Within the context of diet, weight loss researchers have focused primarily on the level of caloric restriction and the degree of structure in the diet. Typically, behavioral weight loss programs recommend a low-calorie, low-fat diet. Participants are instructed to eat 1000-1500 kcals/day (low-calorie diet), depending on their initial body weight, and to reduce dietary fat to 20-25% of calories. There are no specific foods that are required or prohibited, but consumption of complex carbohydrates and guidelines based on the Food Guide Pyramid are stressed. Participants are instructed to self-monitor the calories and fat grams in all foods they consume. Self-monitoring is recommended daily for the first 6 months, and 1 week per month thereafter. Adherence to self-monitoring has been shown to be one of the best predictors of maintenance of weight loss.

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Increased weight loss

Increased maintenance

Combined increased weight loss and maintenance

Years from Randomization

Figure 2 Ways to increase average long-term weight loss maintenance achieved in experimental research.

Very low-calorie diets Very low-calorie diets (VLCDs) are dietary regimens that provide approximately 400-600 kcal per day usually as a liquid formula. VLCDs have been shown to produce excellent initial weight losses (-20 kg at 12 weeks); this effect is due in part to the degree of caloric restriction and in part to decreased dietary variety and the use of portion-controlled foods in these regimens. Given the large initial weight loss produced by VLCDs, it was hoped that combining these diets with behavioral approaches would maximize long-term weight loss. Although VLCDs improve initial weight loss, they do not appear to produce better long-term weight loss than low calorie diets (LCDs). Difficulty with weight maintenance in programs with a VLCD appears to occur during the transition from the VLCD to a diet composed of conventional foods.

Since VLCDs have been very effective at decreasing intake, the effect of intermittent use of VLCDs (initiating weight loss with a VLCD, transitioning to conventional foods, and then returning to a VLCD) on long-term weight loss has also been investigated, but results have been less than promising. During a 50-week behavioral obesity intervention, in which a VLCD was prescribed for weeks 1 through 12 and 24 through 36, weight loss at week 50 was not significantly different between an intermittent VLCD and a LCD.

VLCDs with caloric levels between 400 and 800kcals/day have been compared to examine if greater caloric restriction produces better weight loss. One study compared two outpatient groups with different caloric prescriptions, 420 and 800 kcals/day, and found that weight loss was not significantly different between the groups. This suggests that VLCDs may produce greater initial weight loss not only by restricting calories but also by increasing the structure of the diet.

Structured low-calorie diets Several studies have investigated different ways to increase the structure of LCDs. Structure in the diet can be strengthened by decreasing variety and/or food choices and by controlling portion sizes consumed. A study examined whether providing food to participants, which controls portion size and decreases food choice, improved long-term weight loss during a standard behavioral intervention using an LCD. Participants were provided all of the food they should eat for five breakfasts and dinners each week for 18 months. Participants receiving the food provisions had greater weight loss at 6 months (-10.1 vs. -7.7kg), 12 months (-9.1 vs. -4.5 kg), and 18 months (-6.4 vs. -4.1 kg) than those participants receiving a standard intervention, even though both groups had identical calorie goals (1000-1500 kcals/day). However, even with the greater dietary structure, participants still regained weight during the maintenance phase.

Structure in the diet, by decreasing food choices, can also be increased by providing structured meal plans and detailed grocery lists. One investigation that provided meal plans and grocery lists along with a standard intervention showed greater weight loss than the standard intervention alone. The weight losses achieved with the meal plans were similar to those achieved with food provisions.

Using portion-controlled foods available in the marketplace, such as frozen entrees and meal replacement products such as Slim-FastR, also increases dietary structure. When an LCD composed of conventional foods was compared to an LCD using two Slim-FastR meal replacements, two Slim-FastR snack bars, and a healthy dinner, the diet using the Slim-FastR portion-controlled foods produced better weight loss at 3 months (—7.1 vs. —1.3 kg). For the next 24 months, both groups were instructed to consume one Slim-FastR meal replacement and snack bar per day. At 27 months, the Slim-FastR group still had better weight loss ( — 10.4 vs. —7.7kg), and the greater weight loss was maintained at 4 years (—9.5 vs. —4.1kg) in those participants available for follow-up.

Food provisions have also been used during a maintenance intervention as a rescue strategy. However, used in this manner, food provisions were not helpful in improving weight loss maintenance compared to a maintenance program without food provisions.

Consequently, increasing dietary structure by decreasing variety and food choices and/or using portion-controlled foods appears to improve long-term weight loss. These changes in the diet may increase adherence to an LCD, thereby producing greater weight loss, especially during the first 6 months of obesity treatment.

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