Food intake gives us energy when it is properly digested. Physical body system maintains its health as a default program as long as it gets energy in the required quantity. Health deteriorates only when the energy depletes from the physical body.
The equation is total energy total health, less energy not so healthy and vulnerable to a disease and depleted energy total collapse.
So all that depend what type of food one takes, at what intervals and in what quantity. As the body constitution is a mixture of DNAs from both the sides of the parents, a physical body becomes a complicated end product that is unique in its system.
The health tips usually given are general tips but the result differs from person to person. The way a physical body is allergic to a particular medicine, a physical body draws energy from some food items, reject from some and become neutral some other.
So it is not easy to draw a line between food to take and not to take in case of natural diet without influence of chemicals used for preserving in cans or coloring them.
That is the reason why some precautionary measures are prescribed in Ayurveda like fasting or changing the food pattern.
Fasting is suggested once a week to make the body lose unnecessary accumulations in the body like toxins, fats and other waste material from the digestive system. Fasting sets right the body system after which the normal food the body is accustomed to should be taken in limited quantities initially.
When the body gets some sort of discofort, our Ayurvedi doctors asked the patients to change the diet. First of all they used to suggest fasting if the stomach is upset and there is no appetite. If the patient is in a position to take some food, fruits were suggested or some other alternative food is suggested. For example if a person is having fever and cannot digest his normal food of rice in South India, they are asked to eat bread or rotis made of wheat. On the contrary a person suffering fever is suggested by the doctors in the Northern belt to take rice in the same circumstances. Rice is a light food item for North Indians and Roti for South Indians.
The logic is your taste buds are used to certain type of food items. Mind gets satisfied if the food item for which the body is used to is supplied to the body. If the food item changes, it becomes unpalatable and he will consume less quantity which is second best to fasting.
But as the medication system is changed to chemical drugs in Allopath way of treatment, fasting is prohibited as those medicines on empty stomach will make holes in the digestive system.
If a person is not on medication, then the normal food in small quantities with more number of intakes will keep the body healthy.