If you're interested in losing weight, dieting may immediately spring to mind as the best way to slim down. Before you pick out a diet, you should consider how most dieting efforts play out.
You body burns calories all the time, whether you're sleeping, working, or playing – although your burn rate can be much higher while working or playing. Makes sense, right? Your body's ‘base' burn rate, or basal metabolic rate, is related to your total muscle mass, which is somewhat related to your weight. In other words, if you're overweight, your body will maintain enough muscle to move your body around for normal day to day activities. Keep in mind that we're not talking about strenuous activity here – just moving from the couch to the refrigerator, for example.
If you lose weight by dieting, you need less muscle for your couch potato lifestyle, and your muscle mass decreases accordingly. As you lose muscle mass, your ‘base' burn rate decreases, until your weight, muscle mass, and caloric intake all come into balance again. Good job! You've lost weight, right? High fives all around??
If all you want to do is slim down and look good around the office or at a high school reunion, congratulations – mission accomplished. Keep your caloric intake down and you'll maintain your balance between overall weight, muscle mass, and diet.
Note that if you'd like to go dancing, play a game of volleyball, or participate in a fun-raising 5K run, you'll need to step up your activity level a notch with an exercise program. As you begin to exercise, try to reduce your caloric intake, even just a bit. Your exercising will help maintain or even build muscle mass (depending on the type of exercise program you choose), even as you lose weight. If you exercise regularly, you'll be able to enjoy your new lifestyle without having to stop and catch your breath.
If you're goal is to live longer and to maintain a higher quality of life long into your golden years, however, you need to know about things that happen as you age – loss of bone density and loss of muscle mass. You can address the loss of bone density by engaging in weight bearing exercises, and you can maintain or increase your muscle mass by stressing your muscles. By that I mean that while cardio exercises are good and necessary (walking, swimming, bicycling, etc), you also need to exercise your muscles to the point of failure to ‘encourage' more muscle growth.
I don't want to re-invent the wheel, so let me suggest that you watch a five minute video, and leapfrog from it to a webinar that explains the physiology of muscle growth. Take a look, and let me know what your thoughts are in the comments below!