It’s been 30 years since the glycemic index was proposed as a measure of which foods are likely to be healthiest in diabetes. A low glycemic index (low GI) diet favours those carbohydrates that break down slowly, releasing glucose more gradually into the bloodstream.
The therapeutic potential of low GI index foods has only recently been tested in sizeable studies. Last year, a Canadian study1 compared a low GI diet, a high GI diet, and a high-fat, low-carb diet, prescribed over one year to 162 type 2 diabetes subjects treated with diet alone. This study showed no difference between groups in glycemic control, hardly unexpected given that the mean study HbA1C for the group was already very low at 6.1%. But the low GI diet group did see significant reductions in postprandial glucose and in C-reactive protein, which is increasingly linked to heart disease.
A second study last year2 did demonstrate improved glycemic control from a low GI diet, in type 2 diabetic patients treated with anti-hyperglycemic medications who had a mean starting HbA1C > 7.1%. With over 200 subjects, this study was the largest yet of its kind. It also showed a small but significant rise in HDL-C, plus significant reduction in CRP and, in some analyses, in body weight and diastolic BP. Taken together, these two studies suggest potential benefits from low GI diets for diabetes patients with HbA1C above 6.5%, and also in cardiovascular (CV) disease.
Fighting off radicals
Studies have implicated high postprandial glucose levels in promoting oxidative damage to tissues by free radicals. Sharp rises in blood glucose are also associated with the creation of advanced glycation end products (AGEs), which have been linked to kidney damage.
Improved glycemic control has been shown to reduce microvascular complications of diabetes and in the longer term to improve CV outcomes. But recent trials aimed at tight short-term glycemic control have failed to show expected CV benefits and may actually have increased the risk of these events in some patients. There’s also concern that at least one anti-hyperglycemic therapy, an insulin analogue, may increase cancer risk, and this has fuelled concerns about all therapeutic approaches that markedly increase circulating insulin levels. This can only increase the current interest in drugs such as metformin, which improves glucose disposal, and acarbose, which inhibits carbohydrate digestion and absorption, converting the diet to a low GI one. These approaches have been shown to improve CV outcomes, strengthening the rationale for the use of low GI diets.
One reason such diets aren’t used more is the belief that they’re unpalatable and unduly restrictive. But in many instances they involve simply reverting to foods which were popular in earlier times: peas, beans, lentils, pasta, parboiled or “converted” rice, barley, cracked wheat, and temperate climate fruit (apples, oranges and berries). These shouldn’t be too difficult to substitute for white bread, bagels and so on, which form the bulk of modern carbohydrate foods. In effect, the low GI diet simply turns the clock back to a time before type 2 diabetes became the problem that it is today.
David Jenkins, MD, PhD, FRCPC is Director of the Risk Factor Modification Centre at St. Michael’s Hospital and a professor of Medicine and Nutritional Sciences at the University of Toronto.