Fat vs Muscle:

One of the big things that people have issue with when dieting is thinking that because they are losing weight that they are becoming healthier by that alone.  Unfortunately, it's not quite that simple.  Losing weight when one is overweight is always good if the majority of the weight lost is in fat.  Unfortunately, for those who merely cut out calories and don't do more exercise, or worse, do even less exercise than before, they're also losing muscle mass.  Muscle is more dense than fat, and as a result, someone who thinks they are overweight might be mostly carrying muscle with just a little bit of fat, but if they lose weight too quickly they'll lose muscle with it as a result, and barely notice the difference.  By losing that muscle, however, they make it even harder to burn fat.  Additionally, a certain amount of fat is needed for a body, especially when it is in cold weather, for cushioning the joints and general energy available if one is going through periods of starvation.

When considering the average person's height, age, gender and other factors, doctors make estimations on what weight they feel a person should drop down to, and some more specialized than others (primarily dietitians, nutritionists, and trainers) will discuss the Body Mass Index or BMI.  This is a measurement of the amount of body-fat and individual has compared to the rest of their weight.  One way some measure it is to put hold a weight on a scale underwater and compare it to the weight being held out of the water.  The reason this has any effect at all is because fat floats and therefore the individual holding the weight won't be pressing as hard on the scale as otherwise.  There are more modern ways of testing BMI, but that is one of the oldest ways of testing it, and still fairly accurate for those that have waterproof scales available for testing it.  Highly athletic individuals should aim for a lower BMI, but the average healthy person typically should have a BMI score of 25 to 19.  If you're wanting to estimate your own BMI and find what your ideal weight should be, take a look at this chart for reference: