Actually, strict vegetarians when fed meat won't even produce TMAO, because they don't have the gut bacteria that produces it as its a meat-eating bacteria.

Note that depression is also linked to food consumption and impaired gut health, which is linked to a diet heavy in animal products.

In the blue zones, clearly the biggest factor is the diet which is almost 100% plant based. There is this correlation between health and plant-based diets everywhere.

In the US for example, meat eaters have higher body weight than vegetarians, which have a higher body weight than strict vegetarians. Actually, strict vegetarians are the only ones of those groups that have a normal body weight.