Why Does My Workout Cause Weight Gain?

1. Water Retention After Exercise

Water can alter your weight by as much as 10 pounds (or more).

Think you just lost a few pounds from that serious spin class? Don’t get too excited—it’s just water loss due to sweat. And if you’re seeing a higher number, that could be due to water retention (that sometimes happens after exercise). The takeaway: The amount of water in your system has a heavy influence on the number you see on the scale.

“Water makes up approximately 65 to 90 percent of a person’s weight, and variation in water content of the human body can move the scale by ten pounds or more from day to day,” says Jeffrey A. Dolgan, a clinical exercise physiologist at Canyon Ranch in Miami Beach, Florida. This is one of the main reasons diuretics are so popular—they flush the water out of your system, resulting in only a short term weight “loss”—but they don’t help to change your body composition in any way.

 

2. Weight Gain Immediately After a Workout

A lot of factors can influence your weight—including your workouts.

So, you’re working out but gaining weight? Have you ever noticed that right after (or even a day or two after) an intense workout the scale goes up? That’s normal, and it doesn’t mean you’re actually gaining weight, says Dolgan.

“A person’s scale mass is a combination of muscle, fat, bone, the brain and neural tract, connective tissue, blood, lymph, intestinal gas, urine, and the air that we carry in our lungs,” he says. “Immediately after a workout routine, the percentage of mass in each of these categories can shift as much as 15 percent.” Intense workouts cause variability on the scale due to factors like hydration status, inflammation from muscle damage repair (we call this delayed onset muscle soreness), even the amount of intestinal by-product or urine and blood volume, says Dolgan. So there you have it: if you’re gaining weight while working out and eating healthy, it’s probably not the type of weight gain that you think it is.

3. Weight Gain with Strength Training

Muscle does NOT weigh more than fat.

“A common comment when looking at the scale is that ‘muscle is heavier than fat,’ which is misleading,” says Dolgan.” A pound of fat weighs the same as a pound of muscle; however, the volume of muscle is denser than the volume of fat and therefore, heavier.”

When you start to change your body composition with your workouts—by building more dense muscle mass and decreasing your body fat—your scale weight may increase, while your body fat percentage may decrease. These changes happen over weeks and months (not hours or days) so the scale is useless when tracking them, says Dolgan.

4. Weight Gain from Muscle vs. Fat

The scale says nothing about your fitness level or body composition.

As noted above, the scale can’t tell you how much of your body weight is muscle versus fat, which means if your goal is to improve your fitness level, it’s not the best tool for measuring improvements.

“If someone is trying to improve their fitness, they should ignore the scale and pay more attention to objective measurement tools like body composition to track their progress,” says Dolgan.

While weighing yourself can be one way to track your progress, it shouldn’t be the only way. And it certainly isn’t worth obsessing over with daily weigh-ins (and, as a result, fretting about gaining weight while working out and eating healthy). Don’t forget, Dolgan says, losing pounds on the scale does not mean that you are more fit—it just means you are lighter, which doesn’t mean much at all. And keep in mind that if you’re exercising but gaining weight, it could be that your workouts are effective, but you need to get your diet in check to see weight loss results. (That’s just one of the reasons you aren’t losing belly fat.)