The Real Reasons You Gain Weight While Training for a Marathon (And What You Can Do About It)

blurry picture of runners in a marathon

Most of us equate exercise with some weight loss. Increase the amount of exercise (by committing to something crazy like training for a marathon), and you’re bound to lose weight. Right?

Not necessarily. Even for plant-based athletes.

And maybe more surprisingly, marathon training is probably not the best time to set weight loss goals.

As a dietitian who frequently works with runners, I’ve seen many clients — especially women — put on a few pounds throughout training, and I’ve dealt with signs of it myself while training for anything from a half marathon to ultramarathon.

But why does it happen — and more importantly, what can you do about it?

Well, that depends on your goals …

Important: Now Might Not be the Time to Set Weight Loss Goals

I hate to say it (and as disappointing as it may be), now might not be the time to set weight loss goals, especially if this is your first marathon.

Someone who wants to lose weight aims for a calorie deficit each day, which may result in under-fueling your workouts. On the other hand, in order to recover properly from one workout to the next, a marathon runner needs to replace most (if not all) of the calories they lost. You can see how chasing both goals simultaneously can be counterproductive.

That’s not to say that you need to gain weight while training for a marathon, but I advise that instead of weight loss, make the primary goal your performance.

Now that I’ve gotten that out of the way, let’s explore why weight gain happens, and how you can avoid unnecessarily packing on the pounds.

3 Reasons Why You Might Experience Weight Gain During Marathon Training

When I look at why a client is gaining weight while training for an endurance event, a few reasons stand out:

1. Increased Muscle Mass

Build endurance, strength, and muscle stamina, and you’ll gain muscle mass. This is a good thing.

You’ve likely heard that muscle weighs more than fat, but that’s kind of misleading … a pound of muscle weighs a pound, just like a pound of fat!

However, muscle is denser than fat, and it’s possible to gain weight from gaining muscle. A better measure of fat loss and lean muscle gain is inches, not pounds, but how do your jeans fit? Yeah, I bet they look good.

Dense muscle will trim your waistline, even at the same weight.

That is good news too, because lean muscle is more metabolically active, which means you will burn more calories even at rest. Win – win.

2. Shift in Fluid Balance

Never trust any single weigh-in.

Our weight can fluctuate a lot day to day, particularly due to our hydration. One pound of weight loss is equivalent to one pint of fluid loss, so if you’re losing weight after a long run, you could just be dehydrated.

If you notice weight gain after a tough workout, it could be due to DOMS (delayed onset muscle soreness). With DOMS, your muscles become swollen with fluid and stiff, causing temporary weight gain.

Not to worry, this is all part of your body’s healthy response to your new level of effort, and your muscles are getting stronger. Any weight gain you experience after a tough effort will disappear within a few days.

3. Lack of Sleep and Stress

Aside from the obvious benefits of helping you to not be a monster, did you know adequate sleep is also important for maintaining a healthy weight?

Studies show that tired brains crave sugary treats and aren’t as good at making smart food decisions, which leads to mindless snacking or late-night trips to the fridge.

You know what I’m talking about. We’ve all been there.

Sleep also impacts our hormones. When we’re sleep deprived, we place ourselves under stress, which causes a spike in cortisol, a stress hormone. A small increase in cortisol is a good thing, since it helps us to meet deadlines, stay focused, and train harder, but if we continue to push our body with a tough training schedule and without proper recovery, cortisol builds on itself and starts to work against us, causing weight gain, energy crashes, bad moods, and sugar cravings.

Your body is already under stress from the training, and without adequate rest you will hang on to weight and experience increased inflammation, impacting not just your training but your overall health

But there’s good news: A few extra ZZZs can solve this problem.

Adequate rest helps produce leptin, which promotes a feeling of satiety and keeps the munchies at bay. Aim to get at least seven hours of sleep each night to stay fully rested throughout training.

How to Manage Marathon Training Weight Gain

Unfortunately, there’s not much we can do about the three causes above. That’s just part of training for a marathon.

But carrying extra pounds on race day will impact your performance, and nobody wants that. So how do you manage mid-training weight gain without sacrificing your primary goal, the training?

