Cold Thermogenesis

Cold Thermogenesis

Michael Phelps reportedly ate up 12,000 (kilo)calories per day during his most intensive training periods for the Olympics. Although it appears that the actual number may be 8-10,000 this remains an astonishing amount of food (calories) per day. His approximate daily routine involved up to 4 hours in the pool and 1-2 hours in the gym and based on his likely basal metabolic rate (BMR) and number of calories burned through exercise it still appears that there is a surplus of daily energy. And we know that excess energy would be stored as fat. But there was certainly no excess on this elite swimmer’s body and he didn’t gain weight from Olympiad to Olympiad. So what’s going on?


Well, the likely answer is that those unaccounted for calories were burned through a process known as COLD THERMOGENESIS. This is the body’s attempt to keep warm in cold environments by revving up metabolism. The 4 hours that Phelps spent each day in the pool would impose significant cold stress on his body (lap pools are typically heated to 24 degrees Celsius while body temp is approximately 37 degrees).  While 24 degree air temperature wouldn’t have a significant impact, a liquid of that temperature will cause much more rapid cooling (20 times) due to conductive heat loss. To produce more body heat, energy (calories) are needed and this is the only explanation for why Phelps could eat so much and remain in energy balance each day.


How does the body produce this heat?

Here the answer lies in BROWN FAT; a healthy type of fat that lies mostly in our upper back and chest. Brown fat gets its name from the fact that it contains mitochondria (little energy producing plants) and these rely on a blood supply to make energy. That blood contains iron and iron gives the fat its brown colour. Brown fat is actually able to burn calories to keep us warm. This thermal regulatory action kicks in well before shivering and like many of the processes within our body it gets better at doing its job the more it is asked to do so (the more regularly we get cold). So, it is now well recognised that deliberate cold exposure can promote improvements in body composition if used regularly. But, if you can’t swim for 4 hours in a cool pool what are the options?


  • Turn the heat down in your home, open windows etc… one of the most effective methods may be to control the bedroom temperature to around 16-17 degrees and sleep with only a thin sheet rather than thick padded duvet. This would give you around 8 hours of living in cool conditions every day without really noticing it.
  • Take a cold bath or shower. Bath will work best and its likely 5-20mins would be as much as you can manage with water taken from the cold tap (approx 12 degrees) or a little warmer.
  • Take an ice bath or cold water (lake) dip. Wild swimming and ice bathing have become popular for a multitude of reasons. This will cause the most rapid cooling of the body and with a spike in sympathetic ‘fight or flight’ nervous system activity brown fat is very strongly triggered to do its thing. This needs to be safely and can be progressed up to the point that shivering begins (shivering is a secondary attempt to produce heat through muscular activity and so there’s no need to continue at this point). Note that shivering can continue for a long time after immersion too. This too is calorie burning but can be distressing to those who have not experienced it before.
  • Drink cold water. Ice cold water taken internally will have a similar cooling effect. If those 2 litres (8 glasses) you aim to drink were ice cold then you’d burn an extra 100kcals per day.


Finally, consider that our body’s are finely tuned to maintain equilibrium (aka. homeostasis) and should you begin to burn more calories through cold thermogenesis then you will likely feel more hungry too. Picture school children raiding the tuck shop after swimming lessons up and down the country!

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