Eli5: Why do “empty” calories provide less energy?

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Why do 100 calories in candy feel less sustainable than 100 calories in fruit? Aren’t they energetically equivalent?

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Like firewood. The more substance, the longer it takes to burn. Even if it looks like the same amount.

Calories themselves are measurement of energy, so 100 Calories of anything is 100 Calories. “Empty” calories just refers to something having little to no nutrient value. I think your question has more to do with why some foods feel more filling than others though, and the answer to that is fiber, fats, and protein content. Some foods help us feel more satiated because they bulk up with water in our bellies, or we take longer to eat them so our brains have time to let us know we’re full, or they take longer to digest so we do they another hunger signal as quickly.

They are the same amount of energy. So, your question is about a subjective feeling of one being different than the other

The subjective feeling is exactly that a subjective feeling and not based on anything else.

Your belief is what makes the difference.

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Calories get digested different ways depending on what you eat. Think about how gunpowder burns. Now think about how paper burns. Now think about how a candle burns. Each of those three things might release the same amount of energy when they burn, but you probably imagined dramatically different rates, right?

Calories tend to come from something we call “carbohydrates”. It’s a lot easier if I just explain that as “sugar”, even though that’s an oversimplification. We divide carbohydrates into “simple” and “complex” versions. It’s probably no surprise that complex carbohydrates are harder for your body to process. Remember this for later.

The calories in candy are primarily in either sugar (sucrose) or corn syrup (fructose). These are simple carbohydrates our body breaks those down into the glucose it wants for energy. Fructose is one of the fastest chemicals your body can break down, with sucrose being close to it. So a lot of candy is like a firecracker: you get all of the calories in one blast. This causes your blood sugar to spike really high, which upsets your pancreas, so it produces more insulin to try and get things back to normal. If you get *way* too much sugar too fast, the pancreas will overreact, your body will lose too much sugar, and you’re going to feel a “crash” that makes you either want more food or to stop trying to use energy. Diabetes is a condition where the pancreas overreacts to *everything*, and causing this situation too much can cause people to develop diabetes later in life.

The calories in fruit are also stored in sugar, but a lot of fruits are also high in a complex carbohydrate we call “dietary fiber”. Your body can break this down into glucose, but it takes a long time. Curiously, if a lot of dietary fiber is present, your body spends so much effort breaking it down it can’t spend so much effort breaking down simple carbohydrates, so you absorb them more slowly too. This means even if the fruit has the same amount of calories as the candy, your blood’s glucose level doesn’t spike, your pancreas doesn’t overreact, and you feel a nice, long-burning supply of energy like the firewood or a candle.

When something is “empty” calories, we mean it doesn’t come with anything else of nutritional value and is usually simple carbohydrates. This will give you a quick burst of energy, but also might make you crash later and since you aren’t getting vitamins or fiber or other nutrients from it you’re not helping your body very much. It’s better to eat meals that have complex carbohydrates and other nutrients, so you’re nourishing your body instead of providing the bare minimum.