Carbohydrates are one of three macronutrients your body uses for energy, alongside protein and fat. They’re molecules made of carbon, hydrogen, and oxygen atoms, found in foods ranging from bread and fruit to beans and milk. Your body breaks them down into glucose, its preferred fuel source, which powers everything from brain function to muscle movement.
Simple vs. Complex Carbohydrates
Carbohydrates fall into two broad categories based on their chemical structure and how quickly your body can break them down.
Simple carbohydrates are small molecules, either single sugar units (like glucose and fructose) or two sugar units linked together (like the lactose in milk or the sucrose in table sugar). Common sources include honey, fruit juice, syrup, soda, cookies, cakes, and candies. Refined grains like white bread, white rice, and white pasta also count as simple carbs because processing has stripped away their fiber. That said, simple carbs aren’t automatically unhealthy. Whole fruit and dairy foods contain simple sugars alongside vitamins, minerals, and other beneficial nutrients.
Complex carbohydrates are long chains of sugar units bonded together. Because your body has to work harder to dismantle these chains, they release energy more gradually. Examples include starchy vegetables (potatoes, peas, corn), legumes (beans, lentils), and whole grains like oats, brown rice, and quinoa. These foods tend to come packaged with fiber, which slows digestion further and provides its own health benefits.
How Your Body Digests Carbohydrates
Carbohydrate digestion starts the moment food enters your mouth. Saliva contains an enzyme called amylase that begins breaking starch into smaller sugar fragments. Once you swallow, no carbohydrate digestion happens in the stomach. The real work picks up in the small intestine, where pancreatic amylase continues splitting starches and a set of specialized enzymes finishes the job. One enzyme breaks the bond in table sugar to release glucose and fructose. Another splits milk sugar into glucose and galactose. A third handles maltose, producing two glucose molecules.
The end result is always the same: carbohydrates get reduced to single sugar units small enough to cross the intestinal wall and enter your bloodstream. From there, glucose travels to cells throughout your body to be used as fuel.
What Carbohydrates Do in Your Body
Glucose is your body’s go-to energy currency. Cells convert it into ATP, the molecule that directly powers virtually every biological process. Even at rest, each muscle cell contains roughly a billion ATP molecules, all of which get used up and replaced every two minutes. During intense exercise, ATP production can spike a thousandfold to keep up with demand.
Your body doesn’t burn all the glucose it absorbs right away. Some gets stored as glycogen, a compact form of glucose tucked into your liver and muscles for later use. The liver holds over 80 grams of glycogen and uses it to maintain the roughly 4 grams of glucose circulating in your blood at any given time. When you exercise at moderate to high intensity, muscle glycogen and blood glucose become the primary fuels your body draws on.
When glycogen stores are full and there’s still excess glucose in the blood, your body converts the surplus into fat for longer-term storage.
The Role of Fiber
Fiber is a carbohydrate your body can’t fully digest, which is exactly what makes it valuable. It comes in two forms that do different things.
Soluble fiber dissolves in water and forms a gel-like material in the stomach, slowing digestion. This helps moderate blood sugar spikes after meals. Soluble fiber found in beans, oats, flaxseed, and oat bran can also block some cholesterol absorption, lowering LDL (“bad”) cholesterol levels. Good sources include oatmeal, apples, citrus fruits, and legumes.
Insoluble fiber doesn’t dissolve in water. Instead, it adds bulk to stool and helps material move through the digestive system, which is why it’s helpful for constipation. Whole wheat, nuts, and vegetables like cauliflower and green beans are rich in insoluble fiber. High-fiber diets in general are linked to lower blood pressure and reduced inflammation.
How Different Carbs Affect Blood Sugar
Not all carbohydrates hit your bloodstream at the same speed. The glycemic index (GI) is a scale that ranks foods by how quickly they raise blood sugar compared to pure glucose. Foods with a GI of 55 or less are considered low glycemic. A GI of 56 to 69 is moderate. Anything at 70 or above is high glycemic.
White bread and sugary cereals tend to score high, meaning they cause a rapid blood sugar spike followed by a crash. Lentils, most whole fruits, and non-starchy vegetables score low, providing a steadier release of energy. Pairing high-GI foods with protein, fat, or fiber can blunt the spike, which is why a slice of white bread with peanut butter behaves differently in your body than a slice of white bread alone.
How Much You Need
The Dietary Guidelines for Americans recommend that carbohydrates make up 45% to 65% of your total daily calories. On a 2,000-calorie diet, that works out to roughly 225 to 325 grams per day. The range is wide because individual needs vary depending on activity level, age, and metabolic health.
The type of carbohydrate matters as much as the amount. The World Health Organization recommends keeping free sugars (added sugars plus sugars in honey, syrups, and fruit juice) below 10% of total daily calories, with additional benefits from dropping below 5%, which is about 25 grams or 6 teaspoons per day. For context, a single can of regular soda contains around 39 grams of sugar, already exceeding that stricter target.
Prioritizing complex carbohydrates and whole food sources over refined and heavily sweetened options is the simplest way to get the energy your body needs without the downsides of rapid blood sugar swings and empty calories.