L OW - VS .H IGH -C ARBOHYDRATE D IET FOR P REVENTION OF W EIGHT G AIN

9.6.1 L OW - VS .H IGH -C ARBOHYDRATE D IET FOR P REVENTION OF W EIGHT G AIN

It has been estimated that about one third of adults in the U.S. are attempting to maintain their weight and avoid weight gain. 100 Intervention studies to prevent weight gain in nonoverweight individuals are, by their nature, more difficult to design and conduct than treatment studies of individuals who are already overweight 124 . There- fore, it is not surprising that few such studies have been undertaken. A systematic review of the literature between 1992 and March 2003 revealed 16 prevention trials that targeted or measured changes in carbohydrate intake. Unfortunately, none of these studies were specifically designed to determine the relative effectiveness of low- vs. high-carbohydrate diets for the prevention of weight gain, and none eval- uated the effectiveness of very low carbohydrate diets. The majority of these studies aimed to reduce the risk of chronic disease (cancer, heart disease, etc.) and improve health rather than prevent weight gain per se. Furthermore, all of the studies aimed to measure the effectiveness of a diet consistent with a prudent diet, compared to a control group consuming their usual diet.

All but three of the studies were randomized controlled trials. Two of these three were not randomized; 137,140 the other was randomized but did not include a control group. 125 The interventions among adults involved counseling and education in a variety of settings, including worksite, clinic, home, and community. None men-

Carbohydrates and Obesity

tioned the use of other environmental, mass media or institutional change strategies. All but two of the interventions among children were school based. The two that were not included a clinic-based, family-oriented counseling program 126 and a par- ent-focused behavioral intervention. 125

None of the seven identified adult intervention studies (Table 9.3) targeted total carbohydrate intake, although one did aim to increase grain consumption 127 and

another 128 aimed to increase the intake of complex carbohydrates. Both of these studies observed reductions in the weight of subjects relative to controls. Six did measure and detect increases in percent of calories from carbohydrate relative to controls. 127,129–133 Three of these six studies detected a significant decrease in the weight of subjects relative to controls, 127,129,131 all three of which targeted a reduction in dietary fat in addition to an increase in fiber or complex carbohydrates. Two of them also targeted an increase in fruits and vegetables. 127,129 Two other interventions 132,133 that only tar- geted fruit and vegetable intake (thereby resulting in an increased percent of energy as carbohydrate), however, did not have a significant impact on weight. It appears, therefore, that increases in percent calories from carbohydrates among adults can, but do not always, have a protective effect on weight gain. This effect may be larger when the increase in proportion of calories from carbohydrates comes from whole grains as well as from fruits and vegetables.

As with adults, none of the nine prevention trials conducted among children (Table 9.4) specifically aimed to change the intake of total carbohydrates, although

a few interventions aimed to decrease certain high-carbohydrate foods. Generally speaking, however, when fat intake decreases (as a percent of energy intake), car- bohydrate intake increases; therefore, our conclusions regarding the impact of car- bohydrate intake could be the reverse of those regarding fat intake: a relative increase in carbohydrate intake (as a percent of energy intake) is sometimes, but not always, associated with lower adiposity. However, the only three studies 126,134,135 that actually measured and detected an increase in the intake of total percent of calories from carbohydrates relative to controls did not observe any impact of the intervention on any measure of adiposity. Of the seven studies that aimed to increase total carbo- hydrates, only two observed an impact on adiposity, 138,139 and this impact was only observed with regard to skinfolds and not BMI. Again, there are many possible reasons why these interventions were not effective. Therefore, it is not possible to come to a conclusion regarding the independent impact of carbohydrate intake on these results.