Whether you call them breakfast food, marathon fuel, or diet culture pariah du jour, one thing we can all agree on is that people have strong opinions about carbohydrates. Somewhere along the way, it’s almost as if we’ve forgotten that “carbs” is just a word we use to talk about certain kinds of foods. And that those foods actually play a huge role in giving our bodies the energy we need.
In order to clear up some of the confusion surrounding the oft-maligned, ever-tasty carbohydrate, we’re breaking it down to the very basics: What carbs actually are, and what they do in your body when you eat them.
What carbs actually are
Technically speaking, carbohydrates are one of the three macronutrients (the nutrients we need in large quantities) in our diet, along with fat and protein. Carbs are the body’s most important energy source, according to the U.S. National Library of Medicine.
Most of the foods we eat—fruits, grains, legumes, vegetables, nuts, sugars, and dairy products—contain some carbs. The main exceptions would be oils and meats. We measure the amount of carbohydrates present in a food in terms of grams—e.g. “This apple has 20 grams of carbs.”
When a specific food is relatively high in carbohydrates, as opposed to fat or protein, we call that entire food a carb—e.g. “An apple is a carb.” We do the same thing for fats and proteins: an avocado is “a fat” and a steak is ”a protein.” (And no, in case you’re wondering, butter is not a carb.)
The different kinds of carbs
Let’s talk chemistry 101 for a hot second. The simplest, most fundamental unit of a carbohydrate is a monosaccharide—a single sugar molecule—made of carbon, hydrogen, and oxygen atoms. These monosaccharide building blocks can be stuck together and arranged into different structures of varying size, shape, and complexity, which all have specific scientific names that describe how they look on a molecular level. These compositions help determine how these different molecules taste in our mouths and work in our bodies.
Unless you spend your days looking at carbs under a microscope, though—which, hey, cool gig if you do—what you really need to know is that carbs can be split up into three main types based on their chemical structures: sugar, starch, and fiber, according to the U.S. National Library of Medicine. While something like white sugar is purely made of sugar, many foods contain two or three types of carbohydrates.
Sugars are often referred to as simple carbohydrates because their chemical structure is, well, simple, and their size is small, Merck Manual explains. They come in the form of monosaccharides (single sugars) or disaccharides (two sugar molecules joined together), the FDA explains, and are naturally found in fruits, dairy, and sweeteners like honey or maple syrup.
Starches and fibers are called complex carbohydrates, because—you guessed it—they look more complicated and large under a microscope. They are generally made of long strings of those simple sugars, called polysaccharides (i.e. many sugars). Starches can be found in foods like beans, whole grains, and some vegetables, like potatoes and corn, while fiber is found in fruits, vegetables, beans, legumes, nuts, and seeds, according to the U.S. National Library of Medicine.
Why we even need carbs
The human body needs all three types of carbs—sugar, starch, and fiber—to function well, according to the U.S. National Library of Medicine because they all get used by our bodies in different ways. (A quick note if you’re wondering, “Well, what about the keto diet?” Keto is indeed based on the fact that your body has a plan B when your carb intake is extremely low: ketosis, a process of converting fat into energy. But there are concerns about these kinds of diets, as SELF previously reported, including the fact that you’re missing out on all the nutrients in carb-containing foods and the lack of data about the safety of fueling your body via ketosis long term.)
Now, broadly speaking, sugars and starches get broken down for energy usage and storage in our cells, tissues, and organs, per the U.S. National Library of Medicine. But fiber is the odd carb out: It actually passes through the body mostly undigested, but helps regulate things like digestion, blood sugar, and cholesterol. (You can read more on why fiber is so important and how it works, here.)
The body is a little like a fancy car that only takes diesel gas, though. Its preferred form of fuel is a type of monosaccharide, or single sugar, called glucose. “Glucose is like our body’s currency for energy,” Whitney Linsenmeyer, Ph.D., R.D., nutrition and dietetics instructor in the Doisy College of Health Sciences at Saint Louis University and spokesperson for the Academy of Nutrition and Dietetics, tells SELF.
