A lot has been said about the link between sun and cancer. In spite of it, the truth might be more nuanced than what you’ve been told. Today we’ll dig deep into the truth about sunshine and cancer.
As a whole, cancer is one of the most common diseases nowadays. In fact, according to the World Health Organization cancer is the second cause of death worldwide [4] after heart conditions. Nowadays, researchers know cancer incidence has two major components: genetic and lifestyle factors. Genetics have to do with cancer risk passed from one generation to the next. Since not all cancers have genetic markers and scientists don’t know every marker for every type of cancer, a genetics-only diagnosis is not currently possible. On the other hand, lifestyle factors modulate the expression of genetic markers and are considered to be the key for the progression of the disease.
Luckily, you can generally control lifestyle risk factors and help lower your cancer risk. Researchers know these are the top risk factors for all types of cancer:
On the other hand, chronic exposure to sunshine can also increase your risk for certain cancers, particularly of the skin.
Skin cancer is a specific type of condition that involves uncontrollable growth of skin cells. This type of cancer is very common in the US. In fact, 1 in 5 Americans will develop skin cancer by age 70 [3].
Nowadays, scientists agree that the main risk factors for skin cancer is exposure to the sun. According to the American Cancer Society, most skin cancers are a result of exposure to the UV rays in sunlight [1].
Of course, the general cancer risk factors outlined above still apply to skin cancer incidence. However, there are other factors that might increase your risk for this specific form [3], including:
Overall, exposure to UVB and UVA rays from natural and artificial sources is considered the biggest risk factor for skin cancer. Following this train of thought, one would expect warmer, sunnier areas should have a higher skin cancer rate. After all, people tend to show more skin and be outside more, right? Well, this isn’t exactly right. In fact, according to a 2020 study, people from warmer states actually have lower skin cancer rates.
In spite of the known evidence pointing out at sun exposure and increased skin cancer rate, the link might not be as straightforward as experts once thought. A 2020 study took official statistics covering cancer incidence and mortality. Then, they compared the data to certain geographical regions and more specifically, to climate zone. In this study, a climates zone is ‘a variable that combines temperature and moisture level in a given area’ [7].
After comparing 15 states in the US, the east coast showed a higher incidence and mortality rates associated to cancer. This falls in accordance with previous data sets showing that the east coast has significantly higher cancer rates than the rest of the country, followed by the Midwest. However, these higher cancer rates include melanoma on the skin and other types of skin cancer. How can this be, if these areas are cloudier?
According to the CDC’s state cancer profiles, some of the sunnier states actually have a lower skin cancer incidence than other ‘colder’ states. As of march of 2021, Puerto Rico, Texas and New Mexico are among the 5 states with a lower cancerous melanoma incidence. Using information from the last 5 years, Puerto Rico has a melanoma incidence of less than 4 diagnosis for every 100,000 people, Texas has 13 for every 100,000 people and New Mexico has 16 per 100,000 people [2].
In contrast, Utah, Vermont, New Hampshire and Minnesota are the top 4 states of the list. These count more than 30 cases per 100,000 people, and Utah actually has more than 40 diagnosis per 100,000 [2]. That’s actually 10 times more skin cancer diagnosis than Puerto Rico!
In the study mentioned above, researchers found a consistent link between higher precipitation and the incidence of all types of cancer [7]. This falls in line with the CDC’s statistics of higher cancer incidence in the east coast.
However, this doesn’t mean cold weather will automatically increase cancer risk, or that dry, warm weather will ‘protect’ you from cancer. In fact, the study showed lung cancer was actually higher in warmer areas, and it’s important to remember these results only considered 15 states. These conclusions might not hold true when looking at the country as a whole.
Because of it, the study’s results need to be further investigated. To explain the relationship between rain and cancer incidence, researchers have some theories, mainly based on the effect of continued precipitation in the environment. For example, in the east coast the soil lacks both magnesium and potassium, making soil more acidic and in some cases, releasing nitrous acid into the atmosphere. According to health authorities, nitrous acid is considered a carcinogenic compound [10]. Another reason might be vitamin D deficiency which, ironically, happens less often among populations living in warmer climates.
One of the theories proposed by researchers to explain higher cancer incidence in the east coast is vitamin d deficiency. This nutrient is in fact a group of fat-soluble compounds that your body can turn into hormones. This means vitamin d has important functional effects to ensure your body is working properly. Up until recently, doctors and researchers believed the role of vitamin d was mainly to help the body use calcium and phosphorus, strengthening the bones. However, the latest research shows vitamin d could also contribute to a lower cancer risk.
As such, cancer incidence in southern latitudes is statistically lower than in northern latitudes. Researchers have hypothesized that the increased exposure to sunlight -which allows the body to produce vitamin d- could be the main factor affecting these stats.
In other studies, mice with tumors showed a slower tumor growth when provided with proper vitamin d intake, in contrast to mice in a deficit. According to the national cancer institute, vitamin d is currently under study but it’s shown preliminary influence over cell growth, cell death and tumor growth, which could influence cancer progression [8].
In spite of these promising links, there isn’t firm evidence on whether or not vitamin d can definitely lower cancer risk. For example, it’s possible higher vitamin d levels in the blood correlate to other healthy behaviors, like walking outside, that could have a greater influence on cancer incidence.
Vitamin d was linked to a statistically lower colorectal cancer incidence rate [8] but results are still inconclusive. Among other types of cancer, researchers haven’t been able to establish a stronger link.
What experts do know is that vitamin d deficiency, although relatively uncommon, can be dangerous to your health beyond possibly increasing your cancer risk. To prevent it, get checked for vitamin d blood levels and ask your doctor about possible supplementation.
As of the writing of this piece, research on the link between sunlight exposure and breast cancer is suggestive but inconclusive. According to a 2020 meta-analysis paper, there seems to be a link between exposure to solar ultraviolet radiation like UVA and UVB light and a slightly lower breast cancer risk. This link was even stronger among women over 40 years old [9]. Luckily, the study didn’t find sunscreen use modified this relationship, meaning you could use sunscreen and still reap the benefits of sunlight [9].
These results, however, are based on statistics, and haven’t been trialed with people or animals. Furthermore, the studies didn’t account for other risk factors such as lifestyle changes, ethnic factors, occupational exposure to carcinogenic compounds, and genetic predispositions.
Like all aspects of your lifestyle, sunlight can affect your cancer risk. However, researchers are still on the fence about how the sun interacts with the human body. What we do know is that too much or too little sunlight seem to have negative effects on your health and your long-term cancer risk.
Because of it, it’s best to follow general protection guidelines to avoid the harsh effects of sunlight. This means wearing sunscreen and avoiding midday sun rays. On the flip side, you should also take care to keep your vitamin D levels stable by going out under the sun in the early morning, and supplementing as advised by your medical team.
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