Weight fluctuations are common and occur for a variety of reasons. It is possible to weigh more the day after you strength train, though it is typically because of temporary changes. It is unlikely that overnight you can "gain" four pounds of either muscle or fat. It is more likely that the weight fluctuation is due to a cause other than your strength routine the day prior.
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True weight gain occurs when you eat more calories than you burn. One pound is equal to 3,500 calories. So unless you sleep walk and go on an overnight binge, it is unlikely that you can consume enough calories in one day to truly gain weight. If your weight has fluctuated, it is more often than not just a temporary response to something your body experienced.
Post-Strength Training Weight
Strength training, WWLP.com explains, causes mini tears in your muscles, which is how muscle growth occurs. These tears, however, can lead to fluid retention in the muscle, which may last for a few days and could explain why you weigh more after you lift weights. Muscle also weighs more than fat, and the body can temporarily respond to weight training as it balances the muscle gain with the body fat reduction.
People typically drink more water while they exercise and after, which leads to a temporarily increased water content in the body. On the contrary, if you didn't drink enough water while you lifted weights and you are dehydrated, your body may be retaining fluid. Additionally, hormonal changes in the body, especially in women, can cause daily water fluctuations, which doesn't reflect fat loss or muscle gain.
Weight gain can occur from eating or drinking more than you think you are. Especially after working out hard, you may sometimes think you "deserve" a bigger portion. Sodium-filled foods can also cause a jump in the scale. For the best indication on where your weight stands, weigh yourself only once a week instead of every day. Do it consistently in the morning, preferably without clothing, and monitor changes in your measurements as well. True weight gain will occur by gaining muscle or fat and consuming enough calories to support it.