The healthiest person is thin.

A. Different lipid substances have different functions. Not all lipids can make people fat. Lipids play an important role in the human body and cannot be ingested. A is wrong;

B, the most abundant compound in cells is water, and the most abundant compound in lean meat is also water. B is wrong;

C. Seeds such as wheat and various beans in the eight-treasure porridge contain starch, which belongs to sugar, and C is wrong;

D, sugar is the main energy substance, and the organic matter that the human body needs most every day is sugar, so the staple food should be food. D is correct.

Therefore, choose: d.