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.

Question: Does Vitamin C really prevent or treat colds? My mother swears by it, but I’m not so sure.
Unfortunately, no. It would be marvellous if vitamin C (ascorbic acid), or some other compound, could prevent or treat colds. The medical profession would promote it endlessly if it worked. Exhaustive trials have not shown vitamin C to be any more effective than placebo (sugar pills) in dealing with colds.
The placebo effect can be very strong, and if a patient sincerely believes that a particular medication will work, they may rapidly recover.
A well-balanced diet containing food from all the groups (vegetables, fruit, cereals, meat and dairy products) will give you adequate vitamins, and with a reasonable amount of exercise and adequate rest, is probably the best way to promote your own health and prevent viral infections.