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  Latest update: 12/17/2021

Basics of Nutrition Physiology

Digestion, absorption, and metabolism

All physical activity requires energy. And that energy is provided by the food we eat. Although we often view the bakery stop after a ride as just a pleasant reward, smart eating is essential to enjoying our riding and optimal performance.

The Building Blocks of all Foods - Carbohydrates, Fats, and Protein

All foods are composed of three nutritional building blocks - carbohydrates, fats, and protein - plus water and indigestible fiber. Carbohydrates contain 4.1 Calories per gram and are the primary energy source for athletes involved in short, maximum performance events. Fats play a bigger role as an energy source for slower, endurance events. Protein, is used to maintain and repair cells and only rarely as an energy source in certain unique situations such as malnutrition.

The energy content of a food is, by convention, expressed in Calories (note the capital "C") not calories (small "c") or kilojoules (kj) which is the way energy is quantified in the scientific literature. One nutritional Calorie is equivalent to 1000 calories (a kilocalorie) or 4.18 kilojoules.

Foods vary in their energy content per ounce (or gram) depending on their composition. Fiber provides bulk and weight but little Caloric value. Pure carbohydrates and protein each contain a little more than 4 Calories of energy per gram while a gram of fat has more than double the energy value at 9 Calories per gram.

Carbohydrate Calories supply the majority of the energy for muscles during vigorous activity. Fats are important for less strenuous, endurance type activities. Proteins is, in general, not used as energy for muscle activity.

Glucose, the carbohydrate used by all cells, is provided by either 1) the food you are eating or 2) glycogen, the storage form of carbohydrate in muscle and liver cells. If you have been eating a normal diet, you will have enough stored glycogen to support 2 hours of intense exercise before the reserves are depleted and the bonk occurs. You can extend these internal stores if you eat or drink carbohydrate containing foods while exercising. Using carbohydrate goos or drinks for any exercise expected to last more than 2 hours is a smart strategy to improve your endurance.

In the well fed and rested state, it has been estimated that the human body contains approximately 1500 carbohydrate Calories (stored in the form of glycogen) in the liver and muscle tissue, and over 100,000 Calories of energy stored as fat. The muscles are the main glycogen repository containing anywhere from 300 - 600 grams of glycogen (which equates to 1200 to 2400 Calories) while the liver contains 80 to 110 grams (300 to 400 Calories). The exact numbers are less important than the concept that internal carbohydrate stores are only adequate for several hours of brisk cycling.

In order to avoid the "bonk" (reflecting a depletion of glycogen and a shift to fat metabolism with a deterioration in performance), supplemental carbohydrates need to be used during the early stages of rides that will be more than longer than 1 to 2 hours in length to supplement (and thus spare) the body's internal glycogen stores. It's best to start eating early as food carbohydrates are much less effective when the athlete is trying to catch up after glycogen stores have been depleted.

The bonk in cycling (hitting the "wall" in running) occurs when the body's internal carbohydrate stores are depleted and a shift towards fat metabolism as the prime energy source for the exercising muscle (with maximum energy output generally limited to approximately 50% VO2 max.) occurs. It would be logical to assume that if adequate carbohydrates (to offset those expended) were replaced during a ride, the cyclist could maintain his or her pace indefinitely. Unfortunately this is not the case. Cyclists with low muscle glycogen stores but high blood glucose levels still experience fatigue, even though the time to onset can be delayed by taking carbohydrate supplements. Unknown factors, perhaps related to physical changes in the muscle cell itself, are thought to be responsible for this second wave of fatigue as it is more common in the untrained athlete.(see also Overtraining)

In addition to extending the time to fatigue for longer, moderate activity events, several studies have suggested that it is also possible to improve performance in a 1 hour, high intensity (time trial, ~80% VO2max) event with an oral carbohydrate supplement. In those studies, drinking a liter of a 7% carbohydrate solution at the beginning and during the event improved times by 2%.

All carbohydrate must be converted to glucose before they can be metabolized by a muscle cell. Complex carbohydrates (combinations of single sugar molecules) are broken down into single sugar molecules in the intestinal tract before being absorbed. Then any non glucose monosaccharide must be converted into glucose by the liver before it can be used by the muscle cells. Aside from taste, studies have not revealed any additional metabolic benefit from using glucose polymers, fructose, or sucrose (common table sugar, a dimer of glucose and fructose) as a source of carbohydrate energy. However, some of these alternative carbohydrates can cause digestive tract side effects such as bloating and diarrhea.

Although carbohydrates are required for high intensity performance, fat can be an additional energy source at lower levels of exertion. In fact, the "second wind" that occurs during exercise may reflect that shift - internal carbohydrate stores have been used, fatigue sets in, the body shifts towards fat metabolism, and a feeling of a renewed energy returns). But there is a trade off. Fats can not sustain the exercise intensity that is possible with carbohydrate as the energy supply for the muscle. However there is enough stored fat to continue to cycle at a reduced speed (50 - 60% VO2@max) for days.

