Vitamin C

Vitamin C was first discovered in 1912, isolated in 1928, and became the first vitamin to be chemically produced in the year 1933. By 1934, it was being mass produced and sold as a dietary supplement under the brand name “Redoxon,” which is now owned by the company Bayer.

Although many animals can produce their own vitamin C from glucose in their kidneys or livers, humans (along with many other primates) lost the ability to do so approximately 61 million years ago. This was due to the inactivation of the gene that produces L-gulono-lactone oxidase (GLO), an enzyme that’s needed to complete the last step of vitamin C synthesis from glucose. Scientists still speculate about the exact reasons this gene became inactivated, but the leading theory is that the habitat of our ancestors was abundant in vitamin C-rich fruits, making it unnecessary to produce the vitamin internally. So, there was no selective pressure to preserve GLO gene function, and random disabling mutations of the GLO gene eventually displaced the functional copy as a result of genetic drift.

The foods with the highest vitamin C content include: citrus fruits (oranges, grapefruit, lemons, limes), kiwis, berries, red peppers, guavas, papayas, broccoli, Brussels sprouts, tomatoes, cantaloupe, and leafy greens. Some organ meats (adrenal glands in particular) are also good sources of vitamin C. Cooking can reduce vitamin C levels in foods, with boiling being the worst offender, due to this vitamin being both water-soluble and sensitive to heat; steaming and microwaving lead to the highest vitamin C retention. Prolonged storage, too, causes vitamin C levels in foods to diminish over time.

Vitamin C in Health and Disease

Higher intakes of vitamin C are linked to reduced risk of heart disease, some forms of cancer, type 2 diabetes, cataracts, age-related macular degeneration, and gout. Vitamin C can also help regulate the stress response and reduce anxiety, and there’s preliminary evidence that it may also help prevent Alzheimer’s disease.

Vitamin C and Cardiovascular Disease

Across studies, higher vitamin C status has been linked to a reduced risk of cardiovascular disease and cardiovascular disease risk factors, including coronary heart disease, high blood pressure, and stroke. These cardiovascular benefits may be a result of vitamin C improving the function of endothelial cells, which are the cells that form our blood vessels (particularly with daily intake of vitamin C above 500 mg), reducing LDL “bad” cholesterol and triglyceride levels, reducing inflammation, and protecting LDL cholesterol from oxidation. Large prospective cohort studies have found that every 20 μmol/L increase in plasma vitamin C is associated with a 9% lower risk of heart failure, and combined analyses of vitamin C intake and heart disease show that those who consume at least 700 mg of vitamin C per day have a 25% lower risk of developing heart disease over the course of 10 years.

Vitamin C and Immunity

Due to its role in the immune system, vitamin C can reduce the length and possibly the severity of the common cold, particularly if taken as a supplement before symptoms set in (although science doesn’t support the claim that it reduces the risk of getting sick in the first place). Intravenous vitamin C may also benefit critically ill patients with sepsis (an extreme and life-threatening response to infection), lowering markers of inflammation and endothelial injury and reducing the odds of mortality. Several trials also suggest that vitamin C dietary supplements (1 – 5 grams daily) could reduce the incidence of asthma induced by respiratory infections.

Vitamin C and Cancer

Evidence for the effects of vitamin C on cancer development or progression is mixed and generally shows little benefit, though some observational studies suggest an inverse relationship between vitamin C intake and breast cancer (as well as reduced mortality risk for breast cancer patients with the highest vitamin C intakes), colon cancer, and non-Hodgkin lymphoma. Likewise, vitamin C may help protect against stomach cancer, due to vitamin C inhibiting carcinogenic N-nitroso compounds from forming in the stomach, as well as inactivating an enzyme called urease that the stomach-cancer-linked bacteria H. pylori uses for survival and colonization in the gastric mucosa. Intravenous vitamin C, in particular, may also reduce the side effects of chemotherapy and radiotherapy.

Vitamin C and Diabetes

Some research also suggests that vitamin C could reduce the risk of developing type 2 diabetes. In cross-sectional clinical trials, plasma vitamin C concentrations are inversely associated with markers of insulin resistance and glucose intolerance (such as glycated hemoglobin), and observational studies have linked higher vitamin C intake and higher plasma vitamin C levels to lower incidence of diabetes. 

Vitamin C and Eye Health

Scientific studies also show that vitamin C can prevent or slow the development of cataracts. In addition, vitamin C can slow the progression of age-related macular degeneration, especially when taken in combination with other essential ocular health nutrients, including vitamin A, vitamin E, selenium, zinc, anthocyanins, and carotenoids (especially lutein, zeaxanthin, and β-carotene).

