My best friend sent me a text on the weekend that gave me pause. He asked: "When keto diets make general claims about staying under so many net carbs, they never consider a person’s size or activity. I assume someone who runs a daily 10k could eat way more carbs and stay in ketosis since they’re all getting used."
It is a brilliant question. And scientifically, he is absolutely right. The "20g Net Carb" rule of strict keto is not a universal law of physics; it is a safety buffer designed for the average, sedentary metabolism. When you introduce high-intensity activity (like a 10 kilometer run), the biochemistry changes.
Here is the science of why athletes can "get away" with more carbs, and the mechanism that makes it happen.
The Two-Tank System
To understand this, we have to look at where your body stores carbohydrates (glycogen). You have two distinct fuel tanks:
The Liver (The Thermostat): This tank controls Ketosis. If your liver glycogen is full, your body burns sugar. If it is empty, your liver starts producing ketones.
The Muscles (The Gas Tank): This tank fuels movement.
Here is the key: Muscle Glycogen does not directly control Ketosis. You can have full muscles and an empty liver, and you will still be in ketosis.
The "Sponge" Effect (GLUT4)
When you sit at a desk, your muscle cells are "locked." To get glucose (carbs) into them, your body has to release Insulin to unlock the door. High insulin shuts down fat burning.
However, exercise changes the lock. Muscle contraction activates a transporter called GLUT4. This transporter moves to the surface of the cell and opens the door for glucose without needing insulin. Essentially, exercise turns your muscles into a dry sponge. If you run a 10k, you wring that sponge out. If you then eat some carbohydrates, your muscles soak them up immediately to refill the tank. The glucose never stays in the bloodstream long enough to spike insulin or refill the liver.
The Numbers: Sedentary vs. Active
The Sedentary Person: Their muscle "sponge" is already full. Any carbs they eat spill over into the liver, stopping ketosis. Limit: ~20g.
The Athlete: They have created a deficit. They can eat carbs to refill that deficit without disrupting the liver's ketone production. This is the basis of the Targeted Ketogenic Diet (TKD), where athletes consume carbs right around their workout.
The Verdict
My friend is correct: You cannot treat a marathon runner and a desk worker with the same metabolic math. Activity buys you metabolic flexibility. However, a word of caution: It is easy to overestimate how much you burned and underestimate how much you ate. Think of exercise not as a license to cheat, but as a tool to deepen your metabolic engine.
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