Like most nutrients, our genes play a role in how vitamin C is absorbed, transported, and used by the body. This can influence your risk for certain diseases, and it can make a difference in how much vitamin C you need to consume, at a minimum.
Vitamin C has a variety of functions in the body. It is an antioxidant, as well as a co-factor in many important enzyme reactions, including the synthesis of collagen, carnitine, and some neurotransmitters. It is also important in regulating the absorption of iron. [ref]
Absorbing and transporting vitamin C:
Most mammals actually make vitamin C innately, but humans can’t make vitamin C and have to rely on food sources. Our bodies have vitamin C transporters that are involved in the absorption of ascorbic acid (vitamin C) in the intestines.
Disease due to deficiency
There are several major diseases associated with vitamin C levels as well as with genetic polymorphisms of the vitamin C transporters, SLC23A1 and SLC23A2.
Scurvy is usually the first thing to come to mind with vitamin C. Made famous by sailors who went months at sea with no fresh fruits or vegetables, scurvy is caused by a vitamin C deficiency that lasts a month or more. Symptoms of scurvy include fatigue, bleeding gums, easy bruising, poor wound healing, and bone pain. Vitamin C is needed in the synthesis of collagen, so the symptoms of scurvy are almost all related to defective collagen in the skin, blood vessels, and bones.
Lower levels of vitamin C consumption are also associated with increased risk of several major diseases;
- Higher intake of vitamin C is associated with a reduced risk of cardiovascular disease. Higher plasma vitamin C levels (whether due to genetics or due to higher fruit and vegetable intake) is associated with a reduced risk of both heart disease and overall mortality. [ref]
- Higher intake of vitamin C is associated with a reduced risk of stomach cancer.[ref] Stomach cancer is now the third leading cause of cancer deaths worldwide.[ref]
- Higher plasma vitamin C levels are linked to lower plasma urate levels, and lower risk of gout.[ref]
SLC23A1 and SLC23A2 are the genes that code for vitamin C transporters. Variants of these genes affect the plasma levels of vitamin C. All of these variants are very common; some are associated with higher plasma vitamin C concentrations and some with lower concentrations.
Check your genetic data for rs6133175 (23andMe v4; AncestryDNA):
- G/G: 24% higher (on average) plasma vitamin C concentrations.[ref]
- A/G: normal vitamin C
- A/A: normal vitamin C
Check your genetic data for rs6053005 (23andMe v5 only)
- T/T: 24% higher (on average) plasma vitamin C concentrations.[ref]
- C/T: normal vitamin C
- C/C: normal vitamin C
Check your genetic data for rs33972313 (23andMe v5 only)
Check your genetic data for rs10063949 (23andMe v4, v5):
- T/T: greater vitamin C transport, lower risk of Crohn’s [ref]
- C/T: less vitamin C transport (most common genotype)
- C/C: less vitamin C transport
rs12479919 (v. 4 only): The T allele is associated with lower risk of gastric cancer, with those carrying the T/T genotype at half the normal risk of gastric cancer.[ref] [ref] Since higher vitamin C levels = lower gastric cancer risk, it makes sense that those people with the T allele will have higher levels of vitamin C.[ref]
rs33972313 (v. 5 only): Those with a T allele have an average decrease in plasma vitamin C concentration of 24%.[ref] Another study found that those with the C allele have a reduced risk of heart disease (implied that it is due to increased vitamin C transport) [ref]
SLC2A1 (also known as GLUT1) is the gene that codes for the enzyme that transports glucose across the cell wall. This same enzyme also transports the oxidized form of vitamin c, dehydroascorbic acid, into cells where it is then reduced to ascorbic acid. [ref] While there are studies linking GLUT1 polymorphisms to diabetes in some populations, I didn’t find any relating to vitamin C levels in the cell.
I did find one interesting study that I wanted to share about vitamin C, glucose, and cancer: A study in 2015 looked at the levels of glucose and ascorbic acid in thyroid cancer cells. “The results showed that in thyroid cancer cells high glucose inhibits both transport of AA [ascorbic acid] and DHAA [dehydroascorbic acid]. Inhibition of vitamin C transport by glucose had a cytotoxic effect on the cells. However, stabilization of vitamin C in one of 2 forms (i.e., AA or DHAA) abolished this effect. These results suggest that cytotoxic effect is rather associated with extracellular accumulation of vitamin C and changes of its oxidation state than with intracellular level of ascorbate.”
How much do you need?
The US RDA for vitamin C is 60mg per day, which is just a little higher than what is needed to prevent scurvy (46mg/day). Many sources recommend getting more than the RDA, and the studies on cardiovascular disease indicate that a higher intake is needed for heart health.
At the high end of the recommendation scale, the Vitamin C Foundation recommends 3000 mg/day.[ref] A little more conservatively, the Linus Pauling Institute recommends 400mg/day.
Vitamin C is a water-soluble vitamin and non-toxic, so excess vitamin C is eliminated and doesn’t build up. You (and your bathroom) will know when you’ve had too much and you exceed your personal bowel tolerance…
Eat your fruits and vegetables:
Excellent food sources of vitamin C include oranges, grapefruit, kiwi, strawberries, tomato and red peppers.
More to read:
Oregon State University: Vitamin C