The study involved 39 young adult men and women of normal weight, who ate 750 extra calories per day for seven weeks. The goal was for them to gain three per cent of their starting weight. The project received considerable attention when it started in 2011, partly because the extra calories were ingested in the form of muffins with high fat content, baked in the lab by Fredrik Rosqvist, a doctoral candidate and first author of the study.

One half of the subjects were random to eat surplus calories from polyunsaturated fat (sunflower oil), while the other half got their surplus calories from saturated fat (palm oil). Both diets contained the same amount of sugar, carbohydrates, fat, and protein; the only difference between muffins was the type of fat.