How much sunshine do we need to produce adequate vitamin D?
Research suggests an association between increased colon cancer risk and low vitamin D levels in the body, which may also be tied to other forms of cancer and heart disease.
Aside from that, the role of vitamin D -- commonly known as the sunshine vitamin -- in keeping bones strong is already well-established. Ideally, people should get enough vitamin D from their food.
Foods that are high in vitamin D include egg yolk, fortified milk and yogurt, liver, mushrooms and sardines.
However, it's difficult for people to obtain the vitamins they need strictly from food.
What's not as well known is how much time you need to spend in the sun for vitamin D. During the spring and summer months, there's plenty of daily sunlight to boost vitamin D levels in your body. The challenge lies in balancing skin protection from sun overexposure while reaping vitamin D's health benefits. Concerned about skin cancer risks and sun-aging, many health care professionals recommend using sunscreen for all outdoor activities in the sun.
It can be tricky to strike the right balance. Dark-skinned individuals produce less vitamin D than those with fair skin, and older adults make less vitamin D than younger people.
It's important to keep in mind that a number of factors can influence your ability to make vitamin D from sun exposure, says Dr. Michael Holick, a professor of medicine, physiology and biophysics at the Boston University Medical Campus.
These factors include time of day, season, latitude, altitude, skin color, weather conditions and air pollution.
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
Holick 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.
Some nutrition experts believe it is possible to 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 2020-2025 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 meta-analysis, released in 2018 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% 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 12 years 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.
However, spending too much time in the sun poses health risks. For example, overexposure to ultraviolet radiation from being in the sun can cause damage to the DNA in your skin cells and lead to skin cancer, according to Cancer Research UK. The organization is a cancer research and awareness charity in the United Kingdom and Isle of Man.
Overexposure to the sun and use of sunbeds are risk factors for melanoma, the most serious kind of skin cancer, according to the group. For example, in the UK, nearly 90% of melanoma cases could have been prevented by taking the proper precautions when it comes to being in the sun and not using sunbeds. Suffering a sunburn even once every two years can triple your risk of contracting melanoma, according to Cancer Research UK.
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 -- wearing a hat that shades your face, applying sunscreen and avoiding or minimizing your time in the sun between 10 a.m. and 4 p.m., when UV rays are generally the strongest. "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."
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, a former epidemiologist at Australian National University. She's now retired and holds the title of emeritus professor.
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."
Even if you jog in the early morning on a bright, sunny day, you may not produce enough vitamin D because there are not sufficient UV rays early in the morning, Lucas said. Instead, she suggested, "Taking a stroll during lunchtime is your best bet."
(Source: US News)