In order to test whether or not weight gain is due to the excess calories in meat, hundreds of thousands of men and women spanning 10 countries were followed over a 5 year period in the EPIC-PANACEA study. This study was one of the largest studies on human nutrition ever performed. The study controlled calorie intake, so that they could determine whether or not consuming meat was associated with weight gain. This means that the group including meat in their diet ate the same number of calories as the group that did not consume any meat.
Even after controlling for calories, the group that included meat gained weight. After five years and adjustment for total energy intake and underlying dietary patterns, the researchers concluded: "Total meat consumption was positively associated with weight gain in men and women, in normal-weight and overweight subjects. Our results are therefore in favor of the public health recommendation to decrease meat consumption for health improvement [and weight management]."