I read somewhere that the body is incapable of losing more than 3 lbs of body fat a week & that everything else is just lean muscle & water???
That just about aligns with the math from this study. It's basically saying that your body can "burn" 31 calories per pound of stored body fat you have, each day.
So, if you're 150 pounds with 30% body fat, you have 45 pounds of fat. That means your body can "burn" (45 * 31) = 1395 calories worth of stored fat each day. That means (1395*7) = 9765 calories worth of fat a week.
Since we know that ~3500 calorie deficit is equal to one pound, then (9765 / 3500) = 2.79 pounds a week of fat your body can "burn".
Of course, if you're 250 pounds with 50% body fat, this would suggest you could burn about 8 pounds of fat each week. But you'd have to run a daily calorie deficit of almost 4000 calories... if the math is right.
This also aligns with what they do on Biggest Loser. ~1500 calorie/day diets and 3 hours of intense physical training every day. They are probably running 3000-4000 calorie deficits daily...