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Last Word is Â鶹´«Ã½â€™s long-running series in which readers give scientific answers to each other’s questions, ranging from the minutiae of everyday life to absurd astronomical hypotheticals. To answer a question or ask a new one, email lastword@newscientist.com
When we eat ice cream on a hot day, does it cool us down or do the calories in it warm us up?
Ron Dippold
San Diego, California, US
A typical scoop of ice cream weighs 60 to 100 grams (let’s call it 80g). For generic vanilla the energy content is about 2 calories (kcal) per gram, so 160 calories per scoop. Let’s say you did two scoops (I would!), so 320 calories. Say you weigh 60 kilograms and your normal body temperature is 37°C. Those two scoops are typically at -10°C, but weigh only 0.16 kilograms. In theory, to warm that ice cream to your body temperature would take 15 kilojoules (kJ) of energy, which would drop your body temperature by 0.07°C.
On the other hand, the ice cream contains 320 calories – how much will that warm you? That is the same as 1339 kilojoules, so in theory if all that energy is released, it could raise the temperature of your 60 kg body by 6.4°C. But our bodies don’t work this way at all. Which is kind of good, right? If your body were suddenly 43.5°C after you ate an ice cream cone, that would be very bad. In reality, your digestive system does a lot of work to convert the ice cream into sugars in the bloodstream and then your body will just decide, “Oh, I am slightly cold now, better warm up a bit.” It expends 1/91th of those calories to warm you up by 0.07 °C. The rest could be stored as fat. Or you could walk 7 kilometres to burn them. Thankfully, our bodies use energy just to exist – about 1 calorie per kg per hour, even while watching television. So, no, the calories don’t warm us up.
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If body temperature suddenly rose to 43.5°C after you ate an ice cream cone, that would be very bad
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Eric Kvaalen
Les Essarts-le-Roi, France
The heat obtained from burning the calories would definitely be more than the cooling, but you don’t burn the calories just because you ate them. You may burn them eventually (if they don’t go into making body fat), but eating the ice cream will probably result in your eating less of something else later, because of our system of regulating appetite.
There is another effect, however, called diet-induced thermogenesis. This is heat generated after eating, largely due to the catabolism, or breakdown, of proteins to amino acids. This effect is 20 to 30 per cent of the caloric value of the protein. If ice cream is, say, 3 per cent protein, then this thermogenesis would be about 30 kcal per kilogram of ice cream, whereas to melt a kilogram of ice cream would require around 80 kcal. So the net effect would probably be to cool you down.
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