New Delhi, Nov. 5 -- In India, arguments about food rarely stay about food. Vegetarianism is bound up with faith, family and region, and newer imported camps have added their own certainty. Strict vegans insist that removing animal products is the only clean and moral way to eat. Carnivore-diet enthusiasts insist the opposite: that virtually every plant is armed with toxins the human body was never built to handle. Both camps argue with total certainty. Both are, biologically speaking, half right.

The real question is not which side wins. It is how the human immune system actually responds to what lands on the plate, and what that means for how food should be chosen, combined, and prepared.

An Old Kinship, and its Cost

Humans are anima...