By Susie Rockway, Ph.D.
Vitamin C, or ascorbic acid (ascorbate), is the most frequently taken dietary supplement in North America. Yet, despite its widespread use, national surveys report that 15 percent of the population still doesn't get enough vitamin C to meet recommended amounts for health.
Initially, the Recommended Daily Intake (RDI) for vitamin C was set at 60 milligrams, the amount required to prevent scurvy. The deficiency disease—commonly characterized by bleeding gums and loosened teeth helped identify the vitamin as having an essential role in collagen formation.
However, more recent research has now made it clear that more dietary vitamin C is needed to saturate body tissues such as the brain, heart, liver, and adrenal glands.
Consequently, in 2000, the RDI was raised to 90 and 75 milligrams daily for men and women. The new values were based on studies suggesting that plasma vitamin C should be maintained at specific levels (~80 µmol/L) for good health and antioxidant activity.
This is how the "debate on how much of vitamin C is needed" began—with scientists discussing two questions:
- How much vitamin C is really needed to make an impact on plasma levels?
- Are plasma levels really the best indication of tissue saturation levels?
While there is still not any consensus on this matter, a new, carefully designed study just published in the February issue of American Journal of Clinical Nutrition provides some clues for answering these questions.
In the study, New Zealand researchers compared "normal" mice vitamin C tissue saturation levels with "knockout" mice that were genetically engineered to lack the enzyme needed for synthesizing vitamin C in the body.
Without dietary vitamin C, they saw that tissue concentrations of ascorbate became deficient weeks before the onset of scurvy symptoms in the mice, similar to humans. During these weeks, the scientists observed that the mice exhibited signs of impaired collagen synthesis, changes in aortic wall structure, formation of atherosclerotic plaque, and activation of inflammatory processes.
Sustaining Tissue Concentration
The scientists observed that each tissue became deficient at different rates. For example, the brain maintained its vitamin C longer than other organs; however, after just two weeks of deficiency, vitamin C in the brain became depleted. To replenish these tissues to normal levels, plasma concentration had to be maintained saturated with daily ascorbate intakes.
In addition, they found that tissue levels of ascorbate became saturated more quickly when the mice ate kiwi fruit versus sodium ascorbate in water—the flavonoids present in kiwi are thought to enhance absorption of vitamin C by maintaining it in a reduced state (versus an oxidized state).
Lastly, based on the new evidence, the researchers wrote that the current RDI should receive no less than a 60 percent boost—from 75 milligrams per day to at least 120 milligrams per day—to keep not only plasma levels saturated, but also tissue levels of the brain, and organs that are unable to be measured with current methods.
The vitamin C researchers stressed the need for people to maintain an ongoing and constant vitamin C intake to sustain tissue concentration—which means consuming the scientists' recommended amounts regularly.
What Weight Has to Do with Vitamin C
Another less well-known role of vitamin C is its involvement as a cofactor in the biosynthesis of carnitine, a molecule required for burning fat as energy fuel (fatty acid oxidation).
Interestingly, there is a direct inverse relationship with obesity (adiposity) and plasma vitamin C concentrations—as plasma ascorbate levels are decreased, fatty tissue increases (adiposity increases).
This association has led researchers to wonder whether or not decreased vitamin C levels cause less carnitine to be synthesized and if there is a relationship between less carnitine and less burning of fat for fuel.
Preliminary findings are that subjects who are vitamin C depleted have lower levels of carnitine leading to a 25 percent decrease in fatty acid oxidation (per kilogram body weight) than individuals whose vitamin C status is adequate.
The researchers speculate that reduced fat oxidation seen with vitamin C depletion may result in weight gain by two mechanisms:
- Indirectly, decreased carnitine leads to increased fatigue during exercise and this may lead to exercise intolerance
- Directly, by lipid (fat) accumulation
- Vissers MCM, Bozonet SM, Perason JF and Braithwaite LJ. Dietary ascorbate intake affects steady state tissue concentrations in vitamin C-deficient mice: tissue deficiency after dietary ascorbate intake affects steady state tissue concentrations in vitamin C-deficient mice: tissue deficiency after suboptimal intake and super bioavailability from a food source (kiwifruit). AJCN 93(2):292-301, 2011.
- Johnston CS, Corte C and Swan PD. Marginal vitamin C status is associated with reduced fat oxidation during submaximal exercise in young adults. Nutr & Metab 3(35):1-5, 2006.