Remember that episode of The Biggest Loser when contestant Daris George gained weight during marathon training and trainer Jillian Michaels bit his head off? Runners worldwide gave Jillian the virtual finger, knowing that it’s, in fact, very common to pack on some pounds while training for the big “M.”

I remembered a conversation I had with a friend, whom I am coaching towards her first marathon. I asked how training was going; she said she was plowing through with consistency and strength and had remained injury-free. She then added (while we stuffed guacamole and chips in our pieholes), “But, I’m totally shocked. I’ve gained like 7 pounds. I thought for sure I’d lose weight while training, and this sucks.”

Enough already. I decided I needed to get to the bottom of this weight-gain conundrum. Seriously, if you burn on average 100 calories per mile (depending on age, weight, sex, etc.) and run about 30 miles per week, then technically you have 3,000 extra calories of eating to do just to maintain your weight. Right? Wrong.

Here are some reasons you may gain weight while marathon training:

1. You’re building more muscle mass, which is more dense than fat. So while that may translate to the overall weight gain, your body fat percentage has decreased, and you’re more toned than you were before.

2. Your body is storing carbohydrates as fuel (glycogen) for your long runs. Those glycogen stores are important to completing your long runs and marathon without “hitting the wall,” but you may see a couple extra pounds on the scale on certain days. Your body also requires additional water to break down and store the glycogen, so that need will also add extra weight.

3. You may have been increasing your calorie intake. Running a lot should not be an invitation to eat gallons of ice cream and trays of Oreos. The basic principle for weight loss still applies: you must burn more calories than you consume.

4. You are drinking too many calories. Just because you’re training for a marathon doesn’t mean that you need to constantly drink sugary sports drinks. While it’s important that you replace electrolytes during your long runs, you don’t need to constantly have a sports drink at your fingertips the rest of the time. Plain water is fine for staying hydrated during the week.

5. It’s important to eat enough food to keep you energized. Dieting while marathon training can be dangerous and lead to all sorts of issues like fatigue, illness, and injury. It is a slippery slope: if you eat more calories than you burn, you’ll gain weight. If you eat too few calories, you won’t have the energy to train.

Do you gain weight while training for races?

For more from Beth Risdon, visit Shut Up and Run!