Why You Might Gain Weight After Starting to Work Out

Last Updated on March 12, 2025

So, you’ve finally committed to work out—hitting the gym, running, lifting weights, or following a structured fitness routine. But instead of seeing the number on the scale drop, you notice it’s creeping up. This can be frustrating and confusing, especially if weight loss was one of your primary goals.

The good news? This weight gain is often temporary and can be explained by several physiological changes your body undergoes when you start a new exercise regimen. Below, we’ll break down the most common reasons why you might gain weight after starting to work out and what you can do about it.

1. Water Retention Due to Muscle Repair

When you engage in strength training or high-intensity workouts, your muscle fibers experience microscopic tears—a normal and necessary process for muscle growth. Your body responds by repairing these tears, leading to stronger and sometimes larger muscles.

However, this repair process causes inflammation in the muscles, which leads to water retention. Your body holds onto extra water to help with muscle recovery, which may reflect as an increase in weight on the scale. This is not fat gain—it’s simply your body’s way of healing itself.

How Long Does This Last?

Water retention from muscle repair can last a few days to a couple of weeks, depending on the intensity of your workouts and how quickly your body adapts.

What Can You Do?

• Stay hydrated—drinking more water actually helps flush out excess retained fluids.
• Get enough rest and sleep to allow muscle recovery.
• Continue exercising consistently—your body will adapt over time, and water retention will subside.

2. Increased Muscle Mass

If your workouts include strength training, it’s possible that you’re gaining muscle while also losing fat. This can lead to weight gain or cause the scale to remain the same, even though your body composition is improving.

Muscle is denser than fat—it takes up less space but weighs more per volume. This means you may look leaner even if the number on the scale doesn’t budge or goes up slightly.

How Long Does This Last?

Muscle gain is a gradual process, typically happening over weeks or months, depending on your training intensity and nutrition.

What Can You Do?

• Shift your focus from weight loss to body composition. Taking body measurements or tracking progress through progress photos can be more reliable indicators of fat loss than the scale.
• Use a body fat percentage calculator or get a DEXA scan to track real progress.
• Continue strength training—building muscle helps burn more calories at rest, making it easier to maintain long-term fat loss.

3. Glycogen Storage in Muscles

When you exercise, your body needs energy, and one of its primary sources is glycogen—a form of stored carbohydrate in muscles. As you increase your workout intensity, your muscles store more glycogen to meet the energy demand.

For every 1 gram of glycogen stored, your body retains about 3 grams of water. This means an increase in stored glycogen can lead to an increase in water weight, which can make it seem like you’re gaining weight.

How Long Does This Last?

This weight gain is temporary and fluctuates based on your diet and exercise level.

What Can You Do?

• Don’t panic—it’s not fat gain. Glycogen storage is a sign that your body is adapting to your workouts.
• Continue exercising and eating a balanced diet—your body will regulate itself over time.

4. Changes in Your Eating Habits

Starting a workout routine often increases your appetite. Exercise burns calories, and your body naturally seeks to replenish lost energy. If you overestimate how many calories you’re burning or start eating larger portions than before, you may unintentionally consume more calories than you’re burning, leading to weight gain.

Additionally, some people may reward themselves with high-calorie foods after working out, negating their calorie deficit.

How Long Does This Last?

Your appetite should regulate over time, but it varies from person to person.

What Can You Do?

• Be mindful of post-workout hunger—opt for high-protein, nutrient-dense foods instead of processed snacks.
• Track your food intake if necessary—apps like MyFitnessPal can help ensure you’re not overeating.
• Focus on protein and fiber-rich meals to keep you full longer.

5. Increased Stress Hormones (Cortisol)

Intense workouts can increase levels of cortisol, the stress hormone. While some cortisol is necessary for energy regulation, excess cortisol can lead to water retention and bloating. Additionally, high cortisol levels can increase cravings for sugary or high-fat foods, leading to potential weight gain.

How Long Does This Last?

Cortisol levels should stabilize as your body adapts to regular exercise. However, overtraining can keep cortisol levels elevated.

What Can You Do?

• Avoid overtraining—incorporate rest days and active recovery like stretching or walking.
• Manage stress—practice relaxation techniques like meditation, yoga, or deep breathing.
• Prioritize quality sleep—poor sleep increases cortisol production.

6. Your Body Composition is Changing

Weight gain doesn’t necessarily mean you’re getting fatter. If you’re losing fat and gaining muscle at the same time, the scale might not reflect your progress accurately.

What Can You Do?

• Use a measuring tape to track your waist, arms, and thighs instead of relying solely on the scale.
• Take progress photos every 2-4 weeks.
• Notice how your clothes fit—are they looser even though the scale hasn’t changed?

7. Digestive Changes

If you’ve started eating more fiber-rich foods (like vegetables, fruits, and whole grains), your body may take time to adjust. Fiber adds bulk to stool and retains water, which can temporarily increase weight.

How Long Does This Last?

A few days to a couple of weeks, depending on how quickly your digestive system adapts.

What Can You Do?

• Increase fiber intake gradually to allow your gut to adjust.
• Stay hydrated—water helps fiber move through the digestive system smoothly.

Final Thoughts: Weight Gain is Not Always Fat Gain

If you’ve gained weight after starting to work out, don’t panic! The scale doesn’t always tell the whole story. Weight fluctuations are normal, especially when your body is adjusting to a new routine.

Key Takeaways:

✅ Water retention, glycogen storage, and muscle repair can cause temporary weight gain.
✅ Muscle is denser than fat—gaining muscle while losing fat can make the scale misleading.
✅ Increased appetite or overeating may contribute to real weight gain, so tracking food intake can help.
✅ Stress and cortisol can cause bloating and cravings, so managing stress is important.
✅ Focus on how you feel, how your clothes fit, and your overall strength and endurance, not just the number on the scale.

If you’re consistent with exercise, nutrition, and recovery, the initial weight fluctuations will even out, and you’ll start seeing results. Keep going! 💪