The energy to generate heat has to come from somewhere. Is it the friction of the blood pumping through the veins and tissues in your body? Is there some kind of chemical heat generation at the cellular level? Where does the heat come from?
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The heat comes from the energy in the food you eat. Your body burns calories to generate heat. You burn around 2000 calories a day just by existing.
We use the expression to “burn calories” for a reason: your cells are harnessing the very same combustion process as fire to turn the food you eat into energy. In addition to the chemical energy created by your cells in this reaction, thermal (heat) energy is also released.
Your body burns sugar to make energy, including heat. That’s not just a euphemism: the chemical process of converting sugar to energy is the same in your body as it is if you set it on fire. Sugar breaks down in the presence of oxygen to make carbon dioxide and energy.
Think of food as a mix of atoms that are bound together in ways that store energy. Your body breaks down the food, and captures some of the energy to do work (build other molecules, move things) but the rest of the energy is released as heat. When your body is cold, it can break down the food in such a way that more heat is released and less energy is captured to do work.
For example, sugar is a molecule made of carbon, hydrogen and oxygen. The way those atoms are joined together stores energy – especially the carbon-carbon connections (bonds) and the carbon-hydrogen connections. When the sugar is broken down, the products are carbon dioxide (CO2) and water (H2O). All of the carbon-carbon and carbon-hydrogen bonds have released their energy.
Ultimately, it is sunlight that heats the body, because sunlight is used in photosynthesis to convert water and carbon dioxide into sugars. Plants use energy from sunlight to create the complex energy storage molecules that we use for food.
Breaking the food down happens in every cell in your body, in the mitochondria. The food is first broken into small molecules in your stomach and intestine, and then the smaller molecules are transported through the blood to your cells. They can also be stored, for example as fat, for later use. Fats have lots of high energy carbon-carbon and carbon-hydrogen bonds.
Breaking down foods we eat releases energy, so does the breakdown of ANY molecule. the energy is stored in the chemical bonds and released.
In order for you to stay alive, our bodies constantly break and re-make this chemical called ATP. It’s used for literally everything. Every muscle contraction, for your heart to beat, every process your conscious mind isn’t aware of, all require the breakdown of ATP.
So we break down ATP and the energy released is used for many different things, including the manufacture of more ATP. (Breaking bonds releases energy, making bonds requires energy).
Our efficiency for using this energy is pretty bad. We only utilize roughly 30% of the energy released from the breakdown of an ATP. That remaining 70% is lost to heat
For the most part, we do NOT burn calories just for the heat, although burning calories for cellular function always releases heat as a byproduct.
If your body gets cold enough, shivering is its way of making more heat, but it’s not burning calories for heat directly. The muscles are moving, which requires calories, which comes with extra heat.
Point being, you don’t burn more calories in the cold. That’s why you’ve never heard of a diet where you sit in a refrigerator all day. Once you get cold enough to start shivering, the body is already cold enough that it’s actually slowing metabolism overall and adding shivering is still lower calorie draw than being warm.
For the most part, the body regulates temp by controlling how heat is lost. Particularly how much 98.6F blood goes near the skin surface. If you’re in 60F wind and you’re losing heat faster than you’re making it, you don’t start burning more calories, rather, your skin blood vessels constrict letting the skin get relatively cold, so less heat overall is shed through the skin. However, note that your skin getting cold and clammy is just a surface effect. The blood will remain around its normal temp, just flow differently.
Also, nervous system being what it is, we add or remove extra clothing based on whether we feel cold or hot.
Your body generates heat as an indirect result of its normal functions, but it can also undergo a more direct heat generation, especially to adapt to colder temperatures.
From the total energy your body spends on a day, from 50 to 75% is used to keep everything running throughout your organs and tissues.
From the rest, most of it directly or indirectly contributes to heat generation. Around 10 to 15% is used for physical activity, and all that energy used especially for muscle movement does end up eventually converted into heat.
Another chunk of that total energy spent might be used for more “wasteful” heat generation – that is, heat for the sake of heat. At least 10% of your energy is spent for increased heat protection after you’ve had meals.
Also, adaptive heat generation in your brown fat tissue increases heat production to compensate for colder surroundings, with “chilly” non-freezing weather increasing your heat production by up to 30%.
As to how this last process works, I’ll keep it ELI5 by saying that the usual chemical reactions your body uses to get energy from food and convert it into usable energy for body processes are in a way made more inefficient by your brown fat tissue. Usually you’d want those reactions to be as efficient as possible without losing much energy in its least practical form – heat, but in this case, that is exactly what you want in order to produce more heat. That is the job (in this case) of brown fat.
Y’all say sugar. Then why sugar is bad for you?
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