The oft-cited “Mediterranean diet” – rich in olive oil, fruit and vegetables, enlivened with dairy, fish and, of course, a little wine – is held up as a paragon of virtue. But there is no lack of dietary advice out there. Low-GI, low-carb, high-protein? American Diabetes Association (ADA) or European Association for the Study of Diabetes (EASD)?
So does the evidence stack up? Are these diets fact or fad? Fortunately, a recent systematic review looked at whether they really do help us to control blood sugar in type 2 diabetes.
In patients with type 2 diabetes, do special diets improve glycaemic control, body weight, cholesterol and triglycerides?
The reviewers conducted search of Medline, EMBASE and Google Scholar for studies comparing dietary interventions in type 2 diabetes. They also conducted citation searching from key publications, including guidelines.
Individual outcome data was extracted from each of these studies for people with diabetes and combined in a meta-analysis comparing different dietary combinations.
20 studies were found, comprising 3,460 participants. These studies explored various combinations of dietary interventions:
- Low-carbohydrate diet versus control (including low-fat, low-GI and traditional Mediterranean diet) (9 studies)
- Mediterranean diet versus low fat or ADA diet (4 studies)
- Low-GI diet versus ADA, high-GI or high-fibre diet (3 studies)
- High-protein diet versus low-protein or high-carbohydrate diet (2 studies)
- Other comparisons: Vegan versus ADA, vegetarian versus EASD, high carbohydrate versus high-Mono Unsaturated Fatty Acid (MUFA), high fibre versus low-fat diet (1 study each).
Meta-analysis revealed that Mediterranean diet had the greatest impact on HbA1c (-0.47%, 95% CI -0.64% to -0.30%). It also reduced body weight, unlike the other diets, and had a beneficial effect on lipid profiles.
The reviewers concluded:
Low-carbohydrate, low-GI, Mediterranean, and high-protein diets reduced Hb A1c by 0.12–0.5% compared with comparison or control diets.
- The reviewers provide a useful – and interesting – comparison of the dietary guidance given by different guidelines for different countries.
- However, the literature search was carried out in August 2011. It is likely that important studies have been published since.
- They excluded any study with less than six months of follow-up. Thus, any short-lived changes would not affect the results.
- It is not clear whether multiple independent reviewers carried out the study selection, evaluation and data extraction.
- Some studies were rated as having a higher risk of bias than others. The reviewers did not explore the potential impact of bias within the studies. This could be a serious problem as many of the studies were quite small, and produced very wide confidence intervals.
- The potential impact of heterogeneity was commented on but not formally addressed. It seems likely that important differences existed between the participants, diet and control conditions between the studies. Study populations were substantially different from one another, for example. This might not matter, but it needs to be considered.
- Further, better quality studies are needed. This review provides an important pointer for dietary advice in type 2 diabetes, but is not the final word.
Ajala O, English P, Pinkney J. Systematic review and meta-analysis of different dietary approaches to the management of type 2 diabetes. Am J Clin Nutr. 2013 Mar;97(3):505-16. doi: 10.3945/ajcn.112.042457.