When it comes to diet habits, what you eat directly influences your body - down to the molecular genetics level. Researchers studying vegetarians and meat-eaters found that vegetarians are more likely to have a specific mutation that enhances the production of omega-3 and omega-6 fatty acids.
These fatty acids are important in brain development and controlling inflammation. However, foods rich in these nutrients, nearly all types of fish, aren't vegetarian friendly. So while meat-eaters can easily derive the nutrients from the animals that we eat, vegetarians have to metabolize plants into fatty acids. After generations of consuming only plants, it turns out that some populations developed the mutation as a way to enhance the fatty acid conversion from plants.
Of note, the mutation arose only after being selected for from many generations; that is to say, the mutation is not a metabolic switch that would get turned on if we were to stop eating meat tomorrow. In addition, while the mutation helps plant-eaters get the necessary fatty acids from their diets, it's also associated with increased heart disease and lung cancer. However, these risks could be diminished by eating a diet that includes balanced amounts of omega-3 and omega-6 fatty acids.