Carbohydrate intake recommendations during exercise are expressed in grams per hour and depend on the duration of the activity. They are not expressed per unit of body mass and thus the recommendation is the same for a 50 kg (110 lbs) female runner and a 90 kg (198 lbs) athlete. This seems counterintuitive but there is a reason.
Figure 1: No correlation between body mass and exogenous carbohydrate oxidation achieved after 2 hours of exercise, based on a number of studies.
Generally the longer the activity the more carbohydrate should be ingested during that activity. For an hour bike ride very little carbohydrate is required (up to 30 grams/hour), but small amounts have been shown to improve exercise performance. When exercise is more than 2 hours and certainly when the activity is longer than 3 hours, a carbohydrate intake of 60-90 grams per hour is often recommended (with certain combinations of carbohydrates recommended when intake exceeds 60 grams per hour). The absolute exercise intensity also plays a role. For example a slower athlete who uses less total energy per hour will not require 90 grams of carbohydrate per hour. Body mass, however, is not important. The advice for carbohydrate intake during exercise does not take into account the runner’s body mass. The advice is the same for a 50 kg (110 pound) female runner and a 90 kg (198 lbs) athlete.
This may seems strange at first, but there is a simple explanation. Carbohydrate from a drink is used at a rate that is dictated by the rate at which it can be absorbed. Absorption in the intestine is the limiting factor. Simply put, if you do not absorb it rapidly, you cannot use it rapidly. The intestines of light and heavy athletes seem to absorb carbohydrate at similar rates. Therefore, carbohydrates are also used at the same rate by the muscle.
The figure above shows this. A similar figure was published in a review paper in 2010. It is based on data obtained from a number of studies. In these studies the use (or oxidation) of carbohydrate from a drink was measured and expressed in grams per minute (On the vertical axis). Exogenous carbohydrate in the graph refers to oxidation of carbohydrate from outside the body (the ingested carbohydrate), as opposed to endogenous carbohydrate, which is the carbohydrate stored within the body.
The body mass of participants in these studies (on the horizontal axis) varied from around 60 kg (132 lbs) to 95 kg (209 lbs). It becomes immediately clear that there is no correlation between body mass and carbohydrate use from the drink.
So the advice for carbohydrate DURING exercise is in grams per hour! Not in grams per kilogram per hour. This makes it much easier from a practical point of view.
It is important to note that recommendation before and after exercise, because they relate to glycogen stores which are related to muscle- and body mass, are expressed per kilogram body mass.
Jeukendrup Curr Opin Clin Nutr Metab Care 2010 Jul; 13(4):452-457