David Ludwig is author of Always Hungry (Grand Central, 2016). He’s a practicing endocrinologist at Boston Children’s Hospital and a professor of nutrition at the Harvard School of Public Health in Boston, Massachusetts.

IRA FLATOW: This is Science Friday, I’m Ira Flatow. If you’ve made dieting part of your New Year’s resolutions, you are facing a challenge. Which diet to pick? The paleo? The keto? The Mediterranean? Seems like a new one comes by every day.

And some of the diets count calories, some of them count carbs. In a recent study in the British Medical Journal, scientists tested out the low-carb idea, and they saw that reducing carbohydrates did have an effect in increasing metabolisms, which the researchers said was crucial.

So why is metabolism so important in studying diets? Just one of the many questions we all have about dieting, including what about caffeine and artificial sweeteners? And should we rethink our idea of the calorie count?

Do you have questions about carbs, fats, what you eat affects your metabolism? Our number is 844-724-8255, 844-SCI-TALK. Or you can tweet us at @scifri.

My next guest is here to walk us through it all. David Ludwig is an author on the BMJ study. He’s also a professor of Nutrition at Harvard University, co-director of the New Balance Foundation Obesity Prevention Center and Boston Children’s Hospital. Welcome back, Dr. Ludwig.

DAVID LUDWIG: Great to be with you this first week of the New Year.

IRA FLATOW: Yeah, that’s when everybody did their resolutions, right?

DAVID LUDWIG: Exactly.

IRA FLATOW: There’s so much confusion, when it comes to dieting. Is the public justified in being confused?

DAVID LUDWIG: Well, of course. I’ve said that dieting isn’t nutrition, it isn’t rocket science. It’s actually more complicated. Because you think of diet and nutrition as including hundreds, if not thousands of different individual factors. Carbohydrate, protein, and fat. We’ve heard about fiber.

But the hundreds of vitamins and minerals and other components of our food, the gut microbiome. And the many different ways that these compounds interact with our hormones, our metabolism, even the expression of our genes. And we haven’t even gotten to human behavior, yet, which is why it’s so difficult for one diet study to ever answer every question.

IRA FLATOW: In your study, you found that lowering carbs and replacing them with fats actually increased metabolism. Can you explain the mechanism, and why increased metabolism is important?

DAVID LUDWIG: Yeah. Well, the standard approach to weight control considers all calories are alike. Just eat less and move more. We’ve heard it 1,000 times. Of course, the food industry loves this message, because it means that 100 calories from junk food is, theoretically, no more fattening than 100 calories from Brussels sprouts.

The problem is, when you restrict calories, the body fights back in predictable ways. Hunger increases, metabolism slows down, and that’s why most diets ultimately fail.

But according to another way of thinking, the processed carbohydrates that flooded our diet during the low-fat years have driven up insulin levels, programming the body to gain weight. But by cutting back processed carbs, we might be able to speed up metabolism, making it easier to keep the weight off.

And that’s exactly what we tested. We looked at 164 adults who were at least overweight. Brought their weight down by about 10%, so their body would be under some stress. Their metabolism would be slowing down. And then we randomly assigned them to low, moderate, or high-carbohydrate diets. 20%, 40%, or 60% carbohydrate. We adjusted their calories to keep their weight the same.

After about five months, we found that people on the low-carbohydrate diet, their metabolism was running about 200 or 250 calories a day faster than people on the high-carbohydrate diet. So if that difference were maintained over the long term, it could give a serious boost to our ability to maintain weight loss for the long term.

IRA FLATOW: So actually, your body was burning more.

DAVID LUDWIG: Yeah. The type of calories that you eat may affect the number of calories you burn. And if this is true, then simply considering all calories are alike misses an important lever, an important tool that we can use to help us succeed with long-term weight loss.

IRA FLATOW: Now, a lot of times, when people cut the carbs, they add fat. Adding fat, is that counterintuitive to what you’ve learned?

