Tropical dwellers have higher levels of R globulin, but lower levels of protein; cold dwellers have higher levels of protein, but lower levels of R globulin. This is similar to cold dwellers' meat-based diets, while tropical Residents are predominantly vegetarian. What does this passage mainly mean? ['People have different eating habits in different regions.', 'Food sources vary from region to region.', 'Food composition affects the proportion of blood components in the human body.', 'Vegetarians are healthier and live longer.']