Diet is an essential factor for maintenance of life. Veda, the most ancient science proclaims that food is responsible for existence of living beings on the earth-Annad Bhavanti Bhutani.
Ayurveda believes that Body is born of the Food. Acharya Caraka asserts that the life of all living beings is food. Complexion, gracefulness, good voice, long life, understanding, happiness, growth, strength, intelligence, are all established by food. Whatever is beneficial to worldly life, whatever pertains to Vedic practices leads to heaven and whatever action leads to spiritual salvation and all are established ultimately by food. “Ayurveda, the oldest science of life, deals with body ,mind and spirit,” says Aharah Mahabhaisajyam (Kashyap). Ayurveda sees medicine and diet as complementary to each other rather than separate. The physical health, mental clarity and spiritual progress depend on proper diet. Diet is categorized into three qualities, or gunas:

1. Sattvika (Pure or Goodness)
2. Rajasika (Passion)
3. Tamasika (Darkness)

Sattvika Guna (Pure or Goodness)
Sattwa is beneficial to all creatures in the world, and constitutes the conduct of those that are good. Joy, satisfaction, nobility, enlightenment, and happiness; absence of stinginess, absence of fear, contentment, disposition for faith, forgiveness, courage, abstention from injuring any creature, equability, truth, straightforwardness, absence of wrath, absence of malice, purity, cleverness, prowess- these pertain to the quality of Goodness (Sattwa).
The foods that increase the above qualities are known as Sattvika food which should be fresh and as natural as possible, preferably organically grown, not genetically modified, and kept without preservatives or artificial flavorings

Examples: Juicy fruits, fresh vegetables that are easily digestible, fresh milk, and butter, whole soaked or also sprouted beans, grains, and nuts, many herbs and spices in the right combination with other foods.

Rajasika Guna (Passion)
Injuring others, beauty, toil, pleasure and pain, cold and heat, lordship (power), war, peace, dissatisfaction, endurance, might, valor, pride, wrath, exertion, quarrel, jealousy, desire, malice, battle, protection of others, slaughter, bonds, and affliction, cruelty, vilifying, pointing out the faults of others, thoughts entirely devoted to worldly affairs, anxiety, animosity, false speech, false or vain gifts, hesitancy or doubts, boastfulness of speech, praise and criticism, prowess, defiance, attendance (as on the weak and the sick), obedience (to the commands of preceptors and parents), service or ministrations, cleverness or dexterity of conduct, possessions, and diverse decorations that prevail in the world among men, women, animals, inanimate things, houses, grief, incredulousness, vows and regulations, actions with expectation (of good result), diverse acts of public charity, making of gifts, acceptance of gifts, auspicious acts, affection generated by the merits of the object for which or whom it is felt, treachery, deception, disrespect and respect, theft, killing, desire of concealment, wakefulness, ostentation, attachment, devotion, contentment, gambling, indulgence in scandal, attachment to dancing, instrumental music and songs – all these qualities have been said to belong to the quality of Passion (Rajas).
The food that are bitter, sour, saline, excessively hot, pungent, dry and burning are known as Rajasika.

Examples: Sattvic foods that have been fried in oil or cooked too much or eaten in excess, specific foods and spices that are strongly exciting.

Tamasika Guna (Darkness)
Complete delusion, ignorance, illiberality, indecision in respect of action, sleep, haughtiness, fear, cupidity, grief, censure of good acts, loss of memory, absence of faith, violation of all rules of conduct, want of discrimination, blindness, vileness of behavior, boastful assertions of performance when there has been no performance, presumption of knowledge in ignorance, unfriendliness (hostility), evilness of disposition, absence of faith, stupid reasoning, incapacity for association, sinful action, senselessness, absence of self-control, degradation, - all these qualities are known as belonging to Darkness (Tamas).
That food which is stale, tasteless, rotten and impure is known as Tamasika food.

Examples: Foods that have been strongly processed, canned or frozen and/or old, stale, or incompatible with each other - meat, fish, eggs are especially tamasic.

All known nutrients in our diet can influence the function of our brains and the Central Nervous System. Some of the important ones are:

Proteins & Amino Acids
They play an important role in the normal functioning of the brain, particularly before birth when the concentration in the blood of developing foetus can be three times greater than in the mother. Most of the essential elements in the brain and CNS – the so-called neurotransmitters such as dopamine and serotonin – are produced in our bodies from amino acids.
In adults, any condition that affects adequate protein intake or increases the intake can lead to an imbalance in the supply of amino acids to the brains and CNS, resulting in neurological fall-out and damage.

Vitamin-B
Thiamin
Its deficiency can interfere with the production of certain neurotransmitters like glutamate as well as glucose metabolism which the brain is so highly sensitive to.
Pronounced deficiency can lead to sensory disturbances with paralysis, muscle weakness, loss of memory and permanent brain damage can also occur.

Niacin
This is one of the B-complex vitamins that may be linked to neurological damage. Mild deficiency is associated with anxiety and depression and severe deficiency may lead to delirium, dementia and death – the dreaded three D’s of pellagra.
Diet & Depression
Diet and depression tend to be reciprocally linked to each other. Firstly, a deficient diet that is lacking in B-vitamins, minerals, protein and carbohydrates can either cause depression or contribute to depressive states. In addition, alcohol intake can lead to severe depression which is in part due to the lack of vital nutrients.
Digestion and the effect on the body depend upon the state of mind. A wholesome diet may be incompatible if the state of mind is not proper. Food does not come to us merely by our mouths. It comes through all our senses and through our minds.
When Sri Krishna visited Hastinapur before the Great War, He stayed in Vidur’s house even though Duryodhana had kept sumptuously decorated palace food at His disposal. Sri Krishna said: “One may accept the offering with love or the food of beggar. You bear me no love and we are not beggars either.”
So, diet and mind are inter-related. The diet is essential for a healthy mind. In other words, a healthy mind is responsible for the proper digestion and utilization of its nutrients in the body.