Why You Might Gain Weight When Marathon Training

Marathon training is intense, and for me, it came with an unexpected side effect—weight gain. Here’s why it happens and why it’s more common than you think.

Right now, I’m just over two weeks away from running my third marathon, and one thing has become increasingly clear over the past sixteen weeks of training—I’ve gained weight, despite all the running. Looking back at my first two marathons, I remember noticing the same pattern.

There’s growing awareness that marathon training can sometimes lead to weight gain, but the misconception still exists that running long distances automatically makes you ‘skinny’ or that you need to be ‘skinny’ to run marathons.

This time around, I’ve found the weight gain particularly difficult to process and accept. I’m tired of having to ‘carb-load’ every weekend before long runs or forcing myself to take in gels and sugary snacks mid-run. I know this is a privileged problem to have, and I feel guilty even mentioning it, but I want to be honest about my experience. I know others may feel the same way, and I think it’s important to talk about these mental struggles.

It’s hard to wrap your head around the idea that you need to eat more to perform better—and honestly, I haven’t fully mastered this yet. Growing up in a society that constantly pushes diet culture and the idea that smaller is better, it’s exhausting to feel like I have to eat more to run well. To be clear, my goal isn’t to be ‘skinny,’ but I do feel like I’ve lost some of the muscle definition I worked so hard to build in the gym. Logically, I know this is because I’m spending more time running, less time strength training, and prioritizing carbohydrates over protein. But it’s still a mental adjustment.

At the end of the day, you want to look and feel your best on race day—but you also won’t make it to the finish line without fueling properly.

There are several reasons why some runners gain weight during marathon training, and I’ve found that understanding the physiological and psychological factors behind it helps. On days when I feel less confident in my body, reminding myself of why these changes are happening makes it easier to accept that it’s all part of the process.

Why You Might Gain Weight During Marathon Training

1. Increased Glycogen Storage

Marathon training encourages your body to store more glycogen, which is essential for endurance. However, glycogen holds water - around three grams for every gram of glycogen - so this added storage can temporarily increase weight. This isn’t fat gain, but rather an important adaptation for endurance performance.

Tip: Change the narrative - focus less on the number on your scales and rather appreciate your body’s capacity to adapt, grow and carry you on those long runs.

2. Muscle Gain

Running - especially hill work, sprints, and strength training - can lead to muscle growth. Muscle is denser than fat, meaning the scale may go up as a result of increased muscle mass not fat. This is a positive adaptation that can improve strength, endurance, and metabolism.

Tip: Remember muscular strength enhances performance, reduces injury risk, and supports overall fitness.

3. Blood Sugar Roller-coaster

Long runs, especially when fueled with sugary gels and drinks, can cause sharp spikes and crashes in blood sugar levels. When blood sugar drops, the body craves quick energy, often leading to overeating or poor food choices.

Tip: Have a small pre-run meal to help stabilize blood sugar and experiment with fueling strategies to avoid extreme highs and lows.

4. Overestimating Calorie Burn

Running burns a lot of energy, but it’s easy to overestimate just how much. Many runners, as do I, reward themselves with extra food after long runs, thinking they've earned it. However, the calories burned per mile typically range from 80-100, meaning a 20-mile run might burn around 1,600-2,000 calories—less than what’s in a big post-run feast. If you’re regularly eating more than you burn, weight gain can follow.

Tip: Be mindful of portion sizes and focus on nutrient-dense foods that support recovery and performance.

5. Reduced Overall Activity

Ironically, marathon training can sometimes lead to less daily movement. If your long runs leave you exhausted, you might find yourself resting more, reducing your overall calorie expenditure. Running for an hour but spending the rest of the day sedentary could offset the calories burned. I have fallen victim to this, for example prior to marathon training I used to boulder regularly, but now as a way to preserve my energy for running I often skip this activity.

Tip: Incorporate light movement throughout the day—short walks, stretching, yoga, or gentle cross-training can help maintain an active lifestyle without adding excessive fatigue.