Lately there's been a bit of hype about the glycemic index - you know, the chart that tells you scarfing large starchy potatoes and beets is a no-no but oatmeal and nuts are okay.
The index (glycemicindex.com) ranks foods according to the extent to which they raise blood sugar levels, and the theory is that by choosing carefully, much good health and skinniness follows. Claims for the index are quite hefty; it's touted as a way to control diabetes, reduce cholesterol and minimize the risk of heart disease and age-related eye disease.
Developed in 1981 by Thomas Wolever and David Jenkins at our own U of T, the glycemic index is the cornerstone of diets like the South Beach. But how good can any diet actually be that rules out stuff like watermelon, beets and carrots?
As usual, there's some truth and some hype happening here. In short, the glycemic index can be a useful tool, but it's not a substitute for common sense.
What the experts say " It's not a cure all. In fact, the American Diabetes Association for a very long time doubted it had any practical utility. It has relented and given it grudging acceptance. There are studies where a drug has been used to lower food's GI. This has been shown to be associated with lower blood pressure, lower risk of heart disease and less development of diabetes. There have been suggestions that [using the Index] may cause reductions in body weight. These areas are still being researched. There is both positive and negative data. I think people should not use [the Index] as the only classification of food . It's one of many, including proteins, saturated fats, the amount of trans fatty acids, the amount of fibre and all the other nutrients. And overarching everything is the amount of calories.'
DAVID JENKINS , Canada Research Chair, Nutrition and Metabolism, U of T, St. Michael's Hospital
"In theory, a low-GI food will slow down digestion, slowing the rise in blood sugar, and stall hunger. The GI rates individual foods without taking mixed meals into account. For example, mashed potatoes have a high GI, but if your plate also includes a lean protein like chicken or fish and vegetables , protein and fat in the chicken or fish and fibre from the vegetables will slow down digestion and in effect blunt the high GI of that one food. If you're choosing based on the GI alone, you could be missing the mark. A Snickers bar has a low glycemic index, but no one's going to suggest that it would be a better choice than a salad with lean chicken. Though GI can be an effective weight loss tool, I don't like using it, because it can get confusing.'
DOUG COOK , dietitian, Toronto
"The GI by itself will not help you lose weight. For instance, Häagen-Dazs ice cream is low GI, but you also have to take into account the number of calories along with the fat content . It gets confusing, so I have divided foods into red-, yellow- and green-light foods [according to GI and fat content]. Carrots are medium GI, but you'd have to eat buckets of them to gain any weight. Watermelon is one of the few fruits that are high GI. Any diet high in fruits and vegetables and low in saturated fats is going to help prevent heart disease . The GI can be helpful with polycystic ovarian syndrome (PCOS) and PMS, because hormonal changes cause shifts in blood sugar levels.'
RICK GALLOP , author, The GI Diet, former president of the Heart and Stroke Foundation of Ontario, Toronto
"A low-GI diet could be particularly helpful to women with PCOS because the root of the problem is extreme insulin resistance. High insulin levels produce the high androgen levels that disturb fertility, cause acne and facial hair and inhibit weight loss. We know that a low-GI diet will acutely lower postprandial [after-meal] insulin levels in comparison with a conventional low-fat diet. By lowering insulin levels, a low-GI diet allows us to burn more fat and less carbohydrate per unit of work done. Everything I've said can be backed by several studies. But we are still in the process of proving that a low-GI diet will improve insulin sensitivity and facilitate weight loss specifically in women with PCOS.'
JENNIE BRAND-MILLER , professor of human nutrition, University of Sydney, author, The New Glucose Revolution, Sydney, Australia
"We found that consuming foods with a high glycemic index has a positive association with early-stage age-related macular degeneration. We don't entirely understand the mechanism, but it relates to previous studies involving the glycemic index and major systemic diseases like cardiovascular disease and diabetes. We think the studies might show some common mechanism such as the higher the glycemic index, the higher the glucose level in your blood, which may damage your retinal tissue. We call this disease 'age-related' because it happens to people over 55, but we recommend that people of any age manage their carbohydrates . '
CHUNG-JUNG CHIU , Human Nutrition Research Center on Aging, assistant professor, school of medicine, Tufts University, Boston, Massachusetts