It's hard to know what to think of the hazy spring sun, struggling to break through the clouds. Last week, Cancer Research UK launched its annual anti-skin cancer campaign with stark warnings of a sunburn time bomb, with cases of melanoma expected to triple in the next 30 years. But at the same time, new research argues that staying in the shade could be bad for your health, leaving you deficient in vitamin D. Who are we supposed to believe?

The new research, from the Medical Journal of Australia, highlights the importance of getting some sun to trigger your body into making vitamin D. People who are deficient in vitamin D most famously get rickets, but can also suffer from brittle bones. The research warns: "A balance is required between avoiding an increase in the risk of skin cancer and achieving enough ultraviolet radiation exposure to achieve adequate vitamin D levels."

But there are others who argue that lack of vitamin D can have even more dire effects. Oliver Gillie is a campaigning health journalist who has spent three years researching a book on the subject of vitamin D and sunbathing. He, rather alarmingly, compares his campaign against vitamin D deficiency with the anti-smoking message he was writing about 30 years ago. "Studies show that people with vitamin D deficiency are more susceptible to some very serious cancers of the prostate, breast and bowel. Specialists know about the problem with lack of vitamin D, but the dots haven't been joined up."

But unlike the relationship between sunburn and skin cancer, the links between vitamin D and cancer are disputed. Professor Antony Young, of King's College London, cites evidence that the closer you get to the equator, the higher the incidence of skin cancer among light-skinned people, but that the reverse is true of other types of cancer. Prostate cancer, colon and breast cancer, as well as some auto-immune diseases such as multiple sclerosis, decrease as you move closer to the sun.

"The hypothesis is - and this is controversial - that vitamin D production offers protection against certain types of malignancies and auto-immune disorders," he says. "There's an increasing amount of molecular evidence that vitamin D plays a role in this."

Young says the evidence is patchy, but it is garnering attention in acad emic circles. He is currently applying for grants to investigate the problem and a symposium is being arranged for later this year.

Yet the evidence that has been unearthed so far has been leapt on by the sunbed industry as proof that even artificial ultraviolet is good for you. Young says he would be "extremely wary" of using sunbeds as a prophylactic for internal cancers.

The Australian research has caused ripples on the other side of the world, where the "sun safe" message is in its 22nd year. The Cancer Council of Australia distanced itself from the research. "The bottom line is that a small amount of sun exposure either side of peak UV periods is sufficient, ie in summer, by exposing the face, arms and hands or the equivalent surface area to as little as an average of five minutes of sunlight early morning or late afternoon on most days of the week. Most people get this level of exposure just by being outside," the council told the Guardian. "With the UK climate, the requirements may be different," it added.

But skin cancer is by no means unusual in the UK. Over 69,000 new cases are diagnosed here each year, a figure that has more than doubled since the early 1980s. Around 2,000 people a year die from it. Even a British summer necessitates factor 15 sunscreen, according to Cancer Research UK. Keeping out of the sun between 11am and 3pm is also recommended - but a dash of daylight can do you good.

"The amount of sunlight you need is 15 to 30 minutes a day for most fair-skinned people," says Dr Richard Marais, who works with the Institute for Cancer Research and Cancer Research UK. "If you're very fair and red-headed, then the exposure is reduced compared to people with black skin.

"The vitamin D controversy is a bit frustrating," he adds. "Frankly, I don't think there is a problem with vitamin D deficiency. It was really a problem in this country at the beginning of last century when diets were so appalling. Now you find it in fortified cereals and flour and oily fish."

But other experts doubt whether you can get enough from diet alone. Gillie claims you would have to have the diet of an Inuit, eating oily fish three times a day, to get your daily dose of vitamin D.

So, with such a lack of agreement, how much sun is safe, and when should we cover up?

Marais is struggling with this one. He knows at the molecular level how sunburn destroys the DNA in the skin. But he also knows that people are put off if you tell them never to go in the sun. "If you get a suntan, you will increase the risk of skin cancer. But we're not killjoys; we don't want people to have miserable lives. But do it slowly, let your skin get used to it.

"We can't just say six hours is what you need. That's fine for one person, but far too much for another. Everyone has to monitor their own body." For instance, he adds, black or Asian people rarely get skin cancer, but it has been known to happen.

Young says: "The levels [of sun] that you require to get enough vitamin D are probably less than you would require for sunburn. In reality, nobody can tell you exactly what it is because the research hasn't been done to prove how quickly vitamin D production is triggered by the sun. It becomes an important public health issue in terms of working out what the advice should be."

Although the experts argue about the link between the sun, vitamin D and different kinds of cancer, the actual advice does not differ all that much. Gillie says: "You have to be quite optimistic about sun here in the UK and at this time of year it's very difficult to actually burn. But our bodies actually tell us when we've had enough sun.

"When your skin gets hot and you get an uncomfortable feeling, that's the time to get out. You can be deceived by a light breeze, but it's not beyond our wits to control it. Our bodies tell us."