Most birds and animals make their own vitamin C. But a few species, like people and guinea pigs, must get it from their food.

Vitamin C is required for the body to make and maintain collagen, a protein. Collagen forms the base for all connective tissue in the body. If you don't have enough vitamin C in your diet, you might get the disease scurvy. Symptoms include loss of appetite, bleeding gums, loose teeth, swollen ankles and tiny hemorrhages (bleeding spots) in the skin.

Hundreds of years ago, sailors who spent months at sea were as likely to die from scurvy as from shipwreck or battle. They ran out of fresh fruits and vegetables soon after going to sea.

Good sources of vitamin C are citrus fruits like oranges and grapefruit, strawberries, green peppers, broccoli and potatoes.