Being overweight reduces the motivation to exercise – explaining why New Year resolutions to shed the pounds often fail, according to new research.
It has always been assumed obesity makes it harder for people to move around.
But scientists now reckon the mental affect of the condition could be just as vital as the physical.
A study on bloated mice found their lack of activity was caused by altered dopamine receptors in the brain, rather than the extra ounces they were carrying.
The chemical is a common neurotransmitter released in response to rewarding human activity and is linked to motivation, including activities such as eating and sex.
Dr Alexxai Kravitz, of the National Institutes of Health in the United States, said: “We know physical activity is linked to overall good health, but not much is known about why people or animals with obesity are less active.
“There is a common belief 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.”
Beginning a regular programme at the gym is a common pledge at this time of year, but it is one most people are unable to stick with for very long.
The findings, published in the journal, Cell Metabolism could hold the key.
Dr 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 mice with either condition.
He came up with a theory the reason the mice were inactive was due to dysfunction in their dopamine systems.
He explained: “Other studies have connected dopamine signalling defects to obesity, but most of them have looked at reward processing, how animals feel when they eat different foods.
“We looked at something simpler. Dopamine is critical for movement, and obesity is associated with a lack of movement. Can problems with dopamine signalling alone explain the inactivity?”
In the study mice were fed either a standard or high fat diet for 18 weeks. Beginning in the second week, the mice on the unhealthy diet were fatter.
By the fourth week, these mice spent less time moving and got around much more slowly when they did move.
Surprisingly, the mice on the high fat diet moved less before they gained the majority of the weight, suggesting the excess weight alone was not responsible for the reduced movements.
The investigators looked at six different components in the dopamine signalling pathway and found the obese, inactive mice had deficits in a dopamine receptor known as D2.
Co author Dr Danielle Friend said: “There are probably other factors involved as well, but the deficit in D2 is sufficient to explain the lack of activity.”
By studying lean mice genetically engineered to have the same defect in the D2 receptor, the researchers found they did not gain weight more readily on a high fat diet, despite their lack of inactivity.
This suggested that weight gain was compounded once the mice began moving less.
Dr Kravitz said: “In many cases, willpower is invoked as a way to modify behaviour. But if we don’t understand the underlying physical basis for that behaviour, it is difficult to say that willpower alone can solve it.”
He added 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 they face. Future research will focus on how unhealthy eating affects dopamine signalling.
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.
