Members of the same plant family, garlic and onions do so many things for your heart and immune system, it's hard to list them all. Garlic's 70 active phytochemicals may decrease high blood pressure by as much as 30 points, and it lowers rates of ovarian, colorectal, and other cancers, according to the American Journal of Clinical Nutrition.
Onions are the single best source of quercitin, a flavonoid shown to keep your blood healthy and prevent clots. Both are must-haves for natural allergy prevention. To boost garlic's health effects, be sure to crush the cloves and let them stand for up to 30 minutes before heating them. Most of an onion's nutrition is in the skins, so Duke suggests putting the skins in a mesh bag and allowing them to steep in soups or sauces.