Humans Make Up For Vitamin C Shortfall

More than 4,000 mammals can make vitamin C from glucose, but the human version of the necessary enzyme is out of order. To make up for our species' incompetence, humans have evolved a way to efficiently utilize the vitamin C we consume. In particular, researchers led by Naomi Taylor at the National Center for Scientific Research, in Montpellier, France, show that a glucose transport protein on the surface of human red blood cells is altered by another protein called stomatin so that vitamin C's oxidized form, L-dehydroascorbic acid (DHA, shown above right), is preferentially imported into red blood cells instead of glucose (Cell 2008, 132, 1039). DHA is then quickly reduced to vitamin C inside the blood cells. In fact, humans express three orders of magnitude more of these glucose-turned-DHA transporters on their red blood cells than do mammals that produce their own vitamin C. Because of this efficient vitamin C cycling, humans need only consume 1 mg of vitamin C per kg of body weight per day, whereas goats have to synthesize 200 times that amount.