 Boston,
MA -- Over the course of the last decade, numerous studies
have examined the relationship between the consumption
of trans fatty acids found in partially hydrogenated oils
and coronary heart disease (CHD). A comprehensive review
of the scientific evidence confirms that eating trans fatty
acids increases the risk of CHD.
The review, published in the June 24, 1999, New England
Journal of Medicine, is authored by researchers at the Harvard
School of Public Health and the Wageningen Centre for Food
Sciences in the Netherlands.
Lead author, Alberto Ascherio, said "Coronary heart
disease kills 500,000 Americans each year. According to our
estimations, if trans fats were replaced by unsaturated vegetable
oils, we would expect to see at least 30,000 fewer persons
die prematurely from CHD each year."
Trans fatty acids are found in most
margarines, in many
commercially baked goods, and in the fats used for deep-frying
in many restaurants. The commercial advantages trans fats
hold over unsaturated vegetable oils is that they are solid
at room temperature, they can remain on the shelf for a longer
time before becoming rancid, and they allow for deep-frying
at higher temperatures.
"Because of concerns that trans fatty acids increase
risk of CHD," said Ascherio. "The Food and Drug
Administration is considering new regulations for nutrition
labels that will require manufacturers to report the amount
of trans fatty acids."
Under current guidelines, a consumer who is trying to be
heart-healthy might choose a product that is labeled as being
low in cholesterol and saturated fat, but which is high in
harmful trans fats.
The researchers reviewed more than 25 metabolic and epidemiological
studies. The metabolic studies showed that trans fats have
a two-pronged harmful effect on blood cholesterol levels:
trans fats increase low-density lipoprotein cholesterol (LDL--"bad
cholesterol") and decrease high-density lipoprotein
cholesterol (HDL--"good cholesterol").
The epidemiological studies tracked
people's eating habits and examined occurrence of CHD later in their lives. These
studies found a link between consumption of trans fats and
CHD that was higher than expected from the results of the
metabolic studies. "We don't fully understand all of
the ways that trans fats increase risk of CHD," said
Ascherio, "but it seems clear that they do increase
risk."
Ascherio and colleagues urge the
food industry to replace the partially hydrogenated fats used in foods and in food
preparation with unhydrogenated oils: "Such a change
would substantially reduce the risk of coronary heart disease
at a modest cost."
Alberto Ascherio is an associate professor of nutrition
and epidemiology at the Harvard School of Public Health.
See
also: Trans Fatty Acids and Coronary Heart Disease, The
New England Journal of Medicine -- June 24, 1999 -- Vol.
340, No. 25.
For further information, please contact
Bob Brustman /
Harvard School of Public Health /
Department of Nutrition /
665 Huntington Avenue /
Boston, MA 02115 /
Phone: 617-432-3952 /
Email: brustman@hsph.harvard.edu
Harvard School of Public Health PRESS RELEASE
For immediate release: November 18, 1997.
Boston,
MA -- Harvard School of Public Health and Brigham and Women's
Hospital researchers report from the Nurses' Health Study
that it is the type of dietary fat, not total fat, that affects
coronary heart disease risk.
Saturated
fat (found in meats and dairy foods) and trans unsaturated
fat (margarine, packaged cookies, crackers, and fast foods)
increase the risk of coronary heart disease. A relatively
higher intake of polyunsaturated fat (corn or soybean oils)
and monounsaturated fat (high in olive and canola oil)
actually reduces risk. The study is reported in this week's
New England Journal of Medicine.
"Results from previous studies have been mixed concerning
a possible association between fat and risk of coronary heart
disease. This has probably occurred because some studies
have been small and did not take into account different types
of fat. Because numerous metabolic studies have strongly
suggested different fats act in different ways to affect
blood lipid levels, we were very interested in examining
the impact of different types of fat on coronary heart disease
risk," comments Frank Hu, MD, PhD, lead author on the
study and a research fellow at the Harvard School of Public
Health.
"In this large prospective study of nurses, which included
over 900 cases of heart disease, we enhanced our ability
to examine the strength of the associations between fat and
heart disease risk by obtaining repeated measurements of
fat intake," continues Dr. Hu. "We found no association
between total fat intake and coronary heart disease risk.
This probably reflects the counterbalancing of different
types of fat. The picture changes dramatically when we examine
different types of fat. Our results suggest that replacing
saturated and trans fats in the diet with poly- and monunsaturated
sources of fat is an effective way to reduce coronary heart
risk. Reducing overall fat intake is unlikely to affect heart
disease risk."
The
study also finds that Trans Fat is associated with the highest
relative risk of coronary heart disease, twice that associated
with the same intake of energy from carbohydrates.
This large effect is probably explained,
say the researchers, not only by the impact of Trans Fat
on blood lipid levels but its interference with essential
fatty-acid metabolism and ability to elevate triglyceride
levels. While both
monounsaturated and saturated fats are present in meats and
dairy foods, the potential beneficial effect of monounsaturated
fat is counterbalanced by the saturated fat in those same
food sources. Some vegetable oils, including canola and olive
oils, excellent sources of monounsaturated fat, are not yet
widely consumed by Americans.
