What happens if you add fruit to your regular diet, eating three apples or pears as between-meal-snacks every day? I explore this in my video Eating More to Weigh Less, which I titled as a nod to Dr. Dean Ornish’s smash bestseller.

Fruit is low in calories, but it does have some. It isn’t calorie-free. So, if you add food—even healthy food—to people’s diets, won’t they gain weight? No, subjects who ate three apples or pears every day added on top of their regular diet lost a couple of pounds. Was that reduction because of all that fiber since our gut bacteria can create anti-obesity compounds from fiber? (See my Beans and the Second Meal Effect video for more on this.) Good question. That’s why, in addition to the fruit groups, the researchers had a cookie group!

Subjects ate either 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 did not lead to weight loss. The researchers thought the weight-reducing secret of fruit was its low energy density, meaning 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 adults and in children and adolescents…Energy density is defined as the amount of energy [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 water and/or fiber are generally lower in energy density. On the other hand, because dietary fat provides the greatest amount of energy per gram [calories per unit weight], foods high in fat are generally high in energy density.”

The CDC offers some examples. High energy density foods are like bacon, which have a lot of calories in a small package. A medium energy density food is like a bagel, and low energy density foods are typified by fruits and vegetables. In general, the lower the better, but there are two exceptions: Soda is so heavy that by energy density it looks less harmful than it is, and nuts have so much fat that 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 managing body weight.” This is because people tend to eat a consistent weight of food. So, when there are fewer calories per pound, caloric 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 vegetables and fruits to recipes or by lowering the fat or sugar content.” 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 emphasizing 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 1,570—without cutting portions. They simply substituted high energy dense foods with less calorie dense foods. That means subjects ate lots of fruits, vegetables, whole grains, and beans, compared to having high-energy density meals with lots of meat and sugar. They ate nearly half the calories, but they reported enjoying the meals just as much.

Researchers tried this in Hawaii by putting people on a traditional Hawaiian diet with all the plant foods they could eat. The subjects lost an average of 17 pounds in just 21 days, resulting in better cholesterol, triglycerides, blood sugars, and blood pressure. Caloric intake dropped 40 percent, but not by eating less food. In fact, they lost 17 pounds in 21 days while eating more food—four pounds of food a day. But, because plants tend to be so calorically dilute, one can stuff oneself without seeing the same kind of weight gain.

“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 diet quality.” For example, lower energy dense diets are associated with lower risk of pancreatic cancer.

Lower energy-dense diets tend to be of healthier foods, so we get the best of both worlds.

I talk more about the energy density concept in The Ice Diet and Nutrient-Dense Approach to Weight Management. Are There Foods with Negative Calories? Find out in my video!

The amazing Hawaii study was done by Dr. Terry Shintani. Find out more about the natural human diet in What’s the “Natural” Human Diet?.

Doesn’t fruit have a lot of sugar in it? Check out my videos:

In health,

Michael Greger, M.D.