Isoascorbic Acid (Vitamin C5), now called Erythorbic Acid, is an isomer of Ascorbic Acid (Vitamin C). Being an isomer of Ascorbic Acid, Isoascorbic Acid has the same kind and number of atoms as Vitamin C, but the atoms are arranged differently. Isoascorbic Acid (or Erythorbic Acid) has similar applications to Vitamin C as a water-soluble antioxidant. Used as a food additive in the United States, Isoascorbic Acid (or Erythorbic Acid) is readily used and metabolized in the body.

Being water-soluble and akin in molecular structure, Isoascorbic Acid (or Erythorbic Acid) is a close relative of Ascorbic Acid (Vitamin C). Ascorbic acid is chemically close to sugars and is renowned for its importance in the structure of connective tissue, collagen, bones, cartilage, teeth, gums, and eyes. Ascorbic acid is essential in the repair and production of cells.

Ascorbic acid and is 6 stable esters have been shown to lower cholesterol, strengthen the immune system, and reduce the signs of aging. There are 6 stable esters of vitamin C, all of which prove useful in the body in the different ways they are absorbed. These include: Vitamin C1-Ascorbic Acid, VItamin C2-Dehyroascorbic Acid, Vitamin C3-Ascorbyl Palmitate, VItamin C4-Calcium Ascorbate, Vitamin C5-Isoascorbic Acid, Vitamin C6-Calcium Isoascorbate, and lastly, Ascorbign, or provitamin C (the direct precursor of ascorbic acid) is also useful and appears to potentiate, if not accelerate, vitamin C activity.