3 ms·
Simple physics analysis Almost all of the energy we get from food is used to heat our bodies, mostly indirectly via our liver, spleen and brain. [ https://www.
by Hondor 10y ago
Simple physics analysis
Almost all of the energy we get from food is used to heat our bodies, mostly indirectly via our liver, spleen and brain. [ https://www.boundless.com/physics/textbooks/624/work-and-energy-6/power-65/humans-work-energy-and-power-287-6037/ https://www.boundless.com/physics/textbooks/624/work-and-ene... ]
A 40 degC person exposed to 0 degC air would lose heat at twice the rate of normal (40 degC person in 20 degC air). That's because convective heat flow rate is roughly proportional to temperature difference. All that heat has to be made up for by the body's metabolism. So you would burn calories at nearly twice the rate you would normally. It doesn't matter if it comes from brown fat, shivering, waste heat from your brain, or some other process, as long as it's internal to your body, it's taking energy that would otherwise not have been used.
A shower in 0 degC water could be on the order of 100 times more effective than 0 degC air, so you could get the same value from a shorter exposure. [ http://www.engineersedge.com/heat_transfer/convective_heat_transfer_coefficients__13378.htm http://www.engineersedge.com/heat_transfer/convective_heat_t... ]
A cold shower for a couple of minutes a day (0.1 % of your life) would be equivalent to cold air for a couple of hours (10% of your life). That's about 10% more energy used than otherwise. So it sounds like even from a simple heat transfer perspective, cold showers must be effective at losing weight, as long as you don't compensate for that some other way like feeling more hungry afterwards :P
- konceptz 10y agoAnd I always wondered if drinking a glass (16oz) of 0 degrees water was burning the calories required to heat it where you could drink enough water to offset a donut in a morning. (Forgive the lack of math here, I'm about to pass out in bed.)
- ahelwer 10y agoQuite simple math, since a calorie is the amount of energy required to heat a mL of water by 1°C. Thus a food Calorie (1000 calories) is the amount of energy necessary to heat a liter of water by 1°C. Human body temperature is 37°C, so you burn 37 calories for every liter of 0°C water ingested. The average donut has 195 Calories. Thus you must drink 5.27 liters of 0°C water to cancel out the calories gained from eating a donut. Not very realistic.
- tempestn 10y agoNo, but still more significant than I would have predicted!
- credit_guy 10y agoNo, you are off by a factor of about 10. 16 oz ~ 500 ml. Body temperature - 40 celsius. 1ml water needs ~ 1 calorie to go up 1 degree celsius. The product is about 20 kcal. A medium dunkin donut is about 200 kcal.
- sixhobbits 10y agoBerkeley did the maths for you: http://w.astro.berkeley.edu/~gmarcy/thermal/tpteacher/jokes/icecream.html http://w.astro.berkeley.edu/~gmarcy/thermal/tpteacher/jokes/... (notice the word 'jokes' in the URL, but it's pretty well done)
- david-given 10y agoThose numbers seem very peculiar: > Each ounce of beer contains 16 latent calories, but extracts 1,036 calories (6,216 cal. per 6 oz. portion) in the temperature normalizing process. I think it's muddling food calories (which are really mislabelled kilocalories) and temperature calories (which aren't). So that 16 should be 16000. This is why you do this in joules!
- onion2k 10y agoA cold shower lowers your skin temperature, which in turn causes your body to restrict the blood flow to save heat. When you get out of the shower and back in to the warm you start pumping blood back to the surface again. There is no additional work done to warm you up. Your physics approach is betrayed by biology. We've had thousands of years to evolve an efficient solution to the problem of heat conservation.
- ljf 10y agoSo having a cold shower has no effect on core temp? What about a cold bath? What about being in the sea after an accident at the higher latitudes?? Also I think you will find that returning your skin to a normal temperature after chilling in in any way (water, aircon, etc) temporarily decreases core temperature after which you expend energy to return it to normal. We may have had thousands (if not millions) of years to evolve solutions to heat conservation but they aren't perfect, or people would never die in freezing water or conditions.
- Hondor 10y agoIf your core temperature drops, it's probably because you're losing heat faster than you can generate it. That kind of points to how effective this could be in the limit of being nearly fatal - If you can lose more heat than your body can produce, you're surely doing a pretty good job of burning calories! Yes, I agree we're far from perfect at conserving heat. We're optimized for other conflicting purposes instead. We don't even have fur.
- Hondor 10y agoGood point, but it looks like there's still a big effect. Vasoconstriction causes up to an extra 10 degC temperature gradient across the skin compared to when they're dilated (less than 1 degC). [ http://www.healthyheating.com/Thermal_Comfort_Working_Copy/downloads/http___repositories.cdlib.org_cgi_viewcontent.pdf http://www.healthyheating.com/Thermal_Comfort_Working_Copy/d... p572 ] So that halves heat flow rate from what I described. It's like being in a 10 degC environment instead of 0 degC. You could double the length of the cold shower to 5 minutes to compensate.