A new study in mice finds that ursolic acid, a compound naturally present in apple peel, partially protected the animals against obesity and some of its harmful effects such as pre-diabetes and fatty liver disease.

Lead researcher Christopher Adams, of the University of Iowa (UI) in the United States, and colleagues, said although they found ursolic acid increased skeletal muscle and brown fat in the mice, which in turn led to increased calorie burning, they didn’t investigate the underlying biology, and so can’t say if the same would happen in humans.

The team writes about the findings in a paper published online in PLoS ONE on 20 June.

In their background information, they describe how they recently found “ursolic acid increases skeletal muscle Akt activity and stimulates muscle growth in non-obese mice”.

Akt is a protein that plays an important role in a number of cell processes, including burning glucose to make energy.

Muscle is an efficient burner of calories, so this might explain the reduced obesity in the mice treated with ursolic acid, suggest the researchers.

In a statement to the press, Adams, who is associate professor of internal medicine and a Faculty Scholar at the Fraternal Order of Eagles Diabetes Research Center at the UI, said this kind of research is important because it helps to develop ways to treat muscle wasting.

“In this study, we tested ursolic acid in mice on a high-fat diet – a mouse model of obesity and metabolic syndrome. Once again, ursolic acid increased skeletal muscle,” said Adams, adding that “Interestingly, it also reduced obesity, pre-diabetes, and fatty liver disease.”

Adams said they were surprised to find the compound also increased brown fat, another type of tissue that is very good at burning calories. This suggests another reason why ursolic acid may have helped protect the mice from obesity.

For the study, the team fed two groups of mice on the same high-fat diet for several weeks, except that one group was also fed ursolic acid supplement.

They found the mice whose diet included ursolic acid actually ate more food, but gained less weight and their blood sugar stayed at near normal levels, even though they were no more physically active than the non-supplement group.

There was also no sign of fatty-liver disease in the ursolic acid group. Fatty liver, commonly associated with obesity, is an untreatable condition that currently affects about 1 in 5 American adults.

When they examined the mice, the researchers found that the ursolic acid mice had more skeletal muscle, which increased their strength and endurance, and they also had more brown fat, than the mice in the non-supplement group.

And when they investigated the energy that the mice had used, they found the ursolic acid group had burned more calories than the non-supplement group.

The researchers conclude:

“These data support a model in which ursolic acid reduces obesity, glucose intolerance and fatty liver disease by increasing skeletal muscle and brown fat, and suggest ursolic acid as a potential therapeutic approach for obesity and obesity-related illness.”

It was only recently, due to advances in imaging technology, that scientists discovered human adults have brown fat, deposited in small amounts in the neck and between the shoulder blades.

Recent studies have linked higher levels of brown fat to reduced levels of obesity, and healthier levels of blood fat and sugar, which has led some to suggest perhaps it might also prevent obesity and diabetes.

Previously it was thought only babies had brown fat and it gradually disappeared through childhood.

Adams said:

“Brown fat is beneficial and people are trying to figure out ways to increase it.”

“At this point, we don’t know how ursolic acid increases brown fat, or if it increases brown fat in healthy mice. And, most importantly, we don’t know if ursolic acid will benefit people. Our next step is to determine if ursolic acid can help patients,” he added.

Funds for the research came from UI’s Fraternal Order of Eagles Diabetes Research Center, the National Institute of Arthritis and Musculoskeletal and Skin Diseases of the National Institutes of Health, the Department of Veterans Affairs, and the UI Research Foundation.

Written by Catharine Paddock PhD