Long-term effects of increased dietary polyunsaturated fat from walnuts on metabolic parameters in type II diabetes

Background/Objectives: Most dietary interventions have metabolic effects in the short term, but long-term effects may require dietary fat changes to influence body composition and insulin action. This study assessed the effect of sustained high polyunsaturated fatty acids (PUFA) intake through walnut consumption on metabolic outcomes in type II diabetes. Subjects/Methods: Fifty overweight adults with non-insulin-treated diabetes (mean age 54 +/- 8.7 years) were randomized to receive low-fat dietary advice +/- 30 g per day walnuts targeting weight maintenance (around 2000 kcal, 30% fat) for 1 year. Differences between groups were assessed by changes in anthropometric values ( body weight, body fat, visceral adipose tissue) and clinical indicators of diabetes over treatment time using the general linear model. Results: The walnut group consumed significantly more PUFA than the control (P=0.035), an outcome attributed to walnut consumption (contributing 67% dietary PUFA at 12 months). Most of the effects were seen in the first 3 months. Despite being on weight maintenance diets, both groups sustained a 1-2 kg weight loss, with no difference between groups (P=0.680). Both groups showed improvements in all clinical parameters with significant time effects (P<0.004), bar triacylglycerol levels, but these were just above normal to begin with. The walnut group produced significantly greater reductions in fasting insulin levels (P=0.046), an effect seen largely in the first 3 months. Conclusions: Dietary fat can be manipulated with whole foods such as walnuts, producing reductions in fasting insulin levels. Long-term effects are also apparent but subject to fluctuations in dietary intake if not of the disease process. European Journal of Clinical Nutrition (2009) 63, 1008-1015; doi: 10.1038/ejcn.2009.19; published online 8 April 2009