I’ve been eagerly awaiting this study. It was a 12-month randomized controlled trial comparing non-calorie-restricted low-fat and low-carbohydrate diets. Both diets placed a major emphasis on diet quality. I’m not going to review the findings in detail because Examine.com has already beat me to it. But here are some highlights:
- Participants achieved large differences in fat and carbohydrate intake for the duration of the 12-month trial, although the difference narrowed somewhat over time (as it does in every diet trial).
- The first primary (most important) outcome was about insulin. Genes influencing meal-related insulin secretion were unrelated to a person’s response to the low-fat or low-carbohydrate diets. [Correction 2/21/18: These genes aren’t directly related to insulin; they’re more closely related to fat metabolism and previous preliminary evidence suggested they might impact the response to low-fat/carbohydrate diets]
- The second primary outcome was also about insulin. A person’s baseline insulin response to carbohydrate was unrelated to his/her weight loss response to the low-fat or low-carbohydrate diets.
- Both diets caused similar weight and fat loss at 12 months (-5.3 kg [-12 lb] for LF vs. -6.0 kg [-13 lb] for LC). These are better 12-month results than most diet trials.
Here are a few reasons why this study is a big deal:
- It was funded by the Nutrition Science Initiative, a nonprofit involving Gary Taubes that has been funding additional research to test the carbohydrate-insulin hypothesis of obesity. This is the second of three studies NuSI has funded. The first study showed that a very-low-carbohydrate ketogenic diet doesn’t accelerate fat loss or meaningfully increase metabolic rate relative to a high-carbohydrate, high-sugar diet of equal calories and protein, despite large differences in insulin levels (implying that ketogenic diets promote weight loss by causing people to spontaneously eat fewer calories). So far, these studies are doing a remarkably effective job of further refuting the carbohydrate-insulin hypothesis.
- It was large and well-designed. In included 609 people and the research plan was registered in advance, including which outcomes would be the most important (primary). This guarantees that the researchers couldn’t tweak the methods after the fact or use selective outcome reporting to obtain a preferred outcome.
- It didn’t involve deliberate calorie restriction in either group. One of the (valid) complaints of the low-carbohydrate community is that in randomized trials, non-calorie-restricted low-carbohydrate diets are often compared with calorie-restricted low-fat diets. Some have speculated that if low-fat dieters were allowed to eat until full rather than controlling portion size, they wouldn’t lose weight, and they might even gain. This is not the case.
- It focused on diet quality in both groups. All participants were told to focus on unrefined foods and cook at home whenever possible. This is in contrast to the wimpy low-fat control groups in many randomized trials of low-carbohydrate diets, which I call the “Snackwell diet”.
One typical (somewhat valid) critique about randomized trials like this is that the low-fat diet contained too much fat, or the low-carbohydrate diet contained too much carbohydrate. In this study, both arms were started on very-low-fat (20 g/d) or very-low-carbohydrate (20g/d) diets, and told that they should keep fat/carbohydrate to the lowest level they can sustainably maintain. While it is true that fat and carbohydrate intake rose throughout the 12-month period, this isn’t an indictment of the study: It’s a fact about human nature. This trial showed that even with extensive support, the average person is unable to stick with a very-low-fat or very-low-carbohydrate diet. It is true that they probably would have seen better results on both diets if they had been able to stick with a greater degree of restriction, but the participants demonstrated with their forks that it wasn’t sustainable for them.
Sugar intake declined substantially in both groups, although more in the low-carbohydrate group. I predict that those who are constitutionally unwilling to give up the insulin hypothesis will focus on this as a way to discount the results. Please allow me to inoculate you against this weak argument with the following facts:
- Sugar has little impact on insulin levels relative to other types of carbohydrate unless it’s consumed in unusually large amounts.
- High sugar intake doesn’t seem to impair weight loss, both in settings where calories are strictly controlled, and in settings where they aren’t.
Sugar probably does contribute to obesity, but the evidence suggests it’s very unlikely to be the primary driver of changes in body fatness. I have no illusions that this study will change the minds of the diehards, but I do expect it to impact everyone else.