Abstract
Background: Many studies have compared the effects of high protein diets (protein providing 25% or more of total energy) with relatively high carbohydrate diets (carbohydrate providing greater than 50% of total energy) on weight loss and blood lipids. Only five of those studies however, have been undertaken in individuals with type 2 diabetes and three of these studies were of short duration. Longer term studies are of particular interest in people with diabetes since substantial increases in protein intake may accelerate progression of renal disease especially in those who already have early nephropathy.
Aim: The Diabetes Excess Weight Loss (DEWL) Trial was a 2 year randomised controlled trial of 419 subjects to assess whether a high-protein:moderate-carbohydrate diet was more effective than a low-fat:high-carbohydrate diet in reducing weight and optimising glycaemic control, lipid profile and blood pressure in adults with type 2 diabetes. The impact of a high protein diet on renal function was also assessed.
Methods: Participants were recruited from the community by mail-outs, flyers and posters in Auckland, Wellington and Christchurch. The prescribed high-protein:moderate-carbohydrate diet consisted of 40% carbohydrate, 30% protein and 30% fat whereas the prescribed low-fat:high-carbohydrate diet consisted of 55% carbohydrate, 15% protein and 30% fat. Both diets aimed for no more than 10% of fat to be saturated. Both diets were energy reduced by 500kcal but participants could choose to follow either a portion system using 15g portions for protein and carbohydrate, or a prescribed 7-day sample meal plan. Recipes were provided that were specific to each diet. Participants were free living and provided all their own food. Participants were supported by group sessions with a dietitian, fortnightly for the first 6 months, then monthly for the second 6 months, with no intervention in the second year. Measurements (height, weight, waist circumference, blood pressure and body composition (% body fat and lean body mass)) and blood samples (HbA1c, fasting glucose, lipid profile) were obtained at baseline, 6, 12 and 24 months. A 24 hour urine collection was sampled for albumin:creatinine ratio and nitrogen, and an estimated 3-day diet record completed.
Results: Participants were 59% female, 74% NZ European and the average age was 57.9 years. Retention over two years was 70% with 294 participants completing the study. Overall participants maintained a 3kg weight loss in the intention to treat analysis. However, no differences were demonstrated between the diets for any variable. Protein intake in the high protein group was significantly higher than in the high carbohydrate group but did not reach the goal of 30% of total energy. An exploratory analysis of high protein intakes in pooled data using both categorical and continuous variables demonstrated that a higher protein intake was linked to lower triglycerides (multivariate estimate = -0.37 (95%CI -0.67, -0.06) p = 0.02) and better preservation of lean body mass (0.17kg per 1% increase in total energy from protein (0.04, 0.30) p = 0.01) at two years. However, systolic blood pressure at six months was higher in the higher protein categories (6.41 (0.75, 12.07) p = 0.03).
No adverse effects were observed on renal function. Seven individuals triggered the renal surveillance protocol on the study but were evenly distributed across the two diets. No differences between groups were observed for either serum creatinine or urinary albumin:creatinine ratio. In the exploratory analysis higher protein intakes were associated with a lower serum creatinine at two years (-7.97(-13.64, -2.30) p <0.01 and -8.89 (-16.03, -1.75) p = 0.02).
Analysis of qualitative information indicated that the influence of family and friends, eating out and lack of self-control were the major barriers participants faced in following their prescribed diet. Common themes from exit interviews included dissatisfaction with the allocated diet (22% of respondents) and lack of personal accountability (17% of respondents).
Conclusion: This study demonstrated that a high protein or high carbohydrate diet is equally effective for weight loss. Dietary compliance however was poor and drop out rates were relatively high. In pooled analysis, a high protein diet is potentially beneficial in lowering triglycerides and serum creatinine while retaining lean body mass over time, but the effect on blood pressure requires further investigation. Achieving dietary change is difficult, thus in future targeting barriers identified by participants and using more focussed behaviour change approaches to food choices may improve dietary adherence.