A carb, short for carbohydrate, is one of three macronutrients your body uses for energy, alongside protein and fat. Carbohydrates are molecules made of carbon, hydrogen, and oxygen atoms, and they show up in foods ranging from table sugar to whole grains to vegetables. Current U.S. dietary guidelines recommend that 45 to 65 percent of your daily calories come from carbohydrates, making them the single largest source of fuel in most diets.
The Three Types of Carbohydrates
Carbohydrates fall into three categories based on their size and complexity.
Simple sugars (monosaccharides) are the smallest units. Glucose, fructose, and galactose are the most common. Glucose is the one your body uses most directly for energy. Fructose is the sugar naturally found in fruit. Galactose is part of the sugar in milk.
Double sugars (disaccharides) are two simple sugars bonded together. Table sugar (sucrose) is glucose plus fructose. Lactose, the sugar in dairy, is glucose plus galactose. Maltose, found in beer and malted foods, is two glucose molecules linked together.
Complex carbohydrates (polysaccharides) are long chains of sugars. Starch, the main carb in potatoes, rice, and bread, is the most familiar. Glycogen is the form your body uses to store carbs in your liver and muscles. Cellulose is the structural material in plant cell walls, which humans can’t digest but counts as dietary fiber.
How Your Body Processes Carbs
Digestion starts in your mouth and continues in your small intestine, where enzymes break complex carbs and double sugars down into simple sugars, primarily glucose. That glucose enters your bloodstream, and your blood sugar rises.
In response, your pancreas releases insulin. Insulin acts like a key, unlocking your cells so glucose can move inside and be used for energy. In the liver and brain, glucose flows in relatively freely, but most other cells in your body depend on insulin to absorb it. Insulin can increase glucose uptake into cells by ten times or more. Without enough insulin, or when cells stop responding to it properly, glucose builds up in the blood. That’s the core problem in diabetes.
Where Your Body Stores Extra Carbs
When you eat more carbohydrate than you immediately need, your body converts the excess glucose into glycogen and tucks it away. Your muscles store roughly 300 to 700 grams of glycogen, while your liver holds anywhere from almost none to about 160 grams, with an average around 80 grams. That muscle glycogen fuels physical activity, and liver glycogen helps maintain steady blood sugar between meals.
Once those glycogen stores are full, any remaining excess gets converted into fat. This is why consistently eating more carbs than you burn contributes to weight gain, regardless of whether those carbs come from bread, juice, or candy.
Simple vs. Complex Carbs in Your Diet
The distinction between “simple” and “complex” gets thrown around a lot, but the more useful way to think about it is refined versus whole. Refined carbs, like white flour and white rice, have been stripped of their outer layers during processing. That removes fiber, B vitamins, iron, zinc, magnesium, and selenium. What’s left digests quickly, spikes your blood sugar faster, and leaves you hungry sooner.
Whole grain carbs keep those layers intact. The fiber slows digestion, the minerals support dozens of body processes, and the overall effect on blood sugar is more gradual. Fruits and vegetables also count as carbohydrate sources, and they bring vitamins, water, and fiber along with their natural sugars.
The Role of Fiber
Fiber is a carbohydrate your body can’t break down into sugar. It passes through your digestive system mostly intact, but it does important work along the way. There are two types, and most plant foods contain some of each.
Soluble fiber absorbs water and forms a gel-like substance in your gut. This slows digestion, feeds beneficial gut bacteria, and has been shown to lower cholesterol levels, reducing the risk of heart disease. Oats, beans, and apples are good sources.
Insoluble fiber does the opposite: it speeds things up. It adds bulk to stool and helps food move through your intestines more efficiently, reducing constipation. You’ll find it in whole wheat, nuts, and vegetables like cauliflower and green beans.
Resistant Starch: A Carb That Acts Like Fiber
Some starch escapes digestion in your small intestine entirely and reaches your colon, where gut bacteria ferment it. This is called resistant starch, and it behaves more like fiber than a typical carb. You’ll find it in whole grains, seeds, legumes, raw potatoes, and green bananas. Cooked and then cooled starchy foods, like day-old rice or cold potatoes, also develop resistant starch as the starch molecules rearrange during cooling. Because it isn’t absorbed as glucose, resistant starch has a minimal effect on blood sugar.
Glycemic Index and Glycemic Load
Not all carbs hit your bloodstream at the same speed. The glycemic index (GI) ranks carbohydrate-containing foods on a scale from 0 to 100 based on how quickly they raise blood sugar compared to pure glucose, which sits at 100. White bread and sugary cereals score high. Lentils, most vegetables, and whole oats score low.
The limitation of GI is that it doesn’t account for portion size. Watermelon has a high GI, but a typical serving contains relatively little carbohydrate. That’s where glycemic load (GL) comes in. GL multiplies a food’s GI by the actual grams of carbohydrate in a serving, giving a more realistic picture of what that food does to your blood sugar in practice. If you’re trying to manage blood sugar levels, GL is the more useful number.
What “Net Carbs” Means on Labels
You’ll see “net carbs” on many packaged foods, especially products marketed for low-carb diets. The idea is straightforward: take the total carbohydrates listed on the nutrition label, then subtract the grams of fiber and sugar alcohols. Fiber and sugar alcohols don’t raise blood sugar the way other carbs do, so they’re treated as “free” in this calculation. A bar with 24 grams of total carbs but 10 grams of fiber and 8 grams of sugar alcohols would list 6 net carbs. It’s worth noting that “net carbs” isn’t a term regulated by the FDA, so the math can vary between brands.
Common Carb-Rich Foods
- Grains: bread, rice, pasta, oats, cereal, tortillas
- Starchy vegetables: potatoes, corn, peas, sweet potatoes
- Fruits: bananas, apples, berries, oranges, grapes
- Legumes: beans, lentils, chickpeas
- Dairy: milk, yogurt (from lactose)
- Sweets and sugary drinks: candy, soda, juice, baked goods
Meat, fish, eggs, and oils contain little to no carbohydrate. Cheese and butter contain very small amounts. Nuts and seeds have some, mostly alongside fat and protein.