A new study from Brown University shows that the dietary intake of vitamin A from plant sources is associated with a lower risk of a common type of skin cancer called squamous cell carcinoma. This is the second most common carcinoma of the skin in fair-skinned people.
The role of vitamin A in helping skin cells to grow and mature is well known, but its usefulness in lowering skin cancer risk has been a matter of controversy. The use of sunblock, and avoiding exposure to strong sunlight, have been recommended to lower skin cancer incidence. The current study suggests that eating fruits and vegetables rich in vitamin A can be another good way to lower this risk.
The study was published in the Journal of the American Medical Association Dermatology on July 31, 2019.
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The current research evaluated dietary vitamin A intakes and skin cancer detection rates in two large observational studies conducted over several years. These are the Nurses’ Health Study from 1984 to 2012, which looked at over 75,000 American women, and the Health Professionals Follow-Up Study from 1986 to 2012, including over 48,000 American males. The data gathered from these subjects included their dietary intake, history of skin cancer, hair color, severe sunburn incidents, and family history of skin cancer, all of which contribute to the risk for skin cancer.
In total, about 123,000 subjects were found to be negative for a history of skin cancer and had submitted multiple forms on their dietary intakes. All of them were white, which put them at a higher risk of skin cancer. Among these, there were almost 4,000 cases of squamous cell carcinoma over the study up period.
The researchers were looking for evidence of association between skin cancer and vitamin A intake. They classified the subjects into five categories based on average vitamin A intake levels. The conclusion they drew was that those who had the highest intake had a 17% lower risk of squamous cell carcinoma compared to those with the lowest intake. This could be compared to eating two large carrots or one medium baked sweet potato daily, vs a third of a cup of fried sweet potato wedges or one small carrot. However, the startling fact is that even this lowest category is above the US RDA (Recommended Dietary Allowance), which may mean that these recommendations need to be updated.
Another finding was that most of the ingested vitamin A came from fruits and vegetables rather than from supplements or from animal-based products. Plants rich in vitamin A, besides carrots and sweet potatoes, include leafy green vegetables like lettuce, as well as fruits like apricots or cantaloupes. Compounds like vitamin A, such as lycopene, were found in tomatoes and watermelon, and are also found to reduce the skin cancer risk. Animal foods containing abundant vitamin A include milk, liver and oily fish.
Researcher Eunyoung Cho says, “Our study provides another reason to eat lots of fruits and vegetables as part of a healthy diet.”
Vitamin A is a fat-soluble vitamin which is converted into various retinoids, which are bioactive compounds required for proper maturation and differentiation of epithelial cells. Synthetic forms of these compounds are employed to prevent skin cancer in high-risk populations but have significant potential for harm. Hence the current study’s focus on natural sources of vitamin A for the chemoprevention of skin cancer is justified.
The analysis compensated for the presence of the other high-risk factors mentioned earlier. However, because these studies did not ask about measures to avoid the strong mid-day sun, the effect of this factor could not be assessed.
While the research indicated the potential benefit of vitamin A in reducing skin cancer risk, its toxicity was also mentioned. Animal-based sources and supplements can push up the blood levels of vitamin A too high, causing nausea, liver derangement, osteoporosis and hip fracture. In pregnant women, it may cause birth defects in the babies. However, plant sources of vitamin A do not usually result in toxicity.
This study was observational in nature and so no conclusions can be drawn as to the role of vitamin A in reducing the cancer risk. It suggests that a randomized clinical trial with controls should be carried out to establish that the higher vitamin A intake causes the lower risk. Such a trial will also rule out more confounding factors, such as the effect of less alcohol consumption which might be associated with higher vitamin A intake due to the overlap of these habits in the same group of people.
The researchers plan to carry out such a trial to test the preventive efficacy of vitamin A in squamous cell carcinoma of the skin. It is quite difficult to ensure that study participants adhere to the prescribed diet, however, and a failure at this level would make the whole study meaningless. If technical factors make this type of study non-feasible, the best option would be a large prospective study, says Cho.
Kim J, Park MK, Li W, Qureshi AA, Cho E. Association of Vitamin A Intake With Cutaneous Squamous Cell Carcinoma Risk in the United States. JAMA Dermatol. Published online July 31, 2019. doi:10.1001/jamadermatol.2019.1937, https://jamanetwork.com/journals/jamadermatology/article-abstract/2739070