Do You Need Vitamin C for Good Eye Health?

Vitamin C refers to a chemical called ascorbic acid and several of its chemically similar derivatives, such as dehydroascorbic acid, the reduced form of ascorbic acid. Unlike many mammals, humans can’t synthesise ascorbic acid themselves so it must be obtained through the diet. 

Vitamin C is a coenzyme, which means it is a chemical that is essential for enzymes to perform their function. For example, it's required by lysyl and prolyl hydroxylases for the synthesis of collagen, a protein important in connective tissue such as your skin. That’s why scurvy, a severe vitamin C deficiency, can lead to your teeth falling out as your gums degrade, among other unpleasant symptoms. Scurvy affected a lot of pre-industrial sailors since foods rich in vitamin C, such as bell peppers, citrus fruits, and tomatoes¹, would spoil on ships and so were not consumed.

Short wavelength light sources, such as visible blue and UV radiation, deliver higher energy than other forms of light. The eye is constantly exposed to these forms of radiation as a necessity for carrying out its function. However, when high-energy radiation is absorbed by certain molecules in tissues of our eyes, it can produce ROS. Eyes are also highly metabolically active, especially at the retina, where nerve impulses are constantly transmitted to the brain. This metabolic activity also leads to more ROS, causing oxidative stress. Oxidative stress leads to degradation of these tissues, leading to diseases such as glaucoma, cataracts, age-related macular degeneration, and eye floaters. Vitamin C's role as an antioxidant means it might help prevent build-up of ROS in these tissues.

Before looking at clinical evidence for vitamin C as a treatment or preventative for certain diseases, we need to understand how it is taken up from our diet and how it reaches our eyes once in our bodies.

Vitamin C is absorbed in the small intestine using two types of proteins: Sodium-dependent vitamin C transporters (SVCTs) and glucose transporters. SVCTs require sodium to be excreted into the intestines before absorbing both ascorbic acid and sodium together. Glucose transporters only allow dehydroascorbic acid, which is structurally similar to glucose, to be absorbed by the body, where it is reduced to ascorbic acid. 

Both forms of Vitamin C are equally bioavailable, although high glucose concentration can reduce the bioavailability of dehydroascorbic acid as it competes with glucose for available transporters. This may be the reason that diabetes, particularly type 2 diabetes, and obesity are correlated with lower than average vitamin C levels. Smoking also reduces vitamin C levels in the body by 25-50%, although this doesn’t seem to be related to bioavailability and might be due to smoking-induced oxidative stress.

The absorption process is very efficient, with 70-90% of vitamin C absorbed at daily intakes of 40-130mg. Eating a balanced diet with a range of (e.g. 5 different) fruit and vegetables each day can provide >200mg of the nutrient, significantly higher than recommended daily doses, which range from 40mg to 75-90mg, depending on the governing body. Above 1,000mg a day, absorption efficiency is reduced to 50% and unpleasant side effects such as an upset stomach can occur, but there are not thought to be any diseases associated with excessive vitamin C intake. However, recommended doses vary with certain factors such as disease and pregnancy, and vitamin C can interact problematically with medication, so you should consult a doctor before starting vitamin C supplements.

Once absorbed, most vitamin C does not remain in the blood and is taken up by organs using SVCTs and glucose transporters. There are two types of SVCTs: SVCT1 is used by some organs to take up vitamin C from the blood when it is abundant. SVCT2 will take up ascorbic acid almost always and is expressed in most organs, but is essential where vitamin C is an important antioxidant, such as in the brain and eyes^,. SVCT2 is so active that even in cases of vitamin C deficiency, the brain has completely normal levels of vitamin C. This suggests that supplements or a high vitamin C diet may not affect levels in the eyes, since the SVCT2 proteins are already saturated (i.e. constantly pumping at maximum capacity). 

The ciliary epithelium supplies the eyes with aqueous humour, which supplies the front portion of the vitreous with nutrients. SVCT2’s presence in the ciliary epithelium means that the vitreous, as well as the lens, iris, and cornea, are supplied with vitamin C by its inclusion in aqueous humour. One study found that dietary supplements significantly increased vitamin C concentration in aqueous humour, but the supplements contained 2,000mg of vitamin C - enough to cause discomfort and gastrointestinal irritation. Despite over 25x higher vitamin C intake than the recommended daily dose, vitamin C in the aqueous humour only increased 1.4x. 

However, supplementation may be important for groups such as smokers, obese people and diabetics, who are at higher risk of reduced vitamin C levels which could put them at risk of oxidative stress-related eye conditions. Indeed, vitamin C supplementation has been shown to restore blood vitamin C levels in smokers. It’s important to note that most supplements are not thought to be more bioavailable than dietary vitamin C, but might mean that even with poorer absorption efficiency, a healthy level of vitamin C in the body can still be met.

