Can apples build muscles?

“An apple a day really could keep the doctor away – as long as you don’t throw away the peel,” reported the Daily Mail . It said that a chemical in apple skins is being credited with “a host of health benefits”, from building muscle to keeping weight under control.

The story comes from an early-stage laboratory study which explored potential therapies for muscle wasting (atrophy), a common condition associated with ageing and illness. Researchers first identified gene activity that changed in people’s muscles when they fasted, which can eventually lead to muscle wasting. Using a database that shows the effects of chemicals on gene activity, they identified ursolic acid, a compound found in apple peel, as having the opposite effect on gene activity to that seen in fasting.

The researchers tested whether ursolic acid could counteract the effects of muscle wasting in mice. In fasting mice, ursolic acid was found to protect against muscle atrophy. Ursolic acid added to the diet also enhanced muscle growth in normal mice, as well as reducing their body fat.

It is important to note that this study’s findings may not be applicable to humans. Even if ursolic acid did have an effect on muscle wasting, it’s not clear whether eating apples could have the same effects. Like other fruit, eating apples can have health benefits. However, this study alone is not the reason to eat them.

Where did the story come from?

The study was carried out by researchers from the University of Iowa and the Department of Veterans Affairs Medical Center, Iowa City, USA. It was funded by various academic centres and other organisations, including the US National Institutes for Health, the Doris Duke Charitable Foundation and the Department of Veterans Affairs. The study was published in the peer-reviewed journal Cell Metabolism.

The findings of this study were overstated by several newspapers, which seemed to rely on an accompanying press release. The study was in mice, which the newspapers did mention. However, using the phrase “an apple a day keeps the doctor away” may give the wrong impression that the study found that apples had health properties for humans. The Telegraph ’s headline that “An apple a day keeps your body toned and slender” implies an effect in humans which also cannot be concluded from this mouse study.

What kind of research was this?

This laboratory and animal study had two parts. The first looked at the effects of genes (gene expression) in the muscles of humans who were fasting to identify which genes’ activity changed during fasting. The researchers then used a database to identify compounds that had the opposite effect on those genes’ activity. The second part was a controlled experiment carried out on mice, to test the effect of the compound on muscle.

The researchers pointed out that muscle atrophy is a common and debilitating condition, for which there is currently no medical treatment. They say that previous studies have shown that muscle wasting is driven by changes in the activity of genes in skeletal muscle (muscle that is attached to bone). Their theory was that a compound which produced the opposite effects on gene expression might inhibit muscle atrophy. Their aim in this early-stage study was to identify a compound that might be the basis for a potential therapy in humans.

What did the research involve?

In the first part of this experiment, the researchers looked at which genes’ activities might be related to muscle atrophy. To find this out, they studied seven healthy adult humans who fasted in the clinical research unit for 40 hours, forgoing food but not water. Prolonged fasting leads to muscle atrophy. Muscle biopsies were taken from each participant both after the fast and after a first meal.

To determine the effects of fasting on skeletal muscle gene expression, the researchers isolated RNA from the muscle biopsies. RNA contains instructions from genes that tell a cell which proteins to make and how much. They analysed the differences in the RNA, using specialised techniques, to establish the gene “signature” - the characteristic pattern of gene activity - for fasting in skeletal muscle.

Using a database of the effects of hundreds of molecules on gene expression, they identified ursolic acid as a compound which had the opposite effect to the signature for fasting skeletal muscle. Another compound that was identified as having similar potential effects was metformin, a drug used in treating type 2 diabetes.

The researchers then tested in mice the effects of ursolic acid and metformin on muscle. In the first experiment, the mice were injected with either compound or with an inactive ingredient in two doses before and after fasting. A second experiment tested whether ursolic acid might build muscle, rather than just prevent atrophy. In this experiment, normal mice were given either a normal diet or a diet with added ursolic acid for five weeks. The researchers then examined the effects of ursolic acid on muscle mass, fat mass and the levels of certain chemicals in the blood, such as lipids. They also looked at skeletal muscle gene expression in these mice.

What were the basic results?

The researchers found that:

• Ursolic acid reduced muscle atrophy in fasting mice. Without ursolic acid, fasting in mice reduced muscle weight by 9%. Giving these fasting mice ursolic acid increased their muscle weight by 7%. Metformin had no effect on muscle atrophy in fasting mice.
• Ursolic acid also induced muscle growth (hypertrophy) in normal mice. Mice on a diet containing ursolic acid had larger skeletal muscles and skeletal muscle fibres and increased grip strength compared to those on a normal diet.
• The effects of ursolic acid on muscle in normal mice was accompanied by reductions in their body fat, fasting blood glucose, cholesterol and fats called triglycerides.

How did the researchers interpret the results?

The researchers say the findings identify ursolic acid, a major waxy component in apple peels, as a potential therapy for illness- and age-related muscle wasting. They say the compound acts in part by counteracting the characteristic changes in gene activity during muscle atrophy. 

Conclusion

This early-stage study in mice explored the potential of ursolic acid in the treatment of muscle wasting. These results may stimulate further interest in ursolic acid. However, because of the differences between mice and humans, the compound may have a different effect or no effect at all in people. Also, the mice in this study were given ursolic acid as a compound, not as apples. It is not clear whether eating apples or apple skin could provide enough ursolic acid to have the same effect.

We already know that eating fruit and vegetables has health benefits. Apples can form a part of your five a day if you enjoy them, but this study alone is not the reason to eat them.