In this study, the rate of completion of the intervention and the rate of maintenance of weight loss were higher among participants who were assigned to the high-protein diets and to the low-glycemic-index diets than among those who were assigned to the low-protein diets and to the high-glycemic-index diets (with no restrictions on energy intake in any of the diets). In addition, the participants assigned to the diet that was high in protein with a low glycemic index continued to lose weight after the initial weight loss. The higher protein content was achieved by reducing the carbohydrate content, which adds further support to the concept that reducing the glycemic load (defined as carbohydrate content times glycemic index) is important for controlling body weight in obese patients.8,18,19
No differences were detected in self-perceived satiety, though we suspect that the effects were too subtle to be subjectively perceived or measured with the use of visual-analogue scales.
The dietary intervention was carefully controlled to avoid differences in total fat, alcohol, and fiber among the groups but did not fully achieve the targeted difference of approximately 12% of total energy consumed in protein between the high-protein and the low-protein groups or the targeted difference of approximately 15 glycemic-index units between the low-glycemic-index and the high-glycemic-index groups. The differences that were achieved in the study were 5.4 percentage points of total energy in protein content between the high-protein and the low-protein groups and 4.7 glycemic-index units between the low-glycemic-index and the high-glycemic-index groups.
Measurement of urinary nitrogen excretion confirmed adherence to the diets (high-protein vs. low-protein), but adherence decreased toward the end of the study. Suboptimal adherence, perhaps owing to insufficient knowledge of the content of local foods, may have accounted for the failure to reach the targeted differences in protein and glycemic index. However, the results indicate that even a modest increase in dietary protein or a modest reduction in glycemic-index values was sufficient to minimize weight regain and promote further weight loss in obese patients after a successful weight-loss diet. Higher dietary adherence might have resulted in even greater weight loss.
We used families as the unit of randomization, since we thought that adult participants would be more likely to adhere to the diet if the entire family had the same diet. The participants who underwent randomization in our study were probably a more adherent group than participants in other studies, since they had adhered sufficiently to the low-calorie diet (3.3 MJ [800 kcal] per day) for 8 weeks to lose at least 8% of their body weight. Despite issues of adherence, we believe that our results are generalizable to obese people, particularly if diets are facilitated by easy access to low-glycemic-index foods and a culture that supports these dietary changes.
The dropout rate (29%) was higher than the expected rate of 20%,11
possibly owing to difficulty in maintaining motivation in whole families over the course of the 26 weeks of the study. Other dietary studies have reported similar dropout rates.3,18
We conducted a smaller study with the shop model and ad libitum food, and we found that after a 6-month intervention, a high-protein diet resulted in weight loss that was 3.7 kg greater than that achieved with a low-protein diet.19
The current study aimed to investigate whether participants who have had a major weight loss could maintain the lower weight; the results of the study are similar to those of McMillan-Price et al., who found that participants following high-protein diets lost about 0.6 kg more than did participants following low-protein diets over the course of 3 months.20
A reduction in the glycemic index of 4.7 units resulted in a 0.95-kg difference in body weight between the high-glycemic-index groups and the low-glycemic-index groups. Since there were no differences in fiber intake, the difference in body weight, though small, can be ascribed to a true effect of the glycemic index. This difference is consistent with results from a previous study.21
Furthermore, a Cochrane meta-analysis of intervention studies showed that there was a 1.1-kg greater weight loss with low-glycemic-index diets than with high-glycemic-index diets.9
However, previous studies were designed to investigate weight loss, not weight maintenance, rendering direct comparisons difficult. McMillan-Price et al. did not observe a significant difference in weight loss between diets that differed by 20 glycemic-index units.20
Sloth et al. found a 0.6-kg (nonsignificant) difference between diets that differed by 24 glycemic-index units.22
Philippou et al. found no significant differences in weight change between maintenance diets that differed by 14 glycemic-index units (approximately 1.0 kg).23
The effects of protein and the glycemic index on changes in body weight that we found in the shop centers were consistent with those at the instruction centers. However, subgroup analyses suggested that the high-protein diets were more effective at the shop centers, whereas the low-glycemic-index effect was greater at the instruction centers. The control diet was designed according to guidelines in each participating country and provided a slightly higher proportion of calories from protein than that in the low-protein groups (19% vs. 17% of total energy consumed), with a glycemic index between the high-glycemic-index and low-glycemic-index diets. Thus, the change in body weight that was observed in the control group was as expected, given the protein content and glycemic-index value of the diet. Weight regain in our study was relatively low (0.56 kg), and the overall weight loss in all participants who completed the intervention was therefore quite high (10.6 kg), as compared with the total weight loss in most studies of similar length.
In conclusion, in this large, randomized study, a diet that was moderately high in protein content and slightly reduced in glycemic index improved the rate of completion of the intervention and maintenance of weight loss and therefore appears to be ideal for the prevention of weight regain.