Why do you sometimes gain weight after exercising?

If you've ever started an especially vigorous workout regime with good faith and enthusiasm, only to be met with disappointment as the scale goes past the weight you started with, you may have had a lingering question: Why does exercise make me gain weight? Is there a biological explanation, or am I just unlucky? 

The answer is multifaceted, a physical activity expert told Live Science. Any post-workout weight gain is most likely the combination of a few factors, but crucially, it doesn't mean you should give up on exercise.

"People don't understand that doing exercise is good even if you're gaining weight," Corinne Caillaud, a professor of physical activity and digital health at the University of Sydney in Australia, told Live Science.

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"While exercise plays a role in weight control, the other side of the coin is food intake," Caillaud said. If a person notices their weight increasing, it's worthwhile to review the quantity and quality of the food they're eating, she said. Their post-exercise weight gain could be explained by what and how much they're eating. 

"You may be thinking that, 'Well, I've done exercise, and so I'm fine to eat more,'" Caillaud noted. While there's nothing wrong with an occasional treat, exercising is unlikely to offset the effect of increasing the frequency of indulging in junk food. 

But assuming that your diet hasn't changed, there are still a few other biological quirks that could explain the weight gain. If you're not used to a good workout and then go whole hog, you could end up straining your muscles more than you should. In other words, your muscle fibers suffer microtears, but that's not cause for concern because when this happens, your body sends nutrition to the muscles to help repair the damage, according to University Hospitals in Cleveland, Ohio. It's the reason why your muscles ache the next day, but over time it leads to muscle growth.

However, these microtears can kick-start the body's inflammatory process. "That means swelling," Caillaud said. This swelling can, in turn, lead to extra water retention in the body, according to the Cleveland Clinic. In effect, this increased water can be another explanation for after-exercise weight gain. 

"But this might not actually mean a significant increase in overall weight, because not all the fluid comes from water retention," Caillaud said. "Some of it just comes from elsewhere in the body — from the plasma in the bloodstream, for example."

Exercise, particularly weight lifting, can also help to grow muscle mass. "If someone is doing strengthening exercises, then they may see an increase in their muscles," Caillaud said. "But that takes quite serious training, and it's not going to happen in just a few weeks. It takes months to increase muscle size, and to get it to the point where you notice that change on the scales is going to take a year or more."

The other potential explanation comes down to the amount of blood in your body. "When you do aerobic exercise, at some point, there may be an increase in blood volume, which is essentially an increase in aerobic capacity," Caillaud said. Aerobic capacity is a measurement of the maximum amount of oxygen your body can consume whilst exercising. Muscles need oxygen, supplied by blood, and so the more oxygen a person can consume, the better their stamina.

None of these components — minor diet changes, inflammation, increased muscle mass or increased aerobic capacity — is significant individually, Caillaud said. "But​ when you add everything up, it may start to explain things."

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Importantly, people shouldn't quit the gym in a hurry if they experience weight gain. Water retention from inflammation isn't permanent, and sustained exercise will eventually help to burn calories and, therefore, induce weight loss. 

What all of this means, Caillaud said, is that people who have started to work out properly shouldn't be discouraged from continuing, even if they gain a little weight, because pounds and ounces aren't the only important metrics. 

"People may increase body weight after exercise a little because they've increased muscle weight, overall blood volume and so on, but that doesn't mean it's not successful" in improving their health, she said. "It's all [a] positive change to help build healthy muscles and have them function better in terms of metabolism."

Originally published on Live Science.