Eating red onions may help reduce risk of stroke, heart disease and a variety of cancers, including prostate, colorectal and stomach cancers, according to HealWithFood.org and SFGate. For maximum health benefits, retain as much of the fleshy, edible portion of the onion as possible when removing the onion's outermost layer.
Red onions contain a variety of vitamins and minerals that keep the body healthy, including folate, thiamine, calcium, magnesium, potassium, manganese, vitamin C, vitamin K and vitamin B-6. A one-half cup serving of red onion provides 1.5 grams of fiber, which helps prevent constipation and keeps the digestive tract healthy.
Red onions also contain an antioxidant compound called quercetin that has been proven to reduce blood pressure and lower risk of stroke and coronary heart disease, reports The World's Healthiest Foods. Quercetin also has been found to contain antifungal, antibacterial and anti-inflammatory properties. Quercetin has the potential to prevent and control intestinal polyps, suppress rhinoviruses that cause the common cold, treat psoriasis and inhibit the replication of viruses, such as the herpes simplex virus. Quercetin has been found to have cancer-fighting properties as well. According to one study, consuming quercetin in the form of half an onion daily can reduce the risk of stomach cancer by 50 percent.
Red onions are a rich source of chromium, a mineral that helps control glucose levels. Including chromium in the diet helps ward off type 2 diabetes, according to HealthWithFood.org.