Literature

For years, we have been told that whilst most mammals can make Vitamin C from glucose, this ability is absent in humans, higher primates, guinea pigs and fruit bats. This trait, we are told, has an evolutionary explanation: an "inborn metabolic error" associated with an unfortunate mutation in past times. Instead of a gene for synthesising Vitamin C, these animals have a pseudogene. As a consequence, humans have struggled with scurvy whenever our diets have lacked natural sources of Vitamin C. The most vivid example of this involved British seamen in the late 18th century, who were eventually supplied with limes to avoid succumbing to the disease.

For a sailor, some lime juice a day keeps scurvy away

However, the loss of function story does not stop there. New research reveals a fascinating twist.

"[T]he red blood cells of the handful of vitamin C-defective species are specially equipped to suck up the vitamin's oxidized form, so-called L-dehydroascorbic acid (DHA) [. . .]. Once inside the blood cells, that DHA--which is immediately transformed back into ascorbic acid (a.k.a. vitamin C) - can be efficiently carried through the bloodstream to the rest of the body."

Emphasis really should be given to the word "efficiently". Animals making Vitamin C do not appear to be particularly adept at using it. "The recommended daily dose of vitamin C for humans is just one mg/kg, while goats, for example, produce the vitamin at a striking rate of 200 mg/kg each day." The remarkable efficiency of human red blood cells is described as a form of recycling:

"In essence, the red cells of animals that can't make vitamin C recycle what little they've got. Earlier studies had described the recycling process, Taylor said. "Our contribution to the whole story is to show that this process of recycling exists specifically in mammals that don't make vitamin C.""

This research is framed within an evolutionary perspective:

"Evolution is amazing. Even though people talk about this as an 'inborn error'--a metabolic defect that all humans have--there is also this incredible manner in which we've responded to the defect, using some of the body's most plentiful cells," said Naomi Taylor of Universite Montpellier I and II in France, noting that the body harbors billions of red blood cells. "[Through evolution], we've created this system that takes out the oxidized form of vitamin C and transports the essential, antioxidant form."

However, the research is also perfectly capable of a design perspective. Animals generating Vitamin C do not use it very efficiently. The alternative system of efficient recyling of Vitamin C in the diet avoids making for waste and, most of the time, it works very well. The only problems are when diets leave out foods that supply the Vitamin C. Instead of thinking of this mechanism as an evolutionary adaptation, the new research reveals processes that have optimised functionality.

Erythrocyte Glut1 Triggers Dehydroascorbic Acid Uptake in Mammals Unable to Synthesize Vitamin C
Amelie Montel-Hagen, Sandrina Kinet, Nicolas Manel, Cedric Mongellaz, Rainer Prohaska, Jean-Luc Battini, Jean Delaunay, Marc Sitbon, and Naomi Taylor
Cell, Vol 132, 1039-1048, 21 March 2008

Summary: Of all cells, human erythrocytes express the highest level of the Glut1 glucose transporter. However, the regulation and function of Glut1 during erythropoiesis are not known. Here, we report that glucose transport actually decreases during human erythropoiesis despite a >3-log increase in Glut1 transcripts. In contrast, Glut1-mediated transport of L-dehydroascorbic acid (DHA), an oxidized form of ascorbic acid (AA), is dramatically enhanced. We identified stomatin, an integral erythrocyte membrane protein, as regulating the switch from glucose to DHA transport. Notably though, we found that erythrocyte Glut1 and associated DHA uptake are unique traits of humans and the few other mammals that have lost the ability to synthesize AA from glucose. Accordingly, we show that mice, a species capable of synthesizing AA, express Glut4 but not Glut1 in mature erythrocytes. Thus, erythrocyte-specific coexpression of Glut1 with stomatin constitutes a compensatory mechanism in mammals that are unable to synthesize vitamin C.

See also:

Troadec, M-B. and Kaplan, J. Some Vertebrates Go with the GLO, Cell, Vol 132, 921-922, 21 March 2008.

How Humans Make Up For An 'Inborn' Vitamin C Deficiency, ScienceDaily (Mar. 21, 2008)