A small piece of fried chicken, a few fries, and about a cup of cola have 500 calories. So do a bowl of mixed melons, chicken and seasonal tomatoes, rice pilaf, baby arugula salad, and two cups of unsweetened iced tea.
Which dinner is likely to leave you feeling less hungry?
Studies show that, day after day, people eat about the same weight (or volume) of food. So choose low-calorie foods that make you full. That’s the idea behind The Ultimate Volumetrics Diet, a book by Pennsylvania State University’s Barbara Rolls.
We spoke with Dr. Rolls to find out what she had to say about calorie density and how to get full on low-calorie foods.
Why did you start studying calorie density?
Like most scientists in my field back in the 1990s, I was looking at the effects of fat, carbohydrate, and protein on hunger, satiety, and food intake. And I kept noticing that people were eating a consistent amount of food. Rather than regulating their calories, it looked like they were managing the weight of their food.
They ate the same weight or the same volume?
Both, but it’s harder to get a reliable measure of volume, so we would weigh what we gave them before and after a meal. And we found that as long as the food was equally palatable, people tended to eat the same weight of food.
So then we thought, if we lower the calorie density—the calories per bite or per any given portion of food—perhaps we can lower the overall calorie intake of the meal.
It took us about a year to figure out how to make low-calorie foods that make you full without changing the palatability of the recipes.
The high- and low-calorie foods had to taste equally good?
Yes. Once we got that down, we saw that we could lower the calorie density by about a third. And sure enough, people over a couple of days ate about the same amount of food. They were spontaneously reducing their calorie intake by about 25 percent. That was really exciting. It was a new way of thinking about food.
How do you cut the calories per bite?
The key is to change the amount of water in foods, because water adds weight and volume but no calories. So, for example, if you increase the amount of water in a casserole—which we do primarily by adding vegetables, which are mostly water—you get low-calorie foods that make you full.
And the people weren’t trying to lose weight?
Not in those studies. However, in our one-year trial, lowering calorie density did lead to weight loss. We told one group to eat smaller portions and low-fat foods. That lowered calorie density. They lost 15 pounds in the first six months.
The other group got similar information about fat but were told to eat more fruits and vegetables, soups, and other foods with low-calorie density. And they lost 20 pounds in the first six months.
At the end of the year, both groups had kept most of the weight off. They regained only 1 1⁄2 pounds.
Have larger studies looked at weight loss and calorie density?
We’re gradually building up a consistent story. For example, when we pooled all the data on 700-plus people in the Premier trial, we found that calorie density was the main predictor of how much weight people lost. If people were eating a diet that was less calorie dense, they were eating significantly more food—about a pound more food a day—and they were eating fewer calories and losing more weight.
That was consistent with a study that looked at weight loss maintainers— people who have lost at least 10 percent of their maximum body weight and maintained the loss for at least five years. The maintainers ate foods with lower calorie density than people who were either overweight or normal weight.
The average maintainer ate five servings of vegetables a day, while the normal-weight participants ate four a day and the overweight participants ate three and a half.
Does portion size matter?
Yes. In other studies, we served people ordinary portions for 11 days or increased the portion sizes by 50 percent for another 11 days. With larger portions, we saw a huge increase in how much people ate—about 400 calories more a day.
I was stunned. I thought people would slow down when they realized that they were eating more. But they just don’t notice.
Don’t big portions make people feel more full?
Apparently not. Most of the literature indicates that within three or four days, your body should detect a perturbation in calorie balance. Our first study lasted two days, and people said it didn’t go long enough, so we tried 11 days.
People don’t see the two serving sizes side by side?
No. There’s usually a week between tests. But in our macaroni-and-cheese study, we doubled the portions. That’s a huge difference, yet people were unaware.
And in our lab, people are eating alone in a booth. They have nothing to think about except the food in front of them. And they’re still not noticing that we’re varying the portion size
Aren’t people more distracted than that when they eat?
Yes. If you think about people eating out, not paying attention, having some wine, talking, it’s even less likely that they’re going to notice portion size.
We all hear that portion sizes have gotten bigger, but people just don’t pay attention. If the food is in front of them, they tend to eat it. It’s a really robust effect, and it affects everybody, whether they’re plate cleaners or dieters, restrained or unrestrained eaters.
If people eat the same amount of food day after day, why would big portions make them eat more?
We tend to eat the same weight or volume of food, but when we’re exposed to large portions, they can override this tendency and ramp up our intake.
The good news is that if big portions are low in calorie density, they don’t lead to weight gain like big portions that are calorie dense.
Everybody blames portion size for the obesity epidemic. It’s not big portions that are leading to obesity. It’s big portions of calorie- dense foods. If we ate bigger portions of low- calorie-dense foods, that would help us eat less.