Vitamin D: What's Enough?
Many people may need much more
A few minutes of sun exposure on a summer day
can generate huge quantities of vitamin D in a
person's body. A cholesterol-like substance in
the skin absorbs ultraviolet (UV) energy and
creates vitamin D. Then, a cascade of chemical
reactions turns vitamin D into a surprisingly
versatile hormone—one that has long been
recognized to help the body absorb calcium from
the diet to build strong bones. Recent work,
however, indicates that vitamin D also bolsters
muscle strength, insulin action, immune health,
and the body's natural defenses against cancer.
Inhabitants of the tropics typically have
plenty of vitamin D, says Robert P. Heaney of
Creighton University in Omaha, Neb. However,
studies are now showing that people throughout
the industrial world lag far behind. Many in
temperate and colder climates don't reach the
doses currently recommended to protect bone
health, much less the far-higher amounts that
research has been linking to additional
Some scientists are campaigning for
additional vitamin-D enrichment of foods. Others
advocate that people spend more time outdoors to
increase vitamin D–producing sun exposure. Many
hold that the boost must come largely from
What researchers who study vitamin D do agree
on is that many people would benefit from more
of the vitamin. At issue is only how much.
Out of the tropics
The Food and Nutrition Board of the National
Academies in Washington, D.C., currently
recommends that people from infancy through age
50 get 200 international units (IU) of vitamin D
per day, that those ages 51 through 70 receive
400 IU daily, and that anyone over 70 get a net
of 600 IU from sun, food, and supplements.
That's easy enough to do if you're, say, a
white person working outdoors during the summer
in New Jersey. In shorts and a T-shirt, such a
person can soak up enough ultraviolet rays to
produce 12,000 IU of vitamin D within 20
minutes, notes Reinhold Vieth of the University
That production would plummet if the person
stayed indoors or slathered on UV-blocking
sunscreen or covered up with clothing when out
of doors, as recommended for protection against
Global location and skin color also affect
the amount of vitamin D a person's skin
manufactures. UV intensity falls as one moves
from the equator toward Earth's poles,
increasing latitude. Evolution compensated by
selecting for increasingly unpigmented skin in
northern populations, says Boston University
endocrinologist Michael F. Holick.
Melanin pigment protects the skin from the
damage of UV rays but also lowers the skin's
production of vitamin D. In the March
American Journal of Clinical Nutrition,
Holick quantifies this effect: Fair-skinned
people who sunburn easily and rarely tan need
just 2 to 10 percent as much sun exposure to
produce a unit of vitamin D as do people with
the darkest skin.
Season also matters. Holick has found that
from the latitude of San Francisco northward—or
from Buenos Aires southward—for 3 to 6 months a
year, no amount of exposure will generate
substantial vitamin D in even the palest skin.
Holick composed a map of North America that
shows the minutes of exposure each skin type
needs to generate some 1,000 IU of vitamin D
without risking sunburn. For instance, a
dark-skinned individual living in Anchorage can
get that amount in 20 to 30 minutes midday in
July, Holick reports in his new book The UV
Advantage (2004, with Mark Jenkins,
ibooks). A pale person in Honolulu might do it
in 1 minute.
Severe vitamin D deficiency softens bones. In
children, the result is rickets, characterized
by malformed legs. Adults may develop a rare
condition called osteomalacia, distinguished by
weakened muscles as well as bones. Seventy-five
years ago, when the cause of rickets and
osteomalacia was first recognized, the remedy
was vitamin D–rich cod liver oil. Later, the
United States mandated that dairies fortify milk
with 100 IU of vitamin D per 8 ounces, and
rickets essentially disappeared.
However, rickets has staged a comeback in the
U.S. There is no national count, but according
to Laurence Grummer-Strawn of the Centers for
Disease Control in Atlanta, between 1997 and
1999, "5 per million Georgia children were
hospitalized with rickets due to vitamin D
deficiency." All were African American, 8 to 21
months old. Numbers could be higher in
Scientists offer several explanations for
rickets' reemergence. Vieth notes that
breast-feeding has had a revival and that
mother's milk delivers little vitamin D. And
Holick points out that doctors have been
discouraging parents from letting babies get sun
without liberal doses of sunscreen. The Food and
Nutrition Board last reviewed its vitamin D
recommendations in 1997. As part of that effort,
a panel of experts including Vieth, Holick, and
Heaney was charged to define how the vitamin
should be monitored in people. The active form
wasn't deemed suitable because it's manufactured
in the body on demand, so it doesn't directly
correlate with vitamin D intake and production.
The panel concluded that the best way to
evaluate a person's vitamin D status would be to
measure concentrations of an inactive form known
as 25-hydroxy-vitamin D (25-D) that circulates
in the blood.
However, Heaney adds, "we didn't say how much
an individual should have—because we didn't
In North America, a typical 25-D blood
concentration is 40 nanomoles per liter
(nmol/l), and scientists long assumed that
amount was adequate.
Last year, in a roundtable discussion at an
osteoporosis conference in Lausanne,
Switzerland, Vieth, Holick, Heaney, and others
agreed that an optimal 25-D blood concentration
for most people is 75 to 80 nmol/l. Most
panelists, therefore, recommended that people
strive for 800 to 1,000 IU of Vitamin D daily to
That conclusion rests on a variety of
experiments. David Hanley of the University of
Calgary in Alberta cites studies focusing on
parathyroid hormone, one of the factors
regulating the natural breakdown of bone that
constantly occurs throughout a healthy body.
