Do you have questions about vitamin C for our expert Dr. Alexander Michels? See our first vitamin C FAQ, look below, or ask us here and we will get back to you as soon as possible.

Health Effects of Vitamin C

Does vitamin C cause high blood sugar? What about alterations in HbA1C? Is vitamin C contraindicated in diabetes?

On the contrary – High plasma vitamin C levels have been associated with a reduced risk for type 2 diabetes. People who have diabetes may also see benefits from vitamin C, but more studies are needed on this subject. It is generally agreed upon that people with diabetes should be getting at least the RDA of vitamin C from their diets (90 mg/day for men and 75 mg/day for women), if not the LPI’s recommendation of 400 mg/day.

High ascorbic acid levels in the blood is associated with lower glycated hemoglobin proteins (known as HbA1c). Some studies have shown that achieving high plasma ascorbic acid levels with supplementation can reduce HbA1c levels in some diabetics, but the effects may be small. There is also a suggestion that vitamin C can reduce blood sugar, but the magnitude of these changes after a supplement is currently unclear – more work is needed.

Some glucose blood test kits/monitors can give false high readings in people taking large doses of ascorbic acid due to the chemical nature of the blood test. Look for a test that uses the “hexokinase” method or ask your doctor to test your blood using that method. Vitamin C cannot interfere with this method of determining blood glucose.

Why is intravenous vitamin C not a consideration for some types of cancer treatment?

Intravenous (IV) vitamin C is not often a consideration of oncologists because it is an unproven therapy – it is still in its experimental stages. Beyond this, several cancers have been found that are resistant to high doses of ascorbic acid likely because they can resist or counteract the hydrogen peroxide generated by intravenous doses of ascorbic acid. Additionally, there is the possibility that low amounts of hydrogen peroxide may stimulate cancer cell growth and division, rather than cell death. Although this has not been observed clinically, most clinical trials using IV vitamin C have been conducted in conjunction with standard treatments (chemotherapy, for example) to minimize any risk to the patient.

It is also possible that intravenous vitamin C may interfere with conventional treatments. This would depend on the chemical nature of the agents used and should be discussed in more detail with an oncologist.

I have heard that antioxidants like vitamin C should be avoided because they counteract the beneficial effects of exercise. Is this true?

Some studies have shown that the adaptations seen in cells and mitochondria after intense exercise are lost when antioxidants like vitamin C and vitamin E are taken during the exercise bouts. However, the reports have yet to show any functional changes after taking these supplements – such as loss of muscle function or lack of endurance. Oxidants produced during exercise are very specific and stimulate important signaling pathways. Vitamin C is unlikely to have any effect on these signals due to the type and quantity of the oxidant reactions involved. The beneficial effects of antioxidants in exercise are likely to protect tissues from excessive oxidative damage and help resolve inflammation after exercise is completed.

It is good to keep ascorbic acid levels high when intensively exercising, but there has been no documented effect of taking continual large doses of vitamin C during exercise.

Vitamin C is known to increase oxalate in the urine. Should ascorbic acid be a concern to people who have kidney stones?

Excessive amounts of vitamin C should be avoided in individuals who produce oxalate kidney stones. This is not due to the vitamin C itself but rather to the oxalate breakdown product of ascorbic acid when it is oxidized. People who take large doses of vitamin C may also consume large amounts of oxalate inadvertently, as some ascorbic acid will be oxidized and degraded in the intestine. Additionally, there may be oxidative stress in some individuals who get kidney stones that contributes to accelerated levels of ascorbic acid degradation. Therefore, it may be prudent for individuals who are predisposed to calcium oxalate kidney stones to avoid taking more than 1,000 mg of vitamin C a day, and it is likely better to divide any doses of vitamin C supplements into several doses throughout the day to minimize oxalate formation.

Does applying ascorbic acid topically help wound healing?

Topical vitamin C has been poorly studied in wound healing. Wound healing is a complicated process of oxidative stress, inflammation, immune system recruitment, collagen production, and tissue regeneration where vitamin C may play beneficial roles. However, in some stages of wound healing, excessive amount of ascorbic acid may be detrimental. Current studies suggest that it may be of greater benefit to ensure that sufficient vitamin C is present in the diet along with other vitamins and minerals that support wound healing.

That doesn’t mean topic vitamin C can’t do anything to help skin. Protection from sun damage and boosting collagen production are two possible benefits.

What effects does vitamin C have on blood pressure and nitric oxide production?

