A study that has discovered fat cells beneath our skin shrink when exposed to blue light just might help explain why many of us find ourselves with more snug-fitting pants in spring.
Short days and time spent indoors might be making our fat cells a little fatter. That's not to suggest we can melt away the curves by lazing at the beach, but it could be one more factor to consider in our fight against obesity.
Researchers from the University of Alberta, Canada, set out with the intention of engineering white adipocytes – fat cells commonly found under the skin – that could produce insulin for people with Type 1 diabetes.
On setting up the controls for their experiment, they observed a strange phenomenon which hadn't been reported previously.
It was already known that channels in the adipocytes' membranes could open and close under the influence of certain visible wavelengths of light.
To set the groundwork for bioengineering fat cells, the researchers controlled the types of wavelengths the adipocytes were exposed to, and measured the effect on the movements of charged particles across the membrane.
They noticed the highest current was generated when the light's wavelength was between about 450 and 480 nanometres. Or, in everyday terms, when it was bluest.
"We noticed the reaction in human tissue cells in our negative control experiments, and since there was nothing in the literature, we knew it was important to investigate further," explains pharmacologist and senior author Peter Light.
It's not unusual for other cells in our body to react to blue light. Photosensitive cells in our retina not only respond strongly to this colour, they use the colour to signal to the body they're detecting sunlight.
This is useful for helping the body's clock, or circadian rhythm, to keep time with fluctuating periods of day and night.
The researchers were curious to see if the reaction to blue light caused the same genes to activate in adipocytes as in other photosensitive cells.
Not only did they discover fat cells did indeed produce the body-clock protein known as melanopsin, they found the cells also released more of the molecule glycerol and reduced the size of their lipid store by nearly a third.
"When the Sun's blue light wavelengths – the light we can see with our eye – penetrate our skin and reach the fat cells just beneath, lipid droplets reduce in size and are released out of the cell," says Light.
"In other words, our cells don't store as much fat."
In practical terms it implies getting out and walking in the sunlight not only helps us make more Vitamin D, it could serve to help make it harder for our body to store quite as much fat.
"Obviously, there is a lot of literature out there suggesting our current generation will be more overweight than their parents, and maybe this feeds into the debate about what is healthy sunshine exposure," says Light.
That said, we shouldn't promote this as next season's weight-loss fad. Context is everything.
Subcutaneous white adipose tissue isn't as bad as the spare tyre we grow around our organs, which is buried too deep for light to penetrate.
More importantly, we can forget simply sunbaking our fat away - these results were under laboratory conditions, so we need to keep in mind the risk of melanoma from overdoing exposure to UV light.
What this research might do is point the way to future weight loss treatments or help us develop better methods for managing diabetes.
Looking past practical applications, the discovery adds detail to the complexities of the ups and downs of the human circadian rhythm.
"Well, perhaps that pathway – exposure to sunlight that directs our sleep-wake patterns – may also act in a sensory manner, setting the amount of fat humans burn depending on the season," says Light.
"You gain weight in the winter and then burn it off in the summer."
This research was published in Scientific Reports.