1) Definition :
Diet is that – what we eat.
It includes everything we eat or drink.
Diet is the cause of the very existence, Energy , Color & ‘Ojas’ (immunity/Ultimate purified form of all the Sharir Dhatus i.e Body tissues) of every living being on this earth.
2) ‘Diet’ is the cause of the birth, maintenance, & destruction of all forms of life on this earth.
From ‘Diet’ only we acquire following things – constant growth, unhampered growth, Energy , Wellbeing – Health, Colour, Proper functioning of the Indriyas (Sense Organs).

Improper dietis the major cause of all illness according to Ayurveda.
From all above statements one can say that’ In short everything depends on diet.

 
AYURVEDIC CONCEPT OF FORM OF DIET :
According to Ayurveda Body is said to be made up of Panch Mahabhutas & hence the food also needs to be Panchbhautic (Made up of Panch (five) Mahabhutas (great elements)) then only food can get digested & be utilized for the growth of  Panchamahabhutas (the basic elemental constituents of body) which in turn results in growth of body.

The diet should be made up of all the six tastes:
Sweet, Sour, Salty, Hot, Bitter, Astringent.

It has also been stated that according to the properties diet again can be of 2 types:
Heavy to digest , & Easy or light to digest .

 
Ideal Diet
The ideal diet should have following properties:-
1) It should increase the strength of body.
2) It should be tasty/it should be such that it should be liked by the one, who is taking it.
3) It should give energy instantaneously / immediately after taking it.
4) It should have the capacity to withhold the body & give its strength & energy to carry on its regular functions.
5) It should satisfy the one taking it .
6) It should be hot, unctuous ,not dry when taken.
7) Should be taken in proper quantity.
8) It should not be incompatible .(mentioned in uneasy combinations below)
The food (Diet) with all above properties is said to be ldeal diet.
The Diet which is of all the properties explained above can only do the functions prescribed to it.

 
FUNCTIONS OF DIET (food)
1) Instantly satisfying.
2) Gives strength/energy.
3) Maintains the body & maintains the functions of the body .
4) Increases longevity.
5) Increases the Digestive fire, Vitality , Recollecting Power , & Ojas

QUANTITY OF FOOD TO BE TAKEN
It is decided according to digestive capacity of Agni. If Agni is weak then one should take food only once or should take food in less quantity.  Food taken once i.e. at one time which gets digested completely & properly , is said to be Matra/Quantity of food to be taken.
Considering four parts of the Stomach, one should take food such that 2 parts of the Stomach get filled by the food taken, one part to be filled by water or other liquid matter & one part should be kept free for the ‘Vayu’ / Gas to move .
One should take diet in proper quantity. The quantity of diet to be taken depends on the digestion capacity of the person which varies from person to person & so does the Matra. The amount of food, which without disturbing the equilibrium of dhatus & doshas of the body gets digested as well as metabolized in proper time is to be regarded as the proper quantity. The proper quantity of food does depend upon the nature of food articles. If the food article is heavy only three fourth or half of the stomach capacity is to be filled up. Even in the case of light food articles excessive intake is not conducive to the maintenance of the power of digestion & metabolism.

 
IMPORTANCE OF FOOD TAKEN IN PROPER QUANTITY
Taken in Appropriate quantity, food certainly helps the individual in bringing about strength, complexion happiness & longevity without disturbing the equilibrium of dhatus & doshas of the body.

Indicated Food :-
One should regularly ( can take everyday) take Shastika (a kind of rice harvested in sixty days), Sali (oryza sativum), yellow gram , food cooked in Rock Salt, Amalaka (emblica officinalis), Rain Water, Ghee, Meat of animals dwelling in arid climate & Honey.
Contradicted Food :
One should not regularly take heavy articles such as dried meat, dry vegetables, lotus rhizomes & lotus stalk. One should never take meat of a diseased animal. Moreover, one should not regularly take boiled buttermilk, inspissiated milk with pork, beef, meat of buffalo with fish curd, & barley.

 
ORDER IN WHICH ONE SHOULD TAKE DIET
First, one should eat food of sweet taste for the purpose of Vata Pitta pacification. Sweet taste is helpful for pacification of vata dosha present in the digestive tract or intestines.  Then one should take sour & salty substances for they do the function of increasing the digestive juices. Lastly one should take food of bitter and pungent taste for the purpose of pacification of ‘Kapha’ . 

Important Rules And Regulations Related to Diet

1) Eat to live but don’t live to eat.
2) Eat food which is beneficial.
3) Eat food in a moderate quantity.
4) Don’t eat food, properties of which are not known to you.
5) Don’t eat food in a hurry or when you are in a disturbed state of mind, full of tensions, strains, and emotional outbreaks. Regain peace of mind and then enjoy food with happy and hearty mood.
6) Wash your hands, feet and mouth, using clean utensils in a clean place which is well lighted and well-ventilated.
7) Fill half quantity of stomach with food, one fourth quantity with water and liquid substances and keep the remaining one fourth for air.
8) A person who takes only one full meal a day is called Yogee (or Yoga – practitioner). He takes light breakfast in the morning and the healthy and happy life.
9) A person who takes two full meals in a day is called ‘Bhogi” i.e. a lusty and greedy person who lives mostly to eat and suffers from illness and or ill health from time to time.
10) A person who takes full meals more than two times is called Rogee or a sick person. He a gluttonous person and lives only to eat. He is always sick and away from health.
11) Don’t do yogic exercises immediately after taking food. It is safe to do them four hours after taking food. It is better therefore to do Yogic exercise early morning or in the evening when the stomach is empty.
12) Don’t take food immediately after doing yogic exercises, but one hour after doing them.
13) Don’t go to bed immediately after taking supper wait for a minimum of one hour after it.
14) According to Ayurveda well balanced diet consists of food stuffs having six tastes namely Sweet, Sour, Salty, Pungent, Bitter and Astringent.

 
Diet and Mind :
Yogic and Ayurvedic aspects. Standard American Diet (SAD) consideration is only related to body. But it is a uniqueness of Yoga and Ayurveda which shows intimate relationship between diet and mind. Both the sciences assert with proofs that diet affects mentality of a human being. Satva (virtue), Raja (Aggressive desire) and Tama (Ignorant sloth) are the basic qualities of mind. Satvaguna is always strengthening, invigorating and vitalising. Satvaguna expresses essence, understanding, purity, clarity, compassion and love. Rajoguna implies movement, aggressiveness and extroversion and it operates on a sensual level. Tamoguna manifests ignorance, inertia, heaviness and dullness.  Satvaguna is a positive virtue and it strengthens and stabilizes mind. Rajoguna and Tamoguna are negative attributes which destabilise and weaken the mind. Yet all the three attributes are required of course in a balanced state and having a strong control of satvaguna are necessary for normal functioning of the mind. s the final goal of yoga practitioner is elevation and uplifting of mental faculties to attain eternal bliss or moksha he/she should do everything to go on increasing his satvaguna through proper diet and good behaviour.