DAVID LUDWIG: That’s what we did in our study. Although, I want to emphasize that even our low-carbohydrate diet didn’t eliminate all carbohydrates. It really just eliminated the processed carbohydrates. Those are the ones that digest the fastest. White bread, white rice, potato products, sugar. These digest quickly and raise insulin the most.

One advantage of fats, especially the healthy fats we know about– nuts and nut butters, full-fat dairy, avocado, olive oil, even dark chocolate– that they don’t raise blood sugar or insulin at all. And for that reason, they’re very satiating.

Just do a quick mind experiment. Think about bread and butter. Most people would say, between the two, butter is the tastier component. But which is it easier to binge on? People would have no trouble sitting down and eating three, four, five slices of bread.

But imagine eating a quarter stick of butter. After the first bite or two you would just be intensely satiated. In fact, most binge foods tend to be mainly carbohydrates.

IRA FLATOW: And so, the idea then is to eat well. In other words, I hear what you’re saying, don’t count the calories, just count the kinds of food you’re eating.

DAVID LUDWIG: Just to be thinking that the body wants to control its weight. That’s why we didn’t have an obesity epidemic for most of human history, even for populations that had plenty of food around. We aren’t working in the fields 12, 14 hours a day.

The body is designed to regulate weight, just like it regulates body temperature and breathing. But there’s something that happened in the last 30 or 40 years that seems to be driving that body weight set point up for much of the population.

And according to one way of thinking, the processed carbohydrates are an important part of it. Not the whole story, and certainly not the whole story for everybody, but it’s really one of the most likely explanations for why this obesity epidemic kicked off just as we got onto the low fat diet craze.

IRA FLATOW: There were a couple of popular studies out this week that said the Mediterranean diet is still the most-preferred diet. What is your reaction to that?

DAVID LUDWIG: Well, first, there’s no one Mediterranean diet. What the people eat in Italy or Greece was different from Lebanon or Spain. But one component that’s typically considered throughout the Mediterranean diet is– well, there are several.

One is focus on whole foods. So the carbohydrates that are eaten are natural and slower digesting. Fruits, vegetables, minimally processed grains. And there’s typically more fat. Olive oil is almost worshipped in many parts of the Mediterranean.

IRA FLATOW: Let’s go to the phones. 844-724-8255. Let’s see what we’ve got here. Let’s go to Jordan in [INAUDIBLE], California. Hi, Jordan.

JORDAN: Hi. It’s Willits, California.

IRA FLATOW: I’m sorry.

JORDAN: No problem. So my comment is that your guest has been talking about how, if these things were sustained, it could be great for weight loss. But the science shows that 0.8%– that’s less than 1%– of people that start a diet have kept that weight off five years after they started.

And we also know that yo-yo dieting– losing a lot of weight and then gaining it back, and then losing a lot of weight and gaining it back– is incredibly bad for your heart, and otherwise bad for your health. So my question is, if the science shows that dieting doesn’t work period, and we know that it’s unhealthy, why are we still encouraging people to go through this?

IRA FLATOW: OK, that’s– got a question, Dr. Ludwig.

DAVID LUDWIG: Yeah, great question. I love that question. It’s how we define dieting. And in fact, what the caller is saying is perfectly consistent with the findings of our BMJ study. That if you focus on cutting back calories– that’s been sort of the Holy Grail of weight loss over the last half century. Focusing on eating less, moving more, just getting a negative calorie balance, you’ll of course lose weight for a while. But the body fights back against that.

So our study, and not just this study, but many other lines of research suggests that the quality of our foods make more of a difference over the long term than the quantity. If you get the quality right– in other words, if you control blood sugar, hormones, you calm chronic inflammation and insulin resistance, then the body has a better chance to find a healthy weight naturally on its own.

It’s just like breathing. You don’t think about breathing. You can hold your breath, you can speed up your breath or slow it down. But ultimately, your body takes over naturally. So body weight could be the same thing. It’s over a longer period of time, but there are control systems in the brain and in organs throughout the body that are designed to do that.

We’ve got to figure out what has mucked up this mechanism, and restore conditions that will help the body succeed without that struggle.

