Basics of Nutrition Physiology

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

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

• Mouth - begins the mechanical breakdown of food, adding some digestive enzymes from the saliva
• Esophagus - a transportation conduit to the stomach
• Stomach - further mechanical and enzyme breakdown; no absorption
• Small intestine - completes enzyme breakdown of food contents (carbohydrates, fats, and protein) and absorption of nutrients
• Colon - storage and dehydration of residual from processed food; absorption of some molecules produced by the bacteria in the colon (microbiome).

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.

• Solid versus liquid - liquids are emptied from the stomach more quickly than solids.

• Fat content of the food - fat slows stomach emptying and will delay the availability of any energy Calories.

• Concentration of liquids - a liquid with a simple sugar content of more than 10% will slow stomach emptying. The use of complex carbohydrates, due to the decreased osmotic effect, will offset this delay to a degree and offers one strategy to maximize Caloric intake. To be avoided are high fat and protein pre event snacks/meals and hypertonic drinks before and during the event. 40% of those drinking a hypertonic beverage had severe complaints compared with only 11% of those who had used isotonic drinks.

• Intensity of the physical activity - the rate at which material moves out of the stomach and through the small intestine slows with vigorous activity, usually > 70% VO2 max. Except in short, all out events, this is rarely has an impact, and is much less for cycling than for running where the additional factor of mechanical jostling of abdominal organs also slows digestive tract muscle activity.

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:

• avoid electrolyte imbalance (including dehydration)
• avoid riding too soon after eating
• train at a level closer to the event (the more a competition exceeds the maximum levels of training, the more likely you are to develop crampy abdominal pain).

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:

• If heartburn is a problem, timing of the ride to assure an empty stomach needs to seriously considered, and for a competitive rider a 3 to 4 hour fasting period will minimize the chances of distraction from a feeling of fullness or nausea.
• Slow gastric emptying is usually not a problem for a recreational rider, but those with an especially sensitive stomach should plan their last pre ride meal at least 3 to 4 hours before the ride. Then eat small, frequent snacks while on the bike for rides of greater than 2 hours, and avoiding hypertonic sports drinks.
• Stay hydrated. If you are dehydrated, the stomach will empty more slowly and there will be an accentuation of the decrease in blood flow to the small intestine.
• Although some racers will eat a low residue diet for several days before an event to minimize cramps and the "call to stool", this greatly complicates diet planning, and for most of us, slowing the pace will usually decrease the urge until a bathroom is located.

• Carbohydrate loading

• The insulin surge and potential for subsequent hypoglycemia is theoretically possible possibility if sugary drinks are taken to close to a competitive event. It has be seen in sedentary individuals eating sweets, but this type of rebound hypoglycemia is rarely a problem for athletes. But choosing to err on the side of caution, most coaches recommend avoiding all simple carbohydrates for several hours before an event, beginning carbohydrate supplements only in the few minutes immediately preceding the start of the activity.

• Even though there is general consensus that simple carbohydrates should be avoided in the hour or two immediately preceding a ride, there is almost unanimous support for the benefits of a pre ride meal of complex carbohydrates 3 or 4 hours before an event. These carbohydrates not only "top off" muscle and liver glycogen stores, the slow digestion and absorption of the complex carbohydrates can provide an ongoing glucose supplement from the intestinal tract even after the ride has started. Recent studies show no advantage to commercial energy bars or a high fat meal when compared to a more traditional and less expensive complex carbohydrate such as oatmeal.

• Breakfast does make a difference. Those of us who are breakfast eaters know that if we skimp (maybe trying to shed a few pounds?) we generally don't do well on a morning ride. But how about an effect later in the day, on an afternoon spin for example? This tantalizing article from Medscape.com suggests that an afternoon ride is compromised by skipping breakfast even after eating a slightly bigger than normal lunch to compensate.

  Was it placebo effect? Perhaps the riders who skipped breakfast just expected to ride more poorly. It should not be related to muscle glycogen deficiency as these riders were on a normal diet up until the day of the study. A third possibility is suggested in the article: "Average heart rate and total fat oxidation during the cycling test was greater after skipping breakfast compared to after eating breakfast" which raises the possibility that metabolic changes that limited glucose metabolism in favor of fat metabolism may have been triggered by this one meal fast. This is not as unreasonable as it might seem. There have been a number of articles (here is a recent one) documenting a correlation between skipping breakfast and diabetes, so underlying metabolic factors might be in play.

  The take away? Balance your meals and caloric intake throughout the day. Don't skip any of your meals. You might think about shifting calories from supper to midday however. It has been shown that decreasing the size of an evening meal (total calories eaten) helps in losing weight. This is also thought to be a metabolic change that favors conserving eaten Calories as fat when they are eaten after your activities for the day are finished and hey are not immediately needed for muscular activity.

• Maximizing liquid carbohydrate replacement while riding is an important strategy for events lasting more than 2 hours.

  As sugar concentration increases, the risk of nausea and bloating rises as well. Almost everyone can tolerate a 7 to 10% concentration of glucose, but many cyclists will tolerate solutions of up to 15% to 20%. And the use of polymers will allow more carbohydrates to be ingested and absorbed while limiting to some degree the overall concentration of the solution. Although the sugar concentration has an effect on the rate of stomach emptying, the volume of fluid in the stomach plays a role as well. Keeping the stomach filled by frequent drinks will enhance the rate of gastric emptying.

  In extreme events such as the Tour de France, it has been speculated that as much as 50% of a rider's daily energy expenditures can be replaced while on the bike, although experimental data indicating a maximal absorption rate of 70 grams of glucose per hour speaks against this idea.

  Studies have supported that the upper limit to carbohydrate absorption can be pushed to 1.2 grams per minute of a fructose/glucose drink. Glucose alone was absorbed and metabolized at a slower rate than the fructose/glucose combination so the composition of the mixture was key to maximizing oral supplementation.

• Take advantage of the " glycogen window" that is open in the 4 hours immediately following vigorous exercise. During this interval, ingested carbohydrate will be converted into muscle glycogen at over 3 times the normal rate (and "the earlier the better" as data suggests a 50% fall in the conversion rate by 2 hours and a complete return to normal repletion rate by 4 hours). Muscle glycogen repletion (after a 2 plus hour ride) usually proceeds at a rate of 5% per hour, and although it may require up to 48 hours for complete muscle glycogen replacement, most is accomplished during the first 24 hours post event. The athlete who is training daily, or is in a multi day event, can use this glycogen window to their advantage to get a jump on the normal repletion process and minimize the chance of chronic glycogen depletion (and the fatigue that goes along with it). There is also suggestive evidence that the muscle stiffness that occurs after vigorous exercise is related to muscle glycogen depletion, so rapid repletion may have an added benefit of minimizing this day after effect. One caution though - many simple carbohydrate snacks such as chocolate chip cookies are more than 30% fat and if eaten in large quantities might exceed your planned daily fat intake of 20-30% of Calories. In contrast, complex carbohydrate foods such as pasta, bread, and rice offer significantly more carbohydrate per gram or ounce. And there are even special "recovery drinks" available.