Vegans often claim that the ratio of lysine to methionine in the diet is important for preventing heart attacks, and perhaps also preventing cancer. That methionine increases one's risk of heart attack, and that lysine decreases it. Meat-eaters sometimes respond by asking: "If the animal protein is responsible for heart attacks due to its high methionine content, then the protein in grass should arguably be even worse. Grass is high in methionine and it contains little or no lysine. So, how it is that grass-fed cows don't get heart attacks very often?". What is a good answer to that? An obvious answer is that cows have some metabolic path that humans don't have and that converts methionine they don't need to lysine, but, as far as I can tell, there's no evidence of that.
