Those New Year's resolutions are easy to list but difficult to make a reality.

Exercise is a hardy annual: This year I really am going to the gym every week.

Now science may have found a reason why most people are unable to stick to exercise for very long.

Researchers have found physical inactivity results from altered dopamine receptors in obese mice rather than excess body weight, according to a US study published in the journal Cell Metabolism.

"We know that physical activity is linked to overall good health, but not much is known about why people or animals with obesity are less active," says the study's senior author Alexxai V. Kravitz.

"There's a common belief that obese animals don't move as much because carrying extra body weight is physically disabling. But our findings suggest that assumption doesn't explain the whole story."

Kravitz has a background in studying Parkinson's disease, and when he began conducting obesity research a few years ago, he was struck by similarities in behavior between obese mice and Parkinsonian mice.

Based on that observation, he hypothesised that the reason the mice were inactive was due to dysfunction in their dopamine systems.

"In many cases, willpower is invoked as a way to modify behavior," Kravitz says.

"But if we don't understand the underlying physical basis for that behaviour, it's difficult to say that willpower alone can solve it."

He says that if we begin to decipher the physiological causes for why people with obesity are less active, it may also help reduce some of the stigma that they face.

Future research will focus on how unhealthy eating affects dopamine signals.

The researchers also plan to look at how quickly the mice recover to normal activity levels once they begin eating a healthy diet and losing weight.

This article was originally published by Business Insider.

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