Im constantly losing weight

[Jstar1]

your weight won't go up after eating a full course meal, you'd have to wait a few days.

go work out, building muscle increases your body mass

Well you would gain the amount the food wighed. Not long term but if he weighed himself right after he ate he should have gone up.

it takes about 10 hours to digest food and turn it into body fat. Food doesn't weigh a lot anyways.

[0l3n]

your weight won't go up after eating a full course meal, you'd have to wait a few days.

go work out, building muscle increases your body mass

Well you would gain the amount the food wighed. Not long term but if he weighed himself right after he ate he should have gone up.

it takes about 10 hours to digest food and turn it into body fat. Food doesn't weigh a lot anyways.

Tell that to my 5 pound steak

[StealMySoda]

But assuming you haven't been on the shitter, and haven't been exercising and sweating profusely, then you will weight exactly the same as when you ate UNTIL you visit the bathroom.
Just because your food is digested doesn't magically mean it weighs less. Conservation of mass. Unless of course, mass is given off with respiration? I don't know I finished Bio in year 11, I'm looking at it from a Physics POV.