Carb-Loading Bloat and Scale Weight What It Actually Means

Gaining 1-3 lbs (0.5-1.5kg) on the scale during a 1-3 day carb load is normal — it's glycogen and the water bound to it, not fat. Below: the glycogen-water math behind that number, how to run the calculation for your own bodyweight, how to cut the bloat without under-loading, and the specific signs that mean it's something else.

I get a small panic message from a BRT athlete almost every taper week over this exact number on the scale — often within the hour it happens. It's predictable enough that I can usually tell them what's going on before they've finished describing it, so let's cover the mechanism properly, once, so you're not guessing next cycle either.

Why the Scale Moves: The Glycogen-Water Math

Each gram of glycogen your muscles store is packaged with roughly 3g of water. Load 400-600g of carbohydrate over your loading window — which is a fairly standard target — and you can plausibly add 1.5-2.5kg of combined glycogen and water weight. That's not fat accumulating in two days; nothing metabolic works that fast. It's your muscles topping up their fuel tank, and the water is along for the ride because that's how glycogen is stored.

Do Your Own Math for Your Carb-Load Target

You can check this against your own plan rather than taking a generic range on faith. Take your bodyweight in kilograms, multiply by 8-10g of carbs per day — the standard 1-2 day loading target — and that's your daily carb intake in grams. Multiply that figure by roughly 3-4x to estimate the glycogen-plus-water mass it adds.

Take the 65kg example: 520-650g of carbs a day translates to roughly 1.5-2.2kg of combined glycogen and water by race morning. That weight is functional pre-race fuel. Your muscles draw it down gradually across the full race – typical glycogen stores support around 90-120 minutes of race-pace effort before running low, which is exactly why the load (and any fuelling you take on during the race) matters most in the second half, not the first.

Reducing Bloat Without Under-Loading

The bloat itself is usually less about total carbs and more about how they're eaten. Spread intake across 5-6 smaller meals and snacks rather than 1-2 large ones — the pattern I see most often is runners front-loading two huge carb-heavy dinners, then feeling far worse the next morning than someone who spread the same total across more, smaller sittings.

In the final 24 hours, favour lower-fibre, simpler carb sources — white rice, potatoes without the skin, sports drinks, low-fibre bread — over wholegrain or legume-heavy options. Cut raw cruciferous vegetables and legumes specifically in the final 24-36 hours; they're the biggest fibre-driven bloat contributors. And keep fluid intake steady rather than spiking it alongside the carb load — over-hydrating at the same time as loading compounds the bloated feeling rather than easing it.

When It's Not Normal Carb-Load Bloat

If your stomach reacts beyond ordinary fullness, the fix usually isn't eating less carbohydrate — it's training your gut ahead of time so it tolerates the load without protest. And if race day itself is hot or humid, that changes your fluid math on top of all this — see adjusting fueling if race day is hot or humid.

Back to the full marathon taper guide for everything else in race week.

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