送交者: xj 于 2006-2-08, 13:30:04:
回答: dietary fat ha no protective 由 xj 于 2006-2-08, 13:24:00:
http://www.nytimes.com/2006/02/08/health/08fat.html
N.Y. Times
February 8, 2006
Low-Fat Diet Does Not Cut Health Risks, Study Finds
By GINA KOLATA
The largest study ever to ask whether a low-fat diet reduces the risk of
getting cancer or heart disease has found that the diet has no effect.
The $415 million federal study involved nearly 49,000 women ages 50 to 79
who were followed for eight years. In the end, those assigned to a low-fat
diet had the same rates of breast cancer, colon cancer, heart attacks and
strokes as those who ate whatever they pleased, researchers are reporting
today.
"These studies are revolutionary," said Dr. Jules Hirsch, physician in
chief emeritus at Rockefeller University in New York City, who has spent a
lifetime studying the effects of diets on weight and health. "They should
put a stop to this era of thinking that we have all the information we
need to change the whole national diet and make everybody healthy."
The study, published in today's issue of The Journal of the American
Medical Association, was not just an ordinary study, said Dr. Michael
Thun, who directs epidemiological research for the American Cancer
Society. It was so large and so expensive, Dr. Thun said, that it was "the
Rolls-Royce of studies." As such, he added, it is likely to be the final
word.
"We usually have only one shot at a very large-scale trial on a particular
issue," he said.
The results, the study investigators agreed, do not justify recommending
low-fat diets to the public to reduce their heart disease and cancer risk.
Given the lack of benefit found in the study, many medical researchers
said that the best dietary advice, for now, was to follow federal
guidelines for healthy eating, with less saturated and trans fats, more
grains, and more fruits and vegetables.
Not everyone was convinced. Some, like Dr. Dean Ornish, a longtime
promoter of low-fat diets and president of the Preventive Medicine
Research Institute in Sausalito, Calif., said that the women did not
reduce their fat to low enough levels or eat enough fruits and vegetables,
and that the study, even at eight years, did not give the diets enough
time.
Others said that diet could still make a difference, at least with heart
disease, if people were to eat the so-called Mediterranean diet, low in
saturated fats like butter and high in oils like olive oil. The women in
the study reduced all kinds of fat.
The diets studied "had an antique patina," said Dr. Peter Libby, a
cardiologist and professor at Harvard Medical School. These days, Dr.
Libby said, most people have moved on from the idea of controlling total
fat to the idea that people should eat different kinds of fat.
But the Mediterranean diet has not been subjected to a study of this
scope, researchers said.
And Barbara V. Howard, an epidemiologist at MedStar Research Institute, a
nonprofit hospital group, and a principle investigator in the study, said
people should realize that diet alone was not enough to stay healthy.
"We are not going to reverse any of the chronic diseases in this country
by changing the composition of the diet," Dr. Howard said. "People are
always thinking it's what they ate. They are not looking at how much they
ate or that they smoke or that they are sedentary."
Except for not smoking, the advice for a healthy lifestyle is based
largely on indirect evidence, Dr. Howard said, but most medical
researchers agree that it makes sense to eat well, control weight and get
regular exercise.
That is also what the cancer society recommends. Dr. Thun, who described
the study's results as "completely null over the eight-year follow-up for
both cancers and heart disease," said his group had no plans to suggest
that low-fat diets were going to protect against cancer.
Others cautioned against being too certain that a particular diet would
markedly improve health, and said that whether someone developed a chronic
disease might not be entirely under their control -- genetics also plays a
role.
David A. Freedman, a statistician at the University of California,
Berkeley, who is not connected with the study but has written books on the
design and analysis of clinical trials, said the results should be taken
seriously.
"The studies were well designed," Dr. Freedman said, "and the
investigators tried to confirm popular hypotheses about the protective
effect of diet against three major diseases in women."
"But," he added, "the diet studied here turned out not to be protective
after all."
The study was part of the Women's Health Initiative of the National
Institutes of Health, the same program that showed that hormone therapy
after menopause might have more risks than benefits.
In this case, the study addressed a tricky problem. For decades, many
scientists have said, and many members of the public have believed, that
what people eat -- the composition of the diet -- determines how likely
they are to get a chronic disease. But that has been hard to prove.
Studies of dietary fiber and colon cancer failed to find that fiber was
protective, and studies of vitamins thought to protect against cancer
failed to show an effect.
