An apple a day may really help to keep the doctor away, according to a new study which found that apple peel contains a compound which helps keep muscles strong.
Ursolic acid, a waxy substance in apple peel, was found to promote muscle growth and reduce muscle wasting in mice. In addition, it was also shown to reduce fat, blood sugar, cholesterol and triglyceride levels in the animals tested.
Scientists think the compound could be potentially useful for treating muscle wasting and even metabolic disorders such as diabetes.
"Muscle atrophy causes big problems. It's also very common - it affects most people at some point in their lives, during illness or ageing, said Dr Christopher Adams, at the University of Iowa, who led the study.
"It prolongs hospitalisation, delays recoveries and in some cases prevents people from going back home.
"It isn't well understood and there is no medicine for it," he added.
The researchers looked at what happened at the genetic level in muscles as they became weaker, and discovered 63 genes that changed activity during fasting, in both people and in mice.
They then looked at the effect of more than 1300 molecules on cultured cells, and discovered that ursolic acid caused genetic activity opposite to the pattern seen during muscle wasting.
Further research showed that mice fed ursolic acid were protected against muscle wasting. Healthy mice fed ursolic acid developed larger, stronger muscles compared with mice not fed the compound.
The mice also had less body fat and lowered blood glucose, triglyceride and cholesterol levels.
The scientists think ursolic acid boosts the effect of IGF1 and insulin, two hormones involved in muscle growth.
Ursolic acid is particularly concentrated in apple peel but is also found in prunes, cranberries, peppermint, basil, oregano and thyme.
Although it's not yet clear how the findings will translate to human patients, Dr. Adams said: "We know that if you eat a balanced diet like mom told us to eat you get this material. People who eat junk food don't get this."