The metabolic syndrome describes a cluster of risk factors for type 2 diabetes (T2DM). Nutritional approaches which favourably influence these factors might be expected to reduce the risk of T2DM. Two dietary intervention studies have been undertaken to examine the effects of macronutrient composition on clinical and metabolic determinants associated with the metabolic syndrome.
Previous research suggests that moderately high-protein diets may be more appropriate than conventional low-fat, high-carbohydrate diets for individuals at high risk of T2DM. However in most such studies sources of dietary carbohydrate were not specified or may not have been appropriate. Thus in the first study two weight-loss diets – one moderately high in protein and the other high in fibre-rich, minimally-processed cereals and legumes – were compared to determine whether a relatively high-protein diet has the potential to confer greater benefit. Eighty-three overweight or obese women were randomised to either a moderately high-protein diet (HP) or to a high-fibre, relatively high-carbohydrate diet (HFib) for 8 weeks. Energy intakes were restricted. Participants on both diets lost weight (HP: -4.5kg; 95% confidence interval (CI):-5.4, -3.7kg and HFib: -3.3kg; 95% CI: -4.2, -2.4kg), reduced body fat and showed improvement in other markers of metabolic risk. However participants on HP lost more body weight (-1.3 kg; 95% CI: -2.5, -0.1kg) and total fat (-1.3kg; 95% CI: -2.4, -0.1). Diastolic blood pressure decreased more on HP than on HFib.
Dietary approaches to reducing risk of T2DM typically emphasise fat and energy restriction, but for many achieving and maintaining weight loss is difficult. Diets that focus on substantially altering macronutrient distribution rather than energy restriction are promising alternatives. The second study examined the effects on body composition, insulin sensitivity and other metabolic risk factors, of dietary advice including moderate increases in protein and fibre, without specifying energy intake compared with standard dietary recommendations. Eighty-nine women at risk of T2DM were randomised to either a standard low-fat, highcarbohydrate diet (StdD) or to a relatively high-protein, high-fibre diet (HPHFib) for 10 weeks. Participants on the HPHFib diet lost more weight (1.3 kg; 95% CI 0.7, 1.9) and fat (1.0 kg; 95% CI 0.2, 1.8) than participants on StdD. Total and low-density-lipoprotein (LDL) cholesterol were also significantly lower after the HPHFib diet. In contrast insulin sensitivity was reduced on HPHFib (-17.8%; 95%CI -28.6%, -5.3%) compared with StdD after adjustment for weight loss.
In conclusion a moderately high-protein, weight-reducing diet was associated with greater benefits when compared with an appropriate high-carbohydrate, high-fibre diet in high risk women, confirming the advantages of high-protein diets that have been observed by others. An ad-libitum diet high in both protein and fibre also improved body composition and markers of metabolic risk compared with standard dietary advice, although a decline in insulin sensitivity was observed. The consequences of the apparent impairment in insulin sensitivity are uncertain. Further research is needed to better understand the effect of macronutrient composition on insulin sensitivity.
