Why Am I Gaining Weight During Marathon Training?

You're running more than you ever have before. You're lifting at least twice a week to maintain muscle. And yet the scale keeps creeping up.

It feels like you're doing everything right — so why is your body responding in a way that doesn't make sense?

Here's the good news: most likely, you aren't doing anything wrong. Your body is adapting to your training, and sometimes adaptation looks like weight gain. Here's why.

Reasons Why You’re Gaining Weight During Marathon Training:

1. You're storing more glycogen

Glycogen is the storage form of carbohydrate, and as a marathon runner, you've probably been told to "eat more carbs" like it's your job. Here's what's actually happening when you do: for every 1g of carbs you eat, your body holds onto roughly 3–4g of water. That alone can add up to 1–5 lbs of water weight — and it's nothing to be afraid of.

In fact, your body is doing exactly what it's supposed to. On long runs, it needs quick, accessible energy, and the fastest source is stored glycogen. So your body adapts: "We've been running more lately — let's store more glycogen so we have energy ready for next time." It's your body preparing you to run better, not a sign that something's wrong. Biology is pretty amazing like that.

2. You're gaining muscle

Another likely reason: you're actually building muscle. Runners who start out without much leg strength often develop it fast once mileage increases. Every foot strike while running generates an impact force of 2.5 to 4 times your body weight — and over the course of one mile, that's 1,400 to 2,000 strikes. That takes real strength to absorb, run after run.

You're progressively overloading your muscles with every run, especially your long runs. If you're eating enough — or more than enough — to fuel your workouts and support recovery, some muscle growth is a completely normal result.

3. You're in a calorie surplus

The third possibility is a true calorie surplus. Worth noting: for most runners training seriously for a marathon, a calorie surplus is genuinely difficult to hit — training volume alone burns through a lot. If this does seem to be the case, it's usually one of a few things: overestimating your training needs, leaning heavily on hyperpalatable, easy-to-overeat foods, or your energy expenditure being lower than you assume.

Building meals around a mix of protein, carbs, color, and flavor can go a long way toward supporting satiety and helping you eat in a way that actually matches your training.

The bottom line

A few pounds up on the scale during marathon training usually isn't something to worry about — it's often a sign your body is adapting well, not a sign you're doing something wrong.

If the number on the scale has you worried and you want to understand what's actually going on with your training and fueling, book a free 15-minute discovery call with Gabby to see if coaching is the right fit.