A large meta-analysis published today in Annals of Internal Medicine suggests that "bad" fats may not be so terrible after all—and that "good" fats may not actually be all that, well, good. Here's what you need to know.

The world of dietary heart health is thought to be divided into two groups: saturated and trans fats are bad for your heart, while most polyunsaturated and monounsaturated fats are good for your heart. Right?

Not so fast.

New data suggest that the "bad" fats may not be so terrible after all—and that "good" fats may not actually be all that, well, good. In a large meta-analysis published today in Annals of Internal Medicine, researchers failed to find the dramatic differences in health outcomes you may expect between the two fat groups.

The review of 76 trials involved more than 600,000 participants in 18 countries. It found essentially no association between saturated fat—long considered a major culprit in raising LDL or "bad" cholesterol levels—and heart disease. As for trans fats, the study found only a slight trend indicating they had a negative affect on heart health. And only a small trend indicated that "good" omega-3 fatty acids, such as those found in salmon and other fatty fish, might be helpful. Meanwhile, the study, led by Rajiv Chowdhury, MD, PhD, of the University of Cambridge in the United Kingdom, found no heart-health benefits from taking omega-3 supplements, AKA fish oil supplements.

The authors' carefully worded conclusion: Their findings “did not yield clearly supportive evidence for current cardiovascular guidelines that encourage high consumption of polyunsaturated fatty acids and low consumption of total saturated fats."

The American Heart Association (AHA) recommends that daily fat intake account for no more than 35% of total calories. Saturated fats (which are mostly from animal sources, like butter and meat) should account for no more than 5% to 6% of intake and trans fats (in prepared and fast foods, but increasingly being phased out) should be limited to less than 1%. Monounsaturated and polyunsaturated fats from nuts, seeds, oily fish, and vegetable oils should make up the remainder. Guidelines from other prominent health and government organizations are similar.

This Annals study is only the latest installment of questions emerging about the validity of these guidelines. But even the authors of the new study don't recommend ditching current recommendations, and the findings may not be as radical as they first seem.

"This evidence is suggestive of what we've always thought," says Karol Watson, MD, PhD, professor of medicine at UCLA and director of the UCLA Barbra Streisand Women’s Heart Health Program. "The direction of change with omega-3 showed a little benefit. The direction of change with trans fat looked like there's harm. Saturated fat was kind of a wash. It didn't look like a lot of harm or benefit in either direction."

Add to the equation the fact that not all saturated fats are created equal (dairy saturated fat may not be as diabolical as pot-roast saturated fat, for instance) and you have a picture that's "muddy as heck," says Dr. Watson.

Dietary studies are notoriously difficult to conduct well, with this one being no exception. "This meta-analysis should be interpreted with caution," says Suzanne Steinbaum, DO, a preventive cardiologist at Lenox Hill Hospital in New York City. There were only a few trials investigating omega-6 polyunsaturated fats (which are found in soybean and corn oils and many processed foods), trials differed in design, and people may have underestimated their self-reported consumption of different types of fat, she says.

"I don't think there's any clear evidence that we should abandon our limitations on saturated fat," Dr. Watson says. "So much good science has gone into making those recommendations—we shouldn't abandon them so easily."

And consider this point: "This paper does not say saturated fat is good for you. This should not affect guidelines or behavior," adds Linda Van Horn, PhD, RD, professor of preventive medicine at Northwestern University Feinberg School of Medicine in Chicago and an AHA spokesperson.

"This paper does not change the message that the AHA is making regarding reducing saturated fat, reducing trans fat, and emphasizing the fruits, vegetables, whole grains, and fish intake that we know are especially beneficial to heart health,” Dr. Van Horn says. Instead, the study may help us focus more specifically on which saturated fats may be hurting us and which unsaturated fats may be beneficial, she says.

According to the AHA, the death rate from heart disease, although still alarmingly high, has dropped 39% over the past 10 years. Statins and other treatment improvements have a lot to do with the decrease, but no doubt so does diet. Overall fat intake has declined from 42% in the 1970s to about 32% today, while saturated fat intake has dropped from 20% to 11%, says Dr. Van Horn.