In recent years, though, debate over just how much vitamin C is enough has resulted in differing opinions among experts. Cyndi Thomson, research instructor and registered dietitian with the University of Arizona's College of Public Health and the Arizona Cancer Center, said more than 500 milligrams per day is unnecessary. If you take 1,000 milligrams of vitamin C, the body only absorbs 50 percent of it, Thomson said. Also, anything beyond 500 milligrams can do harm, possibly causing kidney stones and diarrhea. Still, most health care providers agree the benefits of vitamin C taken in moderation far outweigh its risks.
Some studies do support the idea of it decreasing the duration of a cold.
Vitamin C also serves as an antioxidant that boosts immune functions. Buffered forms are treated with different minerals to make them less acidic and easier on the stomach. Ascorbic forms are usually derived from corn and are synthetic. While many Americans follow the Recommended Dietary Allowances as a means of measuring sufficient intake of nutrients, Thomson said that is slowly changing. Once you find the C vitamins, chance are you'll find many of them have rose hips or bioflavonoids added to enhance the activity of the vitamin.