We’ve heard it all our lives: Vitamin C fights colds. That’s partially true. Some human studies show that taking vitamin C can lessen the severity and duration of the common cold.1 What’s irrefutable is the role that vitamin C plays in maintaining immune function.2-4 The ABCs of Vitamin C Vitamin C is an essential nutrient in humans.2 Without it we die. Humans don’t internally produce vitamin C like most animals. It must be obtained from diet or other external sources. Severe vitamin C deficiency—medically known as scurvy2—causes..