"Effect of an Ad Libitum High-Protein Ketogenic Diet on Hunger, Appetite, and Weight Loss in Obese Men" (Johnstone et al., 2008)

Dr. Lee The first paper we'll examine is "Effect of an Ad Libitum High-Protein Ketogenic Diet on Hunger, Appetite, and Weight Loss in Obese Men" (Johnstone et al., 2008). The study design left intake amount to the participants' own choice—allowing them to eat freely—while controlling only the macronutrient ratios so that fat content was elevated. Mean age was 38 years, and 17 obese men were given two diets, both high in protein (30 percent). For 30 days they followed a 4 percent-carbohydrate ketogenic diet, and for the other 30 days a 35 percent-carbohydrate diet, in a crossover design.

Zoe Doctor, why use a crossover design where the same group switches diets at different times?

Dr. Lee Because in a crossover study each participant serves as their own control, reducing the number of subjects required. Comparing two or more diets within a single group also lets us estimate the effects more precisely.