By Dr. Mercola

Sunlight: we all feel better when we're exposed to it, but do you know why? Dr. Michael F. Holick, a foremost expert on vitamin D with an impressive list of credentials, is just the right person to explain the healing power of sunlight.

His academic credentials include chief of Endocrinology, Diabetes, and Nutrition at Boston City Hospital and Boston University Medical Center from 1987 until 2000.

He's currently the director of Bone Health Care Clinic, and a professor of Medicine, Physiology, and Biophysics at Boston University Medical Center. He's also the director of the Boston University Heliotherapy, Light, and Skin research lab.

While enrolled at the University of Wisconsin, he worked with Dr. Hector DeLuca on vitamin D, and ended up getting his M.S. by identifying 25-hydroxyvitamin D3 as the major circulating form of vitamin D in the human blood stream.

This is the form you want to measure to determine your vitamin D status.

For his Ph.D. he identified the biologically active form of vitamin D as 1,25-dihydroxyfitamin D3. As a postdoctoral fellow he participated in the first chemical synthesis of 1,25-dihydroxyvitamin D3 that was used to treat bone disease in kidney failure patients.

Later, he became interested in understanding how vitamin D is made in the skin. So he worked out methods to determine how factors such as time of day, season, latitude, skin pigmentation, and obesity influence this cutaneous process.

"I also realized that the skin had vitamin D receptors," he says. "What that means is that the active form of vitamin D was working in the skin, and I wanted to know why. It turns out that active vitamin D inhibits skin cell growth and modulates it in a very important way.

I then realized that maybe you could use it to treat a very common hyperproliferative skin disorder: psoriasis. I basically introduced the concept, in the mid-'80s, of topically applying active vitamin D to treat psoriasis ...

I've also done lots of other studies looking at how vitamin D is absorbed by the body and its impact on your health."

Sunlight for Your Health

When you're exposed to sunlight, all of the sun's energy is hitting your skin. Over the past 40 years, dermatologists have promulgated the idea that you should never be exposed to direct sunlight because it will damage your skin and cause skin cancer.

What they fail to appreciate is that when you're exposed to sunlight, many important biological processes occur in your skin. This is distinct from swallowing oral vitamin D.

While taking an oral vitamin D supplement will improve your vitamin D status, you forgo the benefits sunlight offers beyond vitamin D production.

Unfortunately, the vast majority of people have no alternative in the winter months. But in spring, summer, and fall, it's wise to take advantage of sensible sun exposure, using the app "dminder.info" which Dr. Holick discusses in his PowerPoint presentation, "Sunlight for Your Health."

Not only does sunlight make vitamin D in your skin; it also makes beta-endorphins, which makes you feel better, and nitric oxide, which can help lower your blood pressure, and a whole host of other chemicals that you don't get when taking a vitamin D supplement.

"It's important to realize that when you're exposed to sunlight, you're exposed to huge amount of energy that's penetrating into your body.

It's also important to know that the atmosphere absorbs most of the damaging radiation that the sun is releasing including x-rays, gamma radiation, and ultraviolet C (UVC) radiation.

So, when you look at the solar spectrum for how much UV is actually coming in to planet Earth, it's a very tiny amount. And of all of the energy that's coming in from sunlight, a very tiny amount of less than

1 percent is responsible for making vitamin D in your skin, estimated at about 0.1 percent ultraviolet B radiation ..."

The Difference Between UVA and UVB Radiation

There are two forms of UV radiation that reach the Earth's surface: UVA and UVB. The UVB has lower wavelength, but higher energy than UVA. Interestingly, even though it has the higher energy, it does not penetrate deeper into your skin than UVA.

The reason for this is because proteins DNA, RNA and other chemicals in your epidermis absorb UVB radiation very efficiently.

UVA radiation, on the other hand, is not as absorbed efficiently, so it penetrates deep into your dermis and can cause skin damage, including cross-linking of your collagen matrix, which increases your risk for wrinkles.

As your skin is exposed to sunlight, melanocytes in your skin make melanin, a skin pigment that acts as a natural sunscreen. UV exposure also causes cross-linking of your DNA, but your skin houses enzymes that specifically break apart and repair those cross-linked DNA.

So, your skin is actually perfectly designed to address and heal the DNA damage produced by sun exposure.

"If you look at the solar spectrum and you look at what radiation is responsible for causing burning and what radiation is causing vitamin D production, in fact, the erythema [skin reddening] occurs principally right at the peak level of where the vitamin D is being produced as well.

At noon time, about 15 percent of the redness of your skin occurs from UVA exposure but 85 percent is from the UVB. UVA will cause sun burn independent of UVB radiation.

The bottom line, of course, and the message that I've always been giving is that you never want to get a sun burn. That's the most damaging to your skin, increasing risk for skin cancer as well as wrinkling," Dr. Holick explains.

