You’ve just started a workout program and find that you gained a few extra pounds. Concerned, you discuss it with friends, and they say, “ it’s fat turning into muscle”.
This is a common “fitness myth”. It is impossible for fat to turn into muscle; they are two different types of tissues. It would be like saying, your fingernails are turning into your hair. You can, lose fat and gain muscle, but this will take time and a dedicated program that includes weight training and probably food modifications.
Tied into this myth is why you will initially maybe gain a few pounds when you start an exercise program. A new exercise regimen puts stress on your muscle fibres. This causes small micro-tears and some inflammation. Those two conditions in your muscle fibres are the reason you may gain some weight. Your body responds to the micro-tears and inflammation in two ways that cause temporary water weight gain. The first is a healing response. That stress and micro-tearing damage to the muscle fibres induces water retention in the body. There may be a small amount of inflammation around the micro tear, and your body retains fluid there to try to heal it. These are short-lived changes in the muscle.
Secondly, the way your body provides energy to the muscles also can add weight at first. Glycogen or sugar that your muscle cells convert to glucose is the energy source for your muscles. When you exercise regularly, your body stores more glycogen to fuel that exercise. Stored in water, glycogen has to bind with water as part of the process to fuel the muscle. That water adds a small amount of weight, too. As your muscles become more accustomed to the exercise and more efficient, they need less glycogen to maintain the same level of energy output. Thus, your water retention becomes less, so your weight will start to go down. You will begin to lose that initial water weight gain (roughly one to three pounds) a few weeks or a month after starting an exercise program.
Weight gain from new lean muscle mass can take a month or two (or longer) to show up on the scale. By that point, you will probably be experiencing a good weight-loss trend because of the exercise.
And finally, check your diet. Many people begin to treat themselves when they start working out. If you eat a protein bar after your workout, it can contain upwards of 200 calories. Since a typical workout burns about that, you can easily gain weight.
Stick to it, early water weight gain will disappear, weight loss will kick in and muscles will start to increase, if you give yourself time.
I Just Started Exercising — Why Am I Gaining Weight? – Health Essentials from Cleveland Clinic