Campbell has followed a "99% vegan" diet since around 1990. He does not identify himself as a vegetarian or vegan because, he said, "they often infer something other than what I espouse". He told the "New York Times": "The idea is that we should be consuming whole foods. We should not be relying on the idea that genes are determinants of our health. We should not be relying on the idea that nutrient supplementation is the way to get nutrition, because it's not. I'm talking about whole, plant-based foods."