What the Research Says About Low-Fat Diets (and What Type of Fats You Should Be Eating Instead)

By Dr. Will Bulsiewicz 

In the late twentieth century, dietary fat became the villain of our plates. Grocery aisles transformed into battlegrounds lined with “low-fat” and “fat-free” products, all promising better health and slimmer waistlines. The prevailing wisdom was simple: Fat was the enemy, clogging our arteries and expanding our waistlines. Health authorities, nutritionists, and doctors rallied behind this message, urging everyone to cut fat wherever possible.

Yet amid this crusade against fat, a curious paradox emerged from the sun-drenched coasts of the Mediterranean. In countries like Italy, Greece, and Spain, people were drizzling olive oil over their salads, enjoying handfuls of nuts and fatty fish. Their diets were inherently high in fat—precisely what we were told to avoid. But instead of suffering from rampant heart disease, these populations exhibited some of the lowest rates of cardiovascular issues globally. Study after study from Denmark to the United States confirmed the protective benefits of the Mediterranean diet.

This contradiction couldn’t be ignored. Intrigued by these findings, a team of dedicated researchers in Spain decided to dive deeper. Funded by their government, they initiated the PREDIMED study—a landmark trial designed to compare the high-fat Mediterranean diet directly against a traditional low-fat diet. Their goal was to challenge the entrenched beliefs about dietary fat and provide concrete evidence on how these diets impacted heart health.

To accomplish this, they recruited 7,447 individuals aged 55 to 80 who had either type 2 diabetes or multiple cardiovascular risk factors—like smoking, being overweight, high blood pressure, abnormal blood lipids, or a family history of heart disease—none had experienced a heart attack or stroke themselves. In other words, these were people at great risk for cardiovascular disease. From 2003 to 2011, across eleven teaching hospitals, these participants were followed for an average of five years. They were randomly assigned to one of three diets: a Mediterranean diet supplemented with 4 tablespoons of extra-virgin olive oil per day, a Mediterranean diet supplemented with 30 grams of mixed nuts per day, or a low-fat diet. There were no calorie restrictions; participants could eat as much or as little as they wanted. So to summarize: 7,500 people, eleven medical centers, three diets, five years, resulting in one rockstar study that was a modern nutrition game changer.

The findings from the PREDIMED study painted a vivid picture of the Mediterranean diet’s profound impact on health compared to that of a low-fat diet. Participants who followed the Mediterranean diets, whether supplemented with EVOO or nuts, experienced a significant reduction in heart attacks, strokes, and death from cardiovascular causes compared to those who were on the low-fat diet. The Mediterranean diet literally gave them a second chance at life.

We see this on a biochemical level. Participants saw their levels of LDL cholesterol, often dubbed the “bad” cholesterol, drop lower, and their blood pressure readings improved, both critical factors in cardiovascular health. Heart failure biomarkers improved, suggesting better overall heart function. Cognition also improved, suggesting benefits that extend to brain health. This could have meaningful implications for aging populations concerned about memory and cognitive decline.

But perhaps most striking were the reductions in inflammatory markers. Inflammation is a key player in many chronic diseases, and the Mediterranean diet seems to tackle it head-on. Participants exhibited lower levels of inflammatory markers related to atherosclerosis. Specifically, C-reactive protein (CRP) levels were 35 to 45 % less, and interleukin-6 (IL-6) levels were 90 to 95% less compared to a low-fat diet. Such substantial decreases highlight the diet’s potent and anti-inflammatory effects.

Keep in mind that not all fats are created equal. The Mediterranean diet specifically pushes you to prioritize the higher-quality fats—EVOO, nuts and seeds, seafood—and to limit sources of saturated fat. Instead of fearing fat, it’s about embracing the right kinds—those that nourish the body and support overall health.

The Role of EVOO in a Mediterranean Diet

Perhaps you noticed that extra-virgin olive oil, or EVOO, is a big part of the Mediterranean diet. And this may seem confusing because there are some experts who are highly intelligent and well educated who believe that we are healthiest when we are oil-free. Many of them are my friends whom I respect and cherish.

In the PREDIMED study, the EVOO group was kicking back 4 tablespoons of EVOO daily. That’s not a small amount! That’s about 480 calories per day from EVOO alone. And remember, calories weren’t being restricted, so this was something that they added to everything else they were eating.

The fascinating part is that they did INCREDIBLY well, with a litany of benefits that people on a Mediterranean diet experienced, whether including EVOO or nuts. But there were some health benefits experienced only by those on the EVOO-supplemented diet, including lower rates of breast cancer, lower rates of type 2 diabetes, lower rates of diabetic retinopathy, lower rates of non-alcoholic fatty liver disease, and lower rates of atrial fibrillation when compared to those following the low-fat diet.

There are many paths to health, and you need to find what works for you. You can absolutely be healthy and be oil-free if that’s what you prefer. But I’ve just shown you evidence that people can be healthy with EVOO. The principle is simple: When you add EVOO, you are adding high-quality fat to your diet. One caveat, though—an extra 480 calories per day with zero fiber isn’t ideal, even if it’s from EVOO, if you are obese or trying to lose weight.

Dr. Will Bulsiewicz is a Board-Certified Gastroenterologist, Gut Health Authority, Lifestyle Medicine Pioneer, New York Times Bestselling Author, Founder of 38TERA, and author of Plant Powered Plus: Activate Your Gut-Immune Connection to Tame Chronic Inflammation, Boost Energy, and Reclaim Lifelong Health.

Excerpted from the book Plant Powered Plus: Activate Your Gut-Immune Connection to Tame Chronic Inflammation, Boost Energy, and Reclaim Lifelong Health. Copyright © 2026 by Dr. Will Bulsiewicz. Reprinted with permission.