This recently randomised clinical trial published in the prestigious Journal of the American Medical Association, Effect of Low-Fat vs Low-Carbohydrate Diet on 12-Month Weight Loss in Overweight Adults and the Association With Genotype Pattern or Insulin Secretion: The DIETFITS Randomized Clinical Trial, looked to answer the million dollar question: is low-carb or low-fat better for weight loss given particular participant characteristics? These characteristics were measured via certain genetic variations (SNPs in genes PPARG, ADRB2, and FABP2) that predispose an individual toward carbohydrate or fat metabolism, and also via insulin levels 30 minutes after a glucose challenge as a measure of insulin resistance. Pretty robust study design, in my opinion.
The study was completed by 609 participants, a very good number. When I saw participants were instructed to eat 20g per day of either fat or carbohydrate for the first 8 weeks I was excited (finally a hardcore study, not another lowish-carb/fat one). However, participants were also instructed to add fat/carbs at a rate of 5-15g per day per week until reaching a level they believed sustainable. This translated to 96.6g (at 3 months) to 132.4g (at 12 months) carbs per day for the low-carb group and 42.0g (at 3 months) to 57.3g (at 12 months) fat per day for the low-fat group.
Participants were also instructed to eat a mostly unprocessed diet, which translated to a lowering of added sugars for both groups. Still, the low-fat group ended up consuming an average of 33.1g of added sugars per day, and the low-carb group 22.8g per day. This equals approx. 6-8 teaspoons added sugar per day. Food for thought.
This being a randomised clinical trial, there were people with both carbohydrate and fat-favouring metabolisms in both groups and also with different insulin levels. The study found that the weight loss was roughly the same in both groups, meaning the individual characteristics played no role in whether the diets were effective or not. Meaning the measured genes and insulin test are not reliable predictors of whether low-fat or low-carb would be the right fit for a person.
One interesting secondary measure was blood lipid levels. The low-fat group lowered LDL-cholesterol the most and the low-carb group lowered triglycerides the most. Perhaps current levels and genetic predisposition toward high LDL-cholesterol or high triglycerides are better indicators of which approach is best for an individual.