Fats provide more than half of the Calories for moderate exercise (< 50% VO2 max.), even when adequate glucose (glycogen) is available. As the level of exertion increases, the proportion of the total energy supplied by fat metabolism diminishes. And in maximum performance events, when metabolism becomes anaerobic (> 100% VO2 max.), fat metabolism ceases to provide any muscle energy. Although there has been speculation that using fats in a dietary program both during training and as supplements during competitive events might improve the use of fat as a fuel and thus athletic performance, there is limited data that a high fat/low carbohydrate training diet changes the fuel ratio for high VO2 max performance. But for long distance endurance events at ~ 50 % VO2max, fat can be used effectively. Will training on a low fat diet increase the efficiency of fat as a fuel for this subgroup? The jury is still out.

Protein the third building block of food, is needed to repair cell injuries - including the micro trauma that occurs with exercise. It is NOT used by the body as an energy source except in malnourished states where fat and carbohydrates are unavailable. Even in endurance activities such as the Tour De France, basic protein needs of 1.5 gms protein/kg body wt/day were easily met by a normal (unsupplemented) diet that covered the total Calories expended. A review of the literature failed to demonstrate any advantage with protein supplements (assuming an adequate daily protein intake) over pure carbohydrate supplements alone. And one study actually demonstrated a actual decrease in overall performance from the appetite suppressing effects of a high protein diet, subsequent decreased carbohydrate intake, and the resulting diminished pre event muscle glycogen stores.

The Digestive Tract

Before we go further, let's take a minute to review the role of the various parts of your digestive tract, that is the organs that food and liquids travel through between the time they enter the mouth and are either absorbed or leave the body as feces. These include the mouth, pharynx (throat), esophagus, stomach, small intestine, large intestine, rectum, and anus.

(More detail on the digestive tract.

When designing a nutritional program to supplement the body's energy stores for athletic events, the rate of digestion and absorption of foods must be taken into account. The time needed for the stomach to start the digestive process, empty its contents into the small intestine, and initiate absorption into the bloodstream directly affects how quickly any food will be available to provide supplemental energy for exercise. These four factors will have an impact.

Taking these into account, it is easy to see why the optimal food for a rapid, high energy boost during a ride would be a semi-liquid or liquid carbohydrate with minimal if any fat. On the other hand, an endurance athlete, competing at a lower VO2 max., might prefer a snack with some fat added to improve taste and complex rather than simple carbohydrates which are more slowly digested and would spread absorption over a longer period of time.

Carbonation does not seem to affect the emptying rate of the stomach. Three independent studies found no difference in the gastric emptying rates of water, carbonated water, and carbonated carbohydrate drinks. Carbonated colas, which contain 160 Calories per 12 ounce can and the caffeine equivalent of half a cup of coffee, remain a favorite drink of many cyclists.

Effects of exercise on digestion

It's not uncommon for competitive athletes to develop gastrointestinal (GI) symptoms during training and competition - generally cramps, diarrhea, and nausea (although constipation has rarely been reported). Cramps and diarrhea reflect over activity of the lower intestinal tract (colon) and are much more common in runners (and triathletes) than in cyclists. It has been speculated that the mechanical jostling effect on the abdominal organs explains why runners have more symptoms than cyclists or swimmers. A recent survey of triathletes participating in a half iron man event revealed that 50 % complained of belching and flatulence (gas), and more symptoms occurred while running than at other times.

Increased small and large bowel activity is the reason for abdominal cramps as well as an increase in bowel movements (including diarrhea). It is likely the result of mechanical jostling the bowel. But a fiber rich, pre race meal might also play a role. In a recent post race survey, almost all the triathletes who had eaten a high fiber meal suffered from cramps. To minimize the risk of cramps:

Blood flow to the digestive system also decreases during vigorous exercise - an 80% reduction after 1 hour cycling at 70% VO2max. And appears to correlate directly with symptoms - individuals with the most severe symptoms having the greatest decrease in blood flow to digestive organs.

Changes in GI hormone levels have been documented in vigorous exercise, but a cause and effect relationship with symptoms has not been proven.

Stress factors are probably more important as a cause of pre competition symptoms such as nausea, vomiting, and diarrhea (which in one study were present in 57% of the participants).

Heartburn (esophageal reflux) is more frequent with exercising within 2 hours of eating. This is the result of a combination of factors - a full stomach (especially one with some fat content) diminishes lower esophageal sphincter pressure (this muscular sphincter separates the esophagus and stomach, preventing the reflux of stomach contents into the esophagus), an increased volume of food and acid in the stomach means more material available to reflux. This is generally a minor problem for cyclists and is best handled by delaying exercise for a few hours after eating or using an antacid or one of the over the counter acid reducing medications such as Tagamet or Zantac.

Exercise slows stomach emptying with more vigorous exercise leading to a greater delay. Running has a greater effect than cycling, presumably because of the mechanical jostling of the stomach. In addition to aggravating esophageal reflux (noted above), a delay in stomach emptying can cause a sensation of fullness and nausea as well as limit the immediate availability of any ingested Calories (as will be discussed shortly).

Except for competitive cyclists (and runners) the effects of exercise on the GI tract are minimal. To minimize problems, keep these in mind:

Miscellaneous Absorption and Metabolic considerations

A summary of the important "basics" of nutritional physiology to help you understand the rationale behind an optimal training and performance diet. And key elements in developing your own nutritional plan to maximize your training and performance.

All questions and suggestions are appreciated and will be answered.

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