Doctors have told patients for decades to eat less saturated fat – the predominant kind in red meat, butter and cheese -- as a way to prevent heart disease.
The truth may be more complicated.
A report published Tuesday concludes that people who cut out saturated fat don't necessarily lower their heart disease risk.
That's not because butter is healthy.
It's just that people who reduce their intake of fat usually end up eating more sugar, white flour and empty calories, said co-author Russell de Souza, an assistant professor and registered dietitian at McMaster University in Ontario, Canada. His review, which examined 73 earlier studies, was published in the BMJ, formerly known as the British Medical Journal.
Reducing saturated fat does reduce the risk of heart attacks, however, if people replace their milk shakes and cheese burgers with healthier foods, such as whole grains, fruits and vegetables, or with polyunsaturated fats, which are found in olive oil, sunflower seeds and other foods, the analysis found.
"It's not really meaningful to say that saturated fats are good or bad," said Frank Hu, a professor of nutrition and epidemiology at the Harvard School of Public Health, who wasn't involved in the new study. "If you replace saturated fat with cookies, white bread and bagels, it's not going to do you any good. It's important to talk about the trade-offs."
The Mediterranean diet, with some of the strongest proven benefits for the heart, actually has relatively high fat content, said David Ludwig, director of the New Balance Foundation Obesity Prevention Center at Boston Children's Hospital. This diet includes fruits, nuts, vegetables, whole grains, olive oil and moderate amounts of wine, with small amounts of poultry and fish, but little red meat, dairy or sweets.
Doctors tested the the Mediterranean diet in 7,500 people at high risk for heart disease, who were allowed to consume up to 40% of their daily calories from fat. In less than five years, those randomly assigned to follow the Mediterranean diet reduced their risk of heart attack, stroke or heart disease-related death by about 30%, compared to people who were told to follow a low-fat diet.
Dietitian Bonnie Taub-Dix notes that fats, although high in calories, have benefits. Fats make people feel fuller and help to stabilize blood sugar, said Taub-Dix, author of Read It Before You Eat It.
Many vitamin-rich, healthy foods – such as salmon and nuts -- contain a mix of saturated and unsaturated fats, Ludwig said. And foods with high levels of saturated fat, such as whole milk yogurt, can be very healthy.
For years, federal dietary guidelines advised Americans to limit their total fat intake to no more than 30% or 35% of daily calories. Yet today, two-thirds of Americans are overweight and one-third are obese.
"The intense focus on reducing saturated fat over all other components of the diet has not taken us where we want to go," Ludwig said.
This year, for the first time, the expert panel that advises the government on nutrition opted not to set a specific limit to the total amount of fat in the diet. The Department of Health and Human Services and the Department of Agriculture are scheduled to release their final report, the Dietary Guidelines for Americans, later this year.
But the government still hasn't completely caught up with the science, Ludwig said.
The National Institute of Health's We Can! program, which aims to help children maintain a healthy weight, steers families toward low-fat foods. The program lists diet soda and ketchup as foods to eat "almost anytime," but says low-fat milk should be consumed only "sometimes." The NIH program puts whole milk in the most restricted category as cookies, doughnuts and French fries, to be consumed only "once in a while."
But some saturated fats are clearly better than others, Taub-Dix said. She urges people to choose "fats with benefits," such as those found in almonds, salmon or avocados. While these foods contain a mix of saturated and unsaturated fats, studies also show they're full of vitamins and good for the heart.
"Maybe the butter is fat is not as bad for you as we thought, but is it good for you?" Taub-Dix asked.
Instead of focusing on individual nutrients, physician Dean Ornish said people should strive for healthy patterns of eating, aiming for "whole foods" – instead of processed ones – with lots of plant foods and "good carbs," such as brown rice instead of white, and whole wheat rather than white flour.
Ornish notes there are good reasons to go easy on red and processed meats, beyond their saturated fat content. Both are linked to an increased risk of some cancers.
When it comes to heart health, Taub-Dix said moderation is key. "The French eat a lot of saturated fat, but they don't eat enormous portions and have candy bars as side dishes," she said.
Rather than trying the latest fad diet, Taub-Dix said people who want to lose weight and protect their hearts should focus on the basics: "Watch your portion size. Move your body. Eat fats, but healthy ones. It's such boring information, but it actually works."
In a related topic, the new BMJ analysis found no benefits to industrial trans fats, found in the partially hydrogenated vegetable oils still used in some microwave popcorns, canned frostings and other foods, the new report said. While consumption of industrial trans fats are associated with a 34% higher risk of death, naturally occurring ones – such as those found in trace quantities in beef – are not, the study found.
The Food and Drug Administration announced in June that it will phase out partially hydrogenated oils from food over three years.