22 Scientific American, September 2019THE SCIENCE
OF HEALTHClaudia Wallis is an award-winning science journalist whose
work has appeared in the New York Times, Time, Fortune and the
New Republic. She was science editor at Time and managing editor
of IY_[dj_ÒY7c[h_YWdC_dZ$Illustration by Celia KrampienToo Much of
a Good Thing
Gulping vitamins can have surprising risks
By Claudia Wallis
More than half of American adults take vitamin pills. I’ve watched
in wonder as some of my more health-conscious friends kick off
their morning with an impressive array of multicolored supple-
ments: A, C, D, calcium, magnesium, you name it. And it’s not just
my friends: data from the National Health and Nutrition Ex am-
ination Survey (NHANES) indicate a trend away from all-in-one
multivitamins and toward specific supplements—especially fish
oil and vitamin D. Most of this is self-prescribed. Ac cord ing to a
2016 analysis of the NHANES data, less than a quarter of supple-
ments are taken at the recommendation of a health professional.
Most of this nutritional enthusiasm does no harm—apart from
the budgetary kind—and for those with inadequate diets or spe-
cial health concerns, supplements can do a world of good. But it
is wise to keep in mind that doses that far exceed the recom-
mended dietary allowances (RDAs) set by the Institute of Medi-
cine can be hazardous. A reminder comes from a recent study
linking excessive B vitamins to a heightened risk of hip fracture.
The study, published in May, combed through the vitamin
habits of nearly 76,000 postmenopausal women participating in
the decades-long Nurses’ Health Study. Lo and behold, those who
took high doses of vitamin B 6 (35 milligrams or more daily), to -
gether with B 12 (20 micrograms or more), had a nearly 50 percentgreater risk of fracturing their hip than those taking low doses or
none. High doses of B 6 alone also raised the risk. The study con-
firms similar findings in a large Norwegian trial, published in
2017, that looked at whether these vitamins and folic acid could
reduce heart attacks and strokes in patients with narrowed blood
vessels. Alas, they did not, and to the great surprise of research-
ers, high doses were linked to hip fractures. Taken together, “the
results are quite convincing,” says Haakon Meyer, a professor of
epidemiology and preventive medicine at the University of Oslo
and an author of both studies.
Why these vitamins would have such an effect is not clear. Mey-
er suggests two possible pathways. Too much B 6 can be toxic to
the nervous system, raising the chances of falling and cracking a
hip. The nurses on high doses took 20 to 30 times the RDA, he
notes. “Traditionally, we thought the doses needed to get these
adverse effects would be much higher, but we don’t know for sure.”
Another possibility is that B 6 competes with estrogen in binding
to steroid receptors, compromising the hormone’s role in bone
health. Both ideas, he says, would require more evidence.
The B vitamin findings are reminiscent of a discovery made
some 20 years ago that linked excessive vitamin A (retinol) with
hip fractures. This research, published in 2002, also relied on the
trove of data from the Nurses’ Health Study. Walter Willett, a pro-
fessor of nutrition and epidemiology at the Harvard T. H. Chan
School of Public Health, cites the response from the vitamin
industry as “a situation where things worked right.” Manufactur-
ers quickly reduced the amount of vitamin A in their multivita-
mins, he says, “and without most people being aware of it, a huge
number of hip fractures were prevented.” Willett, who is a co-
author with Meyer of the new study, suggests that something sim-
ilar may be in order for B 6 and possibly B 12 and that an expert
panel review would be a sensible next step. “We might deal with
most of the problem just by bringing down the level of B 6 [in sup-
plements] to the RDA level,” he explains. B 12 is a more complex
matter, however. Ten to 30 percent of adults older than age 50
need extra B 12 be cause of poor absorption. Meyer points out that
excess B 12 alone does not seem to raise the risk of fracture.
The bottom line is that although vitamins and minerals are es -
sen tial for health, more is not necessarily better. Research shows,
for example, that taking large amounts of beta carotene (a vitamin
A precursor) seems to accelerate lung cancer in smokers, even
though the nutrient may have anticancer properties in other con-
texts. Like everything in nutrition, vitamins are complicated. Just
consider the fact that B 6 plays a role in more than 100 different
en zyme reactions. Perhaps because of that complexity, many
seemingly logical uses of vitamins yield disappointing results. For
in stance, even though low blood levels of vitamin D correlate with
greater risk of heart attacks and strokes, taking D supplements
generally does not help, according to a 2019 analysis.
Vitamins are vital when your diet is deficient. Willett thinks a
daily multivitamin is a sensible insurance policy. The irony, he
observes, is that the people most apt to take lots of supplements
are educated folks with a healthy diet—in other words, those who
need them the least.