system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: why can't we just use complex sugars in snacks than simple sugars?
gpt
Well for starters, table sugar - sucrose - is a complex sugar consisting of a glucose molecule and a fructose molecule bound together. In any case, one problem is the body's inability to digest complex sugars. For example, people who do not produce enough of the enzyme *lactase* are unable to digest milk sugar *lactose*. Instead, the lactose *is* able to be digested by lactobacillus bacteria in the intestines, which leads to bloating, cramping, flatulence, and diarrhea. Sucrose and lactose are pretty much the only disaccharides we can digest, and we can't even digest lactose all that well into adulthood. We can't properly digest *any* sugars larger than disaccharides.

Much larger sugars like cellulose are an important part of our diet to feed important gut bacteria, regulate moisture in our stool, and bind together stool into solid pieces. Basically: fiber makes you poop better. The flip side is that cellulose doesn't *taste* like sugar.

So there are the first two problems: we may not be able to digest it, which may cause gastrointestinal problems; and/or, it may not be sweet, which defeats the purpose of using it to replace the fattening simple sugars.

The other problem is where to get the sugars. The sugars that are commonly used are common for a reason: a whole bunch of stuff already naturally makes it. Even in the case of genetically modified crops like corn to make high-fructose corn syrup, scientists didn't make the gene for fructose from whole cloth, it was borrowed from organisms (like fruit) that make a lot of fructose naturally. Unless we can find a crop that's easy to grow in abundance that already produces that sugar, or the genes are easy to insert into an easy crop like corn, it has to be manufactured artificially. Finding a way to chemically synthesize a complex sugar can be prohibitively expensive, especially when you run into the downsides already mentioned above.

Fructose is itself very popular among manufacturers because by weight it tastes sweeter than either glucose or sucrose. That means they can use less of it to sweeten their products (which saves money) *and* label it as having (marginally) fewer calories (which means people are more likely to buy it) *and* it's super cheap to get, because corn.

There are already artificial alternatives, like sucralose which tastes very sweet but isn't able to be digested so it gives no calories. The problem is that it's way more expensive (in the quantities that manufacturers use) than even natural sucrose, and people complain that it's not natural and avoid it. The other alternative is left-handed glucose. That is, glucose can turn one way (the natural form, dextrose) or it can turn the other way. Pretty much nothing on Earth makes left-handed glucose. We can't process it, so again no calories. BUT also again nothing on Earth makes the stuff naturally so it is also relatively expensive.