The three gunas are part of Yogic philosophy, which says if you eat sattvic food, you will become a sattvic being. If you eat rajastic food you become rajastic (ambitious, temperamental, egoistic, etc.). And if you are naturally tamasic, you’ll continue to eat animal products and unhealthy foods and develop an animalistic lazy nature.
The Brahmin caste traditionally ate a sattvic diet, and has been vegetarian for millennia (though a few groups are non-vegetarian). Eating beef, however, was not permissible except for the untouchables.
Read more at “The Timeless Wisdom of the Brahmin Diet.”