Vitamin D is not a true vitamin. It’s a hormone that organisms have been photosynthesizing for over 750 million years as a result of exposure to sunlight. Our human ancestors evolved near the equator and had dark skin appropriate for the intense solar radiation. As humans migrated from the equator to higher latitudes, lighter skinned children had a survival advantage, because light skin allowed more sunlight to penetrate for vitamin D synthesis. This adaptation allowed some people to survive closer to the polar regions, where sunlight is marginal even during the summer months. For most of our history, both children and adults have depended on adequate sun exposure to satisfy their vitamin D requirement.

Studies dating back to the early 1800s had determined that cod liver oil could help to prevent and cure soft, thin, and brittle bones, a condition known as rickets in children and osteomalacia in adults. In the early 1900s, nearly 80% of children in the industrialized, polluted cities of northern Europe and the northeastern United States suffered from the devastating consequences of rickets. Around that time, a compound called “fat-soluble factor D” was isolated from cod liver oil, and this factor turned out to be the substance that we now refer to as “vitamin D.” Subsequent scientific investigation of rickets helped establish the role of sunlight in providing us with vitamin D, and it also helped establish the role of vitamin D in bone health.

Beginning in the mid-1990s, researchers found that vitamin D is not simply a fat-soluble vitamin needed for healthy bones: it’s also a hormone. Vitamins participate in and regulate your metabolism, allowing it to function properly. Vitamin D helps to regulate your bone development, your muscle function, your immune function, your insulin activity, your calcium balance, and your phosphorus balance. Vitamin D also functions as a hormone because it sends messages to your intestines to increase the absorption of calcium and phosphorus. Throughout childhood, your body uses these minerals to produce bones. If you do not get enough calcium, or if your body does not absorb enough calcium from your diet, bone production and bone tissues may suffer. As with other hormones like estrogen and cortisol, there are receptors for vitamin D (called VDRs, or vitamin D receptors) on the cell membranes of most cell types in your body that allow vitamin D to play a role in many aspects of your health.

There are two basic types of vitamin D:

1. Ergosterol is the basic building block of vitamin D in plants and mushrooms. When ultraviolet light from the sun hits the leaf of a plant or the skin of a mushroom, ergosterol is converted into ergocalciferol, or vitamin D2.
2. Cholesterol is the basic building block of vitamin D in humans.  When ultraviolet light hits the cells of your skin, one form of cholesterol found in your skin cells—called 7-dehydrocholesterol—can be converted into cholecalciferol, a form of vitamin D3. In order for your body to come up with the fully active form of vitamin D3, further metabolism is required.
   1. A first step involves conversion of cholecalciferol into hydroxyvitamin D, also called 25-hydroxyvitamin D or 25(OH)D. Hydroxyvitamin D can be formed in your liver, kidney, lung, skin, prostate, brain, blood vessel linings, and macrophage cells of your immune system. An enzyme called CYP27A1 is required for formation of hydroxyvitamin D.
   2. A second step involves conversion of hydroxyvitamin D into dihydroxyvitamin D (also called 1,25-dihydroxyvitamin D or 25(OH)2D). This second step can take place in the lung, brain, liver, stomach, spleen, kidney, colon, thymus, lymph nodes, skin, placenta, and in the monocyte and dendritic cells of the immune system. An enzyme called CYP27B1 is required for formation of dihydroxyvitamin D.

The hormonal functions of vitamin D include regulation of bone health, regulation of muscle health (including both skeletal and heart muscle), regulation of immune response, regulation of insulin and blood sugar, regulation of calcium and phosphorus metabolism, and regulation of mood. Because vitamin D receptors (VDRs) are in so many different tissue types—including your brain and skin—vitamin D deficiency may contribute to many human diseases including depression and autoimmune diseases, and may increase risks of deadly cancers, cardiovascular disease, multiple sclerosis, rheumatoid arthritis, and type 1 diabetes.