IRA FLATOW: Let’s go to the phones to Miles in Portland, Oregon. Hi, Miles.

MILES: Hi.

IRA FLATOW: Go ahead.

MILES: Well, I’ve been hearing a lot lately about the benefits of reducing the number of hours in which you can eat during a day. Where you would only eat within an eight-hour period, and you have 16 hours in which to digest, I suppose.

I’m wondering what your research or your experience might say about that as a beneficial diet for everyone.

DAVID LUDWIG: This is intermittent fasting, and it’s very popular. And I think it’s a great tool for some people, but it’s a two-edged sword. Just look around, many people have difficulty just driving in the car for a half hour without eating. We’re constantly snacking.

And according to this other way of thinking– it’s called the carbohydrate insulin model, that we tested in our BMJ study– that hunger is being driven by the swings in blood sugar and insulin levels.

So yes, fasting can be very helpful to metabolism, but the first thing we’d need to do is eat a type of diet that’s going to calm down those swings, that help fat cells calm down. And once that happens, people will be able to fast more easily. But if you just take a person on a standard low-quality diet, they’re going to struggle just going from lunch to dinner without eating, let alone these more elaborate kinds of fasts.

IRA FLATOW: You touched on a couple of points I want to make, before we run out of time. How does our diet affect inflammation? Because physicians, researchers are always talking about they one of the great villains in our health is inflammation in our body and our blood vessels, things like that. Does the diet affect that?

DAVID LUDWIG: Absolutely. We know that obesity is a big contributor, but there’s nothing specifically wrong with having some extra fat in the body. It’s when that fat becomes inflamed, when the immune’s response floods into fat cells.

The fat cells get inflamed and release a whole series of compounds that produce inflammation throughout the body and other organs, even in the brain. And that sets the stage for the classic chronic complications such as diabetes, heart disease, maybe even Alzheimer’s.

Independent of weight, we can adjust, we can come inflammation down– and its sort of twin troublemaker, insulin resistance– by the quality of what we’re eating. We know that high-quality fats, slow-digesting carbohydrates, the right amount of protein, lots of natural unprocessed foods tend to do that.

IRA FLATOW: OK, let’s go to the phones to Connecticut. Melissa in Connecticut. Hi, Melissa.

MELLISA: Hi, thanks so much for taking my call. I am overweight, I have type 2 diabetes, and I am a vegetarian. So many people have recommended to me a low-carbohydrate diet.

But when I look into them, they never really seem very accessible to vegetarians. They seem to focus more on having a higher protein, which is usually like the lean proteins like chicken breast, and things like that.

So I just wanted to know what you would recommend for somebody like me.

IRA FLATOW: All right, let me just remind everybody that this is Science Friday from WNYC Studios.

DAVID LUDWIG: Right. Well, that’s a great question. And unfortunately, there’s a lot of misunderstanding leading to unnecessary polarization with the vegetarian and vegan community, for ethical reasons. Tending to favor low-fat diets, and then the low-carbohydrate group tending to favor a more animal product-based diet.

But that conflict doesn’t have to occur in the modern food environment. You can eat low-fat with a lot of animal products. Dry chicken breast, and low-fat dairy and egg whites. Or you can eat a rich high-fat diet which is mostly plant or entirely plant-based, with foods like nuts and nut butters, olive oil, rich sauces and spreads, dark chocolate.

So in terms of diabetes, I can’t make any specific recommendations for you. But diabetes is, by definition, carbohydrate intolerance. It means the body can’t metabolize the carbohydrates from a meal.

So for people with that condition, or severe insulin resistance, which is closely related, I think low-carbohydrate diets– which could be plant-based, as we just discussed– are especially appealing.

IRA FLATOW: One last question before we go has to do with coffee. There are studies that are showing that coffee is good for you, and not just caffeinated coffee. Something in coffee, right?

DAVID LUDWIG: Yeah. It doesn’t seem to be the caffeine, per se, because the protection quite consistently is seen once you get above one or two cups of coffee with decaf or with regular. So why is that?

