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Modest sun exposure isn't a risk factor for melanoma. If anything, it is protective. A thread.
There's a lot I'm leaving out, as I'm trying to briefly summarize the information presented within a scientific book I'm writing on the health effects of sun exposure. 1/n
There's a lot I'm leaving out, as I'm trying to briefly summarize the information presented within a scientific book I'm writing on the health effects of sun exposure. 1/n
The idea that sun exposure causes melanoma is based on data showing UV radiation to be carcinogenic. It damages melanocyte DNA directly and indirectly via oxidative stress, while also suppressing immunosurveillance that would otherwise identify and kill cancerous cells. 2/n
The limitation with this data is that it uses high doses of isolated UV radiation. It's not appropriate to extrapolate this to sunlight, which puts out an array of bioactive light wavelengths, nor is it appropriate to extrapolate this out to modest, non-damaging doses of UV. 3/n
For example, we have research throughout the 2000s that shows chronic, low-dose UV exposure can enhance immune defenses within the skin. It’s a form of hormetic stress where a little bit builds us up over time and ultimately makes us stronger. 4/n https://pubmed.ncbi.nlm.nih.gov/28070504/
As another example, exposure to red and infrared light (which make up more than 30% of sunlight), reduces the ability of UV radiation to damage the skin and helps repair any damage that does occur. 5/n
https://pubmed.ncbi.nlm.nih.gov/26745730/
https://pubmed.ncbi.nlm.nih.gov/26745730/
We also need to consider inborn defenses. Regular low-dose UV exposure causes tanning in most people, which enhances the threshold for exposure before damage occurs, and stimulates vitamin D synthesis, which is protective against melanoma development. 6/n
Diet plays another huge role, as carotenoids and other phytochemicals have been shown to deposit in skin tissue and protect against UV damage by both directly blocking UV radiation and by functioning as antioxidants. 7/n https://pubmed.ncbi.nlm.nih.gov/31370257/
The reality is that there are two scenarios that will drastically change how the sun affects your risk of skin cancer:
SCENARIO #1: You spend your life indoors, go outside only on occasion, come home with a sunburn, and eat a typical American diet. 8/n
SCENARIO #1: You spend your life indoors, go outside only on occasion, come home with a sunburn, and eat a typical American diet. 8/n
SCENARIO #2: You have a lifestyle built around regular sun exposure, with ample time spent outside and minimal burning, and you eat a diet rich in phytochemicals.
These are completely different scenarios. 9/n
These are completely different scenarios. 9/n
The reality is that we have known since at least the 1970s that intermittent sun exposure increases melanoma risk because it doesn't allow for inborn defenses like tanning to occur and increases the risk of harmful exposure doses that cause burning. 10/n https://pubmed.ncbi.nlm.nih.gov/28478931/
Consider the fact that water can cause permanent brain damage and even death by simply drinking too much in a short period of time. If researchers force-fed rodents huge amounts of water, you could easily create a narrative that water is toxic and should be avoided. 11/n
That would be an illogical conclusion though, right?
So would concluding exercise is harmful by having an obese sedentary person run a marathon and suffer a heart attack.
So, why do we make these conclusions with sun exposure and UV radiation? 12/n
So would concluding exercise is harmful by having an obese sedentary person run a marathon and suffer a heart attack.
So, why do we make these conclusions with sun exposure and UV radiation? 12/n
The largest and most recent meta-analysis of 57 studies found that the risk of developing melanoma was increased only with intermittent sun exposure and burning, not with regular and frequent sun exposure. 13/n https://pubmed.ncbi.nlm.nih.gov/15617990/
Does sunscreen help? Only if it prevents burning when you need to be in the sun for long periods of time.
Generally speaking, though, a meta-analysis of 25 studies found no link between sunscreen use and rates of melanoma. 14/n https://pubmed.ncbi.nlm.nih.gov/29620003/
Generally speaking, though, a meta-analysis of 25 studies found no link between sunscreen use and rates of melanoma. 14/n https://pubmed.ncbi.nlm.nih.gov/29620003/
None of this applies to indoor tanning, which is associated with melanoma risk. Why? Because the doses of UV radiation are usually 10–15 times greater than that we receive from the sun and provided in isolation. 15/n https://pubmed.ncbi.nlm.nih.gov/22833605/
All-in-all, the well-known link between sun exposure and melanoma exists in only one reality: that of indoor-loving, sun-burning, indoor-tanning people eating a crap diet. 16/n
But for those of us who spend ample time outdoors, eat a phytochemical-rich diet, and take steps to prevent burning, there is simply no good evidence that sun exposure will increase the risk of melanoma. /end
