Harvard review of evidence verifies that eating trans fats increases risk of heart disease
Harvard School of Public Health PRESS RELEASE
For immediate release: Wednesday, June 23, 1999.
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.
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Study shows how different types of dietary fat affect coronary heart disease risk
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.
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Nutrition Researcher Frank Hu: Fat Quality More Important Than Quantity
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.”
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