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Trans fats significantly increase the risk of heart attack and death

But saturated fats don't

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Since the 1970s, people have been given dietary advice about avoiding fats — and it’s wrong, a new analysis published in the British Medical Journal suggests.

Specifically, the meta-analysis found that a class of fats found in cheese and meats — called saturated fats — aren't linked to type 2 diabetes, stroke, heart disease, or an increased risk of early death. However, a diet high in trans fats — the kind of fat that's often added into packaged snack foods or found in margarine — can significantly increase a person's risk for heart disease and early death.

Trans fats significantly increase the risk for coronary heart disease

These findings contradict most of what was said about fat consumption in the '70s and '80s — a time when margarine, which contains trans fats, was thought to be a lot healthier than regular butter. This may have an effect on current nutritional guidelines as well: they suggest that saturated fats shouldn't make up more than 10 percent of a person's diet. The idea motivating that cap is limiting heart disease and stroke risks.

The researchers don’t want to change those guidelines just yet, though. "Our review of the observational evidence does not support calls to increase in the allowable amount of saturated fat in the diet at this time," says Russell de Souza, an epidemiologist at McMaster University in Canada and a co-author of the study. That’s because of limitations with the study design. What the study does definitively say, though, is that trans fats definitely do increase the risk of heart disease and death.

This doesn't "support calls to increase in the allowable amount of saturated fats."

Consuming artificial trans fats is associated with a 30 percent increase in a person's risk for coronary heart disease events like heart attacks. In addition, a diet high in artificial trans fats is linked to an 18 percent increase in a person's chance of dying from the effects of coronary heart disease. On the flip side, eating higher amounts of saturated fats isn't associated with heart disease, type 2 diabetes, stroke, or an early death.

The negative effects of trans fats aren’t a tremendous shock. Previous research found that trans fats can increase the risk of heart disease; it might even affect memory. In response, the Food and Drug Administration announced in January that it was setting a 2018 deadline for the removal of artificial trans fats from the American food supply, because banning trans fats would "prevent thousands of fatal heart attacks every year." Removing trans fats, which will cost about $6 billion, would save around $140 billion in health care and other costs, the FDA said.

The FDA wants to ban artificial trans fats

Today's analysis used data from multiple studies conducted in about a dozen countries, including the US. Scientists often review previous studies together to help minimize bias in results, a process called meta-analysis. But today’s work couldn’t answer all the questions researchers had: there wasn’t enough data to determine whether there’s an increased risk of type 2 diabetes from trans fats, because the question hasn't been studied enough.

There's another limitation, too. Saturated fats are a class of compounds; some researchers think it's possible that some saturated fats are healthier than others. Because the review looked at the bulk effect of saturated fat consumption, it's hard to know certain individual sources might be harmful.

The take-home message is that fats — saturated fats, especially — aren't as bad as nutritionists once thought. And while that doesn't mean that bacon should be served at every meal, it does show that a little butter now and then won't give you a heart attack.