Does glycemic index matter? That’s the key question in a new study, published this week in the Journal of the American Medical Association (JAMA) and reported on yesterday in The New York Times.
Glycemic index and glycemic load (a related system which factors in serving size) are ways of measuring whether foods cause blood sugar to “spike,” and are often put forth as among the ways to differentiate “good carbs” from “bad carbs.” (Because these concepts are poorly understood by the public, Oldways co-organized a gathering of leading glycemic impact scientists for a summit in Italy last year, which resulted in a Consensus Statement on existing research.)
In the JAMA study, 163 overweight, non-diabetic adults spent 5 weeks each on four different controlled diets, all based on the healthy “DASH” diet and varying in carbohydrate and glycemic-index levels. At the conclusion of the 20-week period, researchers, led by Frank Sacks, MD, of Harvard Medical School and the Harvard School of Public Health, found that all four diets resulted in similar insulin sensitivity, systolic blood pressure and HDL/LDL cholesterol levels.
A quick skim of the NY Times article might lead you to conclude that once again, everything we know about eating well has been stood on its ear. But a closer look at the actual study shows it’s not so – and brings out some important points not included in the article.
1. All four diets improved blood pressure and cholesterol. Study participants were overweight people who were put on a healthy DASH diet (fruits, vegetables, whole grains, legumes and lean meats) for 20 weeks. Their LDL (“bad”) cholesterol went down about 9-10% and their blood pressure dropped several points. At a glance it looks like none of these diets made any difference – when the real news is that all variants of a basically healthy diet improved health.
So does glycemic index matter to you? Depends on how you’re eating overall. The study specifically says, “This trial did not address the effect of glycemic index in a typical US diet.” Our conclusion: Just eat well, and tweaks in a healthy diet don’t make much difference. But you need to eat well first.
2. Diabetics weren’t included. Although the Times article touches briefly on this point, it’s worth emphasizing. The study itself says, “We caution that we did not study lowering glycemic index in people with type 2 diabetes to control their hyperglycemia; 2 meta-analyses reported benefits and our findings should not be extended to type 2 diabetes.”
At a time when the American Diabetes Association reports that 29 million Americans (9.3% of the population) have diabetes and another 86 million have pre-diabetes, glycemic index and glycemic load may be vitally important for about one-third of our country. The scientists participating in our Glycemic Summit last year stated specifically that “there is convincing evidence from meta-analyses of controlled dietary trials that diets low in GI improve glycemic control in people with type 2 diabetes.”
3. This is just one study, of one facet of carbohydrates. Note that the quote directly above from the summit Consensus Statement is based on multiple controlled dietary trials. A single study, even a well-done one, should never be used to draw sweeping conclusions – and yet this is what the popular press so often does. The title of the NYT article – “Questioning the Idea of Good Carbs, Bad Carbs” – is itself sensationalist. This is just one study about one way of ranking the quality of carbohydrate foods. There are other measures as well: whether grains are whole or refined, fiber levels, overall nutrient content, etc.
4. Weight loss wasn’t considered. Before you conclude that glycemic index doesn’t affect health, remember that this study only looked at a few specific markers for a few specific health conditions. The researchers themselves state, “We also did not study the influence of glycemic index on weight loss. Lowering glycemic index may improve weight loss or maintenance according to a meta-analysis and some more recent clinical trials, although others did not find an advantage of low–glycemic index diets.”
So should we all do anything differently today, when we sit down at the table? Dr. Sacks said in The NY Times that the takeaway – unless we are diabetic – is that we should eat whole grains, fresh produce and high fiber foods “without wondering if they’re high or low glycemic” and without learning “that system.”
The bottom line, in Oldways' view, is that glycemic index and load can have an important role in our understanding of healthy eating -- but healthy people should, as a general rule, employ common sense rather than “eat by the numbers”– be they calories, glycemic index, or micrograms of various nutrients. There are good carbs and bad carbs (just as there are differences between fats and proteins), and most of us know who’s naughty and nice without resorting to charts. We all know the difference between an apple and a fruit roll-up, or between a bowl of oatmeal and a cheese doodle – and numbers and labels rarely come into play on “real foods.” As David Katz, MD, of Yale’s Prevention Research Center said at our recent Whole Grains: Breaking Barriers conference, “Get the foods right and the nutrients take care of themselves – and getting it right includes whole grains.”
Go forth. Eat well. Enjoy plenty of good carbs, such as fruits, vegetables, legumes, and whole grains. You already know that a whole grain cookie is still a cookie.
-- Cynthia and Sara