My long-running pet theory is that eating a large variety of foods is bad for our gut biome. Between breakfast, lunch, and dinner, our intestinal flora cycles through blooms and die-offs of different bacteria. The blooms interfere with bacteria diversity, while the die-offs create waves of toxins that trigger our immune systems. To give an analogy, it's a bit like an ecology having its climate messed with. Imagine a forest that was hot one day, cold the next, dry as a bone on some days, and wet as a swamp the next.

I reckon most animals eat a steady diet of only one or two things, and people did too for a very long time (on a seasonal scale). It was only recently that we ate so many different foods so frequently, and this coincides with a rise in gut-related illnesses and obesity in first world countries. Many people see success treating these diseases with some diet, but what's interesting is how different and disjoint these diets are. For example, both carnivore diets and vegetarian diets have seen success treating inflammatory bowel disease. Perhaps this success isn't due to eating one food over another, but rather to the reduction in variety a diet brings.