When added to the hair and scalp, onion juice can provide extra sulfur to support strong and thick hair, thus preventing hair loss and promoting hair growth. The sulfur from onions may also help promote collagen production. Collagen in turn helps the production of healthy skin cells and hair growth. Onions have anti-bacterial properties, which may help fight scalp infections .Onions contain antioxidants, such as flavonoids. Antioxidants are believed to protect the body from free radicals. Free radicals contribute to the aging process. For example, free radicals may destroy the hair follicles and lead to thinning and loss of the hair.