Dietary fat is one of the most controversial and complicated topics in medicine. Experts have debated this topic for years, ever since we were told that a low-fat diet was important. There are enumerable questions, such as: Is a high-fat diet good for you? What about low-fat diets? If this is not enough, what type of fats should we be consuming?

There are multiple types of fats and multiple fat sources. For instance, there are saturated fats and unsaturated fats, which include monounsaturated and polyunsaturated. There are also trans fats, which are man-made. However, there are several things that we can agree on, like we need fat since the brain is made of at least 60 percent fat (1), and trans fats are downright dangerous. Trans fats are the Frankenstein of fats; anything created in a lab when it comes to fats is not a good thing.

How have we evolved in the fat wars? Originally we were told that a low-fat diet was beneficial for heart disease and weight loss (2). This started in the 1940s but gained traction in the 1960s. By the 1980s, everyone from physicians to the government to food manufacturers was exclaiming about a low-fat diet’s benefits for overall health. But did they go too far trying to make one size fit all? The answer is a resounding YES!!

There are only three macronutrients: fats, carbohydrates and protein. Declaring that one of the three needed to be reduced for everyone did not have the results we wanted or expected. Americans were getting fatter, not thinner, heart disease was not becoming rare, and we were not becoming healthier.

Some fats more equal than others

The biggest debate recently has been over the amount of fats and saturated fats. The most recent 2015-2020 Dietary Guidelines for Americans do not limit the amount of fat, but do limit the amount of saturated fat to less than 10 percent of our diet (3). Does this apply to everyone? Not necessarily. Remember, it is very difficult to apply broad rules to the whole population.

However, the most recent research suggests that foods containing pure saturated fats are not useful, may be detrimental, and at best are neutral. Meanwhile, poly- and monounsaturated fats are potentially beneficial. You will want to read about the most recent study below.

Sources of fat

Pure saturated fats generally are found in animal products, specifically dairy and all meats. The exception is fish, which contains high levels of polyunsaturated fats. Interestingly, most foods that contain predominantly unsaturated fats have saturated fat as well, though the reverse is not typically true. There are also saturated plant oils, like coconut and palm. Processed foods also have saturated fats. Potentially beneficial polyunsaturated fats include fatty fish and some nuts, seeds and soybeans, while potentially beneficial monounsaturated fats are olive oil, avocados, peanut butter, some nuts and seeds (4). Let’s look at the research.

Saturated fat

takes a dive In the ongoing battle over saturated fats, the latest research suggests that it is harmful. In recent well-respected combined observational study (The Nurses’ Health Study and Health Professional Follow-up Study), results show that replacing just 5 percent of saturated fat with poly- or monounsaturated fats results in significant reductions in all-cause mortality, 27 and 13 percent, respectively (5). There were also significant reductions in neurodegenerative diseases, which include macular degeneration, Parkinson’s, Alzheimer’s and multiple sclerosis.

However, when reduced saturated fats were replaced with refined grains, there was no difference in mortality. There were over 126,000 participants with an approximate 30-year duration. Also, the highest quintile of poly- and monounsaturated fat intake compared to lowest showed reductions in mortality that were significant, 19 and 11 percent, respectively. Not surprisingly, trans fat increased the risk of mortality by 13 percent.

The polyunsaturated fats in this study included food such as fatty fish and walnuts, while the monounsaturated fats included foods such as avocado and olive oil. Eating fish had the modest reductions in mortality, 4 percent. The authors suggest replacing saturated fats with healthy poly- and monounsaturated fats that are mostly plant-based, but not with refined grains or trans fat.

Previous study showed neutrality

This was a meta-analysis (a group of 72 heterogeneous trials, some observational and others randomized controlled trials), with results showing that saturated fats were neither harmful nor beneficial, but rather neutral (6).

However, there were significant study weaknesses. The researchers may have used foods that include both saturated fats and unsaturated fats. This is not a pure saturated fat comparison. What did those who had less saturated fat eat instead — refined grains, maybe? Also, the results in the study’s abstract partially contradicted the results in the body of the study. Thus, I would pay a lot more attention to the above study than to this one. Again, though, even the best outcomes for saturated fats in this study did not provide a beneficial effect.

What about butter?

In a meta-analysis (group of nine observational studies), results showed that butter was neither beneficial nor harmful, but rather neutral in effect (7). Then is it okay to eat butter? Not so fast! Remember, the above study showed that saturated fat was potentially harmful, and butter is pure saturated animal fat. Also, there are study weaknesses. It is not clear what participants were eating in place of butter, possibly refined grains, which would obfuscate the potential harms. It was also unclear whether there were poly- and monounsaturated fats in the diet and what effect this might have on making butter look neutral.

Unearthing a saturated fat study

In a randomized controlled trial (Minnesota Coronary Experiment), this one from 1968 to 1973 and not fully analyzed until recently, results showed that polyunsaturated fat from corn oil, compared to a diet with higher saturated fat, reduced cholesterol level while increasing the risk of mortality (8).

The researchers expected the opposite result. Is this a paradox? Fortunately, no! Corn oil is used in processed foods and has a high amount of inflammatory omega-6 fatty acids that may negate the positive results of reducing cholesterol. Plus, the patients were only consuming the corn oil for a short 15-month period, which is unlikely to be long enough to show beneficial effects on mortality.

The bottom line is this: It’s not about low-fat diets! Saturated fats have not shown any benefits, and could be potentially harmful, but at best, they are neutral. However, foods that contain high amounts of poly- or monounsaturated fats that are mostly plant-based have shown significant benefit in reducing the risk of death and neurodegenerative diseases.

However, there are several caveats. Not all unsaturated fats are beneficial. For instance, some like corn oil may contain too many omega-6 fatty acids, which could contribute to inflammation. Also, replacing saturated fats with carbohydrates, especially refined grains, does not improve health. I told you fats are not easy to understand. It can be helpful to change our perception of fats: They are not “good and bad.” Instead, think of them as “useful and useless.” For our health, we should be focused on the “useful.”

References: (1) Acta Neurol Taiwan. 2009;18(4):231-241. (2) J Hist Med Allied Sci 2008;63(2):139-177. (3) health.gov/dietaryguidelines/2015. (4) https://www.heart.org. (5) JAMA Intern Med. 2016;176(8):1134-1145. (6) Ann Intern Med. 2014;160(6):398-406. (7) PLoS ONE 11(6):e0158118. (8) BMJ 2016;353:i1246.

Dr. Dunaief is a speaker, author and local lifestyle medicine physician focusing on the integration of medicine, nutrition, fitness and stress management. For further information, visit www.medicalcompassmd.com or consult your personal physician.