The days of watching the scale rise and fall every 30 seconds are over - at least for people who make progress. Body Weight has no bearing on Body Composition without actually assessing composition. To lose 10lbs or more is a pretty common fitness goal common for zillions of people. But what do you want to lose 10lbs of?
Body weight is comprised of lean body mass (LBM) and fat mass (FM). Everything in your body that is not actual fat is part of lean body mass. Organ tissue, connective tissue, muscle, fluids, hair, bone & bone density are all part of lean body mass. Fat mass is comprised of essential fat and storage fat on / in the body.
Essential body fat is necessary to maintain life and reproductive functions. The percentage for women is greater than that for men, due to the demands of childbearing and other hormonal functions. Essential fat is 1-3% in men, and 8–12% in women. So out of all of these things, storage fat is the actual stuff that you want to lose.
The density of mammalian skeletal muscle tissue is about 1.06 kg/liter. This can be contrasted with the density of adipose tissue (fat), which is 0.9196 kg/liter. This makes muscle tissue approximately 15% denser than fat tissue. That being said, 1 lb of fat and 1 lb of muscle are still only 1 lb. It's by volume that muscle is denser than fat not by weight. That old saying that muscle is heavier than fat is technically not true - muscle is denser than fat and therefore weighs more by liter. A pound of fat will take up about 18% more space then a pound of muscle. If squeezed into cans, you would need a can 18% bigger to contain the pound of fat. But both cans would still only weigh 1lb. Make sense?
So back to that 10lbs that you want to lose - Out of the 10lbs, how much of that will be fat? You would probably say all of it right? But how do you know? There are calculations that can be done to actually determine approximately how much storage fat that you have on your body. The numbers rarely come out to the number that you say you need to lose. Let's do an example. Mrs. Jones weighs 173lbs. She says she wants to lose 30lbs. Her bodyfat measurement came out to 17.5%.
Body Fat in lbs. =
(Total Bodyweight) (Body Fat Percentage (in decimal form))
Lean Body Mass =
Total Bodyweight - Body Fat in lbs.
So someone who weighs 173 pounds with a body fat percentage of 17.5% would calculate as follows:
|30.3 lbs. of body fat|
173 - 30.3 =
The issue is that's not very realistic and would be hard as hell to maintain year round assuming she is a normal working mom. But we now know the max she can safely lose is 26.6lbs. What would she look like at that weight? Maybe not what she was hoping for. Keeping in mind that the scale is till of no use during this process other than to check for LBM loss. As you add muscle to your frame and lose fat, the scale will only show the net change. If you gained 3lbs of muscle and lost 5lbs of fat, you would only be down 2lbs on the scale but you would look completely different in a good way.
A more realistic approach is to lose bodyfat while building muscle until you look good in the mirror as opposed to reaching a number. Set a smaller realistic goal and see how you look, perform and feel at that point. Then, if you desire to lose more, keep going with another small goal until you give yourself a thumbs up in the mirror.