Nutrition for Athletes

Exercise Physiology

Muscle cells contain enough preformed ATP to power several minutes of exercise. For sustained physical activity, additional ATP in synthesized in the cell's mitochondria in a series of chemical reactions. These oxidation reactions require glucose or fat and oxygen. (2)

Oxygen is delivered to the cells by the cardiovascular system (lungs, heart, blood vessels and capillaries). Maximal oxygen consumption or VO2max , is a measure of the maximum volume of oxygen the cardiovascular system can deliver to the muscles during intense exercise. Lesser levels of exertion are often defined in terms of a percent of VO2max (%VO2max). Training, especially interval training, improves an athlete's VO2max and the amount of oxygen potentially available at maximal exercise levels to manufacture new ATP.

Except for sprints, where performance is limited by oxygen availability, most exercise is done at 60 - 80% VO2max, a level that provides adequate oxygen to the exercising muscles. At this intensity, it is the availability of glucose (a carbohydrate) and fat that limits duration. The muscle cell mitochondria uses both fat and carbohydrates for ATP production. Below 50% VO2max, fat provides the majority of the needed energy. At 50% VO2max, fat and glucose each provide equal amounts of the needed energy. But then, as the level of exertion increases towards VO2max, the ratio shifts significantly until carbohydrates are providing over 90% of the necessary calories at VO2max.

Digestive and Nutritional Physiology (3)

Before a food's energy can be released in the mitochondria, it must first be digested into smaller pieces and absorbed in the small intestine. Four factors slow the rate at at which the stomach empties digested foods into the small intestine and how quickly the energy in the food we eat will be available in the muscle cell.

1. Solid versus liquid - liquids are emptied from the stomach more quickly than solids.
2. Fat content of the food - a high fat content will slow stomach emptying
3. Concentration of liquids - a drink with a sugar content of greater than 10% slows stomach emptying.
4. Intensity of exertion - stomach emptying slows with increasingly vigorous activity above 70% VO2 max.

The only carbohydrate a cell can metabolize to produce ATP is the single molecule glucose. Both complex carbohydrates (starches), composed of multiple glucose and fructose molecules, and two molecule sugars such as sucrose (a single glucose molecule chemically bonded to a single fructose) must be broken into their individual single sugar molecules before the glucose and fructose can be absorbed. Glucose is transported directly to the cells while fructose is first converted into glucose in the liver. (4)

Any glucose (and fat) not used immediately in the cell is stored as the complex carbohydrate glycogen and in fat cells until needed. The body has approximately 1500 carbohydrate calories stored as glycogen and over 100,000 calories as fat. This is enough glycogen for several hours of brisk cycling or running (80 to 100 %VO2max) where cell energy is mainly carbohydrate based, and even longer at reduced levels of exertion where fat calories are used as well. But when this stored glycogen is depleted, the cells are forced to switch to less efficient fat metabolism. The athlete has to slow down and has bonked.

All cells, with the exception of brain tissue, need insulin (from the pancreas) to extract glucose from the blood. The rise in blood glucose after a meal or snack triggers the release of insulin, and higher blood glucose spikes release more insulin. With a small snack or sugary drink, the glucose lowering effect of insulin can continue after all the snack's glucose calories have been absorbed. Blood glucose levels then drop and you notice you are hungry. If the drop continues, you get the weakness and fatigue of hypoglycemia.

Exercising activates an alternate pathway for glucose uptake in muscle cells. This alternate pathway blunts the insulin spike (and risk of rebound hypoglycemia) from sugary drinks during exercise.

Insulin spikes also stimulate production of triglycerides. These increase the risk of heart attacks and strokes and explain the current recommendation to limit simple sugars to exercise periods and eat more slowly digested and absorbed complex carbohydrates to meet the remainder of the daily carbohydrate requirements. For non exercisers on a 2000 calorie diet, this translates into 200 calories 12 teaspoons of table sugar - a particularly difficult goal when you realize that one 12-ounce can of soda contains the equivalent of 9 teaspoons of sugar, almost all of a day's allowance.

Important points for an athlete's diet.

For sprint activities, at or near VO2max, muscle cells are dependent on stored glycogen. Performance is limited by the availability of oxygen. For workouts of less than an hour, interval training is more important in improving performance than what you eat while exercising.

