I've just started a diet and that's got me thinking about dieting and whether there were really foods with negative calories. Celery is the one that's always cited as it allegedly is low in calories and takes more to eat than it contains. Perhaps1.
But here's another comestible which we can calculate with more precision: a glass of Pepsi Max (or any other zero cal drink of your choice). Say it's a can's worth or 330ml and it's been in the fridge at about 5°C. Your body temperature is about 37°C so if you drink it straight out of the fridge you need to heat it by 32°C.
My rusty O Level physics suggests that 1 calorie is sufficient energy to heat 1g of water (or, in practical terms, 1ml of water) by 1°C. So I reckon we need 330 × 32 or 10,560 calories to heat 330ml of water by 32°C.
Now "calories" in food terms are really kcal but that's still 10.56 kcal so a can of zero cal drink straight out of the fridge is really about worth about -10.5 kcal.
The only problem to this is that it assumes that the necessary heat to warm the drink comes from your metabolism. Does some of it come from the environment? I don't think so, assuming that you're not somewhere which is warmer than 37°C then the heat flow is out not in surely?
So, anyone for a Pepsi Max?!
- I discovered after I wrote this article that Snopes thinks there may be some truth in this, and it also commends cold drinks.
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