Recently the New York Times, as well as many other news organizations, reported on a new study comparing the effects of a low carbohydrate diet to a low fat diet. The study concluded that a low carbohydrate diet is better for weight loss and heart health than a low fat diet. On the surface, the study is very impressive and the general media was quick to praise it, but some subtle problems with the study's design create a critical flaw.
Participants in the study were instructed on how to keep either a low carbohydrate diet or a low fat diet. They had regular counseling with a dietitian and conducted several 24-hour diet recalls to verify that they were following their diets. Participants in both groups lost weight, however those in the low carbohydrate diet lost more.
The first problem was that the low fat diet was defined as less than 30% of their calories from fat. The baseline that dietitians commonly use, the Acceptable Macronutrient Distribution Range is 20-35% of an individual's total calories from fat. This means that the study's "low fat diet" group fell within the generally recommended diet.
So the study compares a low carbohydrate diet to a baseline diet, what's the problem with that? Nothing, if it's done properly, which brings us to the critical flaw. The researchers did not control for the total amount of calories eaten by each group, and as a result the low carbohydrate group ate on average over 100 calories fewer per day than the low fat group. Without having calorically equal diets, it is impossible to say whether the weight loss was due to eating fewer calories, the low carbohydrate component of the diet, or some combination.
In the end it is very sad because a lot of money was spent on this research, resulting in little insight into weight loss. So do low carbohydrate diets work? Maybe, maybe not, we will have to wait for a better designed study before we know. The one thing that can be concluded is the media regularly makes the wrong conclusions from nutrition studies.