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发帖数: 23737 | 1 【 以下文字转载自 Vegetarianism 俱乐部 】
发信人: purity (purity), 信区: Vegetarianism
标 题: 对钙片和维他命D的质疑
发信站: BBS 未名空间站 (Tue Nov 30 19:41:49 2010, 美东)
http://www.nytimes.com/2010/11/30/health/30vitamin.html?_r=1&hp
Report Questions Need for 2 Diet Supplements
By GINA KOLATA
Published: November 29, 2010
The very high levels of vitamin D that are often recommended by doctors and
testing laboratories — and can be achieved only by taking supplements —
are unnecessary and could be harmful, an expert committee says. It also
concludes that calcium supplements are not needed.
The group said most people have adequate amounts of vitamin D in their blood
supplied by their diets and natural sources like sunshine, the committee
says in a report that is to be released on Tuesday.
“For most people, taking extra calcium and vitamin D supplements is not
indicated,” said Dr. Clifford J. Rosen, a member of the panel and an
osteoporosis expert at the Maine Medical Center Research Institute.
Dr. J. Christopher Gallagher, director of the bone metabolism unit at the
Creighton University School of Medicine in Omaha, Neb., agreed, adding, “
The onus is on the people who propose extra calcium and vitamin D to show it
is safe before they push it on people.”
Over the past few years, the idea that nearly everyone needs extra calcium
and vitamin D — especially vitamin D — has swept the nation.
With calcium, adolescent girls may be the only group that is getting too
little, the panel found. Older women, on the other hand, may take too much,
putting themselves at risk for kidney stones. And there is evidence that
excess calcium can increase the risk of heart disease, the group wrote.
As for vitamin D, some prominent doctors have said that most people need
supplements or they will be at increased risk for a wide variety of
illnesses, including heart disease, cancer and autoimmune diseases.
And these days more and more people know their vitamin D levels because they
are being tested for it as part of routine physical exams.
“The number of vitamin D tests has exploded,” said Dennis Black, a
reviewer of the report who is a professor of epidemiology and biostatistics
at the University of California, San Francisco.
At the same time, vitamin D sales have soared, growing faster than those of
any supplement, according to The Nutrition Business Journal. Sales rose 82
percent from 2008 to 2009, reaching $430 million. “Everyone was hoping
vitamin D would be kind of a panacea,” Dr. Black said. The report, he added
, might quell the craze.
“I think this will have an impact on a lot of primary care providers,” he
said.
The 14-member expert committee was convened by the Institute of Medicine, an
independent nonprofit scientific body, at the request of the United States
and Canadian governments. It was asked to examine the available data —
nearly 1,000 publications — to determine how much vitamin D and calcium
people were getting, how much was needed for optimal health and how much was
too much.
The two nutrients work together for bone health.
Bone health, though, is only one of the benefits that have been attributed
to vitamin D, and there is not enough good evidence to support most other
claims, the committee said.
Some labs have started reporting levels of less than 30 nanograms of vitamin
D per milliliter of blood as a deficiency. With that as a standard, 80
percent of the population would be deemed deficient of vitamin D, Dr. Rosen
said. Most people need to take supplements to reach levels above 30
nanograms per milliliter, he added.
But, the committee concluded, a level of 20 to 30 nanograms is all that is
needed for bone health, and nearly everyone is in that range.
Vitamin D is being added to more and more foods, said Paul R. Thomas of the
Office of Dietary Supplements at the National Institutes of Health. Not only
is it in orange juice and milk, but more is being added to breakfast
cereals, and it now can be found in very high doses in supplement pills.
Most vitamin D pills, he said, used to contain no more than 1,000
international units of it. Now it is easy to find pills, even in places like
Wal-Mart, with 5,000 international units. The committee, though, said
people need only 600 international units a day.
To assess the amounts of vitamin D and calcium people are getting, the panel
looked at national data on diets. Most people, they concluded, get enough
calcium from the foods they eat, about 1,000 milligrams a day for most
adults (1,200 for women ages 51 to 70).
Vitamin D is more complicated, the group said. In general, most people are
not getting enough vitamin D from their diets, but they have enough of the
vitamin in their blood, probably because they are also making it naturally
after being out in the sun and storing it in their bodies.
The American Society for Bone and Mineral Research and other groups
applauded the report. It is “a very balanced set of recommendations,” said
Dr. Sundeep Khosla, a Mayo Clinic endocrinologist and the society’s
president.
But Andrew Shao, an executive vice president at the Council for Responsible
Nutrition, a trade group, said the panel was being overly cautious,
especially in its recommendations about vitamin D. He said there was no
convincing evidence that people were being harmed by taking supplements, and
he said higher levels of vitamin D, in particular, could be beneficial.
Such claims “are not supported by the available evidence,” the committee
wrote. They were based on studies that observed populations and concluded
that people with lower levels of the vitamin had more of various diseases.
Such studies have been misleading and most scientists agree that they cannot
determine cause and effect.
It is not clear how or why the claims for high vitamin D levels started,
medical experts say. First there were two studies, which turned out to be
incorrect, that said people needed 30 nanograms of vitamin D per milliliter
of blood, the upper end of what the committee says is a normal range. They
were followed by articles and claims and books saying much higher levels —
40 to 50 nanograms or even higher — were needed.
After reviewing the data, the committee concluded that the evidence for the
benefits of high levels of vitamin D was “inconsistent and/or conflicting
and did not demonstrate causality.”
Evidence also suggests that high levels of vitamin D can increase the risks
for fractures and the overall death rate and can raise the risk for other
diseases. While those studies are not conclusive, any risk looms large when
there is no demonstrable benefit. Those hints of risk are “challenging the
concept that ‘more is better,’ ” the committee wrote.
That is what surprised Dr. Black. “We thought that probably higher is
better,” he said.
He has changed his mind, and expects others will too: “I think this report
will make people more cautious.” |
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