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Fat Attack: Carbohydrate Fate

Do carbs really turn to fat?

The bulk of published research on the metabolic fate of carbohydrate foods shows that it’s difficult to get fat simply from eating too many carbs. Nutrition scientists point to several reasons that account for this. Carbs require more energy than fat to be metabolized, and the primary initial use of carbs is to replenish depleted glycogen stores in liver and muscle. Until those stores are completely filled, most carbs are diverted toward glycogen synthesis, which is largely controlled by insulin-activated enzymes.

Another frequently overlooked aspect of carbohydrate metabolism involves the interaction between carbs and exercise. That was scrutinized in a recent study of the effects of rest and exercise on the intakes of pasta meals.1 Pasta is a rich source of carbohydrates. The size of the pasta meals varied from small (150 grams) to large (400 grams), or about one-third pound to nearly a pound of cooked pasta. The pasta was labeled with a metabolic tracer.

The subjects consisted of three groups of six healthy, sedentary males who ate either the small or the large pasta meal. Some subjects didn’t exercise, while others spent 90 minutes to three hours on a stationary bike. Those doing the longer workout used a low exercise intensity, while the shorter workouts featured a higher, though not maximal, level of intensity. The subjects ate the pasta following their workouts, with the fate of the ingested carbs monitored for eight hours afterward.

In those who didn’t exercise but ate the large pasta meal, fat oxidation was totally suppressed and a small amount of glucose was converted into four to six grams of fat. Those who engaged in exercise’either moderate or low intensity’showed significantly elevated levels of fat oxidation, or burning, despite eating the high-carb pasta meals. Glucose oxidation was similar in the group eating the small pasta meal but lower in subjects exercising at low intensity after the larger pasta meal. It was completely suppressed in those exercising at a higher or moderate level of intensity. The lack of glucose oxidation in the exercising groups shows that the extra glucose derived from the larger pasta meal was diverted to glycogen replenishment following exercise. Fat oxidation following exercise was similar in both the low- and moderate-intensity exercise groups.

The conclusion was that fat synthesis is completely suppressed after exercise, even if you eat a huge amount of carbohydrates. Another finding was that fat oxidation after exercise didn’t differ between low- and moderate-intensity exercise. Only those who ate the pasta and didn’t exercise showed any fat synthesis, and even that was small, consisting of an average of 13.3 grams of glucose converted into four to six grams of fat, or only 4 percent of the carb load.

On the other hand, the authors point out that carbs can be converted into bodyfat if you take in more than your body needs for energy. In addition, eating more carbs than you use for energy purposes over several days (the fate of carbs in most sedentary people) can easily result in stored bodyfat. That’s particularly true for people who are already fat and thus often show diminished thermogenesis and insulin perturbations, such as secretion of excess insulin due to insulin insensitivity, which in turn is directly related to bodyfat levels. The conversion of carbs to fat also varies with individuals, and even in this new study, some subjects did convert 35 to 45 grams of glucose derived from the pasta into 12 to 16 grams of fat. The message from the study is that you don’t have to worry about carbs you eat after an intense workout, even if you’re on a carb-restricted diet. Carbs you eat after exercise will be used mainly for glycogen replenishment. If anything, not taking in carbs and protein after a workout will blunt exercise recovery by limiting vital muscle and liver glycogen replenishment.

1 Folch, N., et al. (2001). Metabolic response to small and large 13-C labeled pasta meals following rest or exercise in man. British J Nutr. 85:671-680.

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