Losing weight while simultaneously retaining muscle mass can be accomplished by virtually anyone, with the key variables being your body-fat percentage and nutrition. It’s actually easier for an overweight or obese person to attain this goal than it is for a fit, lean person. Simply burning more calories than you consume, either by eating less, exercising more or a combination of the two, will result in weight loss. You’re not really in any danger of losing muscle while losing weight unless you have nutritional deficiencies or you allow your muscles to atrophy through lack of exercise.
Walk or run. If you already get aerobic exercise, increase the duration or frequency. You don’t even have to pay particular attention to your diet, as long as you’re not deficient in such macro-nutrients as protein and carbohydrates. By keeping your calorie intake stable and increasing physical activity, you’ll lose weight and maintain muscle tone.
Lift weights. There is some disagreement -- misunderstanding, really -- about this anaerobic approach. While it’s technically impossible to lose fat and gain muscle at the exact same time, you can burn fat and maintain muscle simultaneously. The fact is, weight training burns calories -- a lot, depending on your routine -- though not as many as an aerobic workout like running. Not only will you lose weight and retain muscle, you can also actually gain muscle mass by adjusting your diet, including eating more calories. The calorie increase, including more protein, will fuel muscle retention and growth but be less than your daily caloric needs.
Combine aerobic and anaerobic workouts. Again, you may increase your calorie intake and adjust carb and protein levels to support muscle function. Muscle proteins are, indeed, broken down and used for fuel during running and other aerobic activities, but this is a minor event in the scheme of things. Your muscles perform the same task while you sleep at night. It’s sometimes called “protein turnover.” Not only that, but aerobics also improve recovery from weight training by enhancing blood flow and oxygen transport to muscles. Alternating aerobic and anaerobic days can be helpful in muscle recovery.
Eat fewer calories. If you’re fat, you’ll see quicker weight-loss results. A pound of fat contains 3,500 calories, and most people (who aren’t obese) can’t convert much more than 2 lbs. pounds of fat into energy in a week because they can’t afford to slash 1,000 calories a day. They need a minimum calorie number to sustain themselves. But obese people, although their caloric needs are greater, can afford to reduce calorie intake to a greater degree and thus lose weight more quickly. There is a law of diminishing returns involved, meaning that a dramatic calorie reduction could cause the body to burn muscle after a certain calorie-reduction level is achieved, even in people who have excess fat to burn. The body needs water to convert fat to fuel, and even obese people don’t have enough water in their bodies for unlimited fat burning. As long as nutritional needs aren’t slashed, even minimal physical activity will maintain muscle tone.
It’s true that fat isn’t converted into muscle; loss or gain of either is a separate process. But that doesn’t mean the two are mutually exclusive. Don’t wait until you’ve lost X amount of weight before you begin an exercise regimen. The higher percentage of muscle on your body, the better for losing weight. Muscle is more calorie intensive -- more “high-maintenance" -- than fat. Serious athletes, especially bodybuilders, often employ alternating routines of muscle building and aerobic, weight-loss training. These cycles can cover months each and are repeated to achieve slow and steady fat loss, muscle mass and leaner body-fat ratio.
Avoid low-carb diets, if possible. If you’re extremely overweight, a low-carb diet can help initially, but you want to combine at least moderate exercise to maintain muscle tone, and muscles need carbohydrates.