So you have substituted some unhealthy fat you eat for more healthy versions. You’ve substituted butter for olive oil, and you’ve opted for avocado instead of mayonnaise. But did you know that excess consumption of so-called good fats can potentially lead to fatty liver disease, a risk factor for metabolic disorders like Type 2 diabetes and hypertension, according to a new study by scientists at UC San Francisco?

The finding conducted on mice surprised researchers, who anticipated that unhealthy saturated fat to cause more fat buildup in livers than healthy fats found in olive oil and avocados.

Caroline Duwaerts, an associate research biologist at the UCSF Liver Center noted: “The belief in the field for quite some time has been that saturated fat is bad for the liver,” The findings of her and her team were published in the Cellular and Molecular Gastroenterology and Hepatology.

Instead, they discovered that a diet high in monounsaturated fat, when combined with high starch content, resulted in the most severe cases of fatty liver disease. The finding emphasizes how simply counting calories does ensure that a diet is a healthy one.

“A calorie is not simply a calorie,” noted Duwaerts. “What that calorie is made up of is extremely important.”

It’s noteworthy that the study demonstrates that monounsaturated fat may undermine a person’s metabolism through mechanisms not yet well understood.

Researchers paired saturated or monounsaturated fat, with a carbohydrates, sucrose and or starch, to form four different high-calorie diets. All the plans were approximately 40 percent carbohydrate, 40 percent fat, and 20 percent protein. Such a diet resembles the average American diet. The four types of experimental diets were fed to four groups of 10 mice for six months. Afterward, the health of the mice was compared to mice that ate regular mouse food with a much lower fat content.

Not surprising, all the mice on the experimental diets, became obese by the end of six months and all of the rodents developed some degree of fatty liver. But what was a surprise was that the mice on the starch-monounsaturated fat diet were diagnosed with the most severe disease cases of fatty liver. For now, it’s unclear why the pairing of starch and monounsaturated fat seems to exacerbate fatty liver.

As a consequence of the study, scientists warn that the typical consumption of monounsaturated fat may already be excessive.

Duwaerts wants to remind people that a drizzle of olive oil on your salad is acceptable, but excess may lead to harm.”