Read too many headlines and you might think that everything under the sun causes cancer. And, of course, the sun itself can increase your risk of the disease, because the ultraviolet radiation it puts out is the major cause of skin cancer.
But recently, some researchers have suggested that sunlight could also protect us from cancer – you’ve probably seen stories about it in the media. When sunlight shines onto our skin, we produce vitamin D and indeed, sun exposure is our main source of this chemical. We need vitamin D for strong, healthy bones but some studies suggest that it can also reduce the risk of many types of cancer.
Understandably, this is a fairly controversial issue – how do you balance the need for some sun exposure in order to make enough vitamin D with the need to stay safe in the sun and reduce your risk of skin cancer? And is it true that vitamin D can reduce the risk of cancer at all?
This month, the authoritative International Agency for Research into Cancer (IARC) have weighed in on the issue. By gathering a group of expert scientists, they have looked at all the available evidence and published a detailed report on vitamin D and cancer. The massive tome weighs in at 465 pages, but we’ll take a look at the key points in the first of two posts looking at the vitamin D debate.
Does vitamin D reduce the risk of cancer?
Possibly, according to IARC, although to a lesser extent than is commonly suggested. You’ll often see claims that vitamin D protects against a wide range of different cancers including very common types such as breast, bowel and prostate cancer. But the IARC report paints a different picture.
Much of the evidence that vitamin D could protect against cancer comes from studies looking at how rates of different cancers change depending on where in the world you live. These studies are called “ecological studies” and they have mostly found that the further north from the equator you go, the more likely people are to develop, or die from, many types of cancer.
The big idea is that the sun’s rays are weaker at higher latitudes, so people living in northerly places make less vitamin D. And it’s this that, in theory, accounts for their higher rates of cancer.
But the IARC report heavily criticises this type of research for several reasons:
- These studies are based on a faulty assumption – that people have less vitamin D the further north they live. In fact, studies in Europe show the opposite trend; people living further north often have higher levels of vitamin D. And in North America, the link between latitude and vitamin D is a weak one.
- These studies often don’t adjust for other things that vary with latitude and could also explain differences in cancer rates, such a what people eat, how active they are or how wealthy they are.
- These studies don’t account for differences in the way people behave, such as their desire for a tan, whether they work indoors or whether they go on holidays in sunny places. These things have a much greater effect on a person’s vitamin D levels than simple geography does.
So ecological studies tell us very little about vitamin D and cancer – they are like looking at the issue through a fogged-up pair of glasses. For a clearer view, some studies have looked at individual people, actually measuring the vitamin D in their blood and comparing it to their risk of cancer.
These studies have found that people with higher levels of vitamin D in their blood are indeed less likely to get bowel cancer. So an association does exist. However, the evidence for breast cancer is “limited” – lots of studies have looked into a possible link and their results are so inconsistent that any protective effect could well be a fluke. And “there is no evidence” that vitamin D could reduce the risk of prostate cancer.
What does vitamin D actually do?
So all in all, there is growing evidence that vitamin D is linked to a reduced risk of bowel cancer in some way, but the jury’s very much out for other types. Even then, the IARC report says “The key question is to understand whether low vitamin D status causes an increased risk of cancer… or is simply a consequence of poor health status”. Is vitamin D directly influencing the biology of cells in our bodies, or are vitamin D levels a ‘proxy’ for something else?
There are certainly signs that a person’s levels of vitamin D could reflect their health in general. Many things that increase the risk of conditions like cancer and heart disease are also linked to low levels of vitamin D. These include old age, a high body weight, being inactive, smoking or an unhealthy diet. In at least one large study, the researchers adjusted their results to account for these other factors. They found that this weakened the link between vitamin D and cancer or heart disease.
On the other hand, studies using animals or cells in a laboratory have found that vitamin D could well have direct biological effects. The chemical is good at multi-tasking – it does a large number of jobs in our cells. And according to laboratory experiments, many of these – from stopping cells from growing to causing damaged ones to commit suicide – could help to prevent cancer.
But again, the IARC report advises us to interpret these experiment with care. What happens in laboratory cells may not apply to humans and all too often, researchers have been led down dead-ends by these types of experiments.
Other nutrients, such as beta-carotene or vitamin E, have shown similar anti-cancer abilities in a lab, but failed to deliver on this early promise when supplements were tested in humans. Indeed, in some clinical trials, people who received high-dose vitamin supplements actually had higher risks of cancer. It just goes to show that we have to be very careful when making assumptions about what happens in real people based on what happens in small samples of cells.
So what does this mean for us? Should people be actively seeking out vitamin D supplements, or treating them with caution until more research is done? And should we still bother staying safe in the sun? We’ll see how the new report answers these questions in Part Two.