The authors of this study note that a major problem exists with our ability to understand and compare the effects of various weight loss diets based on the studies we have available.
Most of these studies suffer from high dropout rates and short follow-up times. This means that the results of weight loss diet studies tend to only include people motivated enough to stick with the diet for the duration of the study, and excludes people who dropped out due to adverse effects such as weight gain. Also, because the followup times for these studies tend to be short, there is insufficient research into the long-term effects that each of these diets have on health and body composition.
The present study attempts to correct for this latter problem by including a two-year followup period, so that the long-term effects of each diet and also the likelihood of an individual being able to stick to the diet for a long period of time could be taken into account.
The researchers randomly assigned 322 moderately obese, mostly middle-aged people to one of three weight loss diets: a low fat diet with calorie restriction, a “Mediterranean diet” with restricted calories, and a low carb diet without any calorie restriction.
Weight loss diet results
The adherence rate, or the percentage of study participants who stuck with their assigned diet, were quite good: 95.4% of the people were still on their assigned diet at one year, and 84.6% reported that they were still sticking to the diet at the two year mark. However, these figures are based on self-report, and weight loss diet study participants are notoriously bad at accurately reporting their behavior, so these self-report rates are a limiting factor in our ability to interpret the other results of the study.
Regardless, a number of interesting trends showed up in the way people ate on these diets. The group assigned to eat a Mediterranean diet ate the most fiber and the most monounsaturated fat, while the low carbohydrate diet group ate the most fat, protein, and cholesterol. In spite of this, the low carb diet group realized the greatest improvement in their blood lipid profiles. This means that the low carb dieters ate more fat and cholesterol but had less fat and cholesterol in their blood.
Another interested trend that the researchers noticed was in the area of blood sugar control. Thirty-six of the study participants were diabetic, and of these, the Mediterranean dieters showed the greatest improvements in fasting blood sugar level and insulin level.
Average weight loss outcomes for each diet
- Low Fat Weight Loss Diet: 2.9kg
- Mediterranean Weight Loss Diet: 4.4kg
- Low Carb Weight Loss Diet: 4.7kg
So, while the Mediterranean diet and low carbohydrate diet were fairly similar in average weight loss, the low fat diet group fell way behind. The average low carb dieter in this study lost nearly twice as much weight as the average low fat dieter. Additionally, both the Mediterranean diet group and the low carbohydrate diet group showed improvements in measures of overall health that the low fat diet group did not.Citation: Weight loss with a low-carbohydrate, Mediterranean, or low-fat diet. New England Journal of Medicine. 2008 Jul 17;359(3):229-41. Authors: Shai I, Schwarzfuchs D, Henkin Y, Shahar DR, Witkow S, Greenberg I, Golan R, Fraser D, Bolotin A, Vardi H, Tangi-Rozental O, Zuk-Ramot R, Sarusi B, Brickner D, Schwartz Z,Sheiner E, Marko R, Katorza E, Thiery J, Fiedler GM, Blüher M, Stumvoll M, Stampfer MJ