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Cos your body isn't a machine. You have a lot going on in there. There is an answer to this but it is multifaceted.
1) muscle fuel - in and out, levels of glycogen and water inside the muscle
2) Refuelling and hydration level.
3) Repair of muscle cells can cause temporary fluid retention.
Your body weight is not simply fat and muscle. You don't go out - use up calories and fat melts and muscle grows. There is no 'fixed' timescale for how your body processes the food you eat, the exercise you do - the calorie deficit you generate and how your body meets that deficit. It depends on what and when you eat, what type, time, intensity etc of the exercise you are doing, whether you are male or female - already highly trained or just beginning. I am sure there are more factors that I am missing but these are the first few things that spring to mind.
Thanks for reply GA -not straightforward as you say!
As GymAddict says, it is complicated.
Other aspects include that muscle takes more maintenance than fat, so as you get more trained (ie gain muscle) you'll burn more calories while resting, simply maintaining that muscle.
Also muscle weighs more than fat, so along the journey from untrained to trained, you can get smaller (through losing fat) and weigh more (through gaining muscle) at the same time...
40 minutes wrote (see)
Also muscle weighs more than fat...
Sorry, the pedant in me has to challenge that! A pound of muscle weighs exactly the same as a pound of fat.
A pound of fat would have a lot more VOLUME though.
That's some cracking pedantry there.
more factual than pedantry RW12
it's a common error that many people make - as Wilks says, a pound of fat weighs the same as a pound of muscle, feathers, lead or anything else you care to name. the key word that people forget is density which is the measure of mass per unit volume. so a pound of muscle will fill less space than a pound of fat due to the density difference.
and experienced runners should know that - OK 40mins??
Wilkie, fb - At the risk of being a pedant as well, I never said a pound of muscle weighs more than a pound of fat.
When you say this table weighs more than that chair, you are making a comparison between two objects. When you say that a substance weighs more than another, it clearly means that comparison for equal volumes - in other words density. But you'd need to be fairly dense not to realise that...