Even if you aren’t seeing the scale go down, it’s not necessarily a red flag, and you should keep at your exercise routine. But if you’re curious why you’re gaining weight after working out, here’s the lowdown from experts.

Meet the experts: Rachel Pessah-Pollack, MD, is an endocrinologist and clinical associate professor of medicine at NYU Grossman School of Medicine. Pouya Shafipour, MD, is a family medicine and obesity specialist at Paloma Health. Sarah Pelc Graca, CPT, is a personal trainer and founder of Strong with Sarah Weight Loss Coaching.

Why do you gain weight after working out?

You’re gaining muscle.

If you’ve been consistently strength training, it could be a good sign if you gain a little weight, says Pelc Graca. It means you’re building muscle! The volume of muscle is much denser than fat, so it’s therefore heavier, she says. “Even though your body may look leaner, you might notice a slight weight increase or no change on the scale.”

There’s really no reason to be concerned about gaining muscle (it’s a good thing), so it’s important to remember the scale doesn’t paint the entire picture, says Pelc Graca. Instead, focus more on how you feel and how your clothes fit, she recommends.

You could be dealing with inflammation.

Some high-intensity workouts can cause micro-tears in your muscles fibres, which causes a temporary inflammatory response and fluid retention in the body, says Dr. Shafipour. However, these micro-tears will eventually stabilise and can even lead to an increase in lean muscle mass, which again, is not equivalent to fat, he says. The inflammation typically stabilises after a few days, and is usually not a cause for concern, but talk with your doctor or a certified personal trainer if the inflammation persists or leads to pain.

You’re retaining water.

When you work out, your muscles also store more glycogen, which is the simplest form of carbohydrates broken down and used for energy, and glycogen binds to water, she says. “For every gram of glycogen, the body retains water, and this can cause a slight weight gain initially. However, there is no cause for concern since this is how your body metabolises and uses food for energy, she adds.

It can be frustrating to see the number on the scale increasing due to water retention, but that doesn’t mean you should stop drinking water. Any minor water weight usually subsides as your body adapts to a routine, and being well-hydrated is actually connected with weight loss and reduced waist circumference, per a 2021 study in Clinical Nutrition, so remember to sip 11.5 cups (2.7 litres) of fluids a day, per Mayo Clinic.

You’re sleep deprived.

If you’re waking up early or staying up late to get a workout in, your weight gain may be due to a lack of shuteye, says Dr. Shafipour. Sleep deprivation can promote an increase in ghrelin, your hunger hormone, which can trigger late night snacking and a desire for carb-heavy foods, ultimately increasing your daily caloric intake, he says.

Poor sleep can also interfere with your body’s ability to recover after a workout, leading to increased inflammation and risk of injury, so it’s especially important to aim for at least seven to nine hours of sleep a night, says Pelc Graca.

Your medications or supplements may be to blame.

Certain medications like antidepressants, steroids, some forms of birth control, and supplements like creatine can cause water retention or increased appetite, leading to potential weight gain, says Pelc Graca. Stopping the medication may not always be possible, but if you notice significant, unexplained weight gain after starting a new medication or supplement, it’s worth discussing with your doctor to see if adjustments or an alternative drug are needed.

Your appetite is increasing.

Exercise can boost hunger signals, especially if the activity is particularly intense, says Pelc Graca. “If you’re consuming more calories than your body needs for recovery, you may gain weight,” she says.

This is only cause for concern if it leads to long-term, unintended weight gain, but logging your food intake and focusing on a well-rounded, balanced diet can help keep you on track, says Rachel Pessah-Pollack, MD, an endocrinologist and clinical associate professor of medicine at NYU Grossman School of Medicine. Increasing your protein intake can help keep you fuller for longer (aim for 20 to 30 grams of protein at each meal), Dr. Pessah-Pollack says. It can also help to limit sugary drinks or processed snacks that add little nutritional value.

Thirst can also sometimes be mistaken for hunger, so water is non-negotiable, especially if you’re training in hot environments, says Dr. Pessah-Pollack.

You have an underlying medical condition.

If you feel like you’re consistently working out, eating a balanced diet, sleeping well, and still gaining weight, Dr. Pessah-Pollack says an underlying thyroid condition may be to blame. “An underactive thyroid, known as hypothyroidism, can cause water retention, weight gain, and swelling, and if the thyroid is not properly treated, it can affect ability to lose weight despite exercise,” she says. Additional symptoms of hypothyroidism include fatigue, cold sensitivity, dry skin, hair loss, constipation, mental fog, and menstrual irregularities, so check in with a healthcare provider for lab work if you suspect your thyroid is off-kilter.

Hormone-related conditions such as polycystic ovary syndrome (PCOS) can also cause insulin resistance and chronic inflammation, making it difficult to lose weight, so talk with your doctor if you experience additional symptoms such as acne, irregular periods, and excess hair growth.

More fitness news…

Cut through the noise and get practical, expert advice, home workouts, easy nutrition and more direct to your inbox. Sign up to the WOMEN’S HEALTH NEWSLETTER