Essentials of Nutrition for Sports

arms are exercising. At similar higher VO

2 max levels of exercise,

arm exercise uses relatively more carbohydrate than leg exercise.

At low levels of exercise intensity, most energy is supplied from

fats in the bloodstream. At highe

r levels of exercise intensity, fat

calories come from muscle stores. The absolute energy contribution from fat rises somewhat as exercise progresses from low to medium intensity, but the relative contribut

ion declines. Intramuscular fat

(triglyceride) energy at medium-intensity exercise provides less than one-third the energy of muscle glycogen. At high levels of exercise, the absolute amount of fat contribution decreases and the relative amount plummets as glycogen sources predominate.

Blood fat and blood glucose contribute to muscle energy

production even at high exercise intensity levels, but the contribution is relatively small compared with that of glycogen. The contribution may increase if glucose is ingested. It is impractical to ingest fat—its utilization takes too long and fat is

more likely to slow digestion and

cause gastrointestinal upset.

The roughly 50,000 calories of stored fat could fuel the demands

of running about 500 miles or bicyc

ling about 2,000 miles. However,

stored fat cannot be accessed or processed quickly enough to function as the major energy source of medium- or high-intensity exercise.

A maximum of about 250 calories per hour of ingested

carbohydrate may contribute to contemporaneous muscle energy production. Ingesting carbohydrate

spares muscle glycogen and

allows exercise intensity to increase or remain high longer.

As glycogen stores are used up, exercise intensity cannot be

maintained. The relative contributions of fat and protein to energy production rise.

As the physiology adage goes, “Fat burns in the flame of

carbohydrate.” When glycogen is

exhausted, the rate of fat

metabolism also decreases. With glycogen exhaustion, muscle

protein is broken down, metabolized by the liver, and returned to the muscle as blood sugar.

Training may increase the use of muscle fat and the rate of

uptake of blood fat for a given exercise intensity, but at high levels of exertion, glycogen remains the fuel of choice. Without glycogen, high-intensity exercise cannot take place.

For those regularly exercising at high intensities, increasing fat

in the diet is counterproductive—there is no point in sparing glycogen if the net result is that you have none to spare.

Riding Slowly to Burn Fat—Not!

There is a popular misconception that in order to lose weight,

that is fat, one needs to ride slowly, at a low aerobic training pace.

It is true that a greater percentage of the calories burned during

exercise at lower intensities comes from fat. However, fat calories are also burned during resting or basal metabolic activities. If your training time is limited, you will lose about as much fat by riding at a higher intensity. Further, high-intensity training stimulates the body to burn more fat after exercise is

ended, and it also gets you into

better shape.

In order for you to lose weight, your net daily caloric

expenditure must exceed your intake. To lose one pound of fat, you have to have a deficit of 3,500 calories. Therefore, in order to lose one pound a week, you have to use 500 more calories daily than you take in.

If your basal metabolic need is 1,000 calories a day, your daily

caloric deficit (say, 500 calories)—and weight loss—can be met with basal fat calories just as easily as your exercise calories.

A relevant diet question is this: Do those calories come at the

expense of glycogen or fat stores?

If you have relatively unlimited time, a day with a four-hour

low-intensity ride will burn the same number of calories as a day

Nutrition for Sports, Essentials of 44