While the goal in training for a half marathon [1], a marathon [2], or any running race is to feel fit and strong enough to finish the race, some people may have another goal: to lose weight [3]. With all those hours training, it would make sense that getting leaner would just be automatic. But that's not always the case. POPSUGAR spoke with registered dietitian Kristy Baumann, RD [4], who explained that marathon training can sometimes cause weight gain — it happened to me! Keep reading to learn the five reasons behind weight gain and why it may not be a bad thing.

Baumann has been running for over 20 years and has ran numerous half marathons and nine full marathons, including the famous Boston Marathon. She specialises in helping runners, of all levels, fuel their body to run strong and have a healthy relationship with food, so I trust her expertise in this subject.

Why Do I Gain Weight When Marathon Training?

You'd expect to lose weight when running so many miles, right?! It's actually not at all uncommon to gain weight, and here are some of the reasons:

1. You're eating more carbs: Carbohydrates are your body's primary energy source [5] because they're easily and readily available for your body to use. With endurance activity, naturally, your body needs and uses more carbohydrates, Baumann said. When you eat more carbs like bread, pasta, potatoes, oatmeal, and rice, it gets stored in your liver as glycogen, which causes your body to hold onto more water. "For every one ounce of glycogen stored, your body also stores about three ounces of water," she said, and this leads to extra water weight, which you would notice when stepping on the scale. This definitely does not mean you should skimp on the carbs! Remember, you need carbs for fuel.
2. Exercise increases inflammation: When you are doing higher-intensity workouts, long runs, and interval training, this increases inflammation in the body [6], Baumann explained. In order to heal, your body needs to retain water, which causes temporary weight gain.
3. You skip your post-run fuel: If you don't refuel right after your long run or hard workout, it can leave you feeling ravenous later in the day, Baumann warned. When you're overly hunger, it'll cause you to reach for foods that are quick and calorie dense, like sweets and snacky foods. When you're this hungry, Baumann said, you tend to eat past the point of fullness, which can cause the weight gain you see on the scale. Make sure to fuel throughout the day to prevent overeating later.
Related:

I Did This 10-Minute Workout For Stronger Knees For 2 Weeks and It Absolutely Helped Prevent Knee Pain [7]
4. You're lacking sleep: Life is busy, and if you start training for a marathon on top of that, you may be running early in the mornings, which cuts into sleep time. When you're tired, it can lead to mindless eating, Baumann said, in order to make you feel less tired. In addition, lack of sleep increases the stress hormone cortisol, which causes the body to hold onto weight.
5. You gained muscle: When you're running more miles and longer distances and incorporating track workouts, you are training your muscles to get stronger, which promotes muscle growth [8]. Many runners incorporate strength training into their routine, too, and building muscle results in your weight going up. That's because, as Baumann explained, muscle is more dense than fat. This means that one pound of muscle mass takes up less space than one pound of fat mass. The scale may go up, but you'll look leaner and be a stronger runner.