Many cancer researchers have questioned large parts of the diet-cancer
hypothesis, but it has kept a hold on the public imagination. "Nothing
fascinates the American public so much as the notion that what you eat
rather than how much you eat affects your health," said Dr. Libby, the
Harvard professor.
The study found that women who were randomly assigned to follow a low-fat
diet ate significantly less fat over the next eight years. But they had
just as much breast and colon cancer and just as much heart disease. The
women were not trying to lose weight, and their weights remained fairly
steady. But their experiences with the diets allowed researchers to
question some popular notions about diet and obesity.
There is a common belief that Americans get fat because they eat too many
carbohydrates. The idea is that a high-carbohydrate, low-fat diet leads to
weight gain, higher insulin and blood glucose levels, and more diabetes,
even if the calories are the same as in a higher-fat diet. That did not
happen here.
Others have said the opposite: that low-fat diets enable people to lose
weight naturally. But that belief was not supported by this study.
As for heart disease risk factors, the only one affected was LDL
cholesterol, which increases heart disease risk. The levels were slightly
higher in women eating the higher-fat diet, but not high enough to make a
noticeable difference in their risk of heart disease.
Although all the study participants were women, the colon cancer and heart
disease results should also apply to men, said Dr. Jacques Rossouw, the
project officer for the Women's Health Initiative.
Dr. Rossouw said the observational studies that led to the hypothesis
about colon cancer and dietary fat included men and women. With heart
disease, he said, researchers have found that women and men respond in the
same way to dietary fat.
The most recent study follows a smaller one, reported last year, on
low-fat diets for women who had breast cancer. That study hinted that
eating less fat might help prevent a recurrence. But the current study,
asking if a low-fat diet could protect women from breast cancer in the
first place, had findings that fell short of statistical significance,
meaning they could have occurred by chance.
Dr. Rossouw said he was still intrigued by the breast cancer data, even
though it was not statistically significant. The women on low-fat diets
had a 9 percent lower rate of breast cancer; the incidence was 42 per
thousand per year in women in the low-fat diet group, compared with 45 per
thousand per year in women consuming their regular diet.
That could mean that fat in the diet may have a small effect, Dr. Rossouw
said, perhaps in some subgroups of women or over a longer period of time.
He added that the study investigators would continue to follow the women
to see if the effect became more pronounced.
While cancer researchers said they were disappointed by the results, heart
disease researchers said they were not surprised that simply reducing
total fat had no effect, because they had moved on from that hypothesis.
Of course, Dr. Libby acknowledged, the latest advice, to follow a
Mediterranean diet and get regular exercise, has never been tested in a
large randomized clinical trial. "If they did a study like that and it was
negative," he said, "then I'd have to give up my cherished hypotheses for
data."
The low-fat diet was not easy to follow, said Dr. Rowan T. Chlebowski, a
medical oncologist at Harbor-U.C.L.A. Medical Center and one of the
study's principal investigators. Women were told to aim for a diet that
had just 20 percent of its calories as fat, and most fell short.
The diet they were told to follow "is different than the way most people
eat," Dr. Chlebowski said. It meant, for example, no butter on bread, no
cream cheese on bagels, no oil in salad dressings.
"If a physician told a patient to eat less fat, that will do nothing," he
said. "If you send someone to a dietitian one time, that will do next to
nothing." The women in the study had 18 sessions in small groups with a
trained nutritionist in the first year and four sessions a year after
that.
In the first year, the women on the low-fat diets reduced the percentage
of fat in their diet to 24 percent of daily calories, and by the end of
the study their diets had 29 percent of their calories as fat. In the
first year, the women in the control group were eating 35 percent of their
calories as fat, and by the end of the study their dietary fat content was
37 percent. The two groups consumed about the same number of calories.
Some medical specialists emphasized that the study did not mean people
should abandon low-fat diets.
"What we are saying is that a modest reduction of fat and a substitution
with fruits and vegetables did not do anything for heart disease and
stroke or breast cancer or colorectal cancer," said Dr. Nanette K.
Wenger, a cardiologist and professor of medicine at Emory University
School of Medicine in Atlanta. "It doesn't say that this diet is not
beneficial."
But Dr. Freedman, the Berkeley statistician, said the overall lesson was
clear.
"We, in the scientific community, often give strong advice based on flimsy
evidence," he said. "That's why we have to do experiments."
Copyright 2006 The New York Times Company