United States Department of Agriculture

Popular Diets: A Scientific Review 2⁄2
Obesity Research 9(2001):1s

by Marjorie R. Freedman, Janet King, and Eileen Kennedy
Popular Diets: A Scientific Review 2⁄2

Janet King, Ph.D., R.D., is the Director at the U.S. Department of Agricultural Research Service, Western Human Nutrition Research Center, Presidio of San Francisco, CA and is currently the President of the American Society for Nutritional Sciences.

Insulin and Leptin in the Endocrine Regulation of Appetite and Body Weight

Insulin and leptin are hormones that act as medium- to long-term regulators of body weight through their actions to decrease food intake and increase energy expenditure (metabolic rate), ensuring that energy intake and energy expenditure is closely matched (109 –111).

People who do not produce leptin due to a genetic deficiency, or who have defects in the leptin receptor, have dramatically increased appetites and overeat to the point of becoming massively obese (112,113). The effects of leptin deficiency are ameliorated by the administration of recombinant leptin (114).

Insulin, in addition to its effects in the central nervous system to inhibit food intake, acts in the periphery to ensure the efficient storage of incoming nutrients. The role for insulin in the synthesis and storage of fat has obscured its important effects in the central nervous system, where it acts to prevent weight gain, and has led to the misconception that insulin causes obesity (115). It has recently been shown that selective genetic disruption of insulin signaling in the brain leads to increased food intake and obesity in animals (116) demonstrating that intact insulin signaling in the central nervous system is required for normal body weight regulation.

Insulin also has an indirect role in body weight regulation through the stimulation of leptin (117). Both insulin and leptin are transported into the central nervous system, where they may interact with a number of hypothalamic neuropeptides known to affect food intake and body weight (118). Insulin and leptin are released and circulate in the bloodstream at levels that are proportionate to body fat content. Secretion and circulating levels are also influenced by amount and type of foods eaten, with decreased concentrations noted during fasting or energy-restricted diets (119,120). The decrease of leptin during a prolonged energy- restricted diet has been shown to be related to increased sensations of hunger (120) suggesting a role for low leptin levels to increase appetite during dieting in humans, and therefore to the predisposition for weight regain after initially successful dieting.

Circulating concentrations of both insulin and leptin, measured over a 24-hour period, are reduced in women consuming high-fat meals (60% fat, 20% CHO) compared with when equicaloric meals high in CHO and low in fat (60% CHO, 20% fat) are consumed (36,37). Increased insulin secretion has been suggested to protect against weight gain in humans (106). Because insulin also stimulates leptin production, which acts centrally to reduce energy intake and increase energy expenditure, decreased insulin and leptin production during the consumption of high-fat diets could help contribute to the obesity promoting effects of dietary fat (42,44,121).

Recent studies show consuming a high-fat diet induces resistance to the actions of leptin to decrease food intake (122,123), and that increased energy intake and weight gain is related to an impairment of insulin transport into the brain (124). Therefore, dietary macronutrient composition affects not only production of insulin and leptin but also may influence their ability to gain access to the brain to signal target neurons. In studies investigating the efficacy and long-term consequences of weight loss diets, it is important to consider the effects of dietary macronutrient content and composition on the production of insulin and leptin, and their actions to regulate energy intake and expenditure.

5. Performance and Physical Activity

c Does the low-CHO diet affect physical performance? Although reference is made to physical activity and exercise by proponents of low-CHO diets (48, pp. 260 – 267; 49, pp. 187–206; 50, pp.143–144), only one study examined the capacity for moderate exercise in obese subjects after adaptation to a hypocaloric, ketogenic diet. This study was conducted in six slightly to moderately overweight, untrained subjects on a protein-supplemented fast for 6 weeks (e.g., 500 to 750 kcal/d, ,10 g CHO, weight loss, 10.6 kg). Results indicate that subjects adapt to prolonged ketosis and use lipid, rather than CHO, as the major metabolic fuel during prolonged exercise at 60% of maximum oxygen concentration. This shift was confirmed by an respiratory quotient of 0.66 during exercise (125).

Other studies were conducted in physically untrained, but normal weight males who were fed eucaloric low- CHO (,20 g/d), high-fat (80%) ketogenic diets, or nonketogenic, low-, medium-, or high-fat diets (15%, 30%, or 55% fat) (126,127). They report diet manipulation, per se, did not effect maximal or submaximal aerobic performance in untrained individuals. However, one cannot extrapolate results from these studies to typically untrained, sedentary, overweight individual consuming low-calorie, low-CHO diets.

Weight Maintenance

Is There an Optimal Diet for Weight Maintenance?

In light of the current obesity crisis, prevention of weight gain and weight maintenance is critical. Is there an optimal Data support the contention that those consuming lowfat, low-calorie diets are most successful in maintaining weight loss (194 –196) (Table 18). Insulin and leptin responses to dietary CHO may play a role in the effects of these diets to sustain weight loss through long-term signals promoting decreased energy intake, increased energy expenditure, or both. Increased physical activity and decreased consumption of (high-fat) fast food meals are also key variables (46,195,197). Palatability and dietary variety within food groups may predict body fatness. McCrory et al. (198) report that the direction of the association depends on which foods provide the variety (e.g., the variety of sweets, snacks, condiments, entrees, and carbohydrates consumed was positively associated with body fat, whereas the variety of vegetables was negatively associated).

Table 19 summarizes the macronutrient composition of diets reviewed in this article. The last line represents the diet consumed by individuals enrolled in the National Weight Control Registry, who have maintained a 13.6 kg (30 pound) weight loss for at least 1 year but who, on average, have lost 30 kg and have maintained the loss for 5.1 years. Data from the Registry indicate that successful weight maintainers comsume a low-calorie diet containing ;40 g of fat (24% of energy), 200 g of CHO (56% of energy), and 70 g of protein (19% of energy) (195–197). This diet most closely resembles the moderate-fat, balanced nutrient reduction diet promoted by every health organization in the United States. The high vitamin and calcium intakes of successful weight loss maintainers suggest they eat a diet high in fruits, vegetables, and calcium-rich foods (dairy). The low iron intake suggests a low intake of animal products.

The American public needs to be told (and believe) that diets are not followed for 8 days, 8 weeks, or 8 months, but rather form the basis of everyday food choices throughout their life. A diet high in vegetables, fruits, complex CHOs (whole grains and legumes), and low-fat dairy is a moderate- fat, low-calorie diet that prevents weight gain, results in weight loss and weight maintenance. It is associated with fullness and satiety. It reduces risk of chronic disease. It is fast, convenient, and inexpensive. How can we convince people it works, and to try it?

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