Background
The glycaemic index (GI) is a measure of the ability of a carbohydrate (for example sugar or starch) to affect blood sugar levels.
Study characteristics
In this review update, we examined 21 randomised studies that assessed the effects of low GI diets compared to diets with a similar composition but a higher GI on cardiovascular disease events and levels of cholesterol in the blood or blood pressure (major risk factors for cardiovascular disease, such as heart attacks or stroke). Studies were included up to July 2016.
Results
Participants were adults with a mean age of between 19 and 69 years. In most studies, participants had cardiovascular risk factors such as overweight or obesity or abnormal blood fat levels, and one study included participants with existing heart disease. The diets were followed for at least 12 weeks but most studies had unclear of bias and some of the compared diets only had small differences in GI. Cardiovascular disease events were not reported and no evidence of differences in effects of the diets on blood cholesterol and blood pressure were seen. Most studies did not report harms but the two that did found no harmful effects of the diets, however the evidence was poor.
Conclusions
There was insufficient evidence from randomised controlled trials to recommend consumption of low GI diets for the purpose of improving blood lipids or blood pressure.
There is currently no evidence available regarding the effect of low GI diets on cardiovascular disease events. Moreover, there is currently no convincing evidence that low GI diets have a clear beneficial effect on blood lipids or blood pressure parameters.
The glycaemic index (GI) is a physiological measure of the ability of a carbohydrate to affect blood glucose. Interest is growing in this area for the clinical management of people at risk of, or with, established cardiovascular disease. There is a need to review the current evidence from randomised controlled trials (RCTs) in this area. This is an update of the original review published in 2008.
To assess the effect of the dietary GI on total mortality, cardiovascular events, and cardiovascular risk factors (blood lipids, blood pressure) in healthy people or people who have established cardiovascular disease or related risk factors, using all eligible randomised controlled trials.
We searched CENTRAL, MEDLINE, Embase and CINAHL in July 2016. We also checked reference lists of relevant articles. No language restrictions were applied.
We selected RCTs that assessed the effects of low GI diets compared to diets with a similar composition but a higher GI on cardiovascular disease and related risk factors. Minimum trial duration was 12 weeks. Participants included were healthy adults or those at increased risk of cardiovascular disease, or previously diagnosed with cardiovascular disease. Studies in people with diabetes mellitus were excluded.
Two reviewers independently screened and selected studies. Two review authors independently assessed risk of bias, evaluated the overall quality of the evidence using GRADE, and extracted data following the Cochrane Handbook for Systematic Reviews of Interventions. We contacted trial authors for additional information. Analyses were checked by a second reviewer. Continuous outcomes were synthesized using mean differences and adverse events were synthesized narratively.
Twenty-one RCTs were included, with a total of 2538 participants randomised to low GI intervention (1288) or high GI (1250). All 21 included studies reported the effect of low GI diets on risk factors for cardiovascular disease, including blood lipids and blood pressure.
Twenty RCTs (18 of which were newly included in this version of the review) included primary prevention populations (healthy individuals or those at high risk of CVD, with mean age range from 19 to 69 years) and one RCT was in those diagnosed with pre-existing CVD (a secondary prevention population, with mean age 26.9 years). Most of the studies did not have an intervention duration of longer than six months. Difference in GI intake between comparison groups varied widely from 0.6 to 42.
None of the included studies reported the effect of low GI dietary intake on cardiovascular mortality and cardiovascular events such as fatal and nonfatal myocardial infarction, unstable angina, coronary artery bypass graft surgery, percutaneous transluminal coronary angioplasty, and stroke. The unclear risk of bias of most of the included studies makes overall interpretation of the data difficult. Only two of the included studies (38 participants) reported on adverse effects and did not observe any harms (low-quality evidence).