PT on the Net Research

Absorption Rates for Nutrients


Question:

I was wondering if you guys could help me out with a nutritional question. First, is there a difference between the absorption rate of protein for regular non exercising individuals and athletes? Second, how long does it take for the three macronutrients (protein, fat and carbohydrates) to be digested and actually reach their final destination (e.g., carbs stored within liver and muscle, protein beginning repair and excess fat stored after consumption)?

Answer:

In answer to your first question, yes. There is a difference between the absorption rate of protein for regular non-exercising individuals and athletes. This is due to the fact that endurance training increases the stomach emptying rate and can increase the gastrointestinal transit time without a decrease in absorptive capacity. This is an adaptation that occurs because athletes require up to four times the amount of food as sedentary individuals.

In answer to the second part of your question, I will first consider protein. The rate at which the gastrointestinal tract can absorb amino acids from dietary protein varies from 1.3 to 10 grams per hour. This is dependant upon the type of protein consumed. The rate of absorption of raw egg is 1.3 grams per hour, whereas the rate of absorption for whey protein isolate and free amino acids is eight to 10 grams per hour. The general although incorrect consensus among athletes and bodybuilders is that rapid protein absorption corresponds to greater muscle building. This is in fact not the case, and rapidly absorbed proteins are more likely to be oxidized for energy and not contribute to building body protein. Slower absorbed proteins such as casein (six grams per hour) mixed with whey protein (seven grams per hour) promote protein balance and are superior to more rapidly absorbed proteins. There is little information available about protein absorption from real whole foods such as chicken, meat and fish.

The time it takes for carbohydrate to be digested and stored as glycogen depends on a variety of factors. These include timing of carbohydrate intake, amount of carbohydrate intake, type of carbohydrate ingested and the combination with other macronutrients. First, the timing of carbohydrate intake refers to the time since exercise ceased. Immediately after exercise, glycogen synthesis can be up to three times faster than two hours later. The amount of carbohydrate intake is also crucial. Maximal glycogen synthesis occurs when 75 to 90 grams of carbohydrate is consumed each hour in the first four hours after exercise. This provides a muscle glycogen synthesis rate of 0.31 grams per minute. A higher intake of carbohydrate provides no additional benefit. The type of carbohydrate refers to the glycaemic index. The higher the glycaemic index, the faster the carbohydrate can be utilized by the body and stored as glycogen, so foods like potatoes, rice and pasta are best for optimal glycogen synthesis. The combination with other nutrients will also affect the time it takes to digest the carbohydrate. If a carbohydrate food is eaten with fat or fibre, this will slow down the rate of digestion. Protein does not appear to have any effect on the rate.

Fat is the slowest of all the macronutrients to be digested. The stomach can take from one to six hours to empty fully. The higher the fat content and the larger the meal, the longer the process takes. The fat is slowly released from the stomach so that the body can process it properly. Fats are transported to the tissues where they are needed as sources of energy, as components of membranes or as precursors of metabolites. The ability to metabolize fats from the bloodstream varies considerably between individuals due to genetic differences in the receptors. Whereas some people clear fats rapidly from their blood, others may have constantly high circulating levels of fats if they consume three meals a day with a high fat content.