A rigorous recent study sought to settle the debate, and it had results to disappoint both camps. On the hopeful side, as The New York Times noted, people managed to lose weight no matter which of the two diets they followed.
The study is worth a closer look to see what it did and did not prove.
Researchers at Stanford University took more than 600 people (which is huge for a nutrition study) aged 18 to 50 who had a body mass index of 28 to 40 (25-30 is overweight, and 30 and over is obese). The study subjects had to be otherwise healthy. They couldn’t even be on statins, or drugs for Type 2 diabetes or hypertension, which might affect weight or energy expenditure. They were all randomly assigned to a healthful low-fat or a healthful low-carbohydrate diet, and they were clearly not blinded to which group they were in.
All participants attended 22 instructional sessions over one year in groups of about 17 people. The sessions were held weekly at first and were then spaced out so that they were monthly in the last six months. Everyone was encouraged to reduce intake of the avoided nutrient to 20 grams per day over the first eight weeks, then participants slowly added fats or carbohydrates back to their diets until they reached the lowest level of intake they believed could be sustained for the long haul.
Everyone was followed for a year (which is an eternity for a nutrition study). Everyone was encouraged to maximize vegetable intake; to minimize added sugar, refined flour and trans fat intake; and to focus on whole foods that were minimally processed. The subjects were also encouraged to cook at home as much as possible.
All the participants took a glucose tolerance test as a measurement of insulin sensitivity. Some believe that insulin resistance or sensitivity may affect not only how people respond to diets, but also how well they adhere to them. The participants were also genotyped, because some believe that certain genes will make people more sensitive to carbohydrates or fat with respect to weight gain. About 40 percent of participants had a low-fat genotype, and 30 percent had a low-carbohydrate genotype.
Data were gathered at the beginning of the study, at six months and at one year. At three unannounced times, researchers checked on patients to see how closely they were sticking to the instructions.
This was a phenomenally well-designed trial.
People did change their diets according to their group assignment. Those in the low-fat group consumed, on average, 29 percent of their calories from fats, versus 45 percent in the low-carbohydrate group. Those in the low-carbohydrate group consumed 30 percent of their calories from carbohydrates, versus 48 percent in the low-fat group.
They did not, however, lose meaningfully different amounts of weight. At 12 months, the low-carbohydrate group had lost, on average, just over 13 pounds, compared with more than 11.5 pounds in the low-fat group. The difference was not statistically significant.
Insulin sensitivity didn’t make a difference. People who secreted more or less insulin lost no more or less weight in general on either a low-fat or low-carbohydrate diet. Genetics didn’t make a difference either. People who had genes that might indicate that they would do better on one diet or the other didn’t.
In fact, when you look at how every single participant in this study fared on the diet to which he or she was assigned, it’s remarkable how both diets yielded an almost identical, curving range of responses — from lots of weight lost to a little gained. It wasn’t just the averages.
Some have taken this study to prove that avoiding processed foods, eating more whole foods, and cooking at home leads to weight loss. While I’d like that to be true — I have advocated this healthful approach in my Upshot article on food recommendations and in a recent book — that’s not what this study showed. Although that advice was given to all participants, there was no control group in which that advice was omitted, and so no conclusions can be made as to the efficacy of these instructions.
Others have taken this study as evidence debunking the idea that counting calories is the key to weight loss. While that wasn’t the main thrust of this study, nor the instructions given, participants did reduce their intake by an average of 500-600 calories a day (even if they didn’t count them). This study didn’t prove the unimportance of calories.
The researchers also asked everyone, not just those in the low-carb group, to avoid “added sugars.” Therefore, we can’t really say anything new about added sugars and weight loss.
What this study does show is that people who have staked a claim on one diet’s superiority over another don’t have as strong a case as they think. It’s hard to overstate how similarly these two diets performed, even at an individual level.
It shows us that the many people, and the many studies, suggesting that we can tell which diets are best for you based on genetics or based on insulin levels might not be right either. Almost all of the studies that backed up such ideas were smaller, of shorter duration or less robust in design than this one. Granted, it’s still possible that there might be some gene discovered in the future that makes a difference, but those who think they’ve found it already might want to check their enthusiasm.
This study was focused mostly on people who were obese, so people looking to lose just a few pounds might benefit more from one diet or the other; we don’t know. It’s also worth noting that the people in this study received significant support on both diets, so the results seen here might not apply to those attempting to lose weight on their own.
You should be wary of those who tell you that they know what diet is best for you, or that there’s a test out there to tell you the same. Successful diets over the long haul are most likely ones that involve slow and steady changes. The simplest approach — and many have espoused it, including Jane Brody recently here at The Times — is to cut out processed foods, think about the calories you’re drinking, and try not to eat more than you intend to.
The bottom line is that the best diet for you is still the one you will stick to. No one knows better than you what that diet might be. You’ll most likely have to figure it out for yourself.