Endothelial cells are cells that line blood vessels and act as a barrier between the bloodstream and rest of the body. They also regulate the relaxation and contraction of blood vessels that contribute to blood pressure. Many studies have shown that vitamin C administration promotes dilation/relaxation of endothelial cells and lowering of blood pressure.

It does this by supporting nitric oxide synthesis. Ascorbic acid is thought to prevent a cofactor for an enzyme called nitric oxide synthase (NOS) from becoming oxidized under stress conditions, thus allowing it to continually make nitric oxide and contributing to the relaxation of blood vessels.

Low vitamin C has been associated with sepsis, and intravenous ascorbic acid has been shown to effectively reduce inflammation due to sepsis. Would taking vitamin C supplements do the same thing?

Intravenous vitamin C is used in sepsis because oral supplementation in these situations is difficult: septic patients frequently have poor appetite and nausea and may have poor intestinal absorption due to widespread inflammation in the body. Time is also a critical factor in severe sepsis, and intravenous vitamin C provides immediate supplementation of vitamin C to the bloodstream.

Intravenous vitamin C has been shown to be of benefit in people with sepsis, but we don’t know why. The high amounts of ascorbic acid could produce hydrogen peroxide that would help the body fight off the severe infection. If this is the case, then oral vitamin C could not substitute for intravenous administration. However, the severe inflammatory response may warrant the use of vitamin C to support an antioxidant role, so oral vitamin C may be as effective (given the limitations stated above) as intravenous administration of the vitamin. Until additional studies are done to demonstrate what role vitamin C is playing in sepsis, it is unclear if high doses of ascorbic acid by oral dosing will be effective in sepsis.

Does intravenous ascorbic acid neutralize viruses and bacteria and assist the immune system in a similar fashion as a vitamin C supplement?

It has been speculated that intravenous administration of vitamin C can kill viruses and bacteria, and this is likely by the generation of hydrogen peroxide – the same mechanism that has the potential to kill cancer cells. This would not be reproduced by oral administration of vitamin C – normally optimum levels of vitamin C in the blood bolster the immune system only by enhancing the function of white blood cells.

The use of intravenous vitamin C to kill or neutralize viruses and bacteria has not been well tested. There are many anecdotes that intravenous vitamin C has helped individuals fight off serious infectious diseases, and it has been suggested as a safe therapy for HIV, Ebola, influenza, and other serious viral infections. However, until more clinical studies are performed, we cannot conclude anything about its use in severe viral or bacterial infections.

Other Vitamin C Questions

Why do you not recommend taking large doses of vitamin C, as recommended by Linus Pauling?

Linus Pauling made his recommendations prior to the discovery of the sodium-dependent vitamin C transport proteins in 1999 (five years after Pauling’s death). These proteins make up a transport system that governs the uptake and distribution of vitamin C throughout the body. A key finding about this transport system is that it is saturable; that is, the transporter can only transport a limited about of ascorbic acid. If too much ascorbic acid is present, for example, after a large dose of vitamin C from supplements or food, the transport protein will only accumulate a fraction of that dose. Studies in healthy young men have shown that a dose of 200 mg or less of ascorbic acid is 100% bioavailable (absorbed into the bloodstream from the intestine), while the bioavailability is lower for increasing doses: about 75% of a 500-mg dose is absorbed, while only about 40% of a 1,250-mg dose of vitamin C will enter the bloodstream. If vitamin C status is already high, most of it will be excreted in the urine.

There may be some effect of taking large doses (greater than 1 gram/day) of ascorbic acid, but these likely are limited to the gastrointestinal tract. For example, these doses of vitamin C can promote iron absorption. There is speculation that large doses of ascorbic acid will also cause beneficial changes in the gut bacteria living in the intestine, but the effects have not been investigated.

Why don’t I hear more about vitamin C research? Why aren’t more studies being conducted on vitamin C?

There are few studies being conducted at the moment, mostly due to the lack of funding. There is a general perception that there is little more to learn about vitamin C that has not already been discovered. The use of more sophisticated tools and analytical methods may allow us to discover new roles for vitamin C in human health, if we only would look for them.

Making a gift to the Linus Pauling Institute can help fund future studies.

What are the resources for questions about vitamin C?

The Linus Pauling Institute’s Micronutrient Information Center has an excellent page on vitamin C that is reviewed by experts in the field, and there are also pages on the the bioavailability of supplemental forms of vitamin C and the role of vitamin C in skin health.

And lastly, you can always email Dr. Michels at Alexander.Michels@oregonstate.edu with additional questions.