ELI5: How come there's lots of 'zero calorie' drinks, but no 'zero calorie' foods?

There are, sort of.

Celery is a classic example of a calorie-negative food item, meaning that it takes your body more energy to try and digest it than it gains from that digestion. And that's because celery is mostly water and cellulose - water has no calories and your body can't really digest cellulose. Of course, that means you just wind up shitting out all those cellulose, which is good because it can function as dietary fiber and helps keep you regular.

Now with zero calorie beverages, you have water to serve as the bulk of the drink, carrying with it the artificial flavoring agents to make it not taste like water.

To make a zero calorie food means you have to make your food completely non-digestible. So you'd need something to provide the bulk of the food that's not digestible and can carry artificial flavouring agents to make it palatable. Again, we have things like that already but means you'll probably wind up with something that chews like celery or gum.

And the main issue is, what good would it serve? You'll still be hungry in the end because the main receptors that sate a person's hunger respond to the presence of proteins, fats, and sugars, not raw bulk. And you can't really include any of those three things because that's where calories come from.

Okay, there was a zero-calorie fat substitute introduced in the US some years ago. But that had a slight issue because all that oily, non-digestible fat had to go somewhere, and that was 'out the butt.' If you had too much of it, it basically lubricated your colon and made it into a slip n' slide for your poop. It's popularity waned quite a bit, but I think it's still technically available in US stores.

/r/explainlikeimfive Thread