Carbohydrate Guide
What happens to your body on a low-carb diet
Cutting carbs sharply forces your body to switch fuel sources — and that transition comes with a predictable set of effects.
The adaptation period
When carbohydrate intake drops sharply, glycogen stores empty out within a few days, and your body increasingly relies on fat (producing ketones) and protein (via a process called gluconeogenesis) for fuel. This transition period is sometimes called "keto flu" and can include fatigue, headaches, irritability, and brain fog as your body adapts.[1]
Effects to know about
- Reduced high-intensity exercise performance — glycogen is the primary fuel for fast, powerful efforts, so low-carb intake can blunt performance in that specific type of activity
- Increased muscle protein breakdown risk — with glycogen low, the body relies more on amino acids for energy, which can work against muscle-building goals unless protein intake is raised to compensate
- Reduced fiber intake — cutting carbs often means cutting fiber-rich foods (whole grains, fruit, legumes) too, unless you deliberately replace that fiber elsewhere
- Initial water weight loss — glycogen is stored with water, so early weight loss on a low-carb diet is partly water, not fat
None of this means low-carb diets don't work for some people and some goals — it just means the early effects are largely predictable physiology, not a sign something is wrong. Anyone on blood-sugar-lowering medication should talk to a doctor before cutting carbs significantly.
Keep reading: What is glycogen? · What carbohydrates actually do · Simple vs. complex carbs