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Last updated January 1, 2014

Dr. Michael Holick speaks on UV and D3...and iguanas

©1997 Anne Marsden


The following is from notes taken by Anne Marsden from a talk given by Dr. Michael Holick. Anne says "This is a personal view of the lecture written from notes. All quotes are reconstructed from shorthand. Any errors are mine."

On January 16th, 1996 Quest Diagnostics at Nichols Institute was privileged to receive a visit from Dr Michael Holick of Boston University Endocrinology Section. He gave a talk on photobiology and the roles of rPTHtp and vitamin D in cellular proliferation. My own personal interests is the role of vitamin D in the continued health of the common iguana (Iguana iguana). I attended the lecture hoping to be able to ask questions at the end. As you see, the talk more than satisfied me!

Dr. Holick is internationally recognized for his many contributions in the field of vitamin D, calcium, and bone endocrinology and metabolism. He is a member of the Association of American Physicians and the American Society for Clinical Investigation, a merit awardee of NIH, serves as Chairman of a Review Group for NASA, and has served on editorial boards of major journals and on an NIH Study Section. Dr. Holick's research interests include: ( l ) understanding the role that sunlight plays in providing all humans with their vitamin D requirement, (2) the wide spectrum of biologic effects of light for humans, (3) the cellular and molecular mechanisms by which 1,25-dihydroxyvitamin D3 regulates cell proliferation and differentiation, (4) the development of new drugs for treating psoriasis and osteoporosis and (5) the study of calcium and vitamin D metabolism from an evolutionary perspective.

Dr Holick opened with a slide of the Jurassic Park logo, and began his talk with, 'Stephen Spielberg has been responsible for an epidemic affecting more than half a million young children each year. A typical patient suffering from this syndrome presents with osteomalacia and rickets and failure to thrive. They are normally between 1 and 2 years old and are living on a diet of lettuce and flies.'

He showed a slide of a typical patient. It was an x-ray of a juvenile iguana showing the multiple fractures typical of metabolic bone disease. He went on to talk about his own iguana, and how it would be inappropriate for a world-renowned expert in calcium metabolism to own an iguana which developed rickets. He first offered the iguana ground Tums as a source of calcium but the iguana did not eat it. He then offered the iguana cream cheese as a calcium source, and the iguana eats this along with his lettuce. However, he was then concerned with a source of vitamin D. Cream cheese, lettuce (and indeed all vegetables fed to iguanas in their typical diet) contain no vitamin D. (Dr Holick did indeed have a lot to say about so-called 'fortified' vitamin D milk - it invariably contains less vitamin D than it is labeled to contain.)

He bought a reptile light (Repta-Sun brand) and placed it over the iguana's enclosure. He showed a slide of his iguana and his daughter, both looking healthy. The iguana was probably about a year old at the time of the picture. He then went on to one of the main points of his talk: the relationship between sunshine, vitamin D and health.

If you wish to read and in-depth review of the subject, I suggest you point your web-browsers to the document by B. L. Diffey at Here, I'm just going to recap that irradation of the skin by UVB results ultimately in the production of 25(OH)D and 1,25(OH)2D - the active forms of vitamin D. Vitamin D enhances the absorption of calcium from the gut. Without vitamin D only 10 - 15% of dietary calcium is absorbed into the blood. If vitamin D is present, up to 80% can be absorbed. Since a large amount of calcium is lost each day (in gastric juices and in sweat, for example) there is a significant need for calcium in the blood (serum calcium). Without sufficient serum calcium, bone mineralization is poor and deformities and fractures may occur. In severe cases, muscles are unable to contract normally and a form of tetanus occurs, resulting in death.

Humans get vitamin D from foods containing animal fat (one reason why the Inuit, living with little sunshine, are not deformed; they eat large quantities of fish oil). Although milk is supposedly fortified, it is not a significant source of vitamin D. There is only one other source - UVB irradiation from exposure to the sun. In today's society, sunbathing is frowned upon as an harmful activity. This has contributed to an epidemic of vitamin D deficiency in the population, resulting in possibly as many as 50,000 to 100,000 unnecessary hip fractures per year, plus a number of other bone-mass-loss problems. Dr Holick has studies to show that 10 minutes in the open three times each week, exposing hands and face, are sufficient to build a base level of vitamin D. There's one caveat - in most places that has to be in the summer. In Northern latitudes, such as his native Boston, no vitamin D production can take place in the winter. Californians and Floridians can make vitamin D all year round. He recommends for humans at least this level of sun exposure. After that, you can wear sunscreen or go indoors. In Siberia, where no vitamin D production can take place all year round, children are encouraged to sunbathe under mercury lamps once a week for a short period. This is sufficient to meet their needs.

After this, Dr Holick went on to the main part of his talk, which concerned the role of vitamin D in resolving proliferative disorders of the skin (such as psoriasis) and the role of parathyroid related peptide in hair regrowth after the onset of baldness.

At question time I asked Dr Holick if skin pigmentation was a significant factor in vitamin D levels. He replied that it could take up to ten times the exposure to the sun for a heavily pigmented (black) person to make normal levels of vitamin D. Since this rarely occurs, African-Americans are chronically vitamin D deficient.

If this was the case, I argued, reptiles would be unable to meet their UVB needs with fluorescent lighting. I mentioned the figures in Mader which suggest that the reptile lights typically take from 10 to 25 hours to provide the UVB irradiation found in five minutes in the sun at the equator. Dr Holick was quite emphatic: these lights DO provide sufficient UVB to significantly increase vitamin D production, even in reptiles with their heavily keratinized skin. Dr Holick is a consultant for the National Zoo and is working closely with them in ensuring that their indoor collections are healthy. He is collaborating with URI (I am not familiar with them) to produce standardized, dedicated reptile lights. A test kit is in the works which will involve holding up a cardboard strip to your reptile light at basking distance. If there is sufficient UVB for vitamin D production, the strip will go blue.

I asked Dr Holick about the practice of exposing reptiles to mercury vapor lamps or carbon arc lamps for a few minutes each day. He replied that these lamps have a significant output of UVC, which can result in many problems, including weakness of the immune system. What about owners? 'Owners too.'

Finally, Dr Holick said, ' That's enough about iguanas. What about the tens of thousands of elderly in nursing homes suffering the misery of osteoporosis, bone pain, osteomalacia and hip fractures? They do not have to get these symptoms if they are provided with correct lighting.' This did astonish me. I said, 'You mean that people walking around five feet or so from lighting fixtures will be able to meet their vitamin D needs?'

He replied, 'There's no doubt. Even taking into account the inverse square law, they will receive sufficient UVB.'

I have a great deal of respect for Dr Holick and I was encouraged by his entry into the animal health forum. I think we will see a great improvement in reptile lighting and the test kit will provide assurance that UVB output is being maintained. This could result in half a million happier iguanas each year - and 100,000 older people staying out of hospital in the same period.

Related Articles:

See Anne's other article, Sunlight and Reptile UVB Tubes: The Value of UVB Exposure, wherein she discusses the research literature on UVB and D3. It can be found listed with other lighting and Heating articles on the articles on the Captive Environment page.

Vitamin D Deficiency Worse In Winter

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