"Can sunlight damage the skin? I don't think there's any question that excessive exposure, especially on the top of your hands and of your face, can cause what's called actinic keratosis.

These are called pre-skin cancers. If you're constantly getting overexposure to sunlight, they can definitely become either basal or squamous cell carcinomas.

[However] these are skin cancers that are usually easy to detect and easy to treat. Melanoma is the deadliest skin cancer, [but] it turns out that most melanomas occur on the least sun exposed areas. Occupational sun exposure actually decreases risk for melanoma."

When and Where Can You Make Vitamin D in Response to Sun Exposure?

Research by Dr. Holick's team has shown that if you live north of Atlanta, Georgia, you cannot make vitamin D in your skin from about November through February. Even in Miami, Florida you only make about 10 to 20 percent of the vitamin D that you would make in the summer time.

One factor that will influence your ability to produce vitamin D at northern latitudes is your elevation. At about 15,000 feet, you can make robust amounts of vitamin D even in the winter, due to the fact that the sun's rays — which come in at a more oblique angle in the winter — have less atmosphere to penetrate at higher altitudes.

"We've also been taught to go out in the early morning and late afternoon to go jogging, make your vitamin D. It's [said to be] less damaging to your skin. But, it turns out to be probably one of the worst times to be outside because you're getting blasted by UVA radiation, which can alter your immune system and may even increase your risk for melanoma. But you make essentially no vitamin D," Dr. Holick warns.

"Vitamin D is made in your skin from about 10am until 3pm for the same reason that you only make vitamin D in the spring, summer, and fall. In the early morning and late afternoon, the sun's rays are more oblique and get absorbed by the ozone layer. As a result, you don't make vitamin D in your skin."

How Much Sun Exposure Is Required?

To determine this, Dr. Holick conducted a study to see how much vitamin D adults would produce by using a tanning bed just long enough to get a "minimal erythema dose".

They determined that this is equivalent to taking 15,000 to 20,000 IUs of vitamin D. As a general recommendation, Dr. Holick suggests starting your sun exposure in the spring, going out for about one-third to one-half of the time it typically takes for you to turn a slightly darker shade in the middle of the summer.

The app, dminder.info, that Dr. Holick helped develop, provides guidance for sensible sun exposure.

So for example, if you normally get red after 30 minutes of exposure at noontime in June, then start out by exposing your arms, legs, abdomen, and back, for 10 to 15 minutes per day. After that, put on some protective clothing to prevent excess exposure.

Each day, add a couple of more minutes to build up a tan. Keep in mind that the pigmentation of your skin will influence how much sun you can tolerate.

The skin pigment melanin is a natural sunscreen, absorbing UV light coming into your skin. As a result, it markedly reduces the efficiency of your skin to produce vitamin D. As a result, dark-skinned individuals need more sun exposure than light-skinned individuals to produce the same amount of vitamin D.

What's the Ideal Vitamin D Level?

What do you want your blood level to be? A study done in Africa on Maasai warriors, who are outside every day, showed they had a blood level of 25-hydroxyvitamin D averaging around 48 ng/ml. At present, a vitamin D (25-hydroxyvitamin D) level between 40 and 60 ng/ml is thought to be ideal for optimal health and disease prevention.

"The Endocrine Society Clinical Practice Guidelines Committee that makes the recommendations to physicians for how to treat or prevent vitamin D deficiency [say that] your level should be at least 30 ng/ml. Forty to 60 ng/ml is a good range, and we know up to 100 ng/ml is perfectly safe," Dr. Holick says, adding that: "It's estimated that improving vitamin D status worldwide could reduce healthcare cost across the board by about 25 percent."

How Vitamin D Benefits Your Immune Function

It's known that those who live at the lowest latitudes have the lowest risk of developing multiple sclerosis (MS), an autoimmune disease. We also know that if you live north of Atlanta, Georgia for the first 10 years of your life, you double your risk of developing MS for the rest of your life, no matter where you live.

Type 1 diabetes is another chronic condition that seems to be closely tied to vitamin D deficiency. If you live at the equator, you have a 15-fold reduced likelihood of developing type 1 diabetes than if you live in the far North. A 2001 study published in The Lancet showed that children given 2,000 IUs of vitamin D per day during their first year of life had an 88 percent reduced risk of getting type 1 diabetes.

Research has also shown that women who have the highest intake of vitamin D reduce their risk of rheumatoid arthritis by about 44 percent. And having a 25-hydroxyvitamin D level of 38 nanograms per milliliter (ng/ml) has been shown to reduce your risk of upper respiratory tract infections by about 50 percent.

"We also know that immune cells known as macrophages activate vitamin D for the purpose of programming the immune cells to help fight infections such as tuberculosis ...Thus vitamin D is so important in helping to fight infectious diseases."

