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I encountered a person not wearing a jacket in cold weather. He said he does this to lose weight.

He explained his theory: By allowing his body to lose calories (heat energy) to the environment, his body would have to burn more calories to stay warm. As a result of burning more calories, he would lose a little weight.

Truth or fiction?

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    Just to show notability: Ray Cronise (famous in the skeptical community to helping Penn Jilette lose a lot of weight) makes this claim in Wired.
    – Oddthinking
    May 2, 2016 at 13:27
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    Shivering consumes calories.
    – GEdgar
    May 2, 2016 at 13:35

2 Answers 2

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Energy metabolism in humans at a lowered ambient temperature European Journal of Clinical Nutrition April 2002, Volume 56, pages 288-296 does confirm that men dressed identically at 16 degrees C do use up more energy than at 22 degrees C.

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    I don't think this answers the stated question. The fact that you lose energy when being cold doesn't mean that you necessarily lose weight (unless there is some research which demonstrates this, which should then be quoted). Otherwise, I can totally think of a possibility that body forces any excess intake into fat with higher efficiency when repeatedly exposed to uncomfortable temperatures.
    – sashkello
    May 5, 2016 at 1:36
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Dave already provided the scientific source that does confirm that men use up more energy when staying in a colder environment.

Nevertheless, your question puts this question into the specific context of a diet or losing weight and i feel that Dave's answer is missing this context.

At 16°C (EB), EE (total 24 h EE) was increased to 12.9±2.0 MJ/day as compared to 12.2±2.2 MJ/day at 22°C (P<0.01)

The difference is comparingly small and a diet using this technique alone will most likely not lead to the desired effect of losing weight. The average energy that is being consumed by being six degress colder is less than 1 MJ/day. This equals a maximum of 238.85 calories which is less than one ball of vanilla ice cream.

Personally, i'd rather have 238 calories more while being 6°C warmer.

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    A healthy sustainable slimming diet is generally 500kcal less consumption than needed. I'd say 238kcal is significant. It might be a ball of icecream, but is one every day. A diet with a 238kcal deficiency would make you lose 1kg per month. Not bad.
    – Sklivvz
    May 3, 2016 at 10:23
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    1MJ is 238kcal (which Americans confusingly write as 238Cal, with a capital): google.co.uk/…
    – Sklivvz
    May 3, 2016 at 10:45
  • Thanks for the clarification, i didnt know about this difference. As we were both talking about the same ball of icecream though this shouldnt change it too much. May 3, 2016 at 11:36
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    "Personally, i'd rather have 238 calories more while being 6°C warmer." Should that read 238 Calories fewer?
    – Oddthinking
    May 3, 2016 at 12:08
  • @JamesCameron The men wore long pants at 16°C (61°F) and 22°C (72°F). Maybe it's personal preference and the climate someone is from, but many people would rather be wearing shorts. I could imagine somebody who lived in Chennai all their life feeling cold, but ...
    – DavePhD
    May 3, 2016 at 12:37

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