Just about every diet recommends reducing saturated fats, such as butter and cheese, and replacing them with unsaturated fats, such as vegetable oil and fatty fish. The 2015 dietary guidelines advise limiting saturated fats to no more than 10% of daily calories.
There have been only a few studies comparing these two types of fat head-to-head between groups of people eating otherwise similar diets.
In the new study
, researchers revisited data from a trial carried out from 1968 to 1973 that involved more than 9,000 adults living in a nursing home or mental hospitals in Minnesota. About half the participants received diets that contained 18.5% saturated fat and 5% unsaturated fat, based on total calories, and the other half got diets that were 9% saturated fat and 13% unsaturated fat. In the unsaturated fat diet, the researchers increased the level of linolenic acid, an omega-6 polyunsaturated fat, found in margarine and oils such as corn and soybean.
Researchers found, not surprisingly, that the group that got the diet rich in linolenic acid had lower cholesterol, by about 13%, than the saturated fat diet group. For this analysis, the researchers only included the 2,403 participants who were on one of the two diets for at least a year.
What was surprising was that lower cholesterol was actually associated with an increased risk of dying during the study period, and this was true for both the saturated fat and unsaturated fat diet groups.
Generally, a total cholesterol level above 200 mg/dL
is considered high and could increase the risk for heart disease. However the opposite effect was seen in this study; every decrease in total cholesterol by 30 mg/dL was associated with a 22% increase in death rate. A total of 517 of the 2,403 participants died during the follow-up period of the study, which was three years on average.
"This study is less of one showing harm (associated with a diet rich in unsaturated fat), and more of one saying it is surprising how little evidence there is and what there is does not show benefit," said Dr. Christopher E. Ramsden, a medical investigator at the National Institutes of Health and lead author of the study
, which was published Tuesday in the journal the BMJ.
No reason to change the dietary guidelines
"The results should not alter current dietary guidelines that emphasize healthy sources of polyunsaturated fats such as olive oil and other vegetables, nuts, seeds, avocado, fish and seafood in the context of a healthy dietary pattern," said Dr. Frank Hu, professor of nutrition and epidemiology at Harvard T.H. Chan School of Public Health and one of the committee members
that prepared the advisory report for the 2015 dietary guidelines. These guidelines describe three healthy dietary patterns
-- American, Mediterranean and vegetarian -- that consumers can choose from.
The current study has several flaws that keep it from being a strong case against polyunsaturated fats, Hu said. For one, the average amount of time people were on the diet was just over a year, which he said is not long enough to have an effect on heart health.
For another, a "cholesterol paradox" could be at play here, Hu said, because people who are the most frail or unhealthy tend to have lower cholesterol, as well as decreases in blood pressure and body weight. Frailty or other health conditions could have thus explained the association with higher death rates, and not the lower cholesterol and unsaturated fat diet.
Other research supports the heart benefits of unsaturated fats, including a 1979 study
of 676 middle-aged men in mental health hospitals in Finland. One hospital served diets rich in vegetable oils whereas the control hospital served diets rich in milk and butter. After six years on the diets, the men in the unsaturated fat group had lower cholesterol as well as 50% lower rates of heart disease and death.
In addition, an analysis of eight randomized controlled trials
, including the Minnesota and the Finnish studies, concluded that consuming polyunsaturated fats, including omega-3 and omega-6 fats, instead of saturated fats reduced the overall rate of heart disease by 19%.
However, Ramsden and his colleagues carried out their own analysis of randomized controlled trials as part of the new study and arrived at a different conclusion. They found neither a significant benefit nor a significant harm to replacing saturated fats with polyunsaturated fats. Their analysis only looked at five trials and did not include the Finnish study because it separated the participants into different diets based on their hospital, Ramsden said.
The type of unsaturated fat matters
There could be several reasons why the unsaturated fat diet in the study might not really be responsible for the increased risk of death, including the cholesterol paradox, according to Sonya Angelone, owner of a nutrition consulting firm in the San Francisco Bay Area and a spokesperson for the Academy of Nutrition and Dietetics.
However if the unsaturated fat diet really did contribute to heart disease and death, in the study, it could be because of the type of unsaturated fat the participants were eating. The diet in the study ratcheted up the level of omega-6s but did not increase the level of omega-3s, found primarily in fatty fish but also avocados, nuts and their oils.
Angelone recommends people reduce saturated fats in their diet and also try to replace them with omega-3 unsaturated fats.
For example, do not swap butter for margarine, as the researchers did in the 1968 Minnesota study. Use liquid oils, and specifically olive oil, walnut oil and avocado oil, which all contain omega-3s, instead of corn oil and seed-based oils, such as sunflower and safflower, which contain omega-6s.
"Not all unsaturated fats are the same," Angelone said. Although Angelone agrees with the dietary guidelines to opt for oils instead of saturated fats, it is also important to remember to mix up the oils you use and try to consume olive and nut oils as much as possible.