Luckily, we don’t have to sit around guzzling glucose all day because the body is able to break down all the carbs we eat (save fiber) into glucose during the process of digestion and metabolism. It breaks carbs down into smaller and smaller pieces, with increasingly specialized steps along the way, until all that’s left is that readily usable form of energy, glucose, Linsenmeyer explains.
What happens in your body when you eat carbs
While all carbs follow the same track from our mouth to their final destination (cells throughout the body), the steps and length of time it takes them to get there depends on the structure of the molecules you’re starting with.
If you’re eating sugar—which, remember, is made up of single sugar molecules or two sugar molecules bonded together—it’s already pretty close to the body’s preferred form of glucose, so there’s not much work to be done. These small sugar molecules can be digested and absorbed into the bloodstream really quickly, which is why they’re the most rapid form of energy, Merck Manual explains. (This is also why they are associated with a quick spike in blood sugar—your body absorbs all that glucose at once.) When you eat starch, the process of breaking it down into glucose happens over a longer period of time, because of its complex structure, Linsenmeyer explains. (That’s why this type of carb provides a slower and steadier form of energy, and is less likely to cause blood sugar spikes.)
Amazingly, your body actually gets to work digesting some complex carbs before you even swallow them. “Your saliva produces something called salivary amylase, an enzyme that starts to break down [starches] as soon as they hit your mouth,” Colleen Tewksbury, Ph.D., M.P.H., R.D., a senior research investigator and bariatric program manager at Penn Medicine and president-elect of the Pennsylvania Academy of Nutrition and Dietetics, tells SELF. (In fact, Tewksbury says, if you let a starchy food like white bread sit on your tongue for a while, it will start to get sweeter as the salivary amylase starts converting it into sugar.)
After you swallow those carbs, they get churned up with gastric juices in your stomach that contain various acids and enzymes. Then, the stomach passes this appetizing mixture on to the small intestine, where the real work of digestion happens, Tewksbury says. Here, more specialized enzymes and acids are introduced to break it down into even tinier bits.
Again, how long digestion takes depends on the kinds of carbs involved. Simple sugars have the greenlight to speed through the process we just described. If you’ve eaten something like candy or fruit juice, composed of simple sugars, there’s not much for your stomach and intestines to do, so this all happens really fast. Starches (and everything else) have to hang around for much longer at each point while they get broken down into smaller and smaller pieces, so the process is more gradual.
How the body converts carbs into energy
As carbs are converted into nice little bits of glucose, they become ready to enter the bloodstream. First, the glucose molecules travel from the small intestine to the liver via the portal vein, Linsenmeyer explains. The liver then dispatches most of that glucose throughout the body via the bloodstream.
Once it hits the bloodstream, some glucose will immediately get used by cells in need of energy—say, those in our brain or our muscles—thanks to the vital hormone called insulin. Insulin allows the glucose in our bloodstream to enter the body’s cells so it can be used for energy. When we eat carbs, the pancreas automatically secretes the perfect amount of insulin to help the cells use glucose and keep our blood sugar levels nice and steady. (This is why people with type 1 diabetes, whose pancreases don’t produce any or enough insulin, need to take insulin to keep their blood sugars in check.)
But we usually consume more carbs than we need at that exact moment. Rather than letting that excess glucose pile up in the bloodstream, the body stores it in a few ways.
A small amount of the glucose is converted into something called glycogen, our body’s special form of readily available “storage glucose” that get deposited in our liver and muscles as an emergency reservoir of energy to use when we need it, Linsenmeyer says—like when you go a long time between meals or go for a really long run, for instance. The rest of the excess glucose gets stored in our fat cells as body fat, again with the assistance of insulin. It can be accessed down the road when we have an energy deficit (i.e. using more calories than we’re taking in).
It’s worth saying that this is a pretty simplified look at what’s happening in our bodies when we eat carbs. There are a whole bunch of processes happening when we eat carbs (or any macronutrient), and scientists don’t even totally understand them all yet. “Our bodies are constantly spinning like 20 different plates at once every time we eat [food] to be able to break it down and utilize it,” Tewksbury explains. For example, there are a bunch of other hormonal secretions happening when we eat carbs or any food, but insulin is one of the most well-understood and useful to know about.
The bottom line is that carbs are super important—and that our body does a bang-up job of putting them to good use so we can get stuff done.