The rapid uptake of vitamin C by organs means that blood concentrations of vitamin C remain relatively constant. Because most vitamin C is found inside cells rather than in blood, it can be difficult to accurately measure the absorption efficiency or the distribution of vitamin C once absorbed. Clinical studies investigating the effect of vitamin C in the diet on different eye conditions provide a more direct insight into how vitamin C might help you.

Several studies have investigated the use of vitamin C as a preventative for conditions related to oxidative stress. 

One study observed over 4,000 participants over approximately 10 years and found that higher dietary levels of vitamin C, among other antioxidants such as vitamin E, beta-carotene, and zinc, reduced risk of age-related macular degeneration. This paper used self-reporting of diet, which is not always reliable, and most evidence does not support their conclusion: A paper compiling and analysing the results of 12 different studies found no link between antioxidant intake and risk of developing age-related macular degeneration. 

Another study on over 3,500 participants compared a high-dose antioxidant supplement (including 500mg vitamin C) against a placebo over, on average, 11.3 years and did not find a lower risk of developing macular degeneration. However the study found that the antioxidant group had a 28% lower risk of macular degeneration progressing to severe stages compared to the control group.

A preventative effect against cataracts has also been investigated. A Japanese study looked at over 10,000 participants over 5 years, and found vitamin C significantly reduced the likelihood of developing cataracts. Again, vitamin C consumption was estimated from self-reports of diet - less reliable, but still promising. Other reports have supported this claim, suggesting consuming 300 mg/day could reduce risk of cataracts by 70-75%. 

However, one study on almost 25,000 women found that taking supplements of 1,000 mg/day vitamin C actually increased their risk of cataracts by 25%. Furthermore, the same study that found that antioxidants slow the progression of macular degeneration found no effect on developing cataracts. From these studies it is unclear whether vitamin C is carrying out its proposed protective effect.

Vitamin C’s potential in preventing glaucoma is similarly unclear. A paper compiling and analysing 5 different studies on vitamin C and glaucoma risk found that vitamin C may reduce the risk of developing glaucoma. There was a statistically significant correlation between intake of vitamin C and reduced glaucoma risk, but only after omitting some studies from their analysis. However, a more recent but similar analysis of 8 studies found no significant correlation between dietary vitamin C and glaucoma risk. Many studies exist that find a significantly protective effect of vitamin C, but given these meta-analyses, it’s not clear whether they represent a broadly applicable result.

Whilst less widely studied than other eye conditions, the potential for antioxidants to protect against floaters has been investigated. A recent study providing supplements containing antioxidants such as vitamin C saw 77% of participants on the supplement had a reduction in floaters, compared to just 23% of the placebo group. This was based on eye scans and quality of life questionnaires. Whilst this may seem more clear cut than previous investigations, this is just one study that only looked at 61 patients, compared to the thousands included in trials previously discussed, and the study has yet to be repeated. It is promising but requires further scrutiny.

So should you be worried about your vitamin C intake? Vitamin C is an essential part of the diet, found in fruits and vegetables such as bell peppers, citrus fruits, tomatoes, broccoli and spinach. Dietary requirements are relatively low given its abundance in a balanced diet, with >200mg provided by just 5 different fruit and vegetables. Supplements can provide vitamin C if it cannot be obtained from the diet. Vitamin C is essential for important biological reactions, as well as for its role as an antioxidant. However, vitamin C deficiency is relatively rare in developed countries due to its availability. 

It is still up for debate whether vitamin C protects your eyes. Some studies have found that vitamin C offers protection against eye diseases such as cataracts, glaucoma, and eye floaters, but for every paper reporting a protective effect, there’s another finding no significant advantage to supplementation. This is probably because vitamin C transporters in the eyes, SVCT2, are already transporting vitamin C at high levels and increased consumption of vitamin C has a negligible effect on their rate of transport into the eyes. 

There are also difficulties with measuring the impact of supplementation since it is difficult to assess pre-supplementation vitamin C levels - most vitamin C is found inside cells, so blood vitamin C concentrations don’t necessarily reflect the body’s entire pool of vitamin C or absorption efficiency. The conflicting results of studies may be a result of different baseline vitamin C levels which were not accounted for. Those with lower than average vitamin C intake, or poor absorption efficiency (such as smokers, diabetics and obese people) are more likely to see a measurable effect from supplementation.

For the majority of people, however, the efficiency of absorption of vitamin C by the small intestine drops significantly as intake increases beyond 200mg. As long as you’re getting your recommended daily intake, you’re probably protecting your eyes enough.