Regulation of bone health, calcium, and phosphorus: Your bones comprise many different substances, including collagen proteins, keratin proteins, and the minerals silicon, boron, and magnesium. But the two most critical bone components are the minerals calcium and phosphorus, which make up over half of all bone composition. While bone health is regulated by many different substances in the body—including growth hormone, testosterone, and estrogens—the importance of calcium and phosphorus in bone health also points to the special importance of two bone health regulators: parathyroid hormone (PTH) and vitamin D. Your parathyroid glands secrete PTH whenever your blood calcium level gets too low. When that happens, PTH triggers release of calcium from your bones in order to boost your blood level back up to normal. PTH also triggers your kidneys to retain more calcium (keeping it available for your bloodstream) and excrete more phosphorus (thereby helping to create a more favorable ratio of calcium to phosphorus in your blood). If there is too much PTH released from your parathyroid glands, however, you may end up removing too much calcium from your bones and leaving too much in your blood, compromising both your cardiovascular health and your bone health. Vitamin D deficiency is a key risk factor for overproduction of PTH, and optimal levels of vitamin D are associated with healthy parathyroid function and desirable PTH levels. Like PTH, vitamin D helps your intestines absorb more calcium from your food, and it also helps your kidneys hang on to calcium. But unlike PTH, vitamin D also helps your kidneys retain phosphorus. The two hormones work together in order to assure proper balances of calcium and phosphorus in your bloodstream and in your bones. PTH also triggers the conversion of hydroxyvitamin D into dihydroxyvitamin D (the hormonally active form).

Regulation of immune function: Vitamin D plays a critical role in rheumatoid arthritis, multiple sclerosis, Crohn’s disease, systemic lupus erythematosus, and numerous other autoimmune conditions. There are vitamin D receptors (VDRs) on your immune system’s macrophage and dendritic cells. When triggered by vitamin D, macrophage cells are capable of releasing antibacterial peptides (parts of protein) like cathelicidin, and these antibacterial proteins play a critical role in your immune system’s prevention of infection, including tuberculosis and leprosy. Vitamin D deficiency is a risk factor for these diseases.

Regulation of blood pressure and cardiovascular health: Vitamin D plays a direct role in regulating your blood pressure by inhibiting the activity of a system called the renin-angiotensin system, which helps increase your blood pressure whenever it gets too low. The renin-angiostensin system increases your blood pressure by helping your body retain sodium and water (providing more fluid in your blood vessels) and by causing your blood vessels to constrict, thereby increasing the pressure inside them. You need optimal levels of vitamin D to hold this system in check, and to prevent it from raising your blood pressure under inappropriate circumstances. Vitamin D deficiency is a significant risk factor for high blood pressure, including during pregnancy (pre-ecclampsia). When vitamin D is deficient in your body and PTH is released in inappropriately large amounts, too much calcium can accumulate in your cells. Too much calcium in your heart tissue is a problem associated with increased risk of oxidative stress and tissue damage. The ability of heart tissue to heal after an event like heart attack depends on  an optimal amount of vitamin D.

Regulation of insulin and blood sugar: Vitamin D plays an important role in the regulation of insulin metabolism and blood sugar balance. Vitamin D deficiency is a risk factor for developing type 2 diabetes, and vitamin D levels have been associated with insulin secretion by the beta cells of the pancreas as well as insulin activity after it’s released into the bloodstream. When vitamin D deficiency leads to release of large amounts of PTH, too much calcium can accumulate in your cells. When too much calcium accumulates in fat cells, they can produce too much cortisol, a hormone that counteracts the effectiveness of insulin. Similarly, too much accumulation of calcium in your fat and muscle cells can inhibit the formation of a protein called GLUT-4, which helps carry sugar (glucose) out of your bloodstream and into your cells, whenever insulin directs it to do so. Without sufficient vitamin D, too little GLUT-4 is formed, and insulin lacks one of the tools it needs to do its job.

Regulation of muscle composition and muscle function: Vitamin D deficiency plays a key role in preventing muscle weakness and falls, especially in older adults. Vitamin D deficiency is associated with too much accumulation of fat throughout muscle tissue, in such a way that muscle strength is decreased and physical performance is compromised, independent of muscle mass. Because vitamin D is a key regulator of calcium metabolism, and calcium is known to play a key role in nerve firing and nerve triggering of muscle contraction, vitamin D may have a broader role in reducing risk of falls.

Regulation of mood: Vitamin D can increase the levels of serotonin, which controls your moods in your brain.

Prevention of cancer: Vitamin D regulates several genes and cellular processes related to cancer progression, and vitamin D provides powerful protection against common cancers, including bladder cancer, breast cancer, colon cancer, ovarian cancer, prostate cancer, and rectal cancer. About 75% of women with breast cancer are vitamin D deficient. A 2009 analysis revealed that women in the highest vitamin D range reduced their risk of breast cancer by 45%. Another 2009 review found that sufficient vitamin D levels were associated with reduced risk of colorectal cancer. Even after diagnosis with colorectal cancer, higher vitamin D levels are associated with reduced mortality. Cancers of the prostate, pancreas, lung, and endometrium are also associated with vitamin D insufficiency.

