When someone's body is used to burning fat for fuel, it can still keep their muscles stocked with energy (glycogen) by making sugar from protein and fat — even if they eat zero carbs.

What we've found so far is that the evidence does not strongly support the idea that the body can maintain muscle glycogen without carbs, even when fat-adapted. Our analysis of the available research shows that while one claim suggests the body might replenish muscle glycogen from protein and fat in the absence of carbohydrates [1], this view is outweighed by a larger number of studies that refute it. We looked at a total of 74.0 study assertions related to this question. Of those, only 14.0 support the idea that fat-adapted individuals can maintain muscle glycogen without consuming carbs [1]. The majority — 60.0 assertions — found evidence against it. This means the current body of research we’ve reviewed leans toward the conclusion that carbohydrate intake plays a key role in maintaining muscle glycogen, regardless of fat adaptation. We do not yet have enough consistent evidence to say that the body can fully sustain muscle glycogen stores through alternative fuels like protein and fat alone. The process of making glucose from non-carb sources (called gluconeogenesis) exists, but the data we’ve reviewed suggests it may not be sufficient to keep muscle glycogen at optimal levels over time [1]. Our current analysis shows that while metabolic flexibility — the ability to use different fuel sources — improves with fat adaptation, it may not replace the role of dietary carbs in glycogen storage. We’re still building our understanding, and future evidence could shift this picture. Practical takeaway: If your goal is to keep your muscles fueled for performance, especially during intense activity, eating some carbs may still matter — even if you’re fat-adapted.