How much carbs can you (athlete or not) eat before you start converting it to fat?

That’s a question that is on many people’s mind, including athletes fuelling with sugar-heavy drinks, gels and bars.

As this study from the American Journal of Clinic Nutrition shows: the glycogen storage capacity in humans is around 15g/kg.

What does this mean?

When you eat carbohydrates, they get digested and pushed into the bloodstream in the form of glucose (except for fructose which goes straight to the liver).

Once glucose is in the bloodstream, it can only go to one of 2 locations:

  • Stored as Glycogen in the muscles and liver.
  • Converted into Fat by the liver.
  • (being burned immediately is very rare and only in the middle of a hard and long sporting event such as a marathon).

So of course this means that we want ALL the carbohydrates we eat to get stored as Glycogen, and NOT converted to FAT.

As I mentioned earlier: human Glycogen storage capacity is 15g/kg.

This means that a 75kg man can store 1,125g of carbohydrates.

But WAIT! This is TOTAL storage capacity. This is the storage capacity of EVERY SINGLE MUSCLE in your body, from your small toe all the way to muscles controlling your eyelids, when they are ALL EMPTY!

It’s virtually IMPOSSIBLE to COMPLETELY empty your glycogen stores: your body’s self-defence mechanism will force you to stop exercising before that happens: studies on ultra-marathon runners have shown that even when they hit the “wall” 50km into a run, their glycogen stores are still not empty.

So let’s take a practical example:

  • I do a 60min super-hard cycling workout.
  • This means that I’m burning around 800Kcal for that hour.
  • Let’s assume that I was “hammering it” for the full 1 hour.
  • This means that potentially 75% of calories burned were from glycogen (not fat).
  • To COMPLETELY replenish the glycogen I used, I need 600kcal from carbohydrates (75% of 800kcal).
  • This is equivalent to 150g of Carbs (4kcal per g), or 3 bowls of Oatmeal (yes 3!), OR 2 bowls of Pasta, OR 2 bowls of rice!
  • This means that if you eat much more than 150g of carbs throughout that ENTIRE day, most of that excess is likely to get converted into FAT by the liver (because there is no more glycogen storage capacity to absorb it).


Bottom line, my advice and my personal practice:

On non-training days:

  • I assume 2000kcal burned throughout the day.
  • 15% from carbs (since most of the day I’ll be burning fat).
  • To completely replenish glycogen stores, I need 300kcal from carbs, or 75g.
  • (If I’m trying to lose weight, I drop that by another 20-30g).

On moderate intensity training days:

  • I assume 2800kcal burned (including workout).
  • 20% from carbs (since my workout would be mostly in fat burning zones).
  • To completely replenish glycogen stores, I need 560kcal from carbs, or 140g.

On hard training days:

  • I assume 3000kcal burned (including workout).
  • 35% from carbs.
  • To completely replenish glycogen stores, I need 1000kcal from carbs, or 260g.


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