One explanation is that for people eating a typical American diet– very low in quality, very few protective plant substances– antioxidants, vital chemicals, these things that calm inflammation. That for people on that diet, coffee becomes their main source of these plant protective substances.

And so we don’t know yet what would happen with people eating a very high-quality diet, whether you’d get additional protection. But I think we can all feel pretty good, as long as you don’t have any side effects from the caffeine. You’re drinking two or three cups of day.

IRA FLATOW: Two or three cups. What is the protection you’re talking about?

DAVID LUDWIG: The most striking and consistent protection is against type 2 diabetes. And there are some other chronic diseases that seem to be of benefit, as well.

IRA FLATOW: You seem to be saying, don’t go on a temporary diet to lose weight, but basically a lifetime of changing your eating habits.

DAVID LUDWIG: Well that’s the word diet in its original– I think it was Greek. It means a way of life. And I think that our research suggests that a focus on food quality, rather than calories and quantity, may get you there more slowly. But it’ll keep you there over the long term.

And we certainly need more research, and everybody’s different, and one size isn’t going to fit all. But this is an exciting time to be in nutrition research, and I look forward to discussing this with you more through the years.

IRA FLATOW: I’m very happy to have you on. I am actually going to ask you to stay through the break here, because there are so many people who have so many questions. For example, a tweet from Melissa.

Can genetic testing help with knowing–

I’ve got less than a minute to go. I’m going to read the tweets, we’ll answer them after the break.

–where your origins are, and can help with what you are designed to eat?

Another tweet says, what about bloating? Why did this happen, even when we eat quality foods? And Chris says, why do some people crave carbs so much, so much more than others? I have always preferred a high-protein diet low in sweets, and weight has never been an issue.

Well, let’s get to those tweets after the break, talking with David Ludwig who’s the author of a British Medical Journal study. He’s professor of Nutrition at Harvard University. Our number 844-724-8255. We’ll be right back after the break, so stay with us.

This is Science Friday, I’m Ira Flatow. We’re talking with David Ludwig. He is a professor at Harvard and an expert in nutrition. We’re talking about the dieting that people are going through.

So what is your recommendation, in the few minutes I have left, Dr. Ludwig? What is your recommendation for people who say, I’ve got to change my way of eating? I want to lose weight, but I want to keep it in a plan that lasts me for a lifetime.

DAVID LUDWIG: Right. Well, I think keep the focus on quality over quantity. I think we now really clearly understand that when you go on a conventional low-calorie diet, hunger goes up. That’s the very first thing that happens.

And hunger isn’t a fleeting feeling, it’s a primal biological signal that your body wants calories. And even if you could ignore it, which most people can’t even for one day, your body would fight back in other ways, with slowing metabolism.

We need to figure out how to eat that doesn’t exacerbate this body pushback. In that battle between mind and metabolism, metabolism tends to win. So this is the big debate. Does the relative amounts of protein, fat, and carbohydrate, or other aspects of our diet matter there?

My opinion is that the processed carbs are the main problem, here. They drive up insulin. Insulin, you can call it the miracle grow for your fat cells. It programs your fat cells to store extra calories. It may be a subtle effect, but over weeks, months, and years, I think there’s a plausible case to be made that that’s underlying a good part of the obesity epidemic.

So we can focus on cutting back the processed carbs that flooded our diet during the low-fat years– white bread, white rice, potato products, added sugar– replace that with unprocessed carbohydrates– whole fruits, and minimally processed grains, the way that grandparents used to eat it.

And not fear fat. Fat makes food tasty, and it slows down digestion, doesn’t raise insulin. So in my view, fat should be encouraged, not feared in a weight loss plan.

IRA FLATOW: You sound very much like that Michael Pollan. Eat what your grandmother ate, and shop on the outside aisles of the grocery store.

DAVID LUDWIG: Sounds good.

IRA FLATOW: David Ludwig, professor of Nutrition at Harvard University and co-director of the New Balance Foundation Obesity Prevention Center at Boston Children’s Hospital. Have a happy New Year, and thank you for taking time to be with us, today.

DAVID LUDWIG: Great to be with you.