The biggest contributor to weight gain I see in runners who also have weight-loss goals is over-fueling workouts.

When you eat a pre-workout snack, fuel during the run, and follow it up with a post-run meal — you might be consuming more than you need, especially if your workout isn’t a big one.

Let’s take a look at a 60-minute run as an example:

According to this calculator, a 150-pound runner will burn about 680 calories in a one-hour, six-mile run. That may sound like a lot, but if you started with a bagel and peanut butter (400 calories), fueled with an energy gel (100 calories), and had a post workout smoothie when you got home (200 calories), you just ate back all the calories you burned, plus a few extra. Good for recovery, not so good for weight loss.

And, if you weigh 130 pounds, you only burned 590 calories running at the same pace for the same time.

For easy runs under 90 minutes, it’s less important to fuel the workout. If you’re concerned about weight gain, this may be the time to skip the pre-run snack, stick to water during the run, and recover with a small snack like fruit and more water afterward.

If speed work or a tempo run is on the schedule you’ll want a bit more fuel, but it is surprising how little you need, so I typically stick with just a homemade sports drink.

Quick Tip: My favorite home-made sports drink is coconut water, with fresh-squeezed lemon and a bit of maple syrup to taste. I sometimes add a little hot sauce for zing. That should provide the energy you need for harder efforts.

But always remember, your body needs fuel to recovery properly, so any calorie you’re not replacing with clean fuel is a calorie not working towards your performance goal.

Which leads us to what you do outside of the run … one of my favorite topics.

Why? Because I see time and time again that runners in training give themselves a blank check for indulgences.

Hard work deserves a treat — absolutely — but I recommend taking the mindful approach to eating outside your running:

  • Treat yourself! You deserve it. Just make that vegan brownie a once-a-week thing, not daily.
  • Snacks are great for ravenous runners, but avoid late-night trips to the fridge, since that’s when a lot of mindless eating takes place. After dinner, the kitchen is closed. You’ll burn more fat while you sleep if you extend the hours you fast each night to at least twelve.
  • Use non-food treats to reward your training dedication. A massage or new piece of running gear can be a great motivator. And don’t forget, a great run can be a reward in itself!

Simple, mindful decisions around food can help reduce weight gain without limiting what your body needs to refuel.

Focus on the Marathon … Not Your Weight

Your body is working in overdrive, pushing to new limits and learning how to handle the demands of marathon training.

So trust that training — and your body — and give yourself time to adjust.

Instead of focusing on weight, think back to when you first dreamed of and signed up to train for a marathon. What were your original reasons for doing it?

Looking good in your jeans may be one of them, but chances are it’s about much more than that … the mental and physical benefits, accomplishing a major goal, or any number of other motivators.

If you’ve gained weight throughout training, don’t panic.

It may be a natural part of getting through the 26.2.

About the Author: Pamela Fergusson is a vegan Registered Dietitian with a PhD in nutrition. She and her husband Dave have four children, and she loves to speedwalk ultramarathons. Read her nutrition blog and find her in Facebook.



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  1. Thanks for the column on my constant runner’s plight. Though I know I always gain or maintain, I admit that I still hope to have a lower weight while training. One time I under-fueled while marathon training (trying to fight it) and still gained as my body held onto every last gram of fat to sustain myself. Of course my running suffered too. It is a tough balance. These are great reminders to focus on one goal or the other at a time and I appreciate the advice for fueling while training.

  2. This is a great article, Pamela. I haven’t been thinking about run fueling in terms of combining what I’m getting on both sides of the run, but I can see that I definitely should. The trouble I get into is that I under-fuel before a long run, and then overload to compensate when I get back. This is a great reminder that I should develop some better habits in that area to make sure that the calories going in are more evenly aligned with the calories that I’m burning up. Thanks!

  3. I’ve never gained weight during marathon training. I do most of my runs in a fasted state (because the only time I have to run is at 5AM and I am NOT getting up early to eat beforehand). I will take a few gels or a sports drink on a long run, but that’s it.

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