Red onion is mostly popular because of phytochemicals, which are important because they boost our immune system. Red onion is also a rich source of vitamin K, B6, and C, but at the same time, the phytochemicals act as a stimulant to vitamin C within the body. Fiber from red onion helps in functioning of the digestive tract, and it helps prevent constipation. Red onion is rich in minerals, too: it has folate, thiamine, calcium, magnesium, potassium, and manganese.