Megan Markoff, director of nutrition at coaching company Pallas, was scrolling Instagram when she saw a controversial post claiming “running made me fat.” She grabbed her phone and recorded a response video that later went viral, rebutting the statement. “I see this all the time. What usually happens is a mismatch between training and nutrition,” Markoff said to the camera.
Across social media, influencers are increasingly warning that long-distance running leads to weight gain. “I didn’t even understand how prominently this narrative was getting pushed on social media until a new client told me about it,” Markoff tells Runner’s World.
But the reality is nuanced. While it’s perfectly normal—and often beneficial—to gain some weight during a half or full marathon training cycle, Markoff says social media often oversimplifies and misrepresents what’s actually happening in the body.
If you’ve ever been turned off from running in fear of gaining weight or avoided the marathon because of social media chatter, it’s time to clear up the confusion. Experts break down five myths about running and weight gain that runners should stop believing.
Myth 1: Weight gain during marathon training is fat gain.
Kristy Baumann, RD, a registered dietitian who works with marathoners, says it's not uncommon for runners new to conquering long distances to gain weight, but that doesn’t necessarily mean you’ve gained fat. Some of the bodyweight changes runners experience are normal adaptations to endurance training.
As you run more, your muscles become more efficient at storing glycogen, the carbohydrate your body uses for energy during exercise. This is beneficial—the more glycogen your muscles can store, the more energy you have available during your workouts, even before fueling.
But glycogen also binds to water: “For every gram of glycogen stored, the body stores roughly 3 grams of water,” says Baumann. So if you step on the scale and the number is higher than it was the week before, it may reflect temporary water retention—not permanent fat gain—which can ebb and flow throughout a training cycle.
Hard workouts like long runs, tempo runs, and strength training also create microscopic muscle damage, which your body repairs to get stronger. The resulting inflammation causes the body to retain additional fluid as part of the recovery process, says Baumann.
Myth 2: Eating less before and during your runs will result in weight loss.
Markoff says she commonly hears from people who start running with weight loss as their primary goal. When losing pounds is a top priority, runners tend to believe eating less during training will help them lose weight faster. “I think that can often backfire,” she says.
Diet culture has taught many people to treat exercise and eating as transactional—if you eat too much or the wrong thing, your workout doesn’t “count.” As a result, Markoff says she sees many runners trying to eat as little as possible around training.
Underfueling can not only hurt running performance, but it can also make appetite harder to regulate. “If you don’t refuel correctly after a long run, it can leave you feeling ravenous later in the day,” says Baumann. “When you can’t control your hunger, chances are you’re raiding the pantry and eating everything in sight by nighttime.”
Baumann recommends a mindset shift: “Instead of thinking how little can I eat and still get by and feel fine on my run?, think what can I eat to best support my running and training?” she says.
This means fueling before, during, and after your runs, especially runs over 60 minutes. While everybody is different in terms of the amount of carbs they can take in, Baumann recommends aiming for a minimum of 30 grams of carbs (a banana, a few graham cracker sheets, some dried mango) before your run.
During your runs, try to get at least 30 grams of carbs an hour, though experienced runners may be able to push much higher. And while protein gets all the glory as a postworkout recovery macro, both experts say your meal after a run should also have carbs. Aim for a 3:1 or 4:1 carb-to-protein ratio, which you’ll find in chocolate milk, Greek yogurt with fruit, or eggs on toast.
Myth 3: You don’t need to focus on the type of foods you eat during marathon training.
On the other end of the spectrum, some runners throw all healthy eating habits out the window once they start training, adopting an “I ran 10 miles, so I can eat whatever I want” mentality.
While it’s certainly okay to eat the foods you love, even if some people may label them “bad,” research shows that the average person tends to overestimate the number of calories they burn during exercise. Even if you rely on wearable measurements, most of them do not provide accurate data around energy expenditure.
While marathon training does increase calorie needs and you will need extra carbs to fuel your runs, Markoff says that overall food quality still matters. If you only focus on simple carbs, like gels and pastries, you miss out on protein and fiber, like you’ll find in complete meals, such as a burrito bowl with chicken, veggies, and rice.
Myth 4: Running spikes cortisol, resulting in weight gain.
If you've spent any time on wellness TikTok lately, you've probably heard about cortisol, the stress hormone. Many influencers are promoting the idea that high-intensity exercise, like some running, floods your body with cortisol, causing you to store fat and puff up. While that may sound off-putting, if you prioritize fueling and recovery, running itself is not enough to chronically raise cortisol levels.
Yes, cortisol does rise during a run. Exercise is a form of stress on the body, so cortisol increases temporarily. And this increase is necessary: Cortisol plays a role in the breakdown of carbs and fats into energy. A study published in the Journal of Sports Medicine and Physical Fitness in 2015 examined half-marathoners and found that the better an athlete performed, the higher their cortisol levels.
But while cortisol spikes in the short term during exercise, regular physical activity actually helps reduce cortisol levels over time, according to a meta-analysis published in Psychoneuroendocrinology in 2022. Most runners do not need to worry about their cortisol, says Markoff.
However, there is one caveat: In cases of overtraining or under-recovering, people can experience chronically elevated cortisol levels, which can promote weight gain. But that’s not a consequence of running itself—it’s a sign of an unbalanced training load or an unchecked stressful lifestyle.
Myth 5: Weight gain during marathon training will hurt performance.
Many runners believe they need to be as lean as possible to be the most efficient runner. But Baumann says that running to lose weight is the opposite goal of running for performance. “In order to lose weight, you need to be in a calorie deficit,” she says. “And when you’re marathon training, that results in underfueling, which could lead to drops in performance or even injury.”
When a runner isn’t eating enough to match their training load, the body starts to compensate, leading to fatigue, poor recovery, hormonal disruptions, or burnout. A study published in the British Journal of Medicine in 2024 found that among more than 1,000 Boston Marathon athletes, those with “low energy availability” from restrictive eating patterns had worse race time outcomes—and ended up in the medical tent more often—than athletes who didn’t report any restrictive eating behaviors.
Fueling with enough calories (even if it comes with some mild weight gain) will help you maintain more energy throughout your training program, run faster, and recover better.
In a Reel on Instagram, runner and content creator Emma Mailer says that weight gain was the best thing she did for her running. “Years of underfueling and chasing less left me broken, empty, and now dealing with a stress fracture. Even after regaining my [menstrual] cycle and gaining more than 20 kilograms, the years of underfueling eventually caught up to me,” she wrote. “So let this be your reminder: A fueled body is a faster body.”
Hannah Singleton is a freelance journalist who writes about fitness, health, wellness, travel, and the environment. Her work has been in publications such as the New York Times, Wall Street Journal, GQ, Vox, Wired, National Geographic, Forbes, and Fast Company. You can follow her @hannahsingleton.
