Roy Stewart wrote:Here's a question for you: why are the vast majority of fatal melanomas found on parts of the body which receive no sun?
Here's another one: Why have cancer rates gone up as sunscreen use has gone up?
A third question: Why is there more skin cancer at higher latitudes i.e. closer to the poles?
Ok. to answer the first question will require a bit of understanding:
Melanomas in general, are found anywhere on the skin, and don't exclusively occur in areas that receive high levels of sun. The hypothesis for this is that UV light (A, B and C) can act as a strong systemic immunosupressant (the whole body has reduced immune function for a period of hours post exposure). This is a fact, and has been proven in multiple published peer reviewed experiments. It is believed that this allows cells within the body to then progress though their respective cell cycles without immunosurveillance for mutations. In most cases, mutations that occur will not cause cancer, and are "normal" however in a very small percentage of cases the cell will mutate in a way that makes it rapidly divide and grow. This is essentially the cancer. Because the immunosuppressive effect of UV is systemic, the mutant 'cancer' cell could be anywhere on your body, even under a toenail!
A possible explanation for why people die from melanomas in parts of the body that receive no sun, is that people aren't aware of the risk, and therefore pay less attention to moles or freckles in these areas when self-checking for signs of cancers. I also would like to see your source for the data indicating the location of the "vast majority of fatal melanomas".
To answer your second question
Roy Stewart wrote:Here's another one: Why have cancer rates gone up as sunscreen use has gone up?
It should be obvious! Using some basic logic, and looking at the population demographics you will find that the cancers are predominately occurring in people who grew up in the days when you didn't wear sunscreen, but used coconut oil instead to tan more! Plus, it coincides with the development of the hole in the ozone layer leading to far greater UV transmission through the atmosphere.
The people who grew up then have now had a life of sun exposure and are now at the age when cancers develop and need treatment. Because of this, and the awareness derived from the importance of covering up, people now wear hats and sunscreen to reduce the damage and thus long term risk of sun exposure. The people at risk of cancer in the current population are not the people using sunblock and wearing long sleeve rash shirts, but the people in their 50s, 60s, 70s and 80s who grew up in the sun.
The important question will be whether sunblock will significantly reduce the cancers developing later in life (i.e. 30 years from now). It looks promising, stops tumours in animal models, but is as yet unproven to reduce cancers in humans. For a very brief rundown, see:
http://en.wikipedia.org/wiki/Ultraviolet#Sunscreen_safety_debateAs for your third question:
Roy Stewart wrote:A third question: Why is there more skin cancer at higher latitudes i.e. closer to the poles?
Show me the statistics source. I was under the impression that all published health data showed the highest incidence of skin cancers is at
lower latitudes and the World Health Organisation supports this.
http://www.who.int/uv/health/uv_health2/en/index1.htmlRoy Stewart wrote:Unfortunately chronic vitamin D deficiency is normal.
Regarding Vit. D deficiency - it's common in office workers, shift workers, the elderly, and in northern Europeans in winter, but it's definitely
not normal!! These people are receiving a significantly below average amount of sunlight because of their environmental circumstances. I agree that if these people, who are at risk of a vit D deficiency, wear sunscreen they are more likely to get a deficiency.
Chronic Vit D deficiency reduces the ability of your body to absorb calcium, leading to the most common manifestation of low bone mineral density (think Osteoporosis). It's not normal, and should be treated in at-risk populations by dietary supplementation and lifestyle modifications.
If you're spending a lot of time surfing in the sun and worried about your tan, you're clearly not "at risk" so I'd be wearing the sunscreen. As I said, if you're concerned you should get your doc to test your vit D levels.