If you are considering a weight loss program, you need to make sure that you lose fat not muscle. Some people focus exclusively on the scale, paying no attention to anything else, but this is a short sighted approach to losing weight.
There are two main reasons that people want to lose weight. One is to look better, which means having a more trim body shape. If you lose muscle instead of fat, your body will look flabby and you will not achieve your aim. The other main reason that people want to lose weight is to be healthy and reduce their risk of fatal or chronic diseases such as diabetes or heart disease. For this it is important to include some kind of fitness program so again you want to be sure that you do not lose the muscle that you have.
But there is another reason for retaining and preferably increasing your muscular mass. This is that muscle burns energy, i.e. calories. Muscle even burns calories when resting, so the more muscle you have, the more calories you burn even while you are watching TV. This does not mean you have to turn into a Sylvester Stallone iron-pumping giant. Unless you want to build your body, the key is to make sure that you do not lose the muscle you already have.
Low carb diets and other fad or extreme diets all claim fast initial weight loss, but what they don't say is that the initial loss will most likely include muscle loss due to glucose deficiency in the diet. The consequence is that when you stop the diet, you have *less* capability than you did before to burn calories, which is one reason why many people actually put on more weight afterwards. So the key factor if you want to lose fat not muscle is to steer clear of any fad diet claiming speedy results.
Exercise will undoubtedly help your cause. Aerobic exercise will give you a double bonus, because it will burn fat and may also lead to building muscle, depending on the type you do. Exercise will also improve your general health, it will get your heart pumping, more oxygen will get around your veins and to the brain, and the endorphins released will make you feel great.
Remember that muscle tends to weigh more than fat, so in some cases a person's body shape may improve even while their weight goes up, although that is more true of people whose weight was normal or low to begin. If you are overweight then the fat that you lose will usually mean that you will lose weight even while your muscularity increases, but your weight loss according to the scale may be slower than if you were following a crash diet. Give yourself a break here and remember that what you see in the mirror is more important than what you see on the scale.
Try not to focus your exercise on only one area of the body. For example if you want to lose stomach fat, don't just do sit ups and nothing else. It is virtually impossible to concentrate fat loss in specific areas like that, you are better to do more general fat loss exercise.
Obviously if you are significantly overweight, the exercise should not be taken on without proper medical advice. But for those who want to lose those extra curves, you will gain good benefit by making sure you lose fat not muscle.