A new randomized trial found that low-carb diets increase total energy expenditure.

This week we turn to the BMJ to look at one of the best studies I’ve ever seen in terms of measuring the metabolic effects of low-carb diets and one that the media has really, really misinterpreted.

To begin, here is a picture of my friend Austen.

That handsome guy lost 43 pounds on a ketogenic diet last year, and is a big proponent of that whole thing. I, being a hardheaded academician, assumed silently to myself that really what was going on here is that he was eating less calories. Can’t snack on office donuts if you’re not eating carbohydrates.

Calories in, calories out. That’s what weight loss is all about.

You reduce calories in by eating less. You increase calories out by exercising more.

But what if there’s another way? What this study suggests is that the calories you burn may INCREASE based on the type of calories you take in.

In other words, taking in 1000 calories of carbs might be worse than 1000 calories of fat because the former will reduce the rate at which your body burns calories in general. Think of it as a sugar tax, with apologies to Mike Bloomberg.

Researchers took 234 overweight individuals and put them on a strict diet to lose 12% of their body weight.

164 made the cut. But that was only the beginning of the study.

With their bodies in full-on energy-conservation mode, these 164 participants were randomized to three diets – 60% carbohydrate, 40% carbohydrate, and 20% carbohydrate.  All diets were 20% protein, so basically we are looking at high carb diets vs. high -fat diets here.

For 20 weeks, their calorie intake was carefully titrated to ensure they did not lose or gain any weight.

After 20 weeks of regimented meals, their total energy expenditure was assessed via double-labeled water.  This is the gold-standard method of measuring total energy expenditure.

Now, if all calories are equal, and everyone’s weight was kept the same, the total energy expenditure in all three groups should be the same.

They weren’t.

Those in the low-carb diet group had significantly higher total energy expenditure than those in the other groups. They were burning about 209 calories per day more than the high carb group.

That might not seem like a lot, but if you burn 200 extra calories a day and keep your caloric intake stable, you’d lose around 20 pounds a year.

Elegant study, compelling results.

Are we done here?

Not quite.

 Many media outlets are reporting that this study shows that low-carb diets will help you lose weight.

The study does not show that. Everyone in this study lost weight before they were randomized to the diet.

Some outlets are doing a bit better – stating that the study shows that low-carb diets help keep weight off.

The study does not show that either.

Remember, weight was completely controlled in this study because the researchers controlled what the participants ate in order to keep their weight stable.  Everyone in the study, regardless of the assigned diet, maintained weight loss.

This study shows that low-carb diets increase total calories out. That’s it. Could it mean that low-carb diets will help with weight loss? Sure. But we don’t know that yet.

Still – diet researchers, take note, this is how you do a rigorous diet study. Through really careful experimentation and measurement, we’ve learned that, just maybe, all calories are not created equal. It’s a great appetizer – but we’re still waiting for the main course.

This commentary first appeared on medscape.com.