http://www.bengreenfieldfitness.com Kids are not just little adults, and growing children who are strenuously exercising have several defining physiological characteristics that make them different than the older population. The following are 5 ways that training and racing nutrition should be different for kids and young active individuals (up to age 16).
1) Athletic girls can succeed on lower carbohydrate intake than athletic boys. In large nutritional surveys that have been done on young athletes ranging from 12-18 years old, the intake of female athletes is on average 3-4 grams per kilogram of body weight lower than male athletes. This seems to make sense, since studies in adult endurance athletes have shown that female endurance athletes tend to be able to oxidize more fat at higher intensities compared to men. An approximate level of carbohydrate intake that would be appropriate for a young athlete would be about 4 grams per kilogram in girls and 7 grams per kilogram in boys (remember there are 2.2 pounds in 1 kilogram). So a 90 pound young female athlete would need to eat around 165 grams of carbohydrate daily, or about 650 calories of carbohydrate.
2) Fat is the preferred exercise fuel in young athletes. In most studies, exercising children have shown 10-40% higher fat oxidation rates compared to exercising adults. Interestingly, very well trained exercising adults (such as Ironman triathletes) show these same high fat oxidation rates. While this shouldn’t be used as an excuse to “not eat” during exercise, it would be quite interesting to see studies of children given a higher fat intake prior to exercise compared to children given primarily carbs. I suspect that fueling children with avocados, coconuts, nuts and seeds may yield just as good results as filling them up with candy and energy bars.
3) Compared to exercising adults, exercising children burn lower amounts of storage carbohydrate, but higher amounts of carbohydrate from food sources. Rather than tapping into the body’s own carbohydrate stores during exercise, children tend to rely more upon carbohydrate sources from food. This is due to lower levels of the enzymes responsible for breaking down muscle carbohydrate to fuel, and is probably some type of carbohydrate conserving mechanism that leaves more storage carbohydrate for a child’s growth and development. While you might think that this should mean you make sure a child has adequate food available during a training session or race, since they’re less equipped to break down their own storage energy, this does not appear to be the case, as I point out below.
4) During exercise sessions 75 minutes or less, eating carbohydrates does not appear to give extra performance advantages in young athletes. As you learned earlier, children burn fat more efficiently than adults, and it appears that during exercise, this increased fat oxidation serves as a mechanism to stop any drop in blood glucose. Children’s bodies literally have the ability to downregulate the pathways responsible for converting carbohydrates into energy during exercise. Interestingly, free fatty acids, which indicate available fats to burn during exercise, increase in children during exercise, indicating a very strong ability of children to mobilize fat stores for energy and possibly even use energy sources that have higher amounts of fat. Once again, for exercising individuals who are under the age of 16, it may be beneficial to choose fat-based energy sources rather than sugar.
5) Regular adult sports drinks may not empty fast enough from a child’s stomach during exercise. In both children and adults, higher exercise intensities slow the rate at which fluids and fuels will pass from the stomach into the intestine, which means that less fuel is absorbed and utilized. However, this occurs to an even greater extent in children who are exercising at higher intensities, and the maximum amount of fluids (either water or a carbohydrate beverage) that a child can absorb per hour in these conditions will be approximately one bike-sized water bottle (about 20-24 ounces).