This is one of the most commonly asked questions by people who are on a journey to get fit, and especially those who are trying to lose weight. As you start to make changes to your lifestyle, reducing caloric intake, and increasing time for exercise you start to notice some positive changes in your body. If you are at a caloric deficit, meaning your caloric intake via food is lower than your caloric expense from basic metabolic functions and exercise, then you start to lose weight and as a result fat from your body. However, an unfortunate side-effect is that you also start to lose some muscle.
This happens because in a caloric deficit state your body has to get the energy it needs to run various metabolic operations in the body somehow. And it does so by tapping your fat stores and muscle stores. The proportion of fat to muscle burned for energy generation is dictated by many factors, such as genetics, your hormones etc. So controlling this may be somewhat out of our control.
However, we can create a metabolic environment within our body that signals the retention of muscle so that your body leaves your muscles alone or focuses on building new muscle tissue, while fulfilling its energy deficit by burning the fat reserves.
The way to signal body to retain the muscles is primarily done through a muscle building exercise routine. This routine could be focused on hypertrophy, strength, or power. Another important component is creating an anabolic environment through a diet rich in protein that provides the raw materials (amino acids) to retain, repair, and build muscle tissue. If you perform a muscle building exercise routine but are deficient in protein intake then you risk doing more harm than good with continued muscle loss as they are stressed and broken down during exercise with no amino acids for repair.
On the flip side though if you ingest more protein than what is necessary for repair of muscle tissue for your current muscle mass, some of that protein will be converted to glucose through a process called gluconeogenesis. This extra glucose will either be used for energy if needed or get stored as excess fat. This obviously hurts our goal of body re-composition.
So, what is one to do?
The solution is obvious. Ingest only "sufficient" amount of protein, along with required amounts of Carbohydrates and Fats to continue maintaining a caloric deficit to promote fat loss while incorporating a muscle building exercise routine to promote muscle gain. There are formulas available to calculate the right amount of calories and all the macronutrients needed to achieve this goal, and that is a great place to start.
However, everyone's body is different, so those formulas are not universally applicable. The best approach then is to start with the formula and follow the plan for a week and observe how your body changes. If you are not putting on muscle and still losing weight, increase your protein intake by 10 gms per day. If you are starting to put on fat while also gaining strength and muscle, your calories may still be too high, so reduce your caloric intake, starting with lowered Carbs intake.
Its that simple.