 What happens if you have people add fruit to their regular diet? Three apples or three pears a day is snacks between meals on top of whatever else they were eating. Fruit is low in calories, but not zero. So if you add food to people's diet, even healthy foods, won't they gain weight? No, they lost a couple pounds. Maybe it was all that fiber? If you remember, we learned our gut bacteria can create anti-obesity compounds from fiber. That's why they also had a cookie group. Three apples, three pears, or three cookies with enough oats in them to have about the same amount of fiber as the fruit. Despite the fiber, adding cookies to one's diet does not lead to weight loss. They think the weight-reducing secret of fruit, if it is its low energy density, means you get a lot of food for just a few calories, so it fills you up. Energy density is a relatively new concept that has been identified as an important factor in body weight control in both adults and in children and adolescents. Energy density is defined as the amount of calories per unit weight of a food or beverage. Water, for example, provides a significant amount of weight without adding calories, fiber, too. Thus, foods high in fiber and water are generally lower in energy density. On the other hand, because dietary fat provides the greatest amount of calories per weight, foods high in fat are generally high in energy density. The CDC offers some examples of high energy density foods are like bacon, lots of calories in a small package. Medium energy density food is like a bagel and a low density food typified by fruits and vegetables. In general, the lower the better, with two exceptions. Soda is heavy, so heavy that by energy density it looks less harmful than it is. And nuts have so much fat they appear less healthy than they are. Otherwise, though, the science supports a relationship between energy density and body weight, such that consuming diets lower in energy density may be an effective strategy for weight control. This is because people tend to eat a consistent weight of food, so when there's less calories per pound, calorie intake is reduced. A small drop in energy density can lead to a small drop in weight, and the greater the decrease in energy density, the greater the weight loss. Energy density can be reduced in a variety of ways such as the addition of fruits and vegetables to recipes, or by lowering the fat or sugar content. And indeed, that's how we evolved, eating predominantly low energy density foods, such as fruits, vegetables, plants, and tubers, starch-filled roots, like sweet potatoes. The first study to emphasize how fruits and vegetables could affect energy density and food intake was conducted more than 30 years ago. Researchers were able to cut people's caloric intake nearly in half, from 3,000 calories a day down to 1570 without cutting portions, by substituting less calorie-dense foods, which means lots of fruits, vegetables, whole grains, and beans, compared to a high energy density meal with lots of meat and sugar, nearly half the calories, but they enjoyed the meals just as much. They tried this in Hawaii, putting people on a traditional Hawaiian diet, with all the plant foods they could eat, lost an average of 17 pounds in just 21 days, resulting in better cholesterol triglycerides, blood sugars, and blood pressure. Chloric intake dropped 40%, but not by eating less food. In fact, they lost 17 pounds in 21 days, eating more food, 4 pounds of food. But because plants tend to be so calorically dilute, one can stuff oneself without getting the same kind of weight gain. And the energy density of foods is of interest for weight management, not only because it allows people to eat satisfying portions while limiting calories, but also because reductions in energy density are associated with improved dietary quality. For example, lower energy-dense diets associated with lower risk of pancreatic cancer. Lower energy density foods tend to be healthier foods, so we get the best of both worlds.