Despite its familiarity, vitamin D is pretty puzzling. Not really a
vitamin at all, it is actually a fat-soluble nutrient that is a
prohormone (a precursor substance to a hormone). Milk is fortified with
it, but it does not come in most other dairy products like cheese,
yogurt, and ice cream, and it occurs in significant levels naturally in
only a few foods like fatty fish and fish oils. To fulfill the
recommended daily intake with milk alone may take as much as 6 to 8
glasses, far more than most people drink, and so most people get the majority
of their vitamin D from sun exposure. But wearing sunscreen
significantly impairs the sun’s ability to activate vitamin D — wearing
SPF 8 can reduce the skin’s ability to produce vitamin D by about 95
percent.
Until recently, nobody thought much about whether they got enough
vitamin D from drinking milk or other foods or from the sun. But getting the
so-called “sunshine vitamin” does matter because it plays a far more complex role in our
health than anyone could have imagined just a decade or two ago.
Of course scientists have known for a long time that vitamin D is
important to keeping bones strong. Vitamin D signals the body to absorb
calcium from food and so maintains normal blood calcium levels. When
there isn’t enough vitamin D, the body doesn’t absorb enough calcium
from the diet, and it instead removes calcium from stores in the bones,
making them weak or brittle. Bone diseases
linked to vitamin D deficiency include: rickets, the
bone-deforming and potentially fatal condition that afflicted children
for ages; osteomalacia, a
muscle-weakening, bone-softening form of rickets that occurs in adults; and osteoporosis, the brittle bone disease.
Vitamin D’s role in bone health was pretty much the limit of scientific
understanding back in the early 1990s when I met Michael F. Holick,
professor of medicine, physiology, and biophysics at
Boston University School of Medicine. The endocrinologist and vitamin D
specialist met me in his office and told me about the discovery of
vitamin D during the industrial age, when smoggy skies blocked the
sun’s rays from people’s skin and led to an epidemic of rickets. Rickets mostly disappeared once doctors started prescribing
cod liver oil and sunshine, and especially once governments mandated
that milk be fortified with vitamin D in the 1930s. It doesn’t take
much vitamin D to prevent rickets.
In between talking about vitamin D, Holick talked about his schooling
in Wisconsin, about ice skating with his wife on their first date,
about his adventures in industry developing a vitamin D-based psoriasis
lotion, and about the glorious view of Boston from his office window.
He introduced me to patients in his clinic that had bone disorders like
osteoporosis. And he explained how rickets was now mostly confined to
family pets like his own family’s iguana. Holick cured his captive
reptile by serving it cream cheese and lettuce sandwiches, and by
shining a sun lamp into the cage.
I was taken with this charismatic, energetic scientist. For a while I
even tried to sell magazines an article on how to save pet iguanas from
rickets. I was, not surprisingly, unsuccessful. But given time, a far
larger and richer story about vitamin D has emerged, and much of it has
come out of Holick’s office.
For one thing, rickets never really went away. Soon after our meeting, medical journals started
reporting on cases diagnosed among children living in
northern cities, as well as on a rise in cases of its adult form,
osteomalacia. Scientists started to wonder if vitamin D deficiency was
more common than they thought. A research group in Baltimore found that
more than half of homebound adults over the age of 65 were severely
vitamin D deficient.
To get sense of the scope of vitamin D deficiency in the wider
community, Holick and colleagues measured vitamin D levels in younger
people living in Boston. They found that it was surprisingly common.
About 36 pecent of young adult city dwellers that worked office jobs
were vitamin D deficient at the end of winter, and 11 percent were so
at the end of summer. The emerging research also indicated that people
with darker skin were more likely to be deficient than people with
lighter skin, because they needed more time in the sun to convert
vitamin D into an active hormone. Also, people who were overweight were
more likely to be deficient, a result of the vitamin D becoming harder
for the body to access from fat cells.
Suddenly, people again started paying attention to
long-taken-for-granted vitamin D. Holick and others declared that the
country was in the middle of a silent epidemic of vitamin D-deficiency,
and warned that the health effects could be subtle but far-reaching. To
combat the epidemic, Holick came out in 2004 with a book called The UV
Advantage, which recommends sun exposure of 15 to 20 minutes a day
while de-emphasizing skin cancer concerns. Some of his colleagues
recoiled from his advice, feeling that people would take it as
permission to sunbathe. After it also came to light that Holick
received funding from the Indoor Tanning Association, he was forced to
resign from the dermatology department at Boston University Medical
School. (He still holds other positions there.)
Holick defended himself, saying his situation was no different than
other doctors taking pharmaceutical money, and pursued his studies.
Today he is still considered a leading expert in demystifying vitamin
D. Building on what is now more than 30 years of research, last year he published
a capstone review article in the New England Journal of
Medicine that delineated just how important vitamin D is to the healthy
body — and how many people need more of it.
Scientists now understand that most tissues and cells in the body have
vitamin D receptors, including several that possess the machinery to
convert it to the active form in the body. It is also now commonly
believed that vitamin D helps maintain muscle strength and immune
capability while also decreasing the risk for various chronic
illnesses, including cardiovascular diseases, autoimmune diseases such
as diabetes type 1 and multiple sclerosis, and various cancers
including colon, ovarian, breast, and prostate. Among the evidence is
the fact that the further away from the equator — and the sun — that
people live, the greater their chance of developing some of these
health problems. (Some very northern cultures like the Inuit naturally
compensate for the reduced sun exposure with a diet rich in fish oil.)
The work is not without its doubters. A National Cancer Institute study
based on survey data has found no relationship between vitamin D levels
and the overall risk of dying from cancer based on a 10-year follow-up,
although it did find an association between decreased risk of
colorectal cancer death and higher vitamin D levels. The findings are
not completely clear because of the complex contributions to risk from
total diet and lifestyle.
Large studies are also still needed to confirm the strength of the
links between vitamin D and chronic diseases. In recent years such
studies of several other vitamins turned out to be disappointing for
their role in improving health. But remember, vitamin D is not a
classic vitamin, and so these vitamin studies may not be relevant.
Only time and additional studies will elucidate further whether we
should be paying more attention to vitamin D. Maybe eventually it will
become clearer whether we should be worrying less about skin cancer and
more about whether our skin is soaking up enough rays to fulfill our
sunshine vitamin quota.
In the meantime, I wonder if Holick still has that iguana — they can
live for 20 to 30 years — and whether it really beat rickets for good. • 19 March 2008
SOURCES: “Vitamin D deficiency in homebound elderly persons.” Gloth FM
3rd, Gundberg CM, Hollis BW, et al. JAMA. 1995;274:1683-6. “Vitamin D
insufficiency among free-living healthy young adults.” Tangpricha V,
Pearce EN, Chen TC, & Holick MF. Am J Med. 2002;112:659-62.
“Vitamin D Deficiency.” Holick MF. N Engl J Med 357; 266-81.
Prospective Study of Serum Vitamin D and Cancer Mortality in the United
States, Freedman DM, Looker AC, Chang S-C, & Graubard BI, J Natl
Cancer Inst 2007; 99:1594-1602