These studies almost always compare typical western diet omnivores to health-conscious vegans, which in itself is fallacious. Health-conscious omnivores will ea

These studies almost always compare typical western diet omnivores to health-conscious vegans, which in itself is fallacious. Health-conscious omnivores will eat drastically different compared to typical omnivores. Exactly as you put it, people who care for their diet are likely to be of better shape. BMI is a bad stat here though: it is likely one will get bigger from eating proper meat than they would on a vegan diet, without any training involved. BMI doesn't account for leanness or muscle mass. The biggest link between obesity and food is sugar intake. The thing that's making most omnivores fat is the needless sugar flavoring and breading surrounding a bad piece of food (be it vegetable, meat, fish, fruit, whatever) as something delicious. Nutritional sciences contradicts itself at any corner, finding healthy vegans, healthy vegetarians, healthy carnivores and healthy omnivores, except for the part where sugar is undoubtedly spiking insulin. Insulin in turn sets people up to be more easily overeat, as well as storing all those sugars as fat more easily. Considering health-conscious people will avoid these foods, they will eat less isolated sugar, fructose and foods high on the glycemic index, even if vegans end up getting most of their daily energy from fruit. Fruit offsets this somewhat thanks to the fiber you're eating with it (also why people say not to drink juice, or at least drink the pulp with it), but fiber can only do so much and fiber being a "mostly good" thing is also being put under scrutiny.