Most of the people think that empty stomach creates the feeling of hunger but this is not true. Hunger has nothing to do with an empty stomach. For example, a person who has fever may have an empty stomach, but he does not feel hungry. Similarly when a child is born, he does not feel hungry for several days.
When the blood gets depleted of nutritive materials, a message is sent to our brain by our body through nerves. The deficiency of nutritive materials is caused in the blood by the consumption of energy in the routine work. A sick man remains in a state of rest and consumes small amounts of energy. He, therefore, does not feel hungry for several days. Whenever there is a deficiency of nutritive food substances in the blood, a message goes to the ‘hunger centre’ of the brain. This centre acts like a brake on the activities of the stomach and intestines. When the blood has sufficient nutritive substances, the hunger centre stops the activities of the stomach and intestines. On the contrary when there is a lack of nutrition, the intestines and stomach become active. That is why you can hear your stomach rumbling when you are hungry.
When we feel hungry, we do not demand any particular kind of food. A starved person will eat any kind of food. This main constituent of food is: proteins, carbohydrates, fats, minerals, vitamins and water. Each component has a specific role to play. Proteins are nitrogenous compounds which are used for the growth of the body, for the repair of muscles and tissues. Carbohydrates and fats produce heat energy in the body. Minerals build the bones and tissues. Vitamins protect the body from some diseases. Water is the main constituent of the cells. It carries the nutritive substances and oxygen to the different parts of the body. Because of the deficiency of any nutritive substances, we can become ill. Hence we conclude that deficiency of nutritive elements in the blood is the main cause of hunger.

Question: I have read that one should restrict one’s diet according to your blood group, so that anyone with blood group O should avoid wheat and dairy products, tomatoes, melons, olives and cauliflower. Is there much scientific evidence to support this doctrine?
This is one of the weirdest health fads to come along, and a new one does come up every year or so.
The major blood groups (O, A, B, AB) are determined by reactions between protein particles on the surface of red blood cells, and are important when giving blood transfusions to prevent interactions between red blood cells in the donor and patient blood, but have no relationship to any disease or infection.
When treating patients with any medical condition, with any form of medication (tablet or injection), doctors pay absolutely no attention to the patient’s blood group unless there is a possibility that a blood transfusion may be required.
If there is no interaction between any of the thousands of different medications in use (even when directly injected into the blood) and the blood group, there is no logic in the argument that foods, which are broken down into their basic constituents in the stomach before being absorbed into the body, can react in different ways in people with different blood groups.
Probably the most significant blood factor is the rhesus factor which can be negative or positive for each of the ABO major blood groups. There are also numerous minor blood group divisions, including the Kell, Duff, Lewis and Kidd factors, and groups M, N, S and U. All of these can be responsible for reactions during transfusions and may need to be specifically cross-matched. What do the proponents of these diets say about these sub-groups? I doubt that they even know about them.
There is absolutely no scientific evidence to support this diet.