This amazing vitamin serves as the body’s great regulator of calcium and phosphorus metabolism in three major ways:

  • Vitamin D mobilizes calcium and phosphorus for release from bone in the presence of parathyroid hormone.
  • Vitamin D promotes intestinal absorption of calcium and phosphate.
  • Vitamin D increases kidney absorption of calcium and phosphorus and carries them into the blood.

Adequate vitamin D nutrition is crucial at every stage of our lives, from childhood to old age. But for decades both the prevalence and implications of vitamin D deficiency have been grossly underestimated. A simple test can quickly tell you and your healthcare provider whether you have sufficient stores of vitamin D — don’t hesitate to ask for this important test — your lifelong health depends on it!

Simply with respect to bone health, the body cannot properly absorb calcium without vitamin D, and the bones and teeth become soft and poorly mineralized. In young children, a deficiency causes poor mineralization of the collagen matrix, which results in growth retardation and the bone deformity condition known as rickets. In adults, vitamin D deficiency results in a type of bone-softening adult rickets, known as osteomalacia. Inadequate levels of vitamin D also directly affect bone as it causes a condition known as secondary hyperparathyroidism, which stimulates a loss of matrix and minerals, in turn increasing the risk of osteoporosis and fractures.

Recently there has been a veritable explosion of research on vitamin D and its effects throughout the body. Inadequate levels of vitamin D have now been associated with numerous types of cancer, cardiovascular disease, hypertension, stroke, diabetes, multiple sclerosis, rheumatoid arthritis, periodontal disease, macular degeneration, mental illness, propensity to fall, and chronic pain.

The newly identified link between low vitamin D status and cancer has drawn particular attention. A recent study, for example, showed that women with adequate blood levels of vitamin D at diagnosis had a much better outcome and much less metastases of their breast cancer than did those who were vitamin D–deficient at diagnosis.

Also, for years it has been known that osteopenia, osteoporosis, and needless fractures are linked with low levels of vitamin D. Nearly two decades ago one prominent osteoporosis researcher concluded that, in general, the more adequate the state of vitamin D nutrition, the less bone loss among the elderly. We now know that women of all ages can actually halt bone loss, and even increase their bone density over the course of the year by consuming adequate calcium and getting adequate amounts of vitamin D — the “sunshine vitamin.” This is especially true during the dark days of winter.

Most importantly, adequate-dose vitamin D is now proven to reduce fractures significantly. Recently, in fact, three major vitamin D researchers estimated that 50–60% of all osteoporotic fractures are due to insufficient vitamin D. And indeed, as documented in a recent publication by the Better Bones Center, various clinical trials support this amazing fracture-reduction capacity of adequate-dose vitamin D.

Although we refer to vitamin D as a vitamin, it is really a pre-hormone which is transformed into a hormone in the body. While we consume small amounts of vitamin D in our diet, most of our vitamin D supply is produced by our bodies upon exposure to sunlight. Our wondrous capacity to produce vitamin D internally appears to decrease with age, however, and elderly people in even the world’s sunniest places are especially prone to low levels of vitamin D.

Another fascinating thing about vitamin D is that it directly nourishes muscles. Supplemental vitamin D has now been repeatedly shown to help improve muscle mass and strength and thus help in the prevention of falls. Several studies have shown a rapid reduction in falls among the elderly with administration of even 800 IU vitamin D. Two recent clinical trials, for example, showed a reduction in falls of 49% and 72% with 800 IU supplemental vitamin D. If for this reason alone, it is vital that older people obtain higher amounts of vitamin D through their food or supplementation. But again, vitamin D’s effects across all body tissues are far-reaching throughout our lives.

Vitamin D is a very complex substance, with many varied forms and myriad biological functions, many of which we have yet to explore and describe. Regarding its pivotal role in mineral metabolism, it’s important to understand that as a hormone, vitamin D exists in both more active and less active states. It is converted to more active states within the body on an as-needed basis. The most active metabolite of vitamin D, known as 1,25-dihydroxyvitamin D, or calcitriol, is produced by our kidneys and in other tissues from less active precursors. It is this active vitamin D hormone that mediates the many biological effects of vitamin D, including calcium absorption. For example, in the absence of activated calcitriol, less than 10% of our dietary calcium may be absorbed! Our ability to absorb calcium via the intestines is, in fact, directly related to our blood levels of this active form of vitamin D. What’s interesting, as calcium expert Dr. Robert Heaney has demonstrated, is that intestinal calcium absorption was 65% higher when blood levels of vitamin D averaged 34 ng/mL — we now know this level approaches only the barest minimum needed to ensure system-wide health.

It’s also of note that at extremely high levels, supplemental vitamin D can have toxic effects. For most people this is strictly a theoretical concern, and evidence of toxicity in adults consuming more than 10,000 IU/day is absent in the literature. Nevertheless, it is this theoretical risk that made so many so leery of vitamin D supplementation for so long.

Even though vitamin D deficiency is common and vitamin D toxicity is rare, it is important to get professional guidance and testing before supplementing at levels greater than 2000 IU per day as this is the current (albeit outdated) “safe upper limit” set by US Food and Nutrition Board. Today we know precisely what levels in the blood are needed for optimal bone health: a minimum of 34 ng/mL, but more ideally, at least 50–60 ng/mL. In reality, many people will need more than 2000 IU vitamin D daily to achieve this minimum adequate blood level and thus, at Women's Health Network, we recommend everyone have their vitamin D level tested using the 25(OH)D blood test. Testing is easy, and one of the most powerful tools there is to work with to achieve bone health. Luckily, we need not worry about getting too much vitamin D from our body’s own internal production, since our body simply stops producing vitamin D when levels are adequate.

For more information, see our other articles on vitamin D:

So as you can see, the last few years have seen an explosion of research on vitamin D. Hundreds of studies link low vitamin D levels to an array of health concerns ranging from osteoporosis, autoimmune disease, and cardiovascular disease to diabetes, depression, and cancer. For a comprehensive overview of this exciting new research, you may also wish to visit the Vitamin D Council website, a nonprofit organization founded and directed by Dr. John Cannell:

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