In recent years, scientists have come to recognize that a gene found in your pituitary gland called the proopiomelanocortin (POMC) gene is also found in your skin cells. When your skin is exposed to UVB radiation, that gene is turned on. Dr. Holick explains how this gene helps modulate both inflammation and immune function:

"[The POMC] gene ... produces adrenocorticotropic hormone (ACTH). This hormone tells your adrenal glands to make cortisol, which can help to modulate your immune system. It also has other effects including regulating other types of inflammatory activity ... So now we're beginning to understand why sun exposure helps reduce risk for autoimmune diseases."

Vitamin D Lowers Your Cancer Risk

Vitamin D is produced in your skin, and is then activated in your liver to 25-hydroxyvitamin D — the major circulating form. Next, it travels to your kidneys, where it gets activated to 1,25 dihydroxy vitamin D.

As far back as the 1970s and 1980s, researchers knew that your intestine, bone, and kidneys — organs that regulate calcium metabolism — have vitamin D receptors. These vitamin D receptors interact with the active form, which unlocks genetic information that increases intestinal calcium absorption, helps mobilize calcium from your bones, and conserve calcium in your kidneys.

In the '80s and '90s, they began to realize that not only do these cells and tissues have vitamin D receptors, but so does your colon, prostate, breast, brain, heart, and blood vessels, just to name a few. The obvious question was why? As it turns out, vitamin D receptors found throughout your body serve an important anti-cancer function.

"One of the first pieces of scientific evidence was developed by Dr. Toshio Suda, back in 1979. What he showed was that if you took a leukemic cell that had a vitamin D receptor, and incubated it with the active form of vitamin D, that leukemic cell became normal. It transformed back into a normal cell.

This began to introduce the concept that maybe some of the association studies suggesting exposure to sunlight reduces your risk of cancer may be related to vitamin D.

In fact, one of the early association studies done in 1915 showed that if you were working indoors and living in San Diego, you had an eight times higher risk of developing a deadly cancer and dying of that cancer than if you worked outdoors. A very nice study done in Canada showed that ... women who had the most sun exposure as teenagers and young adults had a 69 percent reduced risk of breast cancer later in life ...

This shows, I think very clearly, that exposure to sunlight or sensible sunlight exposure, throughout your life, helps reduce risk of deadly cancers later in life. What is this association? How is it possible that exposure to sunlight and improving your vitamin D status could reduce your risk of cancer?

[We] worked with Dr. Gary Schwartz, and showed that human prostate cells, obtained at the time of prostate biopsy, had the ability to activate vitamin D.

We then began to realize that there's a new major function of vitamin D that we haven't appreciated before, which is ... if you raise your blood levels of 25-hydroxyvitamin D into healthy range of about 30 ng/ml, this 25-hydroxyvitamin D can get activated in your prostate, colon, breast, and brain, and this active vitamin D has the ability to regulate up to 2,000 genes in your body, either directly or indirectly. That's about one-sixth of the human genome."

These genes have been linked to controlling more than 80 different metabolic processes including enhancing DNA repair, having antioxidant activity, regulating cellular proliferation and differentiation and improving immune function."

Ingenious Mechanism Allows Vitamin D to Prevent Cancer Without Negatively Affecting Calcium Metabolism

Dr. Holick's team found that vitamin D works locally at the cellular level, activating and deactivating about 290 genes, and this was key for explaining how vitamin D helps prevent cancer.

Moreover, they found that once vitamin D is activated in say a colon or brain cell to regulate genes to improve the health of the cell, it, at the same time, induces its own self destruction. This is important, because if the active form of vitamin D were to enter your blood stream, it might adversely affect your calcium metabolism.

"That's clever of Mother Nature," Dr. Holick says, summarizing the process again as follows: "So you make active vitamin D in your kidneys for regulating calcium and bone metabolism.

But now, we've realized that a major new component of the vitamin D story is that you're activating vitamin D locally many cells throughout the body, and they're having all these effects on your genes, and then the cell destroys it so it never gets into the bloodstream and it never has an effect on calcium metabolism ... So, my message is clear. Feed your genes right and improve your vitamin D status."

Neuroprotective Abilities of Vitamin D

Vitamin D also plays a major role in neurotransmission, and vitamin D deficiency has been associated with a number of neurological and brain disorders, including the following:

• Cognitive dysfunction and Alzheimer's disease (In one study, those who were most vitamin D deficient had a 31 percent increased relative risk of suffering neurocognitive decline)
• Increased risk for schizophrenia
• Parkinson's disease
• Stroke
• Epilepsy
• Depression

Beyond Vitamin D

There are many health benefits of sun exposure beyond vitamin D production. For example, sunlight is important for the regulation of your circadian rhythm, and light therapy has been shown to be effective against depression, both seasonal affective disorder (SAD) and non-seasonal major depression.

