and fat is the more important fuel source. Stored carbohydrate (muscle and liver glycogen) are subsequently used at a slower rate, thereby delaying the onset of fatigue and prolonging the activity.

Protein

  • Provides energy in late stages of prolonged exercise—When muscle glycogen stores fall, as commonly occurs in the latter stages of endurance activities, the body breaks down amino acids found in skeletal muscle protein into glucose to supply up to 15 percent of the energy needed.
  • Provides energy when daily diet is inadequate in total calories or carbohydrate—In this situation, the body is forced to rely on protein to meet its energy needs, leading to the breakdown of lean muscle mass.

This excerpt is from the book, Endurance Sports Nutrition-3rd Edition. It’s published with permission of Human Kinetics. Please purchase this book from Human Kinetics.

Cycling – Trends in Tour Races

In this excerpt, we learn about “Trends in Tour Races“, reprinted with permission of Human Kinetics.

“Since the beginning of the 20th century, three-week tour races have been extremely demanding. Compared to the old days, the current trend is toward shorter, more intense daily stages. In the years to come, it is expected that the average amount of time a cyclist spends in zone 3 per stage will be more than 30 min.

Physiological Demands of the Different Phases of Tour Races
In general, three-week tour races have three main competition requirements: flat and long parcours (usually ridden at high speeds inside a large group of riders), individual High Tech Cycling - Science of riding fastertime trials (40 to 60 km over level terrain), and uphill cycling (high mountain passes).

Every tour race includes seven or more flat stages of about 200 km, lasting four to five

In this excerpt, we learn about “Trends in Tour Races“, reprinted with permission of Human Kinetics.

“Since the beginning of the 20th century, three-week tour races have been extremely demanding. Compared to the old days, the current trend is toward shorter, more intense daily stages. In the years to come, it is expected that the average amount of time a cyclist spends in zone 3 per stage will be more than 30 min.

Physiological Demands of the Different Phases of Tour Races
In general, three-week tour races have three main competition requirements: flat and long parcours (usually ridden at high speeds inside a large group of riders), individual time trials (40 to 60 km over level terrain), and uphill cycling (high mountain passes).

Every tour race includes seven or more flat stages of about 200 km, lasting four to five hours. Most of the time, cyclists ride in large groups of 150 to 200 cyclists. This considerably reduces the major force—air resistance—to be overcome in this type of terrain. As a result, the energy requirement of cycling can be decreased by as much as 40% (McCole et al. 1990), making the overall exercise intensity low to moderate. The proportion of the total stage time spent in zone 3 barely reaches 5% (Lucia, Hoyos et al. 1999).

A great mastery of technical skills (such as drafting or the ability to avoid crashes) would seem most important in this type of stage, in which most riders are able to finish within the same time. In fact, these stages usually do not determine the final outcome of a tour race.

The high average speeds (approximately 45 kph) at which riders are able to cover these stages require that they push high gears (53 X 12 to 11) during long periods. This inevitably results in some muscle damage. Previous research has reported increased levels of muscle damage markers during cycling tour races (Mena, Maynar, and Campillo 1996). This phenomenon may have a negative impact on performance during the second part of a three-week race, during which accumulated muscle fatigue may considerably limit performance in the phases of competition that determine the winner—the time trials and high mountain passes.

Tour races typically include three time trials (TT) performed over overall flat terrains: a short, opening TT of 5 to 10 km and two long TT of 40 to 60 km. This phase of the competition usually influences the final outcome of the race.

Air resistance is the main force that the cyclist encounters during TT. Thus, aerodynamic factors (the cyclist’s riding posture, the size of the frontal wheels, etc.) play a major role (Lucia, Hoyos, and Chicharro 2000a).

Those who seek top performance (average velocity of 50 kph) must tolerate high constant workloads, mostly in zone 3, during the entire 60 min of the TT (Lucia, Hoyos et al. 1999). Some authors have estimated that the mean absolute power output sustained during long TT averages 350 W, although TT specialists probably generate much higher power outputs (greater than 400 W) (Padilla et al. 2000).

Some mass-start stages of approximately 200 km (the so-called high mountain stages) include three to five mountain passes of 5 to 10% mean gradient, and thus require cycling uphill during several 30- to 60-min periods over a total time of five to six hours.

When climbing at low speeds (about 20 kph), the cyclist must mainly overcome the force of gravity (Swain 1994). Because of its effects on gravity-induced resistance, body mass has a major influence on climbing performance. A high power-output-to-body-mass ratio at maximal or near-maximal intensities (6 or more W/kg) is necessary for professional road riders (Lucia, Hoyos, and Chicharro 2000a; Padilla et al. 1999).

In addition, rolling resistance resulting from the interaction between the bicycle tires and the road surface increases considerably at lower riding speeds and on the rough road surfaces of most mountain routes (Lucia, Hoyos, and Chicharro 2000a). To overcome these forces, cyclists frequently switch from the conventional sitting position to a less economic standing posture to exert more force on the pedals. Climbing specialists perform high mountain ascents at intensities in zones 2 and 3 (Fernández-García et al. 2000; Lucia, Hoyos et al. 1999). Because of team requirements, however, some riders are not required to perform maximally during high mountain stages.”