Obesity Epidemiology

278 EPIDEMIOLOGIC STUDIES OF DETERMINANTS OF OBESITYCross-sectional analyses have shown an inverse association between sugar intake andBMI, 8 but this relationship most likely reflects reverse causation; that is, the fact that overweightsubjects are more likely to attempt weight control by reducing sugar consumption.Overweight individuals also tend to underreport their sugar consumption. The inverseassociation between total carbohydrate intake and BMI observed in cross-sectional studiesmay also reflect confounding by health-conscious behaviors used to control weight. 1Carbohydrate restriction has recently been promoted as an alternative strategy forweight loss. Several clinical trials have evaluated the effects of low-carbohydrate dietson weight loss. A meta-analysis of five randomized controlled trials with 6-12 months offollow-up compared the effects on weight loss of ad libitum low-carbohydrate diets withthose of low-fat, energy-restricted diets on weight loss. 17 The authors found that, after6 months, participants randomized to low-carbohydrate diets had lost more weight thanthose randomized to low-fat diets (weighted mean difference, –3.3 kg; 95% CI, −5.3 to−1.4 kg). However, there was no difference in weight loss after 12 months. This metaanalysisalso compared the effects of the two dietary patterns on cardiovascular diseaserisk factors. After 6 months, changes in triglyceride and high-density lipoprotein (HDL)cholesterol were more favorable in the low-carbohydrate diet group but that changes intotal cholesterol and low-density lipoprotein (LDL) cholesterol were more favorable inthe low-fat group. It is worth noting that although these trials indicate greater short-term(within 6 months) weight loss with low-carbohydrate diets versus low-fat diets, the studieswere very small and suffered from low compliance to the intervention diets and highdropout rates during follow-up.Recently, Gardner et al. 18 compared the effects of four popular diets (Atkins, Zone,Ornish, and LEARN diets) on weight loss in a randomized trial of 311 free-living, overweight/obesepremenopausal women. Mean 12-month weight loss was 4.7 kg for theAtkins group, 1.6 kg for the Zone group, 2.6 kg for the LEARN group, and 2.2 kg forthe Ornish group. At 12 months, the Atkins group had greater reductions in triglycerideswith only a small and nonsignificant increase in LDL cholesterol. Unlike otherlow-carbohydrate dietary intervention trials, this study had a relatively low dropout rateat one year (approximately 20%), although the degree of dietary adherence in all thegroups was generally low. The data from this study provide the strongest evidence so farthat more severe carbohydrate restriction may be moderately effective for weight loss.Whether such a strategy is beneficial for preventing weight gain is unclear.Quality of CarbohydratesTraditionally, carbohydrates are classified as simple or complex on the basis of chemicalstructures. Since simple sugars are thought to be digested and absorbed more quickly thancomplex carbohydrates, and thus to induce a more rapid postprandial glucose response,prevailing dietary recommendations have promoted intake of complex carbohydrates orstarches and avoidance of simple carbohydrates or sugars. 1 However, it is now recognizedthat many starchy foods (e.g., baked potatoes and white bread) produce even higher glycemicresponses than do simple sugars. 19 To quantify glycemic responses induced by differentcarbohydrate foods, Jenkins et al. 20 developed the concept of glycemic index (GI).The index is based on the increase in blood glucose levels (the area under the curve forblood glucose levels) after the ingestion of 50 g of carbohydrate from a test food comparedwith a standard amount (50 g) of reference carbohydrate (glucose or white bread).To represent both the quality and quantity of carbohydrates consumed, Salmeron et al. 21developed the concept of glycemic load (GL, the product of the GI value of a food andits carbohydrate content).