Yes, you will lose muscle if you do not workout. You need to place a demand on your muscles in order to give them a
reason to grow bigger and stronger. If you don’t place any demand on them, they will get smaller and weaker.
A good example of this is with seniors. They get smaller and weaker as they get older and a big reason for this is
because they get less and less active. But seniors who workout regularly are able to maintain their strength and
muscle mass well into their later years.
To lose weight, you must feed your body fewer calories than you burn. Formal exercise helps you burn more calories to make creation of this deficit easier, but it also builds muscle. To avoid gaining muscle, you’ll have to create the bulk of your deficit by just eating fewer calories.