A new study from the Harvard School of Public Health followed more than 120,000 people over the course of as many as 20 years, to see what they ate and how it affected their weight. Some of the findings are surprising, including these: Nuts and yogurt were most closely correlated with weight loss. And potatoes of any kind caused the most weight gain. Michele Norris talks with one of the authors of the study, Walter Willett from the Harvard School of Public Health.
ROBERT SIEGEL, host:
From NPR News, this is ALL THINGS CONSIDERED. I'm Robert Siegel.
MICHELE NORRIS, host:
And I'm Michele Norris.
There is good health news today for people who love to eat nuts - and bad news if you're a regular consumer of potatoes, whether they're baked or french fried.
A new study from the Harvard School of Public Health followed more than 120,000 people over the course of as many as 20 years, to see what they ate and how it affected their weight. And some of the findings are surprising, including these: Nuts and yogurt were most closely correlated with weight loss. And potatoes of any kind, even just boiling them and eating them plain, caused the most weight gain.
For more on the study, we're joined now by one of its authors. Walter Willett is a professor at the Harvard School of Public Health. Welcome to the program.
Professor WALTER WILLETT (Epidemiology/Nutrition, Harvard School of Public Health): Great to be with you.
NORRIS: We mentioned nuts and yogurt and potatoes. But can you tell us a little bit more some of the other foods you looked at, and how they affected weight gain or loss?
Prof. WILLETT: In general, we found that highly refined foods - sugary beverages and potatoes - were related to greater weight gain on a diet. So that would include things like white bread and white rice. Sugary beverages are very important here because many people take multiple servings a day, so that is actually the number one problem if you look at the whole overall diet pattern.
On the other side, foods related to lower weight gain were, not surprisingly, fruits - not counting fruit juices, though; they were on the bad side - vegetables, high-fiber foods, nuts and yogurt.
NORRIS: I just went to go back to potatoes for a minute, because I'm sure there's someone who's listening to our conversation, thinking about serving mashed potatoes for dinner. Why do potatoes or refined carbohydrates lead to weight gain?
Prof. WILLETT: There is a very strong hypothesis that it's because potatoes, since we cook them, are very rapidly broken down into sugar. It's quickly absorbed into the blood, removed rapidly by the action of insulin and in a few hours, we're hungry again. So they're not very satiating in the long run.
Particularly problematic, of course, are potatoes that are made into french fries and potato chips because they're really designed to make us overeat. And unfortunately, many of us are susceptible to that seduction.
NORRIS: But they are good.
Prof. WILLETT: That's the problem.
(Soundbite of laughter)
NORRIS: Well, on the other hand, some of the foods that are good for us, like nuts and yogurt, why do they accelerate weight loss?
Prof. WILLETT: For nuts, for some reason - this has been studied quite a bit. We don't totally understand it, but they seem to be very satiating. It's probably because we chew them for a while, and then they stay in our stomach for quite a while. They're a package of fat and fiber and a lot of micronutrients that's not rapidly absorbed...
NORRIS: But there is a lot of fat in nuts.
Prof. WILLETT: There is a lot of fat there, but as long as they're keeping us satisfied for quite a long time, calorie-for-calorie they make it easier to control our overall caloric intake.
NORRIS: And yogurt?
Prof. WILLETT: Yogurt, that was a bit of a surprise to us - in fact, probably the most surprising finding here. There has been a lot of work in recent years on how the types of bacteria in our colon influence our state of inflammation and weight gain. So it may well be that the healthy bacteria in the yogurt are helping change the mix of bacteria in the colon and influencing weight.
NORRIS: Now, professor Willett, when I go to the grocery store and I look in the dairy aisle, where they sell all that yogurt, a lot of it has a lot of sugar in it.
Prof. WILLETT: That's right, and I think it's better to stay with a natural yogurt without added sugar.
NORRIS: I want to ask about one of your particular findings because I hail from the Midwest, and I happen to like cheese. And I know one of the other producers working on this has a thing for a cheese as well. And I was delighted to see that cheese doesn't actually appear to have that much impact on our overall weight gain - which is surprising in itself, 'cause I know that cheese often does have a very high fat content.
Prof. WILLETT: Right. I'm from the Midwest, too. So that looked pretty good to me.
It is interesting that just because a food has fat doesn't mean that it's going to be fattening. I think one of the lessons out of this analysis is, to a large extent, how satisfied we are - that really helps us control our caloric intake in the long run.
Just counting calories means sometimes, we're fighting against our physiology. But eating foods that keep us satisfied is really going to be important in the long run.
NORRIS: Professor Willett, thank you very much.
Prof. WILLETT: Good to talk to you.
NORRIS: Walter Willett is one of the authors of a new study that followed more than 120,000 people to see what they ate, and how it affected their weight.