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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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.
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