One Half Red Onion per day with meals.

  Pasteur in the mid nineteenth century, declared onion as anti-bacterial and its essences have been proved to kill harmful bacteria, including E-coli and salmonella. One experiment also found that chewing raw onion for 3-8 minutes made the lining of the mouth sterile.

In 1989, a Chinese study found that people who ate the highest amounts of allium vegetables in their diet, i.e.. onions, shallots, leeks, chives, garlic etc., had the lowest rate of stomach cancer. An independent study carried out in the United States at around the same time would seem to reinforce this theory.
The onion is rich in flavenoids, one of which, quercetin, is currently being studied for its apparent ability to deactivate carcinogens and tumour promoters. Those onions that are highest in these flavenoids are red and yellow onions and shallot.
A separate investigation by Eric Block PH.D. of the State University of New York, uncovered a sulphur compound in onion that can actually prevent the biochemical chain of events that lead to inflammatory reactions and asthma.

The ancient physicians used onion to treat diabetes and, once again, modern studies have only served to proved them right, as recently it has been discovered that onion does indeed play a part in reducing blood sugar levels. Cooked onions are good for the cardiovascular system, taking just about 1 tablespoon to reverse the tendency for the blood to clot after a fatty meal. It is now known to reduce high blood sugar levels and to promote an increase in ‘good’ cholesterol levels in the blood. In fact, just half a raw onion a day will boost it by 30%.