Carbohydrates · 4 calories per gram
Carbohydrates: Your Body's Preferred Fuel
Carbs get blamed for a lot, but they're the primary fuel for your brain and the stored energy your muscles draw on during exercise. Here's how glycogen storage actually works, and what happens when carb intake drops too low.
What Carbohydrates Actually Do
Carbohydrates are sugars, starches, and fiber found in grains, fruits, vegetables, and legumes. During digestion, most carbs are broken down into glucose, which cells throughout your body use directly for energy. Any glucose not needed immediately is stored for later use or, beyond storage capacity, converted to fat.[1]
Carbs are described as your body's "preferred" fuel because they can be converted to usable energy faster than fat, and because certain tissues — most notably the brain — rely on a steady glucose supply to function normally.
Glycogen: Your Body's Carb Battery
Glucose your body isn't using right away gets bundled into a storage molecule called glycogen and stashed mainly in your muscles and liver — muscle glycogen fuels the muscle it's stored in, while liver glycogen can be released into the bloodstream to keep blood sugar stable, including for the brain.
During prolonged or intense exercise, glycogen stores get depleted, which is a major cause of fatigue and declining performance late in a workout or endurance event. Sports nutrition guidance recommends carbohydrate intakes as high as 8–12 g/kg body weight per day for athletes with high training volumes, specifically to keep glycogen stores topped up.[2]
Fuel for Your Brain
Glucose is the brain's primary and, under normal conditions, obligate fuel source. Unlike muscle, the brain can't directly use circulating fat for energy because fatty acids can't efficiently cross the blood-brain barrier — which is part of why very low blood sugar causes symptoms like brain fog, irritability, and difficulty concentrating.[3]
Fiber & Gut Health
Fiber is a type of carbohydrate your body can't fully digest. It adds bulk to stool, feeds beneficial gut bacteria, and helps regulate blood sugar and cholesterol. Most adults need roughly 25g/day (women) to 38g/day (men) under age 50, and most people fall short of that target.[4]
What Happens on Very Low-Carb Intake
When carbohydrate intake drops sharply, glycogen stores empty out and your body increasingly relies on fat (via ketone production) and protein (via gluconeogenesis) for fuel. Commonly reported effects during this adaptation period — sometimes called "keto flu" — include:[5]
- Fatigue and low energy, especially during exercise, as glycogen availability drops
- Headaches and irritability during the initial adaptation period
- Reduced high-intensity exercise performance — glycogen is the main fuel for fast, powerful efforts
- Increased muscle protein breakdown — with glycogen low, the body increases use of amino acids for energy (gluconeogenesis), which can work against muscle-building goals unless protein intake is raised to compensate
- Digestive changes — cutting carbs often means cutting fiber-rich foods too
How Many Carbs Do You Need?
The AMDR for carbohydrates is 45–65% of total calories for most adults, with higher intakes generally appropriate for endurance athletes and lower intakes sometimes used deliberately for specific medical or performance goals.[6]