Ordinary stick margarine and anything baked or fried with shortening and other kinds of hardened vegetable oil appear to be the worst foods of all for the heart.
A large new study offers the strongest evidence yet that something called trans fat, which is a primary ingredient of standard stick margarine and shortening, is an especially unhealthy part of the diet.
The mounting mass of scientific data contradicts a generation of advice that switching from butter to stick margarine is a healthful thing to do. On the contrary, the latest study suggests that ordinary stick margarine - though probably not the newer low-fat spreads - is even worse for the heart than butter. However, both should be avoided.
“The worst type of fat appears to be trans fat,” said Dr. Walter Willett of the Harvard School of Public Health. “That’s still unknown to most consumers.”
Most of the trans fat that people eat is made through a process called hydrogenation, in which vegetable oil is altered so it hardens and resists spoiling. Usually these foods list “partially hydrogenated” oil on the label.
Regular stick margarine is typically about 17 percent trans fat. Most of the trans fat that people eat, though, is hidden. The biggest sources in the diet are cookies, crackers and other commercial baked goods as well as french fries and other deep-fried food.
Willett’s research, called the Nurses’ Health Study, was published in today’s issue of the New England Journal of Medicine. It was based on 80,082 female nurses who were outwardly healthy when they filled out dietary questionnaires in 1980. Over the next 14 years, 939 had heart attacks or died from heart disease.
The researchers first raised concerns about trans fat in a report published in the journal Lancet in 1993. The team now adds an additional six years of follow-up and reaches similar though statistically stronger conclusions.
Researchers calculated that people could reduce their risk of heart disease by 53 percent if they replaced 2 percent of their caloric intake from trans fat with calories from unhydrogenated, unsaturated fats. This would require virtually eliminating trans fat from the typical diet.
They could also reduce their risk by 42 percent if they replaced 5 percent of calories from saturated fat with calories from unsaturated fats.
The study also challenges the wisdom that the less total fat people eat, the better off they are. Instead, people might be better off forgoing some carbohydrates and instead substituting monounsaturated or polyunsaturated fats. This could mean putting olive oil on a salad but eating it without bread to save the carbohydrate calories.
Whether fats harm or help the heart depends on what they do to blood levels of the two main kinds of cholesterol, HDL and LDL. Raising HDL is good because this protects against heart disease, while raising LDL is bad, because this increases risk. Trans fat appears to be especially harmful because it raises the bad LDL while it lowers the good HDL. Saturated fat is also dangerous - but not quite so much as trans - because it raises both HDL and LDL.