Yoga is an ancient science. It has all along insisted on a nourishing but non-stimulant. vegetarian diet. Yogic diet satisfies three main conditions: (1) it is nourishing. i.e. it provides all the essential nutrients to the body. (2) it is vegetarian. (3) it is pure and non-stimulant. According to medical science, if a food satisfies the condition No. (1). then it is fit for consumption. But yoga, being a subtle science. makes subtle distinctions. Proper food is the first requisite of a Sadhaka. So says Lord Krishna in the Bhagvad Gita.
"Yoga becomes the destroyer of woes and can be accomplished only by him who is regulated and moderate in diet and recreation. regulated in performing actions. and regulated in sleeping and waking."
So. one who wants to advance on the path of yoga should first pay attention to his diet. There is no conflict between yoga and modern science as far as the principle of balanced diet is concerned. But yoga diet would exclude all those food items which are non-vegetarian and intoxicating. because according to yoga. food is not for physical fitness alone; it also affects mental and spiritual state of a being. So. yogic diet, apart from giving physical fitness. is conducive to mental and spiritual upliftment. It is simple natural and Sattvic.
The Bhagvad Gita which is the most exhaustive treatise on yoga, divides food into three categories on the basis of its effect on the human mind: Sattvic, Rajasic and Tamasic. Sattvic food is the best and the most superior type of food. The medium type of food for worldly man is Rajasic food and the most inferior type of food is Tamasic one. Our ancient scriptures proclaim in one voice that the type of food determines the type of man. It is to repeat the oft-repeated maxim: "As the food, so the mind, as the mind, so the man."