Sun exposure signals the deeper layers of the skin to make melanin. Melanin is then sent up to the surface layers of the skin to protect us, like a little black umbrella. Melanin makes us darker and blocks production of vitamin D in the skin. As the summer wears on, if we slowly get more and more exposure, we become darker in color and make less vitamin D per hour. People who live in countries closer to the equator in Africa or Central America who still live outside stay tanned all year long. The skin is designed to have a connection between the blood level of D25OH and the amount made in the skin. A person with normal melanin, who is still living outdoors or using a tanning bed daily is not able to produce a vitamin D blood level over 80 ng/ml by sun exposure alone, so that appears to be the normal biological upper level.
This does not mean that liberal sun exposure is good for everyone! There are people who have very little or no melanin in their skin. They have specific genetic mutations that became common in parts of the world such as Scotland and the Nordic countries where there are very few months of UVB light in summer and very few clear days for sun exposure. Losing the melanin was a survival advantage, we all need Vitamin D to survive so in low sun environments people with red hair and very white skin survived better than their dark skinned cousins. The people who are not able to make melanin cannot tan and are therefore at risk for severe sunburn, skin damage and skin cancer. People with this skin type, once transplanted to a very sunny environment must use sunscreen or clothing to protect their skin. But people of color who possess their own sun screen only need to protect their skin when they are first exposed in summer. They can still burn if they’re not careful, but each person, with experience, begins to know how much sun they can tolerate without producing a sunburn. It is not sun exposure that makes skin cancer, it is the damage of sunburn and the lack of vitamin D in the skin to repair the DNA damage that leads to skin cancer. Vitamin D enters the nucleus of skin cells and repairs the DNA damage. Squamous cell carcinoma grown in a petri dish becomes a normal cell when exposed to vitamin D!
People who are very dark skinned came from very high sun environments at the equator. Their bodies were made to be able to live outside all day long and be healthy and make enough D to live long and prosper. They will need more sun exposure to make the same amount of D as a lighter skinned person. Dark skinned people did not start to have skin cancers until the last thirty years. This means that the cause and the treatment are the same and like everything else in nature, there’s a balance. Don’t burn your skin is still good advice. Use your judgement and your own experience with your body!
One other warning: Skin cancer is related to damage over time. If I’ve had low vitamin D for the last thirty years of my life it means that the DNA damage done to my skin over that span of time was NOT repaired because there was no vitamin D on the skin to repair it. That means that the state of my skin at the time I start increasing my sun exposure plays a role. Start doing intelligent sun exposure with your kids but if you already have obvious sun induced skin changes you should use sun screen and do the D supplementation orally. I suspect that once a skin cell has turned fully cancerous, vitamin D supplementation does not turn it around the way it does in the petri dish. Since we have easy, cheap supplementation it’s better to be safe if you’re over 40, even if you have dark skin.