When a person's 25-D concentration dips too low,
parathyroid hormone concentration in the blood
rises and triggers excessive bone loss. Hanley
says that several studies indicate that most
people need 75 to 80 nmol/l of 25-D in their
blood to protect their bones.
However, people 70 years old and older may
need more than 100 nmol/l of 25-D to hold
parathyroid hormone at healthy concentrations.
Vieth and his colleagues reported this finding,
which was based on a study of 1,700 people ages
19 to 97, in the January 2003 Journal of
Clinical Endocrinology and Metabolism.
Low 25-D concentrations may identify
apparently healthy individuals who are at risk
for type 2 diabetes as well as for bone
problems. In the May 1 American Journal of
Clinical Nutrition, Ken C. Chiu and his
colleagues at the University of California, Los
Angeles report that the lower the 25-D in study
participants, the less likely they were to
produce adequate amounts of insulin or to show
sufficient sensitivity to insulin. Chiu's team
found that increasing a person's blood
concentration of 25-D from 25 nmol/l to about 75
nmol/l would "improve insulin sensitivity by 60
percent," which is a greater increase than many
antidiabetes drugs provide.
In people over age 60, 25-D blood
concentrations correlate with leg strength,
according to studies by Bess Dawson-Hughes of
the Agriculture Department's Human Nutrition
Research Center on Aging in Boston and her
colleagues. In one study, they examined data
from 4,100 adults representing a cross-section
of the U.S. population. People with 25-D
concentrations of 40 nmol/l or less walked more
slowly and had more trouble rising from a chair
than did people with concentrations higher than
86 nmol/l. The results took into account
differences between the groups in age,
arthritis, weight, and use of a cane, according
to the team's report in the Sept. 1 American
Journal of Clinical Nutrition.
A third recent study of 25-D links low blood
concentrations to colorectal cancer in women.
Diane Feskanich of Brigham and Women's Hospital
in Boston and her coworkers compared blood tests
from 193 cancer patients with those of
age-matched women who were cancerfree. All the
women were participating in the long-running
Nurses' Health Study. In the September
Cancer Epidemiology, Biomarkers & Prevention,
the researchers report that women in the highest
25-D group—with about 100 nmol/l—had only about
half the cancer risk of women in the lowest
group, averaging 40 nmol/l.
Few people have the blood concentrations of
25-D that researchers recommend. For instance,
Hanley described findings from 200 Calgary
adults at the Experimental Biology meeting in
Washington, D.C., last April. A third of the
study's population showed less than 30 nmol/l
during at least part of the year. "The average
level of 25-D through the four seasons was in
the low 60s [nmol/l]," Hanley told Science
News. If 80 nmol/l is taken as the cutoff
for adequate 25-D, "virtually 100 percent of the
population is vitamin D–deficient at least part
of the year," he says.
In the March 2003 Nutrition Reviews,
Mona Calvo of the U.S. Food and Drug
Administration coauthored a review of five
studies on vitamin D status in Canada and the
United States. They described data indicating a
high incidence of vitamin D insufficiency in
almost all populations.
In one of those studies, Calvo notes, 42
percent of African American women were 25-D
deficient, compared with just 4 percent of their
white counterparts. That study defined
deficiency as concentrations below 37.5 nmol/l.
Calvo says that she prefers to use 80 nmol/l as
the minimum adequate blood concentration of
Some researchers propose that fortified milk
and other foods can cover vitamin D shortfalls.
However, the current diet offers, at most, 200
to 400 IU per day. Furthermore, Calvo has new
data showing that "African Americans do not
consume [vitamin-D] fortified foods." She
suspects that many blacks avoid milk, the most
highly enriched food, because they have
difficulty digesting it.
Harold L. Newmark of Rutgers University in
New Brunswick, N.J., and his colleagues propose
a new food-enrichment scheme in the Aug. 1
American Journal of Clinical Nutrition.
They argue that the best way to help vulnerable
groups get enough vitamin D would be to mandate
fortification of grain-based products, such as
wheat flour, corn meal, and pasta.
Newmark and his colleagues estimate that the
cost could be as low as 7 cents per person per
year if U.S. foods were fortified to the maximum
amount allowed by law.
They calculate that this would increase vitamin
D daily dietary intake by 50 to 200 IU.
Vieth and Holick are among the scientists who
advocate increasing "sensible" outdoor activity
so people can boost their sun exposure and thus
vitamin D supply.
The amount of sun required would pose
virtually no increased cancer risk, Holick says.
"We evolved in sunlight, and so our whole system
is dependent on some exposure to sunlight," he
says. In fact, "our health depends on it," he
Most researchers recommend that people get
much of their vitamin D intake from supplements
and recommend that they boost daily vitamin D
intakes to around 1,000 IU.
Holick says that physicians could measure
25-D in blood and prescribe increasing doses of
the vitamin until 80 nmol/l is reached. Such
personalized prescriptions could take into
account lifestyle and pigmentation. For
instance, Heaney's research in Omaha indicates
that elderly, dark-skinned women could require
up to 2,000 IU of vitamin D to keep 25-D
concentrations around 80 nmol/l.
Linda D. Meyers, director of the Food and
Nutrition Board, which sets the government's
recommended daily intake values for all vitamins
and some minerals, agrees that "it really is
time to look at those [intake standards] again
for vitamin D." The standard probably needs to
be higher, she acknowledges.
In December, the board will begin discussions
with nutrition experts on which nutrients need
to be reevaluated. Considering the wealth of
data that has been coming out, "I'm thinking
vitamin D might even offer a case study to help
us," says Meyers.
"[Vitamin D] deserves to be in the first
group reexamined," she told Science News.
"It really is time to look at that one again."