The authors point out that the high carbohydrate diet recommended
by some heart disease prevention programs, which are intended
to lower LDL levels, also lower the "good" HDL
levels. Consequently, an alternative strategy -- changing
the composition of fats in the diet with the dual aims of
lowering LDL and raising HDL levels -- may be a better way
to lower coronary heart disease risk.
The Nurses' Health Study is an on-going
prospective study of women, age 30-55 at enrollment in
1976. The study is directed
by Frank Speizer, MD, Brigham and Women's Hospital and Harvard
Medical School. Subjects were subsequently followed every
two years answering questionnaires concerning their diet,
lifestyle and health.
For further information, please
contact: Beverly Freeman, Director
of Public Affairs /
617-432-3863, email: bfreeman@sph.harvard.edu / Frank
Hu, MD, PhD, 617-432-0113
Around the School: News and Notices of the Harvard School
of Public Health
April 30, 1999.
Frank Hu, research
associate in the Department of Nutrition, has been receiving
a lot of attention lately from the popular media. The reason for this attention is that he has been
lead author of a number of studies that have produced good
news about a popular and necessary activity: eating.
Specifically,
his work has examined the relationship between diet and heart
disease.
In November, 1998, Hu reported in
the British Medical Journal that eating nuts reduced the
risk of coronary heart disease in women. In April, his
paper in the Journal of the American Medical Society showed
that there was no link between moderate egg consumption and
heart disease. Most recently, in the May issue of the American
Journal of Clinical Nutrition, his analysis demonstrates
that linolenic acid, an omega-3 fatty acid found in some
vegetable oils and salad dressing products, may protect
against fatal heart attacks.
Hu's work has comprised a series
of collaborations with Walter Willett, Fredrick John Stare
professor of epidemiology and nutrition, and other colleagues in the Nurses Health
Study and the Health Professionals Follow-up Study. This
same group, in 1997, published an article in the New England
Journal of Medicine indicating that total fat consumption
was less important to heart disease than the type of fat
consumption.
"The problem is that 'total fat' is not a useful term," said
Hu. "There are good fats and bad fats. In the public's
mind, fat has become public enemy number one. Reducing dietary
fat has become a priority. But the truth is that if you reduce
your total fat consumption, you're also reducing the amount
of good fats that you eat--fats that have a protective effect
against heart disease."
Bad
fats are those that are frequently found in dairy, meat,
and other animal products. These are saturated fats that
have been shown to increase levels of low-density-lipoprotein
(LDL) cholesterol in the bloodstream. If the body has more
LDL cholesterol than it requires, the excess is deposited
on the walls of arteries in the form of plaque. Too much
plaque and the arteries become plugged--a condition known
as arteriosclerosis. When arteries in the heart become clogged,
it causes a heart attack. If arteries that lead to the brain
are plugged, then the result is a stroke.
Good fats, on the
other hand, are found in liquid vegetable oils. These include
monounsaturated fats and polyunsaturated fats. These fats
lower LDL cholesterol levels, resulting in lower risk of
cardiovascular disease.
Trans fats muddy the waters.
"Trans fats are vegetable
oils that are partially hydrogenated," explained Hu. "Adding
hydrogen to the oils makes them solid at room temperature,
a characteristic that makes them useful in the production
of baked goods. Products made with hydrogenated oils have
long shelf lives. "Unfortunately, trans fats are more
dangerous than saturated fats. Not only do trans fats increase
LDL cholesterol levels like saturated fats, they also reduce
levels of HDL cholesterol--the helpful cholesterol. Trans
fats do double harm."
Explicating the relationships between
types of fat and risk of heart disease has been the basis of Hu's recent work. "We
did the nut study to prove our point. Many people avoid nuts
because they're notoriously high in fats--up to 80% of the
energy in a nut comes from its fat content. Therefore, many
people assumed that eating nuts would increase risk of heart
disease. But, because nuts contain primarily unsaturated
fats, eating nuts substantially reduces risk of heart disease."
Next, Hu and his colleagues turned to eggs: "Eggs have
been perceived as unhealthy food for many years because of
their high cholesterol content. People have assumed that
egg consumption would lead to increased risk of heart disease."
Hu was not surprised by the results of the study. "Moderate
egg consumption, which we defined as one egg per day, is
not associated with increased risk of heart disease. These
results are consistent with data from previous metabolic
studies that suggested relatively small effects of dietary
cholesterol on cholesterol levels in the bloodstream," he
said. "The slight adverse effect of an egg's cholesterol
content is balanced by the beneficial contents of its other
nutrients."
The researchers did find, however, that egg consumption
is dangerous for people with diabetes, possibly because of
their altered ability to metabolize cholesterol. Moderate
egg consumption led to a 40-to-50% increased risk of heart
disease for diabetics.
Hu's next project is an examination of the relationships
between types of fat consumption and heart disease in people
with diabetes. "Previous studies have demonstrated that
monounsaturated fat has particular benefits on blood lipids
and glucose response among diabetics," said Hu. "But
the effects of monounsaturated fat on risk of heart disease
among diabetics have not been studied."
Around the School
is published weekly by the Office of Academic Communications
Harvard School of Public Health
665 Huntington Ave., 1204
Boston, Massachusetts 02115
617-432-3952
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