I'm working out - why am I gaining weight?

Starting a new exercise routine can feel like a big step. You may be eating better, moving more, and expecting the number on the scales to fall. So, when your weight stays the same or goes up, it can feel frustrating.

But gaining weight while working out does not always mean you are gaining fat or doing something wrong. Your body can hold onto extra water, build muscle tissue, or respond to changes in appetite and routine.

Weight loss is not always linear. The scales can move up and down from day to day, even when your long-term habits are improving.

Exercise itself doesn’t usually cause fat gain. But starting or increasing exercise can temporarily increase your scale weight.

This can happen because your body is adjusting to a new level of activity. Your muscles are working harder, using more stored energy, repairing themselves, and sometimes holding more fluid than usual. If you are doing strength training, you may also begin to build muscle while losing fat.

That is why it is useful to look beyond the scales. Your waist measurement, fitness, strength, energy, sleep, blood pressure, and how your clothes fit can all tell you more than purely weight alone.

If you have only recently started exercising, a small weight increase can be normal. Your body is adapting to the extra demand.

New workouts can cause tiny amounts of muscle damage, especially after strength training, running, or anything your body is not used to. This is part of the repair process that helps your muscles become stronger. During that repair process, your body may hold onto extra fluid.

This is usually temporary. You may notice your weight settles again as your body gets used to your new routine.

If you are doing resistance training, weights, circuits, Pilates, hill walking, running, or other regular exercise, you may be building muscle while losing fat.

Muscle does not “weigh more than fat” in a literal sense. A pound of muscle and a pound of fat weigh the same. The difference is that muscle is denser than fat, so it takes up less space. This means your body shape can change even if the number on the scales does not move much.

Instead of only weighing yourself, try tracking:

• Waist, hip, chest, thigh, and arm measurements
• How your clothes fit
• Progress photos
• Strength improvements
• Walking distance, pace, or stamina
• Resting heart rate or blood pressure, if you monitor them

Water retention is one of the most common reasons people gain weight after starting exercise. When you work your muscles harder than usual, your body sends fluid to help repair them. This can show up as a temporary increase on the scales, especially after a hard workout.

Your body also stores carbohydrates as glycogen in your muscles. Glycogen is used for energy during exercise, and it is stored with water. So, when you begin training more regularly, your muscles may store more glycogen and more water with it.

This is not fat gain. It’s often a sign that your body is fuelling and repairing itself.

Exercise can help with weight management, but diet and nutrition still play a major role. To lose body fat, you generally need to use more energy than you take in over time. This is called a calorie deficit.

That does not mean eating as little as possible. If your calorie deficit is too aggressive, you may feel tired, hungry, irritable, or more likely to overeat later. If your deficit is too small, or if exercise increases your appetite, weight loss may be slower than expected. It can help to look at:

• Portion sizes
• Snacks and drinks
• Alcohol
• Sauces, oils, dressings, and other “extras”
• Weekend, eating out, and takeout eating patterns
• Post-workout hunger
• Whether you are eating enough protein and fibre

If you are struggling with fewer calories, try:

• Eating more protein with meals, such as eggs, fish, chicken, tofu, beans, lentils, or yoghurt
• Adding more fibre through vegetables, fruit, and wholegrains
• Choosing filling meals rather than relying on snacks
• Eating slowly and without distraction whenever possible
• Planning meals around workouts so you are not ravenous afterwards

A routine that mixes cardio, strength training, and general daily movement is usually more effective than relying on one type of exercise alone.

The NHS recommends adults aim for at least 150 minutes of moderate-intensity activity a week, or 75 minutes of vigorous-intensity activity, plus strengthening activities on at least 2 days a week. If your weight loss has slowed, you may benefit from:

• Adding strength training if you only do cardio
• Adding walking or low-intensity movement if you only do intense workouts
• Increasing daily steps
• Changing workout intensity
• Allowing enough rest days
• Trying a class, sport, or activity you enjoy

More exercise is not always better. Recovery is just as important! Poor sleep, high stress, and overtraining can make it harder to stay consistent and may affect hunger, motivation, fatigue, and water retention.

Sometimes exercise is only part of the picture. Weight can also be affected by:

• Menstrual cycle changes
• Constipation
• Eating later in the day
• Higher salt intake
• Poor sleep
• Stress
• Alcohol
• Some medicines
• Thyroid problems or other health conditions

If your weight gain is sudden and unexplained, or comes with symptoms such as swelling, breathlessness, tiredness, changes in your periods, or feeling unusually cold, speak to a GP.

Small changes on the scales are normal. It is common for weight to change by a few pounds across the week due to food, fluid, salt, hormones, and bowel movements. You should consider getting medical advice if:

• You gain weight quickly without a clear reason
• You have swelling in your legs, ankles, face, or stomach
• You feel breathless or unusually tired
• You are eating less but still gaining weight
• Your periods change unexpectedly
• You have symptoms such as constipation, low mood, dry skin, or feeling cold
• Weight gain is affecting your mental health or relationship with food