Vitamin C and Neurodegenerative Disease

Alzheimer’s disease, too, could be impacted by vitamin C levels, with observational research showing that Alzheimer’s patients have low levels of vitamin C relative to healthy controls, and animal models of Alzheimer’s disease demonstrating that vitamin C supplementation (in the context of deficiency) can reduce amyloid deposition in the hippocampus and cortex, reduce mitochondrial dysfunction, and limit blood-brain barrier impairments. Some studies even show that high vitamin C intake has a protective effect on thinking and memory during the aging process.

Vitamin C and Mental Health

Numerous studies on vitamin C have also shown its relationship with stress.  Supplementation of vitamin C (ascorbic acid) in humans is associated with a decreased cortisol response after a psychological or physical stressor.  In a randomized double-blind, placebo controlled 14-day trial, 120 healthy young adults were given sustained-release ascorbic acid (3 x 1000 mg/day Cetebe) or a placebo.  After being subjected to acute psychological stress, consisting of public speaking and mental arithmetic, participants in the ascorbic acid group had lower blood pressure, subjective stress responses and faster salivary cortisol recovery.  

Furthermore, vitamin C deficiency is widely associated with stress-related diseases such as depression and anxiety! Several reports have suggested a relationship between behavior under stress and ascorbic acid.  In a 2015 randomized, double-blind, placebo-controlled study in 42 high-school students, participants were given 500mg vitamin C daily or placebo for 14 days.  Students receiving vitamin C showed a reduction in anxiety levels and heart rate. Vitamin C is also depleted by stress so if you don’t have enough and you’re using it up quickly you can see how low vitamin C levels will quickly become a “snowball of badness”.  So, if you’re having a tough day, reach for foods high in vitamin C! 

Vitamin C and Gout

And, vitamin C has been inversely associated with uric acid levels and gout (although randomized trials have failed to confirm a protective effect of vitamin C supplementation on a shorter-term basis).

Vitamin C and All-Cause Mortality

When measuring plasma vitamin C levels instead of estimating dietary intake (which can be faulty, due to poor recall on questionnaires), studies have found a strong protective effect of vitamin C on all-cause mortality. For example, in the EPIC-Norfolk prospective study, the relationship between plasma ascorbic acid levels and mortality was investigated in 19,496 men and women aged 45-79 over 4 years.  Plasma ascorbic acid concentration was inversely related to mortality from all-causes, cardiovascular disease and ischemic heart disease.  The risk of mortality in the top ascorbic acid quintile was about half the risk in the lowest quintile (52% decrease in men, 50% in women).  Overall, a 20 micromole per liter rise in plasma concentration (equivalent to ~50 grams per day increase in fruit and vegetable intake) was associated with a 20% reduction in risk of all-cause mortality in the general population!

How Much Vitamin C Do We Need?

Only 10 mg of vitamin C per day is necessary to prevent scurvy, but the recommended dietary allowance (RDA) of vitamin C for adult males and females is 90 mg and 75 mg, respectively — and the US Institute of Medicine estimates that smokers need an additional 35 mg of vitamin C per day (due to smoke inhalation increasing oxidative damage and depleting vitamin C more rapidly), and this may also apply to those exposed to secondhand smoke. But, these numbers should be thought of as a bare minimum for avoiding insufficiency symptoms: consuming levels of vitamin C beyond the RDA is associated with its preventative effects.

Up to 10 grams a day (that’s a tolerable upper intake level of 10,000 mg!) can be consumed by adults without concerns about toxicity or health detriments, though there is a recommendation to keep daily intake below 2000 mg to avoid gastrointestinal upset. (In some cases, dietary supplements that contain high doses of vitamin C can also cause headache, sleeping difficulties, skin flushing, and an increased risk of kidney stones, particularly in men.)

Worth noting, vitamin C reaches maximum absorption (100%) when ingested at doses up to 200 mg at a time. At doses beyond 500 mg, increasingly less vitamin C gets absorbed and instead is excreted in the urine. So, it’s smart to spread vitamin C intake throughout the day instead of eating all your vitamin-C-rich foods (or vitamin C supplements or a multivitamin with a high dose of vitamin C) in one sitting! Likewise, because vitamin C is a water-soluble vitamin, it doesn’t get stored internally; we need to ingest it regularly.

Good Food Sources of Vitamin C

The following foods are also excellent or good sources of vitamin C, containing at least 10% (and up to 50%) of the daily value per serving.