Why do some scientist say to eat a certain amount of foods?

To be healthy. . .

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Attempts are made to balance what we know about the nutritional needs of the human body, and what we know about nutrients available in various foods. No one has perfect knowledge of all these things, but there is a lot of good general knowledge about what benefits we can gain from eating a variety of foods. Most of the time, a recommendation to eat a certain amount of a given food is based on the general assumption that the rest of the diet consists of a healthy balance of other foods.