Part of what makes these books so popular--despite all the exclamation points--is that they give dieters a single key, or "secret," on which to focus their efforts. The conventional prescription for losing weight--step up your exercise, eat more fruits and vegetables and fiber, cut down on saturated fats and calories--is just so hard! How much easier it seems to blame America's epidemic of obesity on "rising insulin levels."
But isn't insulin something only diabetics worry about? No. In fact, insulin is the critical hormone that allows everyone to absorb simple sugars like glucose from their food as it is digested. Most people are so good at producing the right amount of insulin that they are never even aware of its vital presence in their bloodstream.
Researchers have long known that eating refined sugar can provoke short-lived spikes in the body's insulin levels. But in the past two decades, they have learned that certain complex carbohydrates--starchy roots like potatoes and carrots and highly refined foods like white bread, white rice and white pasta--are broken down into simple sugars so rapidly by the body that they can trigger a strong insulin response. Nutritionists refer to such foods as having a high glycemic index.
Why might that be a problem? Generally speaking, high levels of insulin inhibit the breakdown of fatty deposits in the body. So, it doesn't take much of a leap to suggest that eating too many of the wrong kinds of carbohydrates leads to too much insulin, which in turn promotes the accumulation of fat, thereby setting up the body for continuous defeat in the battle of the bulge. Or at least that's the theory that has launched a dozen diet books.
Turns out the truth is more complicated. Dr. JoAnn Manson, an endocrinologist at Harvard Medical School, cites growing evidence that refined carbohydrates could indeed pose a problem for some people who are prone to diabetes. But, according to her 1997 study of 65,000 nurses, the greatest danger occurs only if those at risk also fail to consume enough whole grains like whole-wheat bread and rolled oats. Reason: cereal fiber has a counterbalancing effect that keeps insulin levels from rising.
No one has proved that chronically high insulin levels promote obesity. "It's sort of a chicken and egg question," Manson explains. "Is it obesity that leads to higher insulin levels, or do higher insulin levels lead to obesity?" The evidence to date favors the former explanation.
So what does Manson tell her patients about Sugar Busters!? "I think it's overstated. There may be a kernel of truth there, but it's not the miracle cure for obesity." You still have to cut down on saturated fats and excess calories. You still need to exercise (which, by the way, also decreases insulin levels). As a general rule, she adds, "the more processed a food, the less healthy it is. But [the glycemic index] can be carried to an extreme." So, eat your carrots, order up some whole-wheat toast and, for heaven's sake, get off that couch!
For more information on nutrition and weight loss and a list of useful online resources, see our website at time.com.