Which Fats Are the "Bad" Fats?

Replacing saturated fats with unsaturated fats may not reduce heart disease risk

/ Author:  / Reviewed by: Robert Carlson, M.D

(RxWiki News) Standard advice related to fats in your diet has generally ranked unsaturated fats as healthier than saturated fats when it comes to preventing heart disease. But that advice may be changing.

A recent analysis of an older study has found that the evidence may not support the idea of replacing saturated fats with a common unsaturated fat. Saturated fats are generally found in animal fats.

In fact, the study found a higher risk of death in patients with heart disease if they had eaten more omega-6 unsaturated fats. These omega-6 fats are the ones found in vegetables and nuts.

"Ask your doctor about fat consumption."

The study, led by Christopher E. Ramsden, MD, of the National Institute on Alcohol Abuse and Alcoholism and the School of Medicine at the University of North Carolina at Chapel Hill, involved analyzing data from a trial done in 1966-73.

This trial involved 458 men, aged 30 to 59, who had experienced a recent heart-related condition or event. A total of 86 percent had had heart attacks, and 14 percent been diagnosed with angina (chest pain) or acute coronary insufficiency, in which the arteries are blocked.

Previously, missing data in the study had prevented a good analysis. However, Dr. Ramsden and his colleagues were able to recover the missing data and analyze it to look at the death rates among the men.

The researchers looked at how many of the men died from any cause, how many died from heart disease and how many died from another heart problem.

One group of 221 men were told to reduce their total calories from saturated fats to less than 10 percent of their total diet. These are the fats found in animals, margarines and shortenings.

They were also told to increase their linoleic acid intake to 15 percent of their total calories. Linoleic acid is an unsaturated omega-6 fatty acid. It's found in vegetable oils, nuts and egg yolks. It is different from another kind of unsaturated fat called omega-3 fatty acids.

The other group of 237 men were not told to change their diets at all. The two groups were otherwise similar in terms of their demographics, diets and heart-related risks.

The group that increased their omega-6 fats had an overall death rate of 17.6 percent, compared to 11.8 percent in the control group that did not change their diet. There was a 5 percent chance this difference in rates could be due to chance and unrelated to dietary differences.

The group that increased their omega-6 fats died from cardiovascular disease at a rate of 17.2 percent, compared to 11 percent among those with no changes.

The group that changed their fat intake also died from coronary heart disease at a rate of 16.3 percent, compared to 10.1 percent among those who did not change their diet.

There was a 4 percent chance the differences in heart-related deaths were unrelated to the dietary changes. This means the researchers calculated that they were 96 percent sure that the differences in death rates were related to the changes the one group made in their fat intakes.

However, 96 percent is actually a somewhat low rate of certainty in just one study, so more research is necessary to find out whether increasing omega-6 intake and/or decreasing saturated fat intake really makes any difference to heart health.

Currently, most countries' dietary guidelines recommend that people eat more polyunsaturated fats instead of saturated fats to reduce the risk of heart disease. However, there is not good evidence to support this recommendation.

In this study, the researchers found evidence that the opposite might be true, or at the least, that it may not make a difference what the ratio of saturated and omega-6 unsaturated fats in a diet is.

This study also did not address the role of omega-3 unsaturated fats, which are different from omega-6 and thought to be the healthiest of fats. Omega-3 fats are found primarily in oily fish. A smaller amount is found in eggs, lean red meat and turkey.

Sarah Samaan, MD, a cardiologist with Legacy Heart Center in Dallas-Fort Worth and a dailyRx expert, said this study is particularly intriguing because the subjects' diets were so closely managed.

"That makes it especially valuable in helping us to understand  the effects of dietary changes," Dr. Samaan said. She said the study brings up the issue of the ratio of omega-6 to omega-3 fats in our diets.

"The optimal omega-6 to omega-3 ratio is considered by many experts to be 5:1 or less, meaning that you should get no more than 5 times as much omega-6 as omega-3," she said. "However, the average American diet gives a ratio of 14:1 to 20:1."

She said that high doses of omega-6 fats can contribute to inflammation, which increases a person's risk of heart disease and possibly diabetes.

However, she pointed out that another aspect of the study could have contributed to the results: the amount of trans fats that are in margarine. Trans fats can increase a person's heart disease risk, and the participants may have eaten more margarine to increase their omega-6 intake.

"As little as 6 grams of trans fats per day can raise the risk of heart disease by 39 percent," Dr. Samaan said, adding that margarine contains little saturated fat. "When this study was done, the negative impact of trans fats was not understood. While today's margarines contain less trans fat, that was not the case at the time this study was designed."

The study was published February 5 in the journal BMJ. The research was funded by the Life Insurance Medical Research Fund of Australia and New Zealand and the Intramural Program of the National Institute on Alcohol Abuse and Alcoholism at the National Institutes of Health. The authors declared no conflicts of interest.

Reviewed by: 
Review Date: 
February 4, 2013
Last Updated:
August 19, 2013