CPTIPS Nutritional Program

Nutritional Physiology

NUTRITION FOR TRAINING AND PERFORMANCE (more detailed discussion)

Carbohydrates contain 4.1 Calories per gram (120 Calories per ounce).

• Carbohydrates are stored as glycogen in the liver and muscle cells - about 1500 Calories worth (2 hours of exercise)
• For dietary carbs, there are simple 6 carbon sugars such as glucose and complex (multiple glucose units). Complex carbs are digested and enter the circulatory system more slowly.
• Caloric value of CHO depends on whether one is aerobic or anaerobic i.e. is their enough oxygen being delivered to the cell. This is where the concept of VO2 max comes in. Above 100%VO2 max, you no longer have enough oxygen to support efficient use of food energy. For example, carbohydrate metabolism will produce 19 times as many units of ATP per gram when metabolized in the presence of adequate cell oxygen supplies (aerobic) as opposed to its metabolism in an oxygen deficient (anaerobic) environment.

• The disadvantage of fat as a fuel for exercise is the fact that fat is metabolized through pathways that differ from carbohydrates and alone can support an exercise level approximately 50% VO2 max. at most. As performance levels increase, the percentage of Calories provided by fat decreases.
• Even the leanest athlete has plenty of stored fat available (approximately 100,000 Calories worth in a 70 kg male)
• They play only a minor role in short distance, maximum performance events at 90 to 100% VO2 max.
• Fat Calories are fine for lower work intensity which makes fat the ideal fuel for endurance events. They are a particularly important energy source for the endurance cyclist.

The Bonk

When the body's 1500 stored carbohydrate Calories are gone, only fats are left (plus carbs you eat) At that point, the exercise level drops to 50% VO2 max. at most where fat can provide all the needed Calories. One can avoid the bonk by:

• supplementing stored carbohydrates with eaten carbohydrates
• exercising at lower levels of exertion (lower VO2) where less CHO is being used to provide Calories for exercise. Thus slowing the pace increases the % of energy derived from fat and can extend carbohydrate reserves.

DIGESTIVE TRACT PHYSIOLOGY

Before we go any further, let's take a minute to discuss the role of the various parts of your digestive tract.

• Mouth - important to begin the mechanical breakdown of food and add some digestive enzymes in saliva
• Esophagus - transportation to the stomach
• Stomach - further mechanical and enzyme breakdown; no absorption
• Small intestine - completes enzyme breakdown and absorption into the blood with transport to the muscle cell
• Colon - storage and dehydration of already processed food; no absorption

When designing a nutritional program to supplement the body's energy stores for an athletic event, 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 have the food components absorbed into the bloodstream

Emptying of the stomach into the small intestine is the rate limiting step in getting Calories into the blood stream and to the working muscles. Once food molecules get into the small intestine, they are absorbed quite quickly. You have control over four major factors that can delay stomach emptying.

• Solid versus liquid - liquids are emptied from the stomach more quickly than solids.
• Fat content of the food - fat slows the digestive process and delays the availability of any Calories in the food to the muscles.
• Sugar concentration - especially in liquids, a sugar content of more than 10% will slow stomach emptying. ( The use of complex carbohydrates, due to the decreased osmotic effect, will offset this to some degree and offers an alternative strategy to maximize Caloric intake to offset the metabolic needs of exercise.)
• Physical activity level of the cyclist - the mechanical activity of digestion is slowed by any vigorous activity, usually starting at 70% VO2 max. Except in short, all out events, this is rarely an issue, and it is much less so for cycling than for running where the additional component of mechanical stimulation of abdominal contents from the sport itself slows digestive tract functioning.

From the above four points, it is easy to see that 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 complex carbohydrate with some fat added to improve taste (and generally in a solid form), in order to slow emptying from the stomach and even out absorption over a longer period of time.