A landmark research study on low-fat vs. low-carb diets
was published in the Journal of American Medical Association this February, 2018.1 Some of you saw its review in the New York Times2 and had some questions for me.
After reviewing both the study1 and its review in The Times2
I felt the need to comment here, as The Times got a little mixed up on its reporting of the article and its conclusions.
The study was indeed noteworthy
because of its excellent design and its size:
- It was a randomized clinical trial (which means, subjects were put into groups randomly, not selected according to any specific trait).
- There were 600+ participants of which 481 completed the whole study (a very large group for studies in general). In total, there were 263 males and 346 premenopausal females who began the test, with an average BMI of 33 (class I obesity), and the average age was 40±7 years.
- The study was conducted for twelve months ( a very long study, comparatively speaking).
- The study was done by Dr. Christopher Gardner of Stanford University in conjunction with a team of nutrition experts, the US National Institutes of Health (NIH) and the Nutrition Science Initiative (NuSI). (Of specific note here is that NuSI was cofounded by Gary Taubes, a prominent low-carb advocate and champion of the hypothesis that obesity is caused by carbohydrates and insulin.)
- Participants were coached throughout the study to help them stay on track with their diets, which were either healthy low-carb or healthy low-fat as explained below.
The goal of the study was to see whether differences in genetics or insulin production could predict weight loss
in participants eating either a low-fat or a low-carb diet for one year. Reducing calories was not part of the instructions nor was it a primary focus of the study. However, both groups were asked to “maximize vegetable intake; minimize intake of added sugars, refined flours, and trans fats; and focus on whole foods that were minimally processed, nutrient dense, and prepared at home whenever possible1. Participants were also coached on mindful vs. mindless eating1.
During the first two months of the study, the low-fat group was asked to consume only 20 g of fat per day and the low-carb group only 20 g of carbs per day. At the end of the two-month period, they started adding fats or carbs back into their diet until they felt they’d reached the lowest intake level they could sustainably maintain. Neither group was able to stick to the very low starting intakes. (The final dietary recalls reported that the low-fat group had an average daily fat intake of ≈57 g per day and the low-carb group had ≈132 g of carbs per day.)3
Participants ended up eating an average of 500-600 calories less per day,
even though they weren’t trying to.
(along with my notes in parenthesis):
- People’s weight changes differed between individuals, but both groups lost the same amount of weight. Both groups had weight changes ranging from 70 lbs lost (-32 kg) to 24 lbs gained (+11 kg) over a year. But on average, the low-fat group lost 11.7 lbs, and the low-carb group 13.2 lbs (a difference of 1.5 lbs over 12 months (0.125 lbs/month) which was not statistically significant or clinically relevant. (And carbs fill up your muscle glycogen, so “weight loss” doesn’t always equal “fat loss” on a low-carb diet.)
- Each diet had pluses and minuses: The low-fat group ended up eating less saturated fat (good) and the low-carb group ended up eating a lower glycemic index (good) less sugar (good), and much less fiber (not good). (I’d say it was a wash except for the fact that fiber is showing to greatly participate in the health of your gut microbiome. So the low-fat diet came out ahead.)
- The genes tested in this study failed to predict weight changes in either group. (Which means, hold on to your money when approached by gene-testing companies who say they can predict what kind of diet will work for you based on your genes. We’re not there yet, guys).
- Insulin secretion didn’t predict weight loss for either group. (This replicates numerous other studies that disprove Gary Taubes’ insulin model of obesity—a theory that eating carbohydrates and insulin causes weight gain regardless of calories.)
- Both groups improved many health indicators. Both groups improved body mass index, body fat percentage, waist circumference, blood pressure, and fasting insulin and glucose levels. (All of this is good).
- Both diets improved lipid profiles, except the low-carb group increased in low-density lipoprotein cholesterol (not good, because LDL-C is known for association with heart disease). The low-fat group significantly decreased low-density lipoprotein cholesterol (good). The low-carb group showed an increase in high-density lipoprotein cholesterol (HDL-C) and decrease in triglycerides (both good: and these two factors can also be improved with exercise, not only food).
- Mindfulness counts. According to an interview with the study’s author, Dr. Christopher Gardner the most successful participants in both diet groups were those who “changed their relationship to food”3.
What do I think you should take away from this study?
It may be obvious to you now that the takeaway isn’t exactly what The New York Times suggests—that “The Key to Weight Loss is Diet Quality, Not Quantity”. And it’s not that the CDC or any other agency is “wrong” or “outdated” when they say that “weight management is about balancing calories you eat with the calories you burn” (as The Times also suggests). Because certainly participants lost weight and ate fewer calories.
The study suggests that it’s not important which diet you choose—low-fat or low-carb. And it replicates the results of others which have shown that when calories and protein are constant between diets, manipulating the proportion of carbs and fat makes little difference when it comes to weight loss (because lower calories count).
The research set out to test whether insulin sensitivity or specific genes could predict weight loss with a low-calorie or low-fat diet. They did not find such a relationship.
And finally, the study didn’t focus on calories, but it showed once again something you most likely have known for a while: When we eat more high quality, whole foods and more vegetables (plant foods), we get more nutrients (higher nutrient density) and we get satisfied with less, so we eat less. Most importantly, lots of other good things happen to your body when you choose these healthy, real foods.
- Foster, Gary D., et al. “A Randomized Trial of a Low-Carbohydrate Diet for Obesity.” New England Journal of Medicine, vol. 348, no. 21, 2003, pp. 2082–2090., doi:10.1056/nejmoa022207. https://jamanetwork.com/journals/jama/article-abstract/2673150
- Oconnor, Anahad. “Key to weight loss is diet quality.” New York Times, www.nytimes.com/2018/02/20/well/eat/counting-calories-weight-loss-diet-dieting-low-carb-low-fat.html.
- Hull, Michael. “Low-Fat vs low-Carb? Major study concludes: it doesn’t matter for weight loss.” Examine.com, Examine.com, 21 Feb. 2018, examine.com/nutrition/low-fat-vs-low-carb-for-weight-loss/#ref6.