For endurance events (70 - 80% VO2max), oxygen is no longer the limiting factor, it's ATP derived from carbohydrates and fats. If the workout is less than an hour, the cells can draw on 1500 calories of glycogen but for longer, multi-hour sessions, these need to be supplemented by glucose from the digestion/absorption of snacks and drinks.

The following suggestions are for those planning specific to longer walks or bicycle rides (more than an hour).

1. Slow down a bit. At lower %VO2max, more exercise calories are derived from fat and less from glycogen. This extends the time until glycogen reserves will be depleted. And you have plenty of fat calorie reserves.
2. Start from a balanced training diet with adequate complex carbohydrates. This will maximize internal (liver and muscle) glycogen stores. Excessive carbohydrates, carbohydrate loading, does not push extra glycogen into the muscle cells.
3. Don't skip breakfast, which should be high in complex carbohydrates and, if possible, eaten 3 hours before the event to avoid reactive hypoglycemia. This will "top off" glycogen stores.
4. Snacks and drinks high in simple sugars (not complex carbohydrates) will decrease the need to use stored glycogen calories, extending the time until they are depleted.
   • Avoid these high sugar snacks in the immediate 15 to 20 minutes before starting exercise to avoid rebound hypoglycemia just as you start your exercise.
   • Begin supplements early in a ride, even in the first 10 or 15 minutes. Don't wait until your get hungry which means internal glycogen is no longer available and oral sugar alone will not cover the deficit.
5. Liquid sports drinks are preferred as they empty from the stomach more quickly. Avoid concentrated drinks (more than 10%) as they slow stomach emptying and absorption of the calories. Soda drinks are the ideal concentration and offer an easy on the bike supplement.
6. If this is an ultra-endurance event, generally at a low VO2 max, a little fat in the snack will improve taste. And you can add complex rather than simple carbohydrates which will be more slowly digested and spread glucose absorption over a longer period of time.
7. Take advantage of a post exercise glycogen replacement window, the 1 - 2 hours after exercise when absorbed sugar is preferentially shunted towards glycogen storage. This is especially important on multi-day rides to avoid gradual glycogen depletion.

The following are my "Rules of 4" focused on the timing of nutrition for multi-day and longer bicycle rides. They also apply to any long, endurance event such as an ultramarathon.(7)

• Four days prior to the ride. Maintain a diet with enough carbohydrate Calories to cover those used on your rides. You should be tapering your training, so 9 gm carbohydrate/kg BW/day (600 grams/day) should be more than adequate.

• Four hours prior to the event. This will be topping off the tank, so to speak. Carbohydrate calories will have been digested, absorbed, and to a great degree metabolized into glycogen stores before you get on the bike. How big a meal? A 300 gm (1000 Calorie) complex carbohydrate meal (rice, pancakes) will minimize rapid absorption with an insulin spike. A high Caloric density glucose polymer sports drink is another option.

• Four minutes before the ride. You are just getting on the bike and want to have some Calories starting their way through the digestive system. But not enough to just sit in your stomach, You are not worried about simple sugars leading to an insulin spike and hypoglycemia as you get on the bike. A candy bar works well.

• During the ride. The focus should be on regular carbohydrate replacement starting immediately as you begin the ride. Aim for 60 gram of carbohydrate per hour, liquid preferred (i.e. sports drink), 10% concentration optimal (equivalent to a cola drink), with a drink every 10 - 15 minutes to minimize fullness. Four hours post ride. Take advantage of this interval for restocking of muscle glycogen. Start immediately, high carbohydrate, for 3 to 4 hours.

1. Exercise physiology - the skeletal muscle. https://www.cptips.com/muscle.htm
2. Exercise physiology - cell energy metabolism. https://www.cptips.com/bscphys.htm
3. Basics of nutrition physiology - digestion, absorption, and metabolism. https://www.cptips.com/nutrtn.htm
4. Building blocks of all foods - carbohydrates. https://www.cptips.com/cmplxcb.htm
5. Summary - basics of nutrition. Implications for performance. https://www.cptips.com/nutrtns.htm
6. Nutrition for training and performance - develop your own plan. https://www.cptips.com/bsnutpl.htm
7. Nutrition for endurance rides. https://www.cptips.com/nutend.htm

Content based on :

• Cycling Performance Tips (https://www.cptips.com/index.htm)
• Not Just another Diet Book! (https://sciencenutritionhealth.com/)