A handful of walnuts may be an effective weight loss tool.

Walnuts are rich in omega-3 fatty acids and other substances and, in moderation, have been linked to reduced risk of obesity and diabetes. They may also efficiently reduce appetite.

Researchers now may have found out why. They had nine hospitalized obese patients drink, on five consecutive days, either a smoothie containing 48 grams of walnuts (1.7 ounces, or about 14 walnut halves and 315 calories) or a placebo smoothie identical in taste and calorie content. Then, after a month on their regular diet, the patients returned for a second five-day trial, with placebo drinkers on the first trial receiving a walnut smoothie, and vice versa.

The participants underwent M.R.I. brain exams while looking at pictures of high-fat food (cake, for example), low-fat food (vegetables) or neutral pictures of rocks and trees.

The study, published in Diabetes, Obesity and Metabolism, found that when people looked at pictures of high-fat food, activation in the insula, a part of the brain involved in appetite and impulse control, increased among those who drank the walnut smoothie, but not among placebo drinkers. The study was funded in part by the California Walnut Commission.