It’s summer, so it’s time to worry about the sun.

And these days, there’s twice as much to worry about.

On one tanned hand stands the risk of skin cancer. By now, nearly everyone knows that exposure to ultraviolet rays in sunlight raises a person’s risk for skin cancer. And more UV rays filter down in summer to our skins, especially those light white and extra sensitive to sun damage.

The other hand, slick with suntan lotion, weighs a growing pile of evidence that we’re not getting enough Vitamin D, a powerful hormone the skin makes when exposed to the sun.

The body needs Vitamin D to process calcium, build bones and avoid deforming diseases such as rickets. Scientists increasingly think the same vitamin may limit other diseases, including prostate cancer and autoimmune diseases such as multiple sclerosis. While those ideas aren’t proven, a growing chorus warns that our sun-shunning ways have the nation – especially the north – chronically low on Vitamin D.

The controversy

“It’s a big and heated debate,” said Dr. Tomasz Beer, an Oregon Health & Science University Cancer Institute researcher who studies Vitamin D and prostate cancer.

“I have the utmost respect for dermatologists,” he said. “But there’s no question they’re more focused on the skin than on any other organ. On the sun side … there’s a multibillion-dollar industry that looks to benefit from a certain outcome in this debate.”

That’s the tanning industry, whose fierce interest in funding and publicizing research on Vitamin D might be clouding the issues. The clearest case involves Michael Holick, a Boston doctor who has become one of many scientists worried about low Vitamin D levels. His work has won him many admirers and papers in the New England Journal of Medicine warning about the Vitamin D crisis. It’s also won him money from the tanning industry and few fans among dermatologists.

Holick “was literally kicked out of the dermatology department at Boston University” but stayed on with the school’s endocrinology and nutrition faculty, said Victoria Drake, a research associate with Oregon State University’s Linus Pauling Institute.

There is middle ground in the sun debate, which might be the wisest place to stand while science grinds on.

“Most doctors are now agreeing that it’s an underdiagnosed epidemic, Vitamin D deficiency,” said Dr. Erin Allen, with Providence Dermatologic Surgery in Portland. “But I also know skin cancer is epidemic.”

Allen says she’s seeing increasing numbers of skin cancers as people age, but also seeing them “in younger and younger patients,” even down to age 30.

The challenge is to get more Vitamin D but not more sun.

It’s hard to know how much Vitamin D you have or how much you need. The federal Institute of Medicine recommends amounts ranging from 200 international units in kids and younger adults to 600 units for people older than 70. Studies show one-third to one-half of Americans don’t meet the government levels, especially people in northern climates, older adults and people with dark skin who can’t turn sunshine into Vitamin D as quickly as pale people do.

Limits faulted

Most experts think the government recommendations are “woefully inadequate,” said Drake, the Pauling Institute nutritional scientist. Many people urge 800 to 2,000 units a day. That means taking two to five usual supplements, because few foods contain Vitamin D besides fatty fish and fortified milks, juices and cereals. Or getting some sun – the most efficient way to make that hormone.

“Relatively brief exposure to the sun can produce substantially more Vitamin D than we usually get in supplements,” Beer said.

Cloud levels, age and skin color affect Vitamin D production – dark-skinned people and older people generally have lower levels. But most people can make 3,000 to 10,000 international units of D by having their arms and face exposed to the sun for five to 15 minutes.

Whether to spend a few moments in the sun is the toughest call. Allen, the dermatologist, recommends a Vitamin D3 pill but says people should always wear a hat and sunscreen – SPF 30 or higher – that blocks UVA rays, applied every 90 minutes. Even sunscreened skin will make some Vitamin D, she notes, while limiting cancer risk.

The Pauling Institute recommends 400 units of Vitamin D3 daily but suggests spending 10 to 15 minutes with your face or limbs in the sun, three times a week, between 11 a.m. and 2 p.m. in the spring, summer and fall.

So a pill, yes, and maybe a little sun. But you shouldn’t stay out long enough to get a tan, and never get a sunburn.

“They can still have their tan; they need to do a spray-on,” Allen said. “They can still have their Vitamin D; take it in a supplement.”