Perspective | Low-carb diets can work, but not for the reasons people think

To those people I suggest you read the comments and comments on social media and you will find many people with the second objection:

2. Energy conservation does not apply to the human body! They’re hormones or mass or something, and calories are just an invention of the processed food industry.

I suggest to those people that they talk to a physicist.

And now, the interesting argument:

3. Yes, the calorie-in versus calories-in math has to work, but low-carb diets affect both sides of the equation, so you can’t dismiss (as I did) the effect as real but small.

Everyone agrees that the macronutrient content of your diet can affect how many calories you burn (because fats, carbohydrates, and proteins are metabolized differently) and how many you eat (because some foods are more satiating than others). The disagreement revolves around the magnitude of those changes, particularly with respect to low-carbohydrate diets. So let’s talk about that.

The first is that low-carb diets affect the calorie loss part of the equation. Fewer carbs mean less insulin, and because insulin is integral to fat storage, the reasoning is that you end up burning calories instead of storing them. That’s a plausible and testable theory! Take people into a controlled environment and feed some people low-carb diets and some people high-carb diets with the same amount of calories, and see who loses the most weight.

Then there’s the calorie side of the equation. Because low-carb diets are more satiating, the reasoning goes, it’s easier to eat fewer calories. That’s a plausible and testable theory too! Assign people to different types of diets without calorie restrictions and see who loses the most weight.

Luckily for us, both tests were done, so let’s look at the evidence.

Ah, evidence. It’s tricky territory, because you can find “evidence” to support almost any conclusion you want to draw. And, if you’re into the diet discourse, you’ve no doubt seen long lists of evidence supporting the low-carb theory.

When you look at a list like that, this is the question you have to ask yourself: Is someone trying to Test it something is true, or discover if it’s true? As a journalist, my job is B, but I’m as susceptible to confirmation bias as the next person, so I’ll do my best to analyze all the evidence. I’m sure you’ll keep me honest.

Let’s incorporate the calories first. If you really want to know if you eat less on low-carb diets, park at PubMed, the repository of journal articles. I focus on meta-analyses because their job is to evaluate the preponderance of the evidence. If you use “low carb diet weight loss” as your search terms and restrict the results to meta-analyses, you will get 61 results. I read all the ones that seemed relevant.

They all said the same thing, which is comforting because they are mostly meta-analyzing combinations of the same set of studies. The bottom line is that, in short-term tests (a year or less), low-carb diets outperform other diets by a few pounds (less than 10). Examples include Chawla, 2020; Monsoor, 2016; and Nordmann, 2006. However, two meta-analyses find that people lose slightly more weight on the Mediterranean diet than on the low-carb diet. (Disaster, 2013; Bread, 2018).

After a year, any advantage of eating low-carb foods virtually disappears. Some meta-analyses find a small advantage, but we’re talking about two or three pounds (well, 2013; Tobias, 2015). Others find no advantage (Rafiula, 2022; Silvers, 2022; Nude, 2014; Hu, 2012).

My reading is that low carb diets are easier to follow in the short term. Removing entire categories of high-calorie, easy-to-eat foods from the table (bread, pasta, rice, baked goods) works for many people, at least for a while. And there it is some evidence that very low carbohydrate diets (ketogenic type) are more satiating than other diets, but also a compelling but short-term study That found that people on ketogenic diets ate almost 700 more calories per day than people on low-fat diets.

However, in the long term, what matters about a diet is simply whether you can stick to it, and trials indicate that the low-carb diet is no better than other diets in that regard.

Let’s move on to the eliminating calories part of the equation. Do people who follow low-carb diets lose more weight for the same amount of calories because their bodies burn more energy?

In strictly controlled trials in which subjects receive isocaloric diets with varying macronutrient composition, they do not. There have been several of these trials and I couldn’t find any where carbohydrate content made a difference.

Back in 1977, a small studio compared a 14-day diet containing 70 percent carbohydrates with one containing 10 percent carbohydrates. He found that the subjects lost more weight on the low-carbohydrate diet, but regained it immediately when the diet was stopped, indicating that this was the fluid loss expected from very low-carbohydrate diets. A longer one study in 1992 tested diets comprising between 15 and 85 percent carbohydrates and found no differences in the energy needed to maintain body weight.

In 2015, researcher at the National Institutes of Health Kevin Hall found that Subjects on a low-carbohydrate diet burned less fat (53 grams/day) than those on a low-fat diet (89 grams/day). AND in 2016Hall found that a ketogenic diet did indeed lead to greater energy burning, but that the small difference did not correspond to fat loss.

I wanted to make sure I wasn’t missing anything, so I consulted with David Ludwig, a professor of nutrition at the Harvard School of Public Health and a leading proponent of the low-carb diet. He told me (via email) your meta-analysisnot of weight loss but of total energy expenditure (TEE), showing that low-carbohydrate diets lead to greater energy expenditure, but the greatest effects do not occur until after about 17 days.

Hall and Ludwig have disagreed about how to measure TEE and how these studies should be interpreted, and although they see things very differently, I think their back-and-forth is an excellent example of how scientific disagreements should be handled. Congratulations to both of you.

Ludwig hangs his hat on the TEE, which he told me is an “instant measurement” that doesn’t require waiting until the deficit manifests as fat loss. But, as Hall says He has written, sometimes TEE doesn’t add up, especially in outpatient studies. One study found that a low-carbohydrate diet increased TEE by 200 to 300 calories per day, for example, but reported that he consumed 480 fewer calories than he expended, even though average weight remained stable.

I’m going to go out on a limb and suggest that the gold standard for weight loss testing is actual weight loss, and if the amount of excess energy burned on low-carb diets was significant, we’d expect to see it appear in Weight Loss ( or, more particularly, fat). And it is not like that.

Ludwig explained this to me: “In my opinion, anything that lasts less than 3 months is so hopelessly confused by transient effects and so underpowered that it is practically meaningless.”

A three-month study in hospitalized patients, with carefully controlled diets, is probably prohibitively expensive, but I guess that’s where I’ll rest my case. Yes, low-carb diets can increase energy expenditure, but so little that the signal would not overcome the noise for at least 3 months.

I am a supporter of low carbohydrate diets; They eliminate large groups of foods that most of us should probably eat less of, and some people find a way to preserve them for the long term. But there is no metabolic magic.

You lose weight on low-carb diets because… drum roll please… you eat less.