A new mice study indicates that liberal consumption of so-called good fats, like those found in olive oil and avocados, may lead to fatty liver disease, a risk factor for metabolic disorders like type 2 diabetes and hypertension.
"The belief in the field for quite some time has been that saturated fat is bad for the liver," Caroline C. Duwaerts, a specialist at the University of California, San Francisco, Liver Center and first author of the study, published in Cellular and Molecular Gastroenterology and Hepatology.
Duwaerts and her colleagues set out to study the role of different nutrients in the development of fatty liver disease, an ongoing project in the lab of Jacquelyn Maher, director of the Liver Center. They paired a fat, saturated or monounsaturated, with a carbohydrate, sucrose or starch, to create four different high-calorie diets. The diets were roughly 40 percent carbohydrate, 40 percent fat, and 20 percent protein by calorie, a ratio on par with the average American diet.
Four groups of 10 mice were fed the experimental diets for six months and compared to mice fed regular mouse chow, which is much lower in fat.
As a result, all the mice on the experimental diets, free to eat as much as they wanted, grew obese by the end of six months, and all developed some degree of fatty liver. To the researchers' surprise, the mice on the starch-monounsaturated fat diet had the most severe disease, accumulating 40 percent more liver fat than mice on the other three diets. Their livers swelled with extra weight and, when seen under a microscope, appeared crowded with globules of fat.
In addition, the researchers noticed that these mice lost fat around their testes, an area of their bodies normally used for fat storage. When they examined this visceral fat tissue, they saw an unusual degree of fat cell death and signs of inflammation. Perhaps the starch-monounsaturated fat diet somehow induces the fat from these areas to be shuttled into the liver at an abnormally high rate, fattening the liver.
Acknowledging that it's unclear why the pairing of starch and monounsaturated fat seems to exacerbate fatty liver, Duwaerts plans to further study how this transfer of fat is affected by diet.
"A calorie is not simply a calorie," she was quoted as saying in a news release from UCSF. "What that calorie is made up of is extremely important."
A drizzle of olive oil on your salad is fine, but a daily habit of pasta drenched in olive oil could be cause for concern, according to Duwaerts, who believes it's all a matter of proportion. "I always think back to what I was told as a child," she said. "Everything in moderation."