The theory behind the sattvic diet is stated directly in the Bhagavad Gita, whose seventeenth chapter sorts food, along with sacrifice, austerity and giving, into three kinds matched to the three gunas that the text holds run through all of nature. Sattvic food is described as juicy, mild, substantial and pleasing to the heart, food that promotes life, purity, strength, health and joy. Rajasic food is bitter, sour, salty, excessively hot or pungent, prized for its stimulation but held to breed disease and pain. Tamasic food is stale, flat, putrid or left over from another's meal, food that dulls the mind rather than clarifying it.
This is a psychological and spiritual classification before it is a nutritional one: the point is not calories but the effect food has on the mind of the person who eats it. A diet meant to support meditation, self-discipline or ritual purity is built around sattvic foods for exactly this reason, and the classification extends beyond food itself to speech, action and worship in the same chapter.
The scheme reappears in Ayurvedic medical literature, which shares the guna vocabulary with classical yoga and Vedanta and treats diet as one of the primary levers for shaping temperament, alongside more clinical concerns such as digestion and constitution.