No diet has ever been able to defy the laws of thermodynamics. Whether you go low carb, low fat, low this or low that, the only way to lose weight is to burn more calories than you consume. Even the new "it" diet, volumetrics—which uses fancy terms such as energy density and satiety to describe why filling up on certain low-calorie, water-based foods like celery makes you less hungry—can't miraculously melt away fat. But new research indicates that where on your body you pack on extra kilograms may provide a clue to determining which diet will work best for you.
It is already widely accepted that even the most rigorously adhered-to diet will not produce the same results from person to person. Some of us are simply genetically predisposed to burn more calories more efficiently than others. Restricting those calories, as you do on a diet, will similarly lead to differing results. But the biggest wild card in the diet game may be how you crank out insulin.
As digestion breaks down much of what we eat into sugary, energy-rich fuel that helps keep us on the go, insulin triggers the body to store excess sugar floating around the bloodstream as fat. Insulin was particularly important in our caveman days, when we needed the energy from one meal to last as long as possible, until we had hunted down the next. "Insulin is the hormone of feast," says Gary D. Foster, director of the center for obesity research and education at the Temple University School of Medicine in Philadelphia.
But nowadays, with food so plentiful that groups like Weight Watchers are making a fortune promoting portion control, our insulin is often forced to work overtime, sweeping up the excess carbohydrates we pour into our system from candy bars or fruit juice or starchy foods like pasta. Sometimes insulin can do such a good job of responding to a spike in blood sugar that it causes those levels to quickly drop. This in turn can lead to feelings of hunger shortly after a big meal. For this reason, many scientists think insulin's ride on the blood-sugar roller coaster may be a stimulus for overeating and, as a result, weight gain. It would be nice if there were an easy way to determine how aggressive your particular insulin response is, and now it appears there is.
In a study of 73 obese adults published last month in the Journal of the American Medical Association (J.A.M.A.), Dr. David Ludwig, director of the obesity program at the Children's Hospital Boston, and his colleagues looked at high- and low-insulin secretors. People who rapidly secrete a lot of insulin after eating a little bit of sugar tend to carry their excess weight around their waist—the so-called apple shape. People who secrete less insulin carry their excess fat around their hips—the pear shape. Those differences are more than aesthetic. The study found that high-insulin, apple-shaped people will not lose as much weight on a diet that restricts fat calories as they will on a low-glycemic-load diet—one that restricts simple carbohydrates from sugary and starchy foods like cookies and potatoes. Low-secreting, pear-shaped people will do equally well on either type of diet. But the results went deeper than simply how much weight was lost.
Over the course of six months, high-secreting, apple people lost an average of 6 kg on a low-glycemic diet and just 2.3 kg on a low-fat diet. Low-secreting, pear people lost about 4.5 kg on both diets. At the end of 18 months, however, the pear-shaped people had gained back half of the weight they had lost on either diet. Apple-shaped people gained back almost 1.4 of the 2.3 kg they lost on the low-fat diet but kept off all the weight they lost on the low-glycemic diet. While the study is revealing, almost nothing about it is simple. It's not clear just what the mechanism is that links body shape and insulin levels—a crucial detail if scientists are going to understand the full implications of their findings. More important, nothing suggests that apple-shaped people should simply dash out to sign up for an Atkins-type low-carbohydrate diet.
True, a large report published in J.A.M.A. earlier this year showed that regardless of body shape, Atkins produces the greatest short-term weight loss. ("If you want to look good in your wedding gown, I would go for Atkins," says Dr. Anastassios Pittas, assistant professor of medicine at Tufts University School of Medicine.) But adherents tend to fall off the low-carb wagon and quickly gain back unwanted kilograms. What's more, the Atkins diet allows only a small fraction of calories to come from carbs, compared with 40% on the new study's low-glycemic regimen. The more balanced diet allows—indeed, encourages—people to eat whole-grain cereals and other complex carbs that take longer to digest and thus don't cause the rapid fat production that accompanies spikes in blood sugar. Atkins' more restrictive regimen may reduce fat even faster, but people lose weight on both diets. "Atkins just does it with a bludgeon instead of a chisel," says Ludwig.
What's clearer from the study is that apple-shaped people should probably not choose low-fat diets, because the white rice or other types of simple carbs they are still allowed to eat may have a yo-yo effect on blood-sugar levels, making them hungrier sooner. The study didn't evaluate whether these people would do better on an Ornish-style vegetarian diet that restricts fat intake and has dieters make up the difference by eating lots of complex carbs, such as brown rice and oats—which are high in fiber and tend to make people feel fuller longer—as well as low-sugar fruits like blueberries.
For apple-shaped people hunting for the right diet, a blood test to determine insulin levels may help confirm which regimen will work best for them. But for pears, it remains a toss-up. So until scientists find out more about their body shape, they'll have to lose the old-fashioned way: eating less.