Abstract and Introduction
Objective: The aim of this study was to examine the effect of a moderate-fat diet inclusive of walnuts on blood lipid profiles in patients with type 2 diabetes.
Research Design and Methods: This was a parallel randomized controlled trial comparing three dietary advice groups each with 30% energy as fat: low fat, modified low fat, and modified low fat inclusive of 30 g of walnuts per day. Fifty-eight men and women, mean age 59.3 ± 8.1 years, started the trial. Dietary advice was given at baseline with monthly follow-up and fortnightly phone calls for support. Body weight, percent body fat, blood lipids, HbA1c, total antioxidant capacity, and erythrocyte fatty acid levels were measured at 0, 3, and 6 months. Data were assessed by repeated-measures ANOVA with an intention-to-treat model.
Results: The walnut group achieved a significantly greater increase in HDL cholesterol-to-total cholesterol ratio (P = 0.049) and HDL (P = 0.046) than the two other treatment groups. A 10% reduction in LDL cholesterol was also achieved in the walnut group, reflecting a significant effect by group (P = 0.032) and time (P = 0.036). There were no significant differences between groups for changes in body weight, percent body fat, total antioxidant capacity, or HbA1c levels. The higher dietary polyunsaturated fat-to-saturated fat ratio and intakes of ω-3 fatty acids in the walnut group were confirmed by erythrocyte biomarkers of dietary intake.
Conclusions: Structured "whole of diet" advice that included 30 g of walnuts/day delivering substantial amounts of polyunsaturated fatty acid improved the lipid profile of patients with type 2 diabetes.
Walnuts are distinguished from other nuts by virtue of their higher polyunsaturated fat content (and importantly their α-linolenic acid [ALA] content) combined with antioxidants in the form of γ-tocopherol. There are mechanistic explanations for the influence of dietary polyunsaturated fatty acid (PUFA) on insulin action and energy metabolism,[1,2] and cohort studies of women in the U.S. have demonstrated a reduced risk of developing type 2 diabetes with dietary PUFA replacing trans or saturated fatty acids (SFAs).[3,4] Intervention trials have demonstrated the benefits of replacing dietary SFAs with monounsaturated fats (MUFAs),[5,6,7] but the effects of dietary PUFA have been less well studied in people with diabetes.[8]
The literature on recommendations varies to a minor degree, and cultural differences may have an influence,[9] but a total fat level of ≤30% energy[10] and <10% saturated fat is reasonably well accepted.[11,12] There is some concern that PUFAs are more susceptible to oxidation and therefore may be more atherogenic,[11] so a level of <10% energy for PUFAs is seen with greater flexibility in the proportions of carbohydrate and MUFA.[13] Within the PUFA fraction of the diet, the ratio of ω-6 to ω-3 fatty acids is also considered[9] with daily recommendations for a 2,000-kcal diet of 0.65 g for the long-chain ω-3s (EPA + DHA) and 2.22 g for ALA[14] and the ω-6-to-ω-3 ratio reduced to substantially <10.[14a]
Fish are the major source of EPA + DHA, but fish oil supplements produce a decreased oxidative stability of plasma LDL.[15,16] In contrast, consumption of fish itself may be protective against type 2 diabetes.[17] Nuts provide substantial amounts of dietary PUFA, and nut consumption is inversely associated with the risk of type 2 diabetes in women.[18] There is no net effect on glucose homeostasis from nut supplementation, but more interestingly, no net weight gain when nuts were used as a replacement food.[19] In subjects with dyslipidemia (total cholesterol >5.17; LDL >3.36; triglycerides >2.26 mmol/l), a low-fat diet supplemented with walnuts was found to reduce total cholesterol compared with a habitual or low-fat diet, and LDL compared with a low-fat diet alone,[20] with no differential effects on triglyceride levels. In a summary of walnut studies, Feldman (2002)[21] found that there was evidence of decreased total cholesterol and LDL cholesterol in diets of at-risk subjects supplemented with two to three servings of walnuts per day, with no net gain in body weight. However, most studies considered in that review had been conducted over a limited period of time and were not "real world" in context, and "the effect of walnut ingestion at a practical level, i.e., one serving a day, has not been evaluated." The aim of the study reported here was to examine the effect on blood lipid profiles of including 30 g of walnuts/day in a modified-fat diet plan compared with the provision of generic low-fat or low-fat/modified-fat dietary advice.
Diabetes Care. 2004;27(12) © 2004 American Diabetes Association, Inc.
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