So you don't like the gym?
Well, according to new research, there's a new way to stay slim that's as effective as exercise - simply take off your jumper and stand outdoors in the cold.
Shivering, says the study, helps produce hormones that stimulate the creation of calorie-burning "brown fat".
Once thought only to be present in babies, scientists now know adults also possess brown fat - and those with more of it are slimmer.
While white fat stores energy, brown fat burns it up. Around 50 grams of brown fat can burn up to 300 calories per day, the same amount of energy stored by 50 grams of white fat.
The new research involved exposing volunteers to increasing levels of cold until, with temperatures down to around 15C, they began to shiver.
"We identified two hormones that are stimulated by cold - irisin and FGF21- released from shivering muscle and brown fat respectively," said study leader Dr Paul Lee, from Sydney University in Australia.
"These hormones fired up the energy-burning rate of human white fat cells in the laboratory, and the treated fat cells began to emit heat, a hallmark of brown fat function."
Around 10 to 15 minutes of shivering produced as big a rise in irisin as an hour of moderate exercise, the scientists found.
In the laboratory, irisin and FGF21 together transformed white fat cells into brown fat cells over a period of six days.
"Excitement in the brown fat field has risen significantly over the last few years because its energy-burning nature makes it a potential therapeutic target against obesity and diabetes," said Dr Lee.
"White fat transformation into brown fat could protect animals against diabetes, obesity and fatty liver. Glucose levels are lower in humans with more brown fat."
The findings are reported in the latest edition of the journal Cell Metabolism.
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As part of the study, the cold-exposure volunteers were asked to pedal an exercise cycle for an hour. The moderate level activity generated the same amount of irisin from muscles as shivering for 10 to 15 minutes.
"From a clinical point of view, irisin and FGF21 represent a cold-stimulated hormone system, which was previously unknown, and may be harnessed in future obesity therapeutics through brown fat activation," said Dr Lee.Suggest a correction