Because how healthy a food is can vary with gut biome and genetics, and those are hard to control for. Exercise is also big on genetics. Isolating with high con

Because how healthy a food is can vary with gut biome and genetics, and those are hard to control for. Exercise is also big on genetics. Isolating with high confidence whether something is good or not requires large, international datasets with people who consistently report exact quantities of everything they eat and do for decades. Since the budget for that is very rare, the best we can do is to run the same study several times on different populations. Often, studies measuring the same thing will vary in methodology, which requires even more studies to control for those.