Submitted By: various

I get a lot questions on carbohydrate intake in relation to training and sports performance, and it remains a contentious and challenging issue. So I’ll try to offer additional clarification based on NHE and what I’ve learned since writing that book. Rob

Determining the proper amount of dietary carbohydrate requires an individualized assessment. A sedentary person represents one end of the spectrum. He or she requires very little or no carbohydrate to perform daily activities, because virtually all activities are aerobic as opposed to anaerobic and total energy expenditure is minimal. Athletes training and competing at the highest level represent the other extreme. They may train 5 or more hours per day, expending a huge amount of energy, and their training often has a substantial anaerobic component.

For these athletes, the limiting factor to maximal performance is more likely to be insufficient total caloric intake because it is difficult to consume enough calories to fulfill the energy demands of extremely high-volume training. In this scenario, qualitative considerations (proportion of fat, carbohydrate, and protein) take a backseat to quantitative considerations (consuming enough energy). During vigorous activity blood is redirected away from digestion to the working muscles, which makes liquid an ideal means of delivering the calories needed to close the intake/expenditure energy gap. This is where performance drinks such as Gatorade can be useful. As between two athletes starving for energy and partially dehydrated, one who drinks sugar water gains an advantage over one who drinks only water, and an even greater advantage over one who drinks nothing. Optimal provision of electrolytes added to glucose and water confers an additional benefit.

Both the objectives and virtually all metabolic variables are different between someone for whom physical training is the main daily activity and someone on a fat loss diet who exercises a few hours per week. For those in the former category, adequate hydration, potassium, and consuming enough calories are the dominant dietary considerations.

Books discuss general principles, an understanding of which is necessary in order to tailor a diet or exercise plan to specific objectives. However, no book can substitute for individualized training or dietary consultation - there are simply too many variables to consider. For example, the objectives of one who is 5’10’’ 350 pounds are likely to be much different from someone who is 5’10’’ 150 pounds. The former probably has a high bodyfat to muscle ratio, whereas the latter is more likely to be concerned with building muscle. Then there are athletic training considerations, not applicable to the average person.

A central feature of macronutrient cycling is carbohydrate reduction, along with an appreciation of the fact that fat has greater nutritional value than carbohydrate. Alpha-linolenic acid, for example, derived from dietary fat, would qualify as a “vitamin” except that the definition of vitamin excludes substances that provide energy as distinguished from substances that catalyze or regulate metabolic processes involved in energy production. Unlike fat, dietary carbohydrate is not essential to life - the body can manufacture it from protein, through a process called gluconeogenesis. A carbohydrate food may contain essential nutrients, but unlike fat and protein, carbohydrate is not itself an essential nutrient. Carbs may be helpful in terms of performance of high-intensity work, but this is not the definition of “essential.”  

Macronutrient cycling (carb intake lower when fat intake is higher and vice versa) is the optimal framework within which to exchange carbohydrate calories for fat calories. Cycling allows you to maximize the benefits while minimizing the drawbacks of fat and carbohydrate. Within a certain range, replacing carbohydrate calories with calories from fat does not lead to lower energy levels due to the principle of glycogen use efficiency.

Glycogen use efficiency means that, under isocaloric conditions (equivalent calorie intake), marginal carbohydrate reduction does not impair energy levels because the less carbohydrate you ingest the more conservatively your body utilizes stored glycogen. This is due to a metabolic adaptation in which fat-burning enzymes are upregulated, causing fat stores (both adipose tissue and, for higher intensity activities, intramuscular triglyceride) to be more heavily tapped as a fuel source. This adaptation can take weeks, which is why people often feel lethargic within days of converting to a lower carbohydrate/higher fat diet. But if they stick to it and get past the carb-weaning period, they generally experience higher and more stable energy levels.

For athletes, glycogen use efficiency can be capitalized upon to improve performance. This is accomplished by cyclically restricting carbohydrate and increasing fat intake (with an emphasis on healthy fats), which conditions the body to burn fat at a higher rate and spare glycogen, then carb-loading prior to competition. Both athletes - the one who carb depleted then loaded and the one who maintained a chronic high carbohydrate intake leading up to competition - enter the event with high glycogen levels. (Actually, research suggests that muscles depleted of glycogen will store more glycogen as a result of carb repletion due to a compensatory mechanism.) The athlete who switched from high fat to high carb will be better able to utilize fat while having access to an equal or greater supply of glycogen.

This represents an idealized metabolic model for performance enhancement. The specifics of optimal cycling will vary for each athlete, and if improperly implemented this approach can be counterproductive. Consideration must be given to the proportion of aerobic versus anaerobic work. For example, 2 hours per day of high-intensity interval training will require that more carbs be consumed than 2 hours of bike riding performed below the lactate threshold. Also, when energy demands are extremely high, practical concerns related to getting enough calories take precedence over macronutrient cycling.