Our findings demonstrate that adherence to the DASH diet alone, although sufficient to modify BP values12
, resulted in significant improvements in metabolic indices of cardiovascular risk only when accompanied by aerobic exercise and weight loss. In the DASH-WM group, participants lost an average of 19 pounds over 4 months and increased their aerobic capacity by 19%. While both the DASH-A and DASH-WM groups achieved clinically meaningful reductions in BP and improvements in other cardiovascular biomarkers of risk, as described in our earlier publication12
, only DASH-WM participants demonstrated significant improvements in glucose tolerance and insulin sensitivity.
Although the DASH diet has been shown to reduce BP in controlled “feeding” studies7, 8
and in studies of free living individuals9, 12
the present study found that ENCORE participants who adhered to the DASH diet but did not exercise or lose weight achieved minimal improvements in glucose metabolism or insulin sensitivity, and also in lipids, relative to controls. Our findings contrast with results from the PREMIER substudy11
, in which addition of the DASH diet to an established intervention of weight loss, reduced sodium intake, increased physical activity, and moderation of alcohol intake resulted in a significant improvement in insulin sensitivity relative to controls. However, because there was no difference in insulin sensitivity between groups randomized to the established intervention with or without the DASH diet, and there was a trend toward greater weight loss in the DASH group, the added value of the DASH diet is uncertain. The present ENCORE study findings indicate that despite DASH-related reductions in BP12
, the DASH diet by itself produced minimal improvements in insulin sensitivity.
Our study was designed to evaluate only the DASH diet, and it is possible, even likely, that other diets, either alone or combined with exercise, could be beneficial. Many studies have examined the impact of various diets on weight loss23–26
. Sacks et al.23
, for example, randomized overweight adults to one of four diets in which the targeted percentages of energy derived from fat, protein and carbohydrates varied. After 2 years, groups achieved similar benefits in weight loss and lipid-related risk factors and fasting insulin levels. It was concluded that reduced calorie diets result in significant weight loss regardless of the macronutrient content. Foster and colleagues25
reported that a low carbohydrate, high protein and high fat (Atkins) diet was associated with greater weight loss after 6 months compared to a conventional low fat, low calorie, high carbohydrate diet, but that the differences were not significant after 12 months. With respect to body composition, the present findings confirm the results of previous findings suggesting that a low fat, weight loss diet (50% carbohydrate, 30% fat, 20% protein) results in reduced lean body mass. However, very low carbohydrate diets have been found to result in even greater reductions in weight and lean body mass compared to low fat diets27–29
. Lipid changes were generally similar over time, and both diets were associated with lower DBP and insulin response to an oral glucose load.
While weight loss is associated with improved lipids, particularly LDL-cholesterol30
, and increased insulin sensitivity31–33
, diet composition also may affect lipids and glucose metabolism independent of weight loss. For example, with a 4-week, isocaloric weight maintenance diet, both the Ornish diet and South Beach diet have been shown to favorably reduce lipids, while high fat diets may be associated with increased LDL and total cholesterol levels34
. However, the number of calories consumed appears to be more important relative to the content of the calories with regard to the development of diabetes35
Exercise also was a key component of the DASH-WM intervention, but its effects on insulin sensitivity could not be determined independent from weight loss. Although exercise is widely considered to be important for successful weight loss, studies of the effects of exercise in the absence of weight loss on glucose, insulin sensitivity and lipids have produced mixed results. Exercise has been shown to improve insulin sensitivity, either due to chronic effects of exercise training or to the residual effects of acute exercise. Studies of both healthy adults and patients with type 2 diabetes have demonstrated that improved insulin sensitivity is maintained up to 16 hr after a single bout of exercise36, 37
but may be diminished 60 hours after the final exercise training session38, 39
. Some studies have demonstrated that exercise training is associated with reduced glucose levels and improved glycemic control40–44
, while others have not45–50
. Because studies that have shown improvements in glucose control after exercise training have not established that these effects are due to exercise independent of weight loss51
, the extent to which the exercise component of the DASH-WM condition contributed to the metabolic improvements observed in the ENCORE study is not known. The effects of exercise training on lipids also have provided mixed results52
although recent evidence suggests that high levels of exercise without weight loss may be required to achieve improvements in lipid and lipoprotein variables53
Finally, it should be noted that some studies also have suggested that obesity may moderate the effects of exercise training on insulin sensitivity. Poirier et al.48
, for example, reported no improvement in insulin sensitivity in obese type 2 diabetic patients after 12 weeks of aerobic training, although insulin sensitivity was improved in nonobese type 2 diabetic subgroups. Our data, in overweight but non-diabetic patients revealed no evidence that obesity moderated the effects of treatment. Therefore, our findings suggest that the improvements in insulin sensitivity observed in the DASH-WM intervention are generalizable to both obese and non-obese populations.