The Role of Dietary Antioxidants in Melanoma and Nonmelanoma Skin Cancer

Nonmelanoma skin cancer (NMSC) is the most common cancer in the United States, and cutaneous melanoma is projected to be the fifth most common form of cancer in 2022, with increasing incidence and high potential for mortality.^1-3 Estimates indicate that 35% to 45% of all cancers in White patients are cutaneous, with 4% to 5% occurring in Hispanic patients, 2% to 4% in Asian patients, and 1% to 2% in Black patients.⁴ Of the keratinocyte carcinomas, basal cell carcinoma (BCC) is the most prevalent, projected to affect approximately 33% to 39% of White males and 23% to 28% of White females in the United States during their lifetimes. Squamous cell carcinoma (SCC) is the second most common skin malignancy, with a lifetime risk of 9% to 14% for White males and 4% to 9% for White females in the United States.⁵ The incidence of melanoma continues to increase, with approximately 99,780 new cases expected in the United States in 2022.¹

UV-induced DNA damage plays a key role in the pathogenesis and development of various skin malignancies.⁶ UV radiation from sunlight or tanning devices causes photocarcinogenesis due to molecular and cellular effects, including the generation of reactive oxygen species, DNA damage due to the formation of cyclobutane pyrimidine dimers and pyrimidine-pyrimidone, melanogenesis, apoptosis, and the increased expression of harmful genes and proteins.⁶ The summation of this damage can result in skin malignancies, including NMSC and melanoma.^6,7 Dietary antioxidants theoretically help prevent oxidative reactions from occurring within the body, and it has been suggested that intake of dietary antioxidants may decrease DNA damage and prevent tumorigenesis secondary to UV radiation.⁸ Antioxidants exist naturally in the body but can be acquired exogenously. Investigators have studied dietary antioxidants in preventing skin cancer formation with promising results in the laboratory setting.^8-11 Recently, more robust human studies have been initiated to further delineate this relationship. We present clinical evidence of several frequently utilized antioxidant vitamins and their effects on melanoma and NMSC.

Antioxidants

Vitamin A—Vitamin A is a fat-soluble vitamin found in animal sources, including fish, liver, and eggs. Carotenoids, such as beta carotene, are provitamin A plant derivatives found in fruits and vegetables that are converted into biologically active retinol and retinoic acid.¹² Retinols play a key role in cellular growth and differentiation and are thought to be protective against skin cancer via the inactivation of free radicals and immunologic enhancement due to their antiproliferative, antioxidative, and antiapoptotic effects.^13-16 Animal studies have demonstrated this protective effect and the ability of retinoids to suppress carcinogenesis; however, human studies reveal conflicting results.^17,18

Greenberg et al¹⁹ investigated the use of beta carotene in preventing the formation of NMSC. Patients (N=1805) were randomized to receive 50 mg of beta carotene daily or placebo. Over a 5-year period, there was no significant reduction in the occurrence of NMSC (relative risk [RR], 1.05; 95% CI, 0.91-1.22).¹⁹ Frieling et al²⁰ conducted a similar randomized, double-blind, placebo-controlled trial investigating beta carotene for primary prevention of NMSC in 22,071 healthy male physicians. The study group received 50 mg of beta carotene every other day for 12 years’ duration, and there was no significant effect on the incidence of first NMSC development (RR, 0.98; 95% CI, 0.92-1.05).²⁰

A case-control study by Naldi et al²¹ found an inverse association between vitamin A intake and development of melanoma. Study participants were stratified into quartiles based on level of dietary intake and found an odds ratio (OR) of 0.71 for beta carotene (95% CI, 0.50-1.02), 0.57 for retinol (95% CI, 0.39-0.83), and 0.51 for total vitamin A (95% CI, 0.35-0.75) when comparing the upper quartile of vitamin A intake to the lower quartile. Upper-quartile cutoff values of vitamin A intake were 214 µg/d for beta carotene, 149 µg/d for retinol, and 359 µg/d for total vitamin A.²¹ More recently, a meta-analysis by Zhang et al²² pooled data from 8 case-control studies and 2 prospective studies. Intake of retinol but not total vitamin A or beta carotene was associated with a reduced risk for development of melanoma (retinol: OR, 0.80; 95% CI, 0.69-0.92; total vitamin A: OR, 0.86; 95% CI, 0.59-1.25; beta carotene: OR, 0.87; 95% CI, 0.62-1.20).²² Feskanich et al²³ demonstrated similar findings with use of food-frequency questionnaires in White women, suggesting that retinol intake from food combined with supplements may be protective for women who were otherwise at a low risk for melanoma based on nondietary factors. These factors included painful or blistering sunburns during childhood, history of more than 6 sunburns, more than 3 moles on the left arm, having red or blonde hair, and having a parent or sibling with melanoma (P=.01). However, this relationship did not hold true when looking at women at an intermediate or high risk for melanoma (P=.16 and P=.46).²³

When looking at high-risk patients, such as transplant patients, oral retinoids have been beneficial in preventing NMSC.^24-27 Bavinck et al²⁴ investigated 44 renal transplant patients with a history of more than 10 NMSCs treated with 30 mg of acitretin daily vs placebo. Patients receiving oral retinoid supplementation developed fewer NMSCs over a 6-month treatment period (P=.01).²⁴ Similarly, George et al²⁵ investigated acitretin in renal transplant patients and found a statistically significant decrease in number of SCCs in patients on supplementation (P=.002). Solomon-Cohen et al²⁶ performed a retrospective case-crossover study in solid organ transplant recipients and found that those treated with 10 mg of acitretin daily for 2 years had a significant reduction in the number of new keratinocyte carcinomas (P=.002). Other investigators have demonstrated similar results, and in 2006, Otley et al²⁷ proposed standardized dosing of acitretin for chemoprevention in high-risk patients, including patients developing 5 to 10 NMSCs per year, solid organ transplant recipients, and those with syndromes associated with the development of NMSC.^28,29 Overall, in the general population, vitamin A and related compounds have not demonstrated a significant association with decreased development of NMSC; however, oral retinoids have proven useful for high-risk patients. Furthermore, several studies have suggested a negative association between vitamin A levels and the incidence of melanoma, specifically in the retinol formulation.

Vitamin B₃—Nicotinamide (also known as niacinamide) is a water-soluble form of vitamin B₃ and is obtained from animal-based and plant-based foods, such as meat, fish, and legumes.³⁰ Nicotinamide plays a key role in cellular metabolism, cellular signaling, and DNA repair, including protection from UV damage within keratinocytes.^31,32 Early mouse models demonstrated decreased formation of skin tumors in mice treated with topical or oral nicotinamide.^32,33 A number of human studies have revealed similar results.^34-36