6 ChemMatters | DECEMBER 2017/JANUARY 2018 www.acs.org/chemmatters
The outer layers of bones are composed
of collagen and hydroxyapatite, a calcium
phosphate salt (Ca10(PO4)6(OH) 2). These
substances give bones their strength
and resilience. To form strong bones,
hydroxyapatite depends on a good sup-
ply of both calcium ions (Ca2+) and
phosphate ions (PO43–) in the blood.
Vitamin D is needed for both calcium and
phosphate ions to be absorbed by the
intestines and increasing reabsorption
of calcium ions in the kidneys, thus
reducing the loss of calcium in the
Hydroxyapatite mineralizes around the collagen. When vitamin D levels are low, there is
relatively little hydroxyapatite compared with
collagen, so bones become soft. In a growing
child, this is a problem. Soft bones are pliable,
so their shape changes, and it’s difficult for
a child to carry his or her own weight. It can
also lead to permanently deformed bones.
What about getting
vitamin D from the sun?
Most people get at least some of the vitamin
D they need by making it themselves through
exposure to the sun. The tricky thing about
sunlight exposure is balanc-
ing the benefit of vitamin D
synthesis with the risk of
skin damage. Developing a
sunburn or, worse, skin can-
cer, depends on the amount of time you spend
in the sun and the properties of your skin.
Some research suggests that approximately 5 to 30 minutes of sun exposure
between 10 a.m. and 3 p.m. at least twice
per week can produce adequate levels of
vitamin D. But on the other hand, the American Academy of Dermatology advises that protection should be used in the form of clothing
or sunscreen whenever exposed to sun.
Can you overdose on
believe that if a
little bit of some-
thing is good,
then more must
be better. When
this comes to
vitamins, this kind
of thinking can
lead to health problems. Taking excessive
doses of vitamins can be toxic. Symptoms
of vitamin D toxicity, or hypervitaminosis D,
Nearly all cases of hypervitaminosis D
occur in people who ingest megadoses of
vitamin D from supplements. Taking a daily
dose of 50,000 international units (IUs)
of vitamin D for several months has been
shown to cause toxicity. This level is many
times greater than the Recommended
Dietary Allowance of 400 to 800 IUs of
vitamin D per day. Human bodies seem to
naturally moderate the amount of vitamin D
produced by sun exposure, and even forti-
fied foods don’t contain large amounts of
vitamin D. The best way to avoid toxic levels
is to avoid consuming more than the recom-
mended amount, which is summarized in the
Vitamin D Factsheet for Health
Professionals. Office of Dietary
Supplements, National Institute of
Health, Feb 11, 2016: https://ods.
od.nih.gov/factsheets/VitaminD-HealthProfessional/ [accessed Sept
Kamangarl, F.; Emadi, A. Vitamin and
Mineral Supplements: Do We Really
Need Them? International Journal of
Preventive Medicine, March 2012, vol.
3 ( 3); pp 221–226.
David Warmflash is a science writer who
lives in Portland, Ore. His most recent
ChemMatters article, “Preserving Organs:
Saving Lives, Giving Hope,” appeared in
the December 2016/January 2017 issue.
Vitamin D allows the body to deposit calcium ions on the outer layer of bones,
making bones rigid and strong. Those with vitamin D deficiency (rickets) have soft,
weak bones and may have bowed legs as a result.
Definition: an international
unit (IU) is a unit of activity or
potency for vitamins, hormones,
or other substances, defined
individually for each substance
in terms of the activity of a standard quantity or preparation.
Recommended daily vitamin D dose
Age range (years) International units (IU) μg/day
0–1 400 10
1– 13 600 15
14–70 600 15
70+ 800 20
The tricky thing
balancing the benefit of
vitamin D synthesis with
the risk of skin damage.