The endless series of diets that claim to help you lose weight tend to fall into two camps: low fat or low carbohydrates. Some companies even claim that genetics can tell us which diet is better for which people.
A rigorous recent study tried to end the debate and it had results to disappoint both camps. As far as hope is concerned, as the New York Times pointed out, people succeeded in losing weight, regardless of which of the two diets they followed.
The research is worth a look at what it has done and what it has not proven.
Researchers at Stanford University took more than 600 people (who are huge for nutritional research) between the ages of 18 and 50 who had a body mass index of 28 to 40 (25-30 is overweight and 30 and older is obese). The subjects had to be healthy. They did not even have statins or medications for type 2 diabetes or hypertension, which could affect weight or energy consumption. They were all randomly assigned to a healthy low-fat or healthy low-carbohydrate diet and they were clearly not blinded to the group they were in.
All participants attended 22 instruction sessions over a year in groups of about 17 people. The sessions were held weekly in the beginning and were then spread out so that they were monthly in the last six months. Everybody was encouraged to reduce the intake of the avoided nutrient to 20 grams per day for the first eight weeks, after which the participants slowly added fats or carbohydrates to their diet until they reached the lowest level of intake they believed to be long-term. term could be sustained.
Everyone was followed for a year (which is an eternity for nutritional research). Everyone was encouraged to maximize the intake of vegetables; to minimize added sugar, refined flour and trans-fat intake; and to focus on whole foods that are minimally processed. The subjects were also encouraged to cook as much as possible at home.
All participants took a glucose tolerance test as a measure of insulin sensitivity. Some believe that insulin resistance or sensitivity can affect not only how people respond to diets, but also how well they adhere to it. 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. Approximately 40 data were collected at the start of the study, six months and a year. At three unannounced times, researchers checked patients to see how closely they adhered to the instructions.
Percentage of participants had a low-fat genotype and 30 percent had a low-carbohydrate genotype.
This was a phenomenally well-designed trial.
People changed their diet based on their group assignment. Those in the low-fat group consumed an average of 29 percent of their calories from fats, compared to 45 percent in the low-carbohydrate group. Those in the low-carbohydrate group consumed 30 percent of their calories from carbohydrates, compared to 48 percent in the low-fat group.
However, they did not significantly lose other weights. After 12 months, the low-carbohydrate group had on average lost just over 13 kg, compared with more than 11.5 kg in the low-fat group. The difference was not statistically significant.
Insulin sensitivity has made no difference. People who secrete more or less insulin generally did not lose more or less weight on a low-fat or low-carbohydrate diet. Genetics has also made no difference. People who had genes that could indicate that they would do one or the other diet better did not.
If you look at how each participant in this study accomplished the diet to which he or she was assigned, it is remarkable how both diets yielded an almost identical, bending range of reactions – from a lot of lost weight to a little bit. It was not 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. Although I would like this to be true – I have advocated this healthy approach in my Upshot article on dietary recommendations and in a recent book – that is not what this study showed. Although that advice was given to all participants, there was no control group that omitted that advice and therefore no conclusions can be drawn on the effectiveness of these instructions.
Others have taken this study as proof that the idea is invalid that counting calories is the key to weight loss. Although not the main focus of this study, nor the instructions given, participants reduced their intake by an average of 500-600 calories per day (even if they had not counted). This study did not prove the unimportant of calories.
The researchers asked everyone, and not just those in the low-carbohydrate group, to avoid ‘added sugars’. That is why we can not say anything new about added sugars and weight loss.
What this study shows is that people who have placed a claim on the superiority of one diet over another do not have such a strong example as they think. It is difficult to overestimate how similar these two diets have performed, even on an individual level.
This study was mainly focused on people who were obese, so people who want to lose just a few pounds can benefit more from one diet or another; we do not know. It is also worth noting that the people in this study have received significant support for both diets, so the results shown here may not apply to those who only want to lose weight.
You must be wary of those who tell you that they know which diet is best for you, or that there is a test to tell you the same thing. Successful diets over the long term are very likely to bring about slow and steady changes. The simplest approach – and many have joined, including recently Jane Brody in The Times – is processing processed foods, thinking about the calories you drink and trying not to eat more than you plan.
What it comes down to is that the best diet for you is still what you’ll stick to. Nobody knows better than you what that diet might be. You will most probably have to figure it out yourself.