What is the main source of energy for low and moderate-intensity physical activity?

By Arnie Baker, M.D.

Energy for exercising muscle comes from carbohydrate, fat, and protein.

Carbohydrate may come from blood sugar (from the liver by way of stored glycogen or metabolized amino acids or from the intestine by the absorption of carbohydrate) or from glycogen stores in muscle. Typically, athletes with normal stores have about 2,000 carbohydrate calories stored as glycogen: about 1,500 calories of intramuscular glycogen are stored in muscle cells, and about 500 calories in the liver.

Fat energy may come from the bloodstream by way of adipose tissue or the intestine, or from fat stores in muscle. About 2,500 calories of intramuscular fat (triglyceride) energy are stored in muscle cells. Depending upon percent body fat, about 50,000 calories are stored elsewhere as fat.

Protein supplies the least amount of energy and is usually omitted from consideration. If fat and carbohydrate sources are plentiful, protein supplies about 5% of energy sources. Protein contribution to energy production increases after several hours, when intramuscular carbohydrate and fat stores are depleted. As much as 15% of energy sources may derive from protein if muscle fat and glycogen are depleted.

Figure 2 depicts the contribution of carbohydrate and fat to energy production in a cyclist when energy stores are plentiful. Low activity corresponds approximately to a heart rate of 65% of maximum, moderate activity to a heart rate of 75% of maximum, and high activity to a heart rate of 90% of maximum.

Traditionally the amount of carbohydrate energy used was believed to be higher than in Figure 2. Carbohydrate metabolism was originally calculated based on respiratory exchange ratios—determined from the relative concentrations of carbon dioxide and oxygen expired. Newer techniques have suggested that fat contribution is greater than was previously determined.

The most recent studies show that at low levels of exercise intensity, about 85% of calories are supplied by fats; at medium levels, about half. At high levels of exercise intensity, 70% of energy needs are derived from carbohydrate.

Cyclist and runners use their legs, rather than their arms as do kayakers. At mild-moderate exercise levels, the percentage contribution from carbs and fat is the same whether the legs or the arms are exercising. At similar higher VO2 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 higher 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 contribution 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 bicycling 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 with a two-hour high-intensity ride: 3,000 total calories—2,000 calories for the activity and 1,000 calories for basal metabolism. If you do this every day of the week, and if you ingest just 2,500 calories, the deficit of 500 calories will contribute to an average weight loss of one pound a week. If you have only two hours, you will burn fewer calories with low-intensity work than with high-intensity work. Moreover, with a caloric surplus of 500, you will gain an average of a pound a week.

A 2,500-calorie diet that is 65% carbohydrate will provide 1,625 calories toward glycogen replacement. A 40% carbohydrate diet will provide only 1,000 calories of carbohydrate.

If you eat a high-carbohydrate diet, you will be able to better replace your glycogen, and you will be able to train day after day. If you do not, your glycogen tank will not be filled. After a few days you won’t be able to train at as high an intensity level, and you’ll run out of high-performance energy—glycogen. You will have to train more slowly and longer to lose as much fat.

The moral is this: if your time is limited, within the limits of your overall training program, ride hard and eat a high-carbohydrate diet.

Carbohydrate Chemistry

Carbohydrates are simple sugars; complex sugars, or starches; and indigestible sugars, or fiber.

Simple Sugars

Simple sugars are categorized as single- or double-molecule sugars.

Single-molecule sugars include glucose, fructose, and galactose.

Double-molecule sugars include sucrose (table sugar—a glucose and a fructose molecule), lactose (milk sugar—a glucose and a galactose molecule), and maltose (malt sugar—two glucose molecules).

Refined sugars are processed sugars devoid of other nutrients.

Natural simple sugars, found in fruits, juices, milk, and vegetables are associated with vitamins and minerals.

Simple sugars are the building blocks of complex sugars, or starches.

Foods and drinks with a lot of simple sugars or simple carbohydrates are often sweet. They include candies, fruit, and nondiet soft drinks. Simple sugars usually come with few vitamins or minerals and are therefore often referred to as “empty calories.”

Complex Sugars—Starches

When simple sugars form long chains of carbohydrate, they are called “complex.” Complex carbohydrates, or starches, are often associated with other nutrients. Foods consisting primarily of complex carbohydrate are pasta, breads, potatoes, and grains. Ingested complex carbohydrate is digested (broken down) into simple sugars before being absorbed into the bloodstream.

The body re-forms a complex carbohydrate for energy storage called glycogen. Glycogen is the critical fuel for performance in the high aerobic and anaerobic threshold range, and is stored primarily within muscle cells and the liver. When one exercises for a couple of hours at high intensity, it is easy to use up these stores.

Since complex carbohydrate is associated with other nutrients and are critical for glycogen replacement, they form the cornerstone of meal planning.

Maltodextrins or glucose polymers are medium-length chained carbohydrates, partially broken down from naturally occurring complex carbohydrate. They are often found in energy bars and gels. The contention that they provide a more constant source of energy than simple sugar, one that is easier to digest than naturally occurring complex carbohydrate, is only partially true. The discussion of the glycemic index below, explains why.

Fiber includes indigestible complex carbohydrate. Fiber plays a role in overall health but has little bearing on athletic performance.

Glycemic Index

It used to be thought that simple sugars entered the bloodstream rapidly but that their effects on energy production were short-lived. It used to be thought that complex sugars provided a steadier release of food energy.

Studies have shown that the rate of release of sugar into the bloodstream, or glycemic effect, is related to factors other than whether sugars are simple or complex. The rate of digestion of sugars has more to do with cooking, ripening, and the presence of fiber, fats, and proteins associated with the sugar than it does with the presence of simple sugars. For example, a well-baked potato releases sugar into the bloodstream almost as rapidly as glucose. The release of simple sugars in whole milk is delayed by the presence of fat. Bananas release sugar more rapidly when ripe. Simple sugars consumed as part of a meal raise blood sugar more slowly than when consumed by themselves.

Pure glucose is assigned a glycemic index of 100. The rate of release of sugar into the bloodstream caused by other substances is compared with the release rate of pure glucose.

Sugars that have a glycemic index greater than 80 are considered to be released quickly. Sugars that have a glycemic index between 40 and 80 are considered to be released moderately. Sugars with a glycemic index below 40 are released slowly.

Sugars that release quickly and help to spare or replace burned glycogen may be suitable during or after exercise. Sugars that release moderately slowly may be more suitable several hours before or after exercise.