Stronger evidence now links increased colon cancer risk to low vitamin D levels in the body, which may also be tied to other forms of cancer and heart disease. The role of vitamin D – the sunshine vitamin – in keeping bones strong is already well-established.

In the midst of summer, there's plenty of daily sunlight to boost vitamin D levels in your body. The challenge lies is balancing skin protection from sun overexposure while reaping vitamin D's health benefits. Concerned about skin cancer risks and sun-aging, many health authorities continue to warn against any outdoor sun unless you're wearing sunblock or sunscreen.

Ideally, people should get enough vitamin D from their food. However, that's difficult to do, Many folks don't get enough of the nutrient from dietary sources like fatty fish and fortified milk.

It can also be tricky to strike the right sunlight balance. Dark-skinned individuals produce less vitamin D than those with fair skin, and older adults make less vitamin D than younger people.

In the winter, it's virtually impossible to produce vitamin D from the sun if you live 37 degrees above the equator (or north of Atlanta), because the sun never gets high enough in the sky for its ultraviolet B rays to penetrate the atmosphere, according to Harvard Womens' Health Watch. But summer is a great time to stock up on the nutrient. When the sun's UV-B rays hit the skin, a reaction takes place that enables skin cells to manufacture vitamin D.

Safer Sun Strategies

Dr. Michael Holick, a professor of medicine, physiology and biophysics at the Boston University Medical Campus, is a long-time proponent of what he calls sensible sun exposure. However, "you should never, ever get a sunburn," he says – that's what increases the risk for melanoma and other skin cancers. If you decide to get limited, unprotected sun exposure for the sake of vitamin D, he suggests the following rules of thumb:

Always protect your face and top of your ears at the beach, because those are the most sun-exposed and sun-damaged skin areas.

Allow 10 to 15 minutes or so of unprotected sun exposure to your arms, legs, abdomen and back. After that, follow up with good sun protection, like a 30-SPF or higher sunblock.

Choose the right time of day. "If your shadow is longer than your body height, you can't make any vitamin D," Holick says. Between 10 a.m. to 3 p.m. is the usual window for significant sun exposure, he says. He's helped develop the dminder app, which uses multiple factors – time of day, location and skin type – to recommend optimal sun exposure and provide sun-safety warnings.

"You cannot get an adequate amount of vitamin D from your diet," Holick says, even with fortified foods. He recommends vitamin D supplements in appropriate doses for adults and children.

According to other experts, you can get enough vitamin D by eating the right foods – combined with supplements if needed. Fish such as wild-caught salmon, mackerel, tuna and sardines; beef liver, eggs, cod liver oil and mushrooms are good vitamin D sources. Fortified milk and foods such as breakfast cereals, yogurt and orange juice can help boost vitamin D intake.

Beginning in late July, the recommended daily values for vitamin D on food nutrition labels will double from 400 to 800 IUs. The increase is based on newer scientific evidence from the Institute of Medicine and reports like the one used to develop the 2015-2020 Dietary Guidelines for Americans.

Vitamin D and Disease

The sunshine vitamin may protect against a host of diseases, including osteoporosis, heart disease and cancers of the breast, prostate and colon. What's more, sunlight has other hidden benefits – like protecting against depression, insomnia and an overactive immune system.

A large, international study on colorectal cancer and low vitamin D levels offers some of the most definitive results to date. The study, released June 14 in the Journal of the National Cancer Institute, analyzed more than 5,700 cases of colorectal cancer from 17 studies of people from the U.S., Europe and Asia.

With blood samples taken before participants were diagnosed with colorectal cancer, researchers measured vitamin D levels using a consistent method of analysis. A control group consisted of a matching number of people who did not develop colorectal cancer. Over an average period of 5.5 years, participants who were vitamin-D deficient had a 31 percent higher risk of developing colorectal cancer than those with adequate blood levels of vitamin D.

"This study strengthened the evidence that vitamin D may lower the risk of colorectal cancer," says Marji McCullough, senior scientific director for epidemiology research at the American Cancer Society and a study co-first author. It's possible that vitamin D has a protective role in preventing colon cancer by slowing or stopping cells from becoming malignant, she says.

About a decade ago, a study published in the Archives of Internal Medicine (now JAMA Internal Medicine) showed that those with the lowest vitamin D levels have more than double the risk of dying from heart disease and other causes over an eight-year period compared to those with the highest vitamin D levels. The researchers cited "decreased outdoor activity" as one reason that people may become deficient in vitamin D.

The American Cancer Society does not support sun exposure for increasing vitamin D. "Because excessive ultraviolet radiation is a major risk factor for skin cancer, we do not recommend obtaining vitamin D from the sun as a way to boost vitamin D levels in the blood," McCullough says.

If you're concerned about your vitamin D levels, talk to your health care provider. You can also take other preventive measures. "There are many modifiable risk factors that have been established, including maintaining a healthy body weight, being physically active and eating an overall diet that's rich in plant foods and low in red and processed meat," McCullough says. "Very importantly, get your screening done. Follow screening recommendations. That's really critical for reducing your risk of getting colorectal cancer."

Balancing Solar Risks and Benefits

There are upsides to basking at least briefly in the summer sun. Public health messages that advocated sun avoidance now focus on sun protection when the UV rays are very intense, but recommend getting a little bit of sun on the skin most days of the week, says Robyn Lucas, an epidemiologist at Australian National University.

Lucas led a related study published in the February 2008 issue of the International Journal of Epidemiology. Her finding: There may be a much larger risk to health and well-being from not having any sun exposure than from too much that increases the risk of skin cancer. Lucas discussed her results with U.S. News shortly thereafter and recently updated her remarks.

"Sun protection messages arose in response to rapidly increasing rates of skin cancers, and they were an essential public-health message," Lucas said. "But we now recognize that some sun exposure is important for health, at the very least, to maintain healthful vitamin D levels." Taking this into account, she said, the Cancer Council of Australia slightly eased its sun protection message concerning sunscreen, outdoor clothing and hats.

Skin pigmentation affects how much radiation your skin absorbs, Lucas pointed out. "The darker the skin, the more it's protected against skin cancer but the less able it is to absorb UV-B rays," she said. "It also depends on how much skin is exposed and the time of day." If you're a fair-skinned person, she says, spending a few minutes outside in the middle of the day without sunscreen would be enough to get vitamin D. However, she adds, "If you're already tanned or of Hispanic origin, you need maybe 15 to 20 minutes. Black skin may require six times the sun exposure to make the same vitamin D levels as a very fair-skinned person."