As a biologist I know that the perfect human diet is the one that our ancestors adapted to and this included a lot of meat but this meat came from animals who were themselves well-fed and exercised a lot so it was not laden with saturated fat or as low and imbalanced in nutrients as modern farm animals. Also, they ate the organs and brains and bone marrow more so than the muscle, resulting in our need for the healthy oils now.
Aside from Inuits and ancient Europeans as well as some others in certain areas, fruit and non-starchy vegetables also made up a huge proportion of the diet. However, nowadays in order to get enough energy, energy dense but nutritionally weak foods like cereals are often consumed by vegetarians to replace the energy from meat.
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