Vitamin C is a water-soluble antioxidant used to make collagen, support immune function and recycle other antioxidants like vitamin E. The body can't make or store much of it, so blood levels reflect recent intake.
Low vitamin C is more common than expected, especially with low fruit and vegetable intake, smoking or high stress. It affects skin and connective tissue, wound healing, iron absorption and immune defense. Because levels reflect the last few days of intake, retest after any change in diet or supplements.
Optimal Vitamin C: 1-2 mg/dL.
Typically re-tested every 12 weeks.