Endurance Exercise & Weight-Gain: Why It Happens & How to Prevent It


Did you know that eating too little can lead to weight-gain if you’re challenging your body to perform above and beyond normal physical activity?

The human body is a powerful, complex machine. One of its specialties is conserving energy, which is a fancy way of saying “storing fat.” If you’re burning a major amount of calories every day through intense exercise but you aren’t eating enough, you may experience the most unexpected side-effect: weight-gain.

Let’s take a closer look at why this happens and how you can prevent it.

Types of exercise that qualify as “above and beyond” normal…

While endurance (long-distance) exercise definitely calls for a thoughtful increase in calories, other types of intense exercise count, too. It really depends on the intensity, frequency, regardless of how many miles or hours you put in.

The types of intense exercise that can burn significantly more calories than basic daily exercise include:

  • Training for a marathon or triathlon
  • Long-distance cycling, hiking, swimming, running, etc.
  • CrossFit
  • Powerlifting
  • Bodybuilding
  • Spinning

When you’re engaging in regular intense exercise, you’re burning hundreds and potentially thousands of calories each day. Learning how to properly fuel your training with food is just as critical to your long-term success as showing up for each workout.

Your metabolism is a lot like a burning campfire.

When you build a fire, there are two things it needs in order to keep burning: oxygen and a careful supply of more firewood.

When you’re training for a marathon—or another intense type of exercise that burns hundreds of calories daily—your physical activity is the oxygen, and food is your firewood.

The fire itself is your metabolism.

  • Without enough exercise (oxygen), your metabolism slows down. If you’re not exercising a lot, but you’re also eating more food (firewood) than your body needs, you’ll store those extra calories as body fat, like a pile of extra firewood that isn’t being added to the fire.
  • Without enough food (firewood), your metabolism (the fire) slows down in order to conserve energy. Your body is smart! It isn’t going to let you starve to death. Conserving energy means storing more fat and burning fewer calories. If you are exercising a lot, but you’re not eating enough, you’ll lose the balance that maintains that steady fire—and gain weight.
  • Too much junk, regardless of your metabolism, can lead to weight-gain. If you’re using intense training to compensate for a diet full of highly processed junk foods, you can still gain weight. It’s not as simple as “calorie in vs. calories out.” Your body needs quality ingredients to maintain a healthy weight.
  • Don’t starve the fire all morning then overwhelm it with firewood at night. If you’re not spreading your calories throughout the day, you’re more likely going to feel exhausted and gain weight. Yes, intermittent fasting is a cool weight-loss too, but it’s really not appropriate for endurance athletes because of all the metabolic reasons we’ve already discussed. Your body needs frequent meals throughout the day—generally every four or so hours—to keep your metabolism burning.
  • And lastly…you can exercise a lot (adding lots of oxygen to the fire) and still eat too much (adding more firewood than necessary) which can lead to weight-gain.

It is a tricky, complex balancing act of oxygen vs. firewood to keep the fire burning steadily.

Other signs that you’re not eating enough for endurance exercise

Your body needs fuel! When it isn’t getting enough fuel, other aspects of your health start struggling, too.

  • You’re not hungry: Your metabolism has slowed down to conserve energy because you aren’t eating enough. Someone who’s training for a marathon should be hungry for that next meal.
  • You’re extremely tired: Whether it’s hard getting out of bed in the morning or you can’t make it through the afternoon without a nap, your body isn’t getting what it needs.
  • You’re not sleeping well: Physical exhaustion can easily throw other things (like stress-related hormones) out of whack, too.
  • You’re feeling depressed: Combine everything above with a variety of stress-related hormones and your mood is going to suffer!

Don’t ignore these signs and symptoms that are trying to tell you something important about your exercise vs. fuel vs. recovery routine!

How much should you be eating for endurance exercise?

While you don’t need to obsess over every calorie and track your diet for weeks on end, it is definitely helpful and wise to track your calorie consumption for a few weeks.

Tracking your calorie intake for a few weeks can help you:

  • See how much you’re eating
  • Learn more about calorie content of food
  • Adjust if you’re over- or under-eating
  • Create new habits around food

Here are a few calorie calculator tools to help you get an idea of what you should be consuming to fuel your training. Keep in mind that a standard calorie calculator likely doesn’t think you mean “training for a marathon” when you choose “highly active.”

Depending on how many miles you’re putting in, your body could need over 3,000 calories a day—even for a petite woman! Eating that many calories a day is work, and it’s a huge part of engaging in intense exercise training. (Remember, too, that as your mileage increases, your calories may need to increase, too.)

If you’re engaged in a specialized sport—like a marathon, CrossFit, or powerlifting—you should use a calculator that can take that intense training into account.

Once you get a hang of creating meals that contain enough calories and eating constantly throughout the day, you’ll likely find you don’t need to track everything you eat every day. Use these tools to help you build that knowledge and those habits!

More food means more insulin.

While more exercise generally means you become more sensitive to insulin, endurance sports and other intense types of exercise can actually call for more insulin.

The more you exercise, the more glycogen (stored sugar) your liver and muscles are going to use. Keeping those glycogen stores full is yet another reason why you need to consume enough calories for the demands of your intense exercise regimen.

But getting glucose into those glycogen stores requires insulin!

Eating more calories also simply means you have more glucose that needs to be managed whether it’s for immediate energy or glycogen stores.

So, don’t be surprised if your insulin needs actually increase noticeably throughout your endurance training!

Quality matters, even if you’re running two hours a day.

The quality of your food matters, too. Whether you’re eating low-carb, vegan, paleo, omnivore—whatever you choose—what matters most is that you’re giving your body mostly high-quality real food.

If you read articles about the diets of competitive athletes in any sport, you’ll find one thing in common: they eat mostly real food.

These competitors don’t use hours of exercise as an excuse to eat junk, because junk cannot provide your body with the high-quality nutrients it needs to perform and recover.

Your diet doesn’t need to be 100 percent perfect—but striving for a “mostly real food” diet is a big part of your success as an athlete!

WRITTEN BY Ginger Vieira, POSTED 11/30/21, UPDATED 08/04/23

Ginger Vieira is an author and writer living with type 1 diabetes, celiac disease, fibromyalgia and hypothyroidism. She’s authored a variety of books, including “When I Go Low” (for kids), “Pregnancy with Type 1 Diabetes,” and “Dealing with Diabetes Burnout.” Before joining Beyond Type 1 as digital content manager, Ginger wrote for Diabetes Mine, Healthline, T1D Exchange, Diabetes Strong and more! In her free time, she is jumping rope, scootering with her daughters, or walking with her handsome fella and their dog.