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Low-Carb Diets Don’t Work the Way We Thought

By Brenda Goodman, MAAugust 13, 2015
From the WebMD Archives

You may have seen claims that low-carb diets work by melting away fat, especially belly fat. The thinking has been that because these eating plans lower levels of the hormone insulin, which promotes fat storage, they are the most effective diets to help people lose their muffin tops and spare tires.

Now a new study has laid that theory to waste (and waist.)

Scientists at NIH got 19 overweight adults — 9 women and 10 men — to follow very carefully controlled diets for two 11-day stretches. The study volunteers stayed at the NIH, where they were instructed to eat every morsel of every meal provided to them. For part of these visits, they lived inside a special room called a metabolic chamber, which captured all the air they inhaled and exhaled as well as their urine. This careful collection of body gasses and fluids allows researchers to precisely measure how many calories people are burning and what those calories come from—carbohydrates, fat, or protein.

For the first part of the study, volunteers ate a starting diet designed to keep their weight steady and even out their metabolism. That diet was made up of 50% carbohydrate, 35% fat, and 15% protein. For the next six days, they ate a diet with 30% fewer calories. On the low-carb diet, all those calories were cut by reducing carbs, while protein and fat stayed constant. On the low-fat diet, all those calories were cut by eliminating fat, while protein and carbs stayed steady. All 19 people followed both diets with a break of a few weeks in between.

The results were surprising. People lost more total fat on the low-fat diet than they did when they were eating the low-carb diet. And cutting fat didn’t appear to slow metabolism, while cutting carbs did.

What does all this mean for weight loss in real life? Unfortunately, not much. We spoke to study author Kevin Hall, PhD, a senior investigator in the laboratory of biological modeling at the National Institute of Diabetes and Digestive and Kidney Diseases, who explained why.

Q: What was the purpose of your study?

A: What I want people to understand is that this is a really basic physiology study. It’s really designed to, under very carefully controlled conditions …. find out what happens to the body metabolically when you make these large changes in diet. In other words, cutting specifically fat or specifically carbs — How does the body adapt to that? At the end of the day, is there any net difference in body fat changes when you make equal calorie cuts?

And what we found was that indeed, if you cut carbs, insulin goes down, fat burning by the body goes up, and you lose fat. Perhaps the most surprising thing was when you cut fat from the diet, nothing happens to the number of fat calories you’re burning, nothing happens to insulin, and in fact, if you look at the difference between the fat eaten and fat burned, both diets led to fat loss, but the reduced-fat diet led to slightly more fat loss.

Q: Wow. How does that happen?

A: You can think of it this way: The low-carb diet did increase fat burning, but it didn’t increase it enough to top the fat loss that resulted from not eating that fat in the first place.

The other interesting thing we saw was that the reduced-carb diet led to a significant decrease in calorie burn compared to when people were eating the baseline diet. The reduced fat diet had no significant effect on calorie burn.

Q: So you end up with more net fat loss if you don’t eat it in the first place?

A: That’s right. I think that the most interesting part of our study is that when you cut fat from the diet, nothing happens to the amount of fat that the body burns. It’s like it doesn’t even sense that you’ve just cut this stick of butter out of your diet. It just keeps plugging away. It doesn’t adapt. It just keeps burning the same amount of calories and the same amount of fat.

Whereas if you cut carbohydrates, insulin goes down, fat burning goes up, the number of calories that you burn goes down slightly. … (your body is) trying to adapt to this new diet and make it so the main effect is fat loss, and it does that, but it doesn’t do it as completely as when you cut fat. And these are results from the same people undergoing the same calorie reductions under each case.

Q: This isn’t really very natural at all, because when people go on low-carb diets, usually, they don’t worry about total calories so much as cutting carbs. So if they cut carbs they may eat more protein, etc.

A: One of the things about low-carb diets is that because carbs typically make up about half of what people are eating in the first place, when they target carbs to cut them out of their diets, they end up targeting the most calories. So is the effect because of the cutting of the calories or the cutting of the carbs in particular?

Another argument has been that when you’re eating a low-fat diet, you’re choosing different carbs and those have an effect on insulin. We specifically didn’t want that to happen in this study, so we made sure the sugar content of the reduced-fat diet was the same as the starting diet. We had no changes in insulin that took place.

Q: So if you could cut fat out of your diet without adding more carbs or sugar to make up for it, in theory, you could burn more fat, overall, than if you just cut carbs.

A: That’s kind of what this was saying. You can think of this as a test case of, ‘Is a calorie really a calorie?’ and the answer was not quite, but almost. That’s kind of opposite of what the low-carb folks with their theory about insulin and fat burning have suggested in the past.

Q: Tons of articles have been written about foods that help stoke your metabolism. Would your study suggest there’s only so much of that you can do?

A: That’s a little bit of a different question. But one of the things we found is that when we cut carbs from the diet, people’s metabolism slowed down by about 100 calories a day. When we cut fat from the diet, nothing happened to metabolism. That’s another reason why the reduced fat diet led to greater fat loss than the reduced carb diet.

Q: Do you think it’s even possible for real people to take these results and apply them to their lives?

A: No (laughs). I don’t. We as researchers have a very difficult time measuring what people eat, even when they come into the lab. So I think applying this to real life, to people’s day-to-day lives where they’re presented with opportunities to eat all different kinds of foods, nutrition labels aren’t 100 percent accurate, so I think it’s a mistake to translate this in that sense.

What I would say is that the reasons why people have often claimed that low-carb diets work, we tried to dig in a little bit and try to understand the science—that is if you cut carbs and reduce insulin, it should have some sort of metabolic advantage. This study shows definitively that this is not the case.

The other aspect is that maybe people feel fuller when they eat a low-carb diet. Maybe they’re more satisfied, and they choose to eat less. It may explain that when people are prescribed these diets, people on low-carb diets do tend to lose more weight and fat, at least for the first six months.

Q: What kinds of foods might we be treated to if we enrolled in a study at the NIH metabolism lab?

A: It ended up being pretty boring for subjects. Every day, they had the same breakfast, the same lunch and the same dinner. I’d have to go back and look at my notes to see exactly what those meals were. But basically by the end of six or seven days on those diets, people were not salivating to see that same sandwich again for lunch.

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