Why Do I Gain Weight When I Exercise?

As personal trainers in Philadelphia, we always give a fitness assessment to our new clients to develop a baseline measurement and to create realistic fitness goals for them.  During this assessment, we’ll usually have a conversation about the client’s past exercise habits and their success with these habits.  Time and time again, we hear about clients gaining weight when they’ve exercised in the past.  This is fine for someone who is trying to add mass and bulk up, but for the majority of our personal training clients looking to lose weight, this is the opposite of what they want.  If exercise is supposed to help you lose weight, why are there so many people who experience weight gain during periods of exercise?  Philly Personal Training is going to explain 3 very simple reasons why you might gain weight when you exercise.

Muscle Weighs More Than Fat

Most people who have heard this don’t actually believe it, but it’s true.  One pound of fat will take up more space than one pound of muscle, meaning if a mass of muscle and a mass of fat were the same size, the muscle would weigh more.  So if you have two women who are both 5’4’’ and 140 pounds, but one is 20% fat and the other is 30% fat, the woman is 20% fat is going to have smaller measurements.  So even though she is much smaller in size, the woman with 20% weighs the same as the woman with 30% body fat.

With that being said, when you use resistance training in your exercise routine, you will (hopefully) gain some muscle.  Since we now know that muscle weighs more than fat, if you gain some muscle without losing fat, you’ll actually weigh more than you did previously.  Contrary to popular belief, fat does not turn into muscle.  Just because you’ve gained 5 pounds of muscle, it does not necessarily mean you’ve lost 5 pounds of fat.  You still need to make an effort to lose the fat as well.  So if you’re putting on muscle and not losing any fat, the scale will start to climb.  What’s to blame for this adding or muscle without subtracting any fat at all?

Post Exercise Overeating and Food Rewards

One major reason why people gain weight while they exercise is that they overeat after coming home from a hard workout.  While a post workout meal is definitely necessary, it is important to not overeat and consume more calories than you need after a workout.  The last thing you’d want to do is let a great workout go to waste.  Let’s take a 150 pound person as an example.  After a tough workout, this person can expect to burn about 600 calories in an hour.  Most exercisers never even reach this number but we will be generous for the sake of this example.  This same person comes home and overeats or rewards themselves with a hamburger with fries, totaling 650 calories.  It took this person an hour to burn 600 calories, but it took the same person about 10 minutes to gain it all back again because they overate and rewarded themselves after a tough workout.  OK, maybe you don’t reward yourselves with a burger after every workout.  But how many times have you told yourself you can have that extra piece of cake or slice of pizza because you worked out that day?  Start thinking of the workout as a reason not to overindulge when you eat your next meal.  Why would you want to cancel out a great workout?  If you’re like most, you want to start seeing positive changes; a good place to start would be to stop canceling out your workouts with unhealthy rewards.

Hydration Makes You Fluctuate

This is a reason you might gain weight that actually isn’t a negative.  Although you might not be happy with the result on the scale, being hydrated is a good thing.  During and after a tough workout, most will go to the water fountain and rehydrate with a lot of water.  This water can be retained for a few hours so this could be contributing to your added weight gain.  It is important to realize that being hydrated will probably only add a couple pounds at most.  So if you’re continually gaining more than 4 to 5 pounds, there’s most likely another reason you’re gaining weight.


Now you’ve learned that not all weight gain during exercise is necessarily a bad thing.  A few pounds of weight gain might mean you’ve added some muscle or you’re better hydrated.  However, it could also mean you’ve been rewarding yourself with post-workout food too often.  How do you tell which is the culprit for your weight gain?  One easy way to find out is to simply record your measurements.  If you’ve gained 3 pounds but your measurements in your waist, arms, and thighs have gone down, you’ve most likely put on some muscle (and maybe even lost a little bit of fat).  If you’ve gained 3 pounds and those measurements have gone up (while you’ve been exercising), you most likely still need to make more adjustments to your nutrition.

The one 100% foolproof way to tell if it’s muscle or fat is to have someone conduct a fitness assessment on you.  A personal trainer can assess your body measurements and body fat percentage to see if you’re gaining muscle or fat.  Once determined what direction you’re going in, the trainer can develop a workout plan and a meal plan specifically designed for you so you can reverse your exercise-induced weight gain.  Contact us today to get started!

Personal Trainer Philadelphia
Philly Personal Training, LLC
1429 Walnut St. Philadelphia, PA 19102

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