According to this article:
It is good to combine vegetables with either proteins or starches, but:
- Do not combine proteins and starches (meat and potatoes, fish and rice, etc.).
- Generally, eat fruit alone. Acid fruits may be consumed with sweet fruits, in which case, the acid fruits should be eaten first. Acid fruits can also be combined with nuts and seeds.
- Do not combine melons with anything (even other fruits, except other melons).
Are there any studies that back these claims?
This is different from Are there any toxic or dangerous food pairings?, in which he specifically mentions that he is not asking about general food categories as being unhealthy, but rather specific foods that can produce toxins when combined.