Your body makes vitamin D when your skin is directly exposed to the sun. That is why it is often called the “sunshine” vitamin. Most people meet at least some of their vitamin D needs this way. Your skin has a large capacity to make vitamin D.  Just 10-15 minutes in the sun is the equivalent of taking between 15,000 and 20,000 IU’s of Vitamin D, but without the potential for toxicity that is becoming more common as people take larger doses of it orally. there is a difference between Vitamin D synthesized from the sun and Vitamin D taken orally, since there are at least 5-10 additional photoproducts made in response to sun exposure that are not in the supplement form of Vitamin D. People who do not live in sunny places may not make enough vitamin D. Skin that is exposed to sunshine indoors through a window will not produce vitamin D. Cloudy days, shade, and having dark-colored skin also cut down on the amount of vitamin D the skin makes. If you don’t get any sun exposure, you need to take a supplement to maintain healthy levels of 25-hydroxyvitamin D in your blood. 

A 2011 review in the New England Journal of Medicine found “…that a serum level of 20 ng per milliliter of 25-hydroxy- vitamin D would protect 97.5% of the population against adverse skeletal outcomes such as fractures and falls.” Likewise, a 2011 editorial in the American Journal of Clinical Nutrition found that a vitamin D level in children above 12 ng/mL had no benefit on calcium absorption and above 20 ng/mL there was no evidence on improved bone health.

Plants and most animals do not provide a source of vitamin D to humans, with the exception of some fish that live at high latitudes. This exception demonstrates that natural environments support their inhabitants: in this case, eating fish provides the preformed hormone vitamin D to animals such as polar bears who live with very little sunlight almost all year long.

Your enzymes convert both vitamin D2 and D3 into 25-hydroxyvitamin D, and then into the active form of 1,25-dihydroxyvitamin D in our kidneys. This form of vitamin D can bind to receptors in various tissues around your body for use. Vitamin D3 stays in your bloodstream for weeks compared to days for vitamin D2. However, for people taking vitamin D supplements several times a week, there seems to be no significant advantage in taking one form or the other. Most vitamin D3 supplements come from the oil on the wool of sheep, so most vegans avoid them. Vitamin D2 supplements come from mushrooms.

Eating mushrooms packed with vitamin D2 confers many other health benefits. Mushrooms have many helpful nutrients, including beta glucans for immune enhancement, ergothioneines for antioxidative potentiation, nerve growth stimulators for helping brain function, and antimicrobial compounds for limiting viruses.

Organically-grown shiitake mushrooms, have 100 IU of vitamin D2 per 100 grams. When dried outdoors in the sunlight with their gills facing upward for full sun exposure for two days, six hours per day, the vitamin D levels in these mushrooms soars to nearly 46,000 IU per 100 grams. (Their stems produce relatively little vitamin D: about 900 IU.) Mushrooms dried this way preserve significant amounts of vitamin D2 for nearly a year after exposure. This means that you can capture vitamin D in mushrooms and have a ready source of this important vitamin — and delicious mushrooms — through the fall, winter, and spring.

Here is how to do it, according to Paul Stamets:

1. Obtain fresh organic shiitake, maitake, crimini, oyster, shimeji, or other mushrooms.

2. On a sunny day in June, July or August, slice the fresh mushrooms. Place them evenly on a tray exposed directly to the sun from 10 a.m. to 4 p.m.

3. Before nightfall, cover the mushrooms with a layer of cardboard to block moisture from dew.

4. The next clear day repeat exposure to the sun from 10 a.m. to 4 p.m.

5. Remove the mushrooms and finish drying (if necessary in a food dehydrator) until they are crisp.

6. When thoroughly dry, store in a glass jar or sealed container. Adding a tablespoon of uncooked rice as a moisture absorbent will help keep the mushrooms dry. The mushrooms should be good for a year or more, depending upon conditions.

7. Take 10 grams daily per person, about a small handful. Rehydrate in water for one hour. The mushrooms will swell. Then cook as desired.

Too much vitamin D from supplements can make the intestines absorb too much calcium. This may cause high levels of calcium in the blood. High blood calcium can lead to:

• Calcium deposits in soft tissues such as the heart and lungs
• Confusion and disorientation
• Damage to the kidneys
• Kidney stones
• Nausea, vomiting, constipation, poor appetite, weakness, and weight loss

The safe upper limit for vitamin D supplements are:

1,000 to 1,500 IU/day for infants
2,500 to 3,000 IU/day for children 1 – 8 years
4,000 IU/day for children 9 years and older, adults, and pregnant and breast-feeding teens and women
One microgram of cholecalciferol (D3) is the same as 40 IU of vitamin D.