Review question

Are diet or physical activity, or both able to prevent or delay the development of type 2 diabetes and its associated complications in at-risk people?

Background

People with moderately elevated blood glucose (often referred to as ‘prediabetes’) are said to be at an increased risk of developing type 2 diabetes. It is currently recommended that all people with increased risk of developing type 2 diabetes should adjust their eating habits and physical activity levels. We wanted to find out whether these changes in diet, physical activity or both could prevent or delay type 2 diabetes in people at increased risk. We also wanted to know the effects on patient-important outcomes, such as complications of diabetes (e.g. kidney and eye disease, heart attack, stroke), death from any cause, health-related quality of life (a measure of a person’s satisfaction with their life and health) and side-effects.

Study characteristics

Participants had to have blood glucose levels higher than considered normal, but below the glucose levels that are used to diagnose type 2 diabetes mellitus. We found 12 randomised controlled trials (clinical studies where people are randomly put into one of two or more treatment groups) with 5238 participants. The duration of the treatments varied from two years to six years. Most trials included people defined as being at increased risk of type 2 diabetes based on glucose levels measured two hours after ingestion of 75 g of glucose (i.e. ‘impaired glucose tolerance’ (IGT) after an oral glucose tolerance test).

This evidence is up to date as of January 2017. We used a MEDLINE email alert service to identify newly published studies up to September 2017.

Key results

One study compared diet only with physical activity only. Fifty-seven of 130 participants (44%) in the diet-only group compared with 58 of 141 participants (41%) in the physical activity-only group developed type 2 diabetes. Two studies compared physical activity with standard treatment; in one study 58 of 141 participants (41%) in the physical activity group compared with 90 of 133 participants (68%) in the control group developed type 2 diabetes; in the other study 10 of 84 participants (12%) in the physical activity group compared with seven out of 39 participants (18%) in the control group developed type 2 diabetes. Eleven studies compared diet plus physical activity with standard or no treatment. Diet plus physical activity decreased the risk of developing type 2 diabetes, which occurred in 315 of 2122 participants (15%) in the diet plus physical activity group compared with 614 of 2389 participants (26%) in the standard treatment group.

We detected neither an advantage nor a disadvantage of diet, physical activity or both with regard to heart attacks or strokes. Our included studies did not report on complications of diabetes such as kidney or eye disease. The effects on health-related quality of life were inconclusive. Very few participants died in the course of the studies and side-effects were also rare. Future long-term studies should investigate more patient-important outcomes like complications of diabetes, because we do not know for sure whether ’prediabetes’ is just a condition arbitrarily defined by a laboratory measurement or is, in fact, a real risk factor for type 2 diabetes mellitus and whether treatment of this condition translates into better patient-important outcomes.

Quality of the evidence

All included trials had deficiencies in the way that they were conducted or how key items were reported. For diet plus physical activity compared with standard treatment, we found rather good evidence that the development of new type 2 diabetes was reduced or delayed. For the other comparisons the number of participants was small, resulting in a high risk of random errors (play of chance).