What is Carbohydrates?

What is Carbohydrates?

Carbohydrates are one of the three macronutrients and the body's preferred, fastest-available energy source. Chemically they're built from sugar units, ranging from single sugars like glucose to long branching chains like starch and fiber. Foods high in carbohydrates include bread, rice, pasta, potatoes, fruit, and vegetables. During digestion, most carbohydrates are broken down into glucose, which either fuels cells immediately or is stored as glycogen in the liver and muscles for later use.

How it works

Once eaten, digestible carbohydrates are broken down into glucose, which enters the bloodstream and is either used immediately for energy or stored as glycogen in the liver and muscles, with insulin managing that storage process. During exercise, especially higher-intensity resistance training or sprinting, stored muscle glycogen is the primary fuel your muscles draw on, which is why carbohydrate intake directly affects training performance and recovery. Fiber is a type of carbohydrate the body can't fully digest; instead of being converted to glucose for energy, it supports digestion and gut health and is often excluded from "net carb" counts on some labels.

Types

Worked example

Say your calorie target is 2,400 kcal/day and your macro split allocates 40% of calories to carbohydrates. Here's how that becomes a daily gram target.

Carbohydrate needs rise with training volume — endurance and high-volume strength athletes often need more than 40% of calories from carbs to keep glycogen stores topped up.

Simple vs complex carbohydrates

By goal

• Strength/hypertrophy training: Carbs replenish muscle glycogen between sessions — most lifters do well with 3-5 g/kg bodyweight per day.
• Endurance athletes: Higher carbohydrate needs (often 5-8+ g/kg bodyweight per day) support long glycogen-dependent training sessions.
• Low-carb / keto dieters: Very low carbohydrate intake shifts the body toward using fat and ketones for fuel instead of glucose and glycogen.

Common misconceptions

• "Carbs make you gain fat." Weight gain comes from a sustained calorie surplus, not carbohydrates specifically; carbs at any amount within your calorie target don't uniquely cause fat gain.
• "All carbs are equally healthy (or unhealthy)." Whole-food complex carbohydrates (vegetables, whole grains, legumes) provide fiber and micronutrients that refined, sugary carbohydrates largely lack, even though both provide 4 kcal/g.
• "You need to avoid carbs at night." Total daily calorie and carbohydrate intake matters far more than the time of day carbs are eaten for body composition outcomes.

Related terms

Carbohydrates FAQ

How many calories are in a gram of carbohydrates?

Carbohydrates provide 4 calories per gram, the same as protein and less than half of fat's 9 calories per gram.

What's the difference between simple and complex carbohydrates?

Simple carbohydrates are single or double sugar units that digest quickly (fruit, table sugar), while complex carbohydrates are long chains that digest more slowly (whole grains, legumes, starchy vegetables).

How many carbs should I eat per day?

It depends on training volume and goals, but many lifters do well around 3-5 g/kg bodyweight per day, with endurance athletes often needing more (5-8+ g/kg).

What happens to carbs the body doesn't use right away?

Excess glucose is stored as glycogen in the liver and muscles for later use; once glycogen stores are full, any additional excess calories — from any macro — can be stored as fat.

Is fiber a carbohydrate?

Yes. Fiber is a type of complex carbohydrate the body can't fully digest for energy, so it mostly passes through supporting digestion and satiety rather than being converted to glucose.

References

1. Carbohydrates: Getting the Most Out Of Fiber, Starches & Sugars Cleveland Clinic
2. Physiology, Carbohydrates StatPearls / NCBI Bookshelf, NIH
3. Carbohydrates Harvard T.H. Chan School of Public Health, The Nutrition Source
4. Carbohydrates, Proteins, and Fats Merck Manual (Consumer Version)