Why does the scale go up when you start a new workout?

If you’re trying to lose weight, when you first start exercising you want the scale to go down. You’re doing more, so expecting to lose more, right? Not exactly.
If you are new to strength training, your body will make some key adjustments as a response to the new way of moving. Using weights and added resistance during a workout puts a stress on your muscle fibers that they are not used to, which results in micro muscular tears and inflammation.
This sounds worrying, but it’s a normal response to resistance training. This process can cause an increase in weight because both muscle damage and inflammation cause a small amount of temporary water retention. As your muscles are not used to being worked, water is retained to neutralize the inflammation produced by the micro muscular tear.
Water retention can mask true fat loss and make it look as if a diet that is otherwise set up perfectly (and working well) isn’t. Water retention can be caused by a number of factors:
• Hard deficits
• Excessive exercise
• High salt intake
• Hormones associated with the menstrual cycle
• Hot weather
Some people will have no problems with water retention, where others may hold an extra 5-10 lbs. If a client is holding excess pounds in water it may take weeks before we can actually see that the diet is working.
Another reason that you weight may increase is the most known reason: muscle is denser than fat. But realistically in the first few weeks of training, you’re not going to gain muscle that quickly - so how is weight gained? When you exercise more, your body tends to store more glycogen from food in the muscles to fuel the workout. When glycogen is stored it has to bind with water, approximately per pound of glycogen there is four pounds of water.
But this isn’t a permanent retention; as your muscles become used to the exercise, they require less glycogen to maintain the same output of energy. This means that your muscles are becoming more efficient in fueling and recovery, so all the aforementioned water weight will not occur.
After time you may also gain muscle which will cause your weight to increase. However, this doesn’t mean that you should eat less calories to lose weight; in fact, the more muscle mass you have the higher your basal metabolic rate and thus the amount of calories you burn at rest is increased.