How to convey to someone that food is losing heat?
The story is that I want someone to come earlier to the dining table as food is hot now but in ten minutes it would not be.
How in American English can I say to someone to come early, as food on the table is getting cold or losing heat?
Your dinner is getting cold will be quite acceptable I think. Dunno if there are any idiomatic synonyms though.
Get it while it's hot is an alternative to your dinner's getting cold.