Although the efficacy of the DASH diet in reducing blood pressure is well established, less is known about the metabolic effects of this dietary pattern, including its influence on insulin sensitivity. Observational data from the Insulin Resistance Atherosclerosis Study suggest that the DASH diet may prevent the development of diabetes is some individuals [44
]. Habitual dietary intake at baseline was assessed using a semiquantitative, 114-item food frequency interview, from which a DASH adherence score was derived. A significant inverse association of the DASH score with incident diabetes over 5 years of follow-up was observed in whites, but not in blacks or Hispanics and not in the study cohort as a whole.
In an ancillary study of PREMIER, insulin sensitivity was measured in a subset of 52 participants using the frequently sampled IV glucose tolerance test with minimal model analysis [45
]. Over the course of the 6-month intervention, the insulin sensitivity index was unchanged in the advice-only group, improved from 2.32 to 2.97 with the established intervention (P
=0.146 in comparison with advice only), and improved from 1.96 to 2.95 in the patients randomized to the established intervention plus DASH diet (P
=0.047 for established plus DASH vs advice only; P
=0.616 for established plus DASH vs established). The results of this small study suggest that the DASH dietary pattern in combination with a comprehensive lifestyle modification program for hypertension may improve insulin sensitivity. Whether the DASH diet provides incremental benefit over exercise and weight loss is uncertain, however, as participants in the established-plus-DASH group tended to lose more weight than those receiving the established intervention, and the changes in insulin sensitivity index were not significantly different between these two groups. In another secondary analysis of PREMIER, the established and established-plus-DASH interventions both led to significant decreases in fasting insulin levels and in the homeostasis model index of insulin resistance [46
]. However, there were no differences in the insulin resistance index between the active intervention arms.
The design of the ENCORE study permitted an assessment of the effects on insulin sensitivity of the DASH diet alone and in combination with weight loss [47•
]. Glucose tolerance tests using an oral glucose load of 75 g were performed at baseline and at the conclusion of the 4-month interventions. Glucose tolerance was assessed by calculating the area under the glucose concentration curve. Insulin sensitivity was estimated using the quantitative insulin sensitivity check index (QUICKI), as described by Katz et al. [48
]; and also by using the Insulin Sensitivity Index (ISI0,120
), a method based on dynamic glucose and insulin levels [49
]. Both of these surrogate measures provide estimates of insulin sensitivity that correlate closely with glucose clamp measurements and are predictive of the onset of type 2 diabetes [50
Compared with usual-care participants, those who completed the DASH plus weight management intervention showed lower fasting glucose and insulin levels and lower values for area under the glucose concentration curve (). The DASH plus weight management participants also exhibited greater insulin sensitivity, as measured by both QUICKI and ISI0,120, compared with either DASH-alone or usual-care participants (). The DASH-alone group did not differ from the usual-care group on any measure of glucose metabolism. Thus, although participants in the DASH plus weight management condition achieved significant improvements in glucose tolerance and insulin sensitivity, no change in these metabolic parameters was noted in response to the DASH diet alone.
Background characteristics of the sample
Figure 1 Indices of insulin sensitivity, adjusted for age, sex, ethnicity, and pretreatment insulin sensitivity values: comparison of posttreatment means and 95% confidence intervals in the three arms of the ENCORE study. For the Insulin Sensitivity Index (ISI (more ...)
Based upon their glucose levels 2 hours following the oral glucose load, ENCORE participants were classified as diabetic (>199 mg/dL), prediabetic (141–199 mg/dl), or normal (<140 mg/dL). Overall, 72% (n=13) of the 18 participants in the DASH plus weight management group who were either prediabetic or diabetic at study entry improved by at least one category over the course of the trial, compared with 54% (7/13) randomized to DASH alone and 44% (8/18) in usual care. Among participants who were not diabetic or were prediabetic upon study entry, diabetic risk status worsened in only 2% (1/44) of participants in the DASH plus weight management intervention, versus 16% (7/43) in the DASH-alone group and 11% (5/46) in usual care.