(Intro)

A lot of people think that carbohydrates are the enemy. The word might make you think of spaghetti, or white bread, or other foods that will supposedly make you gain weight. But dietary science is more complicated than that, and carbs are also found in fruits, and grains, and vegetables, and milk. They help you make and store energy, poop a little better, and are one of the three classes of macronutrients that you need to live.

Chemically, carbs are pretty simple: just carbon, hydrogen, and oxygen. Hence the name – "carbo" for the carbons and "hydrate" for water. But not all carbs are the same. There are simple carbohydrates – the monosaccharides and disaccharides – and complex carbohydrates, or polysaccharides. These words just describe the structure of the molecules – so, monosaccharides are the basic building blocks of larger carbohydrate chains.

Monosaccharides are also commonly called simple sugars, and taste sweet. Like, you've probably heard of glucose. We can thank plants for glucose – and, really, for most carbohydrates that we eat – because they convert carbon dioxide and water into glucose during photosynthesis. There are also molecules like fructose, which is found in some fruits and honey, and galactose, which is in milk. All three of these molecules are isomers, so they have the same chemical formula, but their atoms are arranged differently. This gives them different chemical properties, like levels of sweetness.

Now, of course, the taste of sugars is great, but carbs are really important because they're a great source of energy. Glucose metabolism is basically when glucose undergoes a bunch of chemical reactions inside of your cells to make these molecules called ATP. And the cells in your body use chemical energy from ATP to do pretty much everything – move, grow, especially keep your brain functioning. So, monosaccharides are small enough to pass from your intestines into your bloodstream, and then into different cells in your body that starts these energy-producing chemical reactions.

But lots of the carbs we eat aren't plain old monosaccharides. When two are bonded together, you get a disaccharide – like maltose, which makes cooked sweet potatoes sweet, sucrose, which is table sugar, and lactose, the sugar in milk. These have to be digested by enzymes before they can enter the bloodstream. Some people can't break them all down, though, like if you're lactose-intolerant. So bacteria might do it instead, which can make you feel all gassy and bloated.

And polysaccharides, or complex carbs, are just a bunch of monosaccharides bonded together. Like, starch is a long chain of glucose molecules that plants use to store energy. You have enzymes that can break down starch from foods like grains or potatoes, but it takes a little longer to digest and absorb than monosaccharide-filled foods, like candy bars.

On the other hand, we store extra glucose in the form of glycogen, mostly in our liver and skeletal muscle cells. See, there's this chemical produced by your pancreas called insulin that helps regulate your blood sugar. When you have too much glucose in your bloodstream, more insulin is released. And it basically tells your body to start making glycogen, to store for later when you need energy. And glycogen is the reason that endurance athletes, like marathon runners, will sometimes eat a bunch of carbs before exercising. They're trying to store more glycogen, so when their body needs energy, their cells can break those polysaccharides down and have lots of glucose to metabolize.

But here's the catch: you can only store so much. And when your liver and skeletal muscle cells can't hold anymore, any extra glucose will be converted into energy-dense fats for storage. Sound familiar? That fear that eating too many carbs makes you fat? So that's why people might go on low-carb diets – to avoid making and storing more fat molecules, even though eating carbs in moderation is not unhealthy.

If your body runs low on carbohydrate fuels, it'll start to metabolize these fats to make energy instead. There are different chemical reactions involved, and different byproducts get created. But if you completely eliminate carbs from your diet, your body might not get enough fiber, which is a group of compounds – including some complex carbohydrates – that we don't have the enzymes to digest. At first, that doesn't seem at all helpful, because typically we eat food to give our bodies energy or chemical building blocks to make more stuff. But some kinds of fiber – like cellulose, a polysaccharide that gives plants structural support – help promote healthy bowel movements by adding bulk to your stool. And other kinds of fiber can dissolve in water, forming a sludgy mixture that can reduce your LDL cholesterol levels – the kind that can build up in your bloodstream and be dangerous. These fiber molecules bind to cholesterol and these chemicals called bile acids – which are made from cholesterol – in your intestines, so you excrete them.

So fiber is your friend, and you shouldn't be afraid of carbohydrates. They're important nutrients, can taste delicious, and give your body lots of energy. But it is possible to have too much of a good thing.

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