Exposure to sunlight also makes you feel good. The reason for this is because UV rays stimulate epidermal cells known as keratinocytes to make beta-endorphins.

A study done at Massachusetts General Hospital showed that when healthy adults were exposed to UVA and UVB, they increased beta-endorphin levels in the blood by about 44 percent. The beta-endorphin also likely enters your brain, and since they're an endogenous opioid, they make you feel good.

Sun exposure also has a number of cardiovascular benefits causing vasodilation and reducing risk for peripheral vascular disease and myocardial infarction. The Framingham Heart Study showed that those who were vitamin D deficient had a 50 percent higher risk of having a heart attack, so vitamin D is thought to be very important for cardiovascular health.

Sun exposure also has a number of cardiovascular benefits beyond vitamin D's effects. Nitric oxide that is produced in your skin in response to sunlight may also play a role, as it helps dilate your blood vessels and helps reduce blood pressure.

Nitric oxide also has a number of other health benefits. For example, it can:

• Induce melanin production
• Alter immune function
• Enhance wound healing
• Have antimicrobial effects

When exposed to sunlight, your hemoglobin molecules also release carbon monoxide, which in tiny amounts can also cause vasodilation and reduce blood pressure. Carbon monoxide can also act as a neurotransmitter, and has beneficial effects on your nervous system. It causes relaxation, and it has anti-inflammatory activity.

"There are other studies that show other health benefits ... They're now beginning to use red and blue lasers to affect fibroblast to produce healthy collagen in your dermis. There have been studies to show that the effect of red, and even infrared light, is effective in reducing wrinkling and fine lines, and to improve your intradermal collagen density.

Blue light has been shown to do the same thing. Studies have also shown that it enhances wound healing. Therefore, there are a variety of biologic effects of light on the skin that are above and beyond vitamin D," Dr. Holick notes.

More Information

If you look at the spectrum of sunlight that reaches the Earth's surface, UVB radiation is responsible for making vitamin D. Meanwhile, UVA radiation helps modulate your immune system, and UVA and UVB in combination improves beta-endorphin production in your skin, which makes you feel good.

Sun exposure on bare skin also produces nitric oxide and carbon monoxide that cause vascular relaxation, improves wound healing, and helps fight infections among other biologic processes. The blue wavelength of sunlight is particularly important for regulating your circadian rhythm and suppressing melatonin levels; it helps improve your mood, and reduces depressive symptoms.

"There is no downside, in my opinion, to improving your vitamin D status and getting some sensible sun exposure," Dr. Holick says. "I don't think you need to be a genius to know that you need sensible sun and vitamin D supplement recommendations. It's not a hypothesis."

In 2004, Dr. Holick wrote his first book for the general public, "The UV Advantage," in which he discusses many of the health benefits of sun exposure covered in this interview. His second book, "The Vitamin D Solution: A 3-Step Strategy to Cure Our Most Common Health Problems," followed in 2010. This book focuses on strategies to help prevent and treat vitamin D deficiency.

"The obvious question is, can you get enough vitamin D from your diet? I hopefully convinced you that you cannot get enough vitamin D from your diet alone ... I get approximately 4,000 units a day from all of my sources. My blood level on average is about 55 to 60 ng/ml ...

The Endocrine Society Clinical Practice Guidelines recommends 400 to 1,000 IUs a day for children during their first year of life. For children 1 to 18 years of age, they should get 600 to 1,000 IUs. My preference for teenagers [is the same dose as] adults; at least 1,500 to 2,000 units a day. You cannot get this from dietary sources.

So, what's the best source? Of course, it's sensible sun exposure ... [but] time of day, season of the year, latitude, and degree of skin pigmentation all have influences. So we developed an app, DMinder.info. It will tell you, anywhere on the globe, anytime of the year, and for any skin type, whether you can make vitamin D, and how much vitamin D you're making.

It will also warn you to get out of the sun so that you don't get a sun burn and don't significantly damage your skin."

Remember, the best time to get sun exposure to optimize your vitamin D (25-hydroxyvitamin D) levels is between 10 am and 3 pm from March through October. November through February, you will not be making vitamin D if you live north of Atlanta, Georgia, and even at southern latitudes, you'll only be making 10 to 20 percent of your summertime norm.

During winter months, your alternatives are to use a tanning bed or to take an oral vitamin D3 supplement. If you opt for a supplement, remember that you also need to increase your calcium intake. There is no need to have your blood level of 25-hydroxyvitamin D determined if you are getting adequate sun exposure and vitamin D supplementation as I have recommended.

However, if you are obese, or have a fat malabsorption syndrome or have had gastric bypass surgery, I strongly recommend getting your vitamin D level tested at least once or twice a year, say during the winter and summer, to make sure your chosen strategy is providing you with enough vitamin D.

Ideally, you'll want your level to be between 40 and 60 ng/ml year-round.