How difficult is it to rank on good with the 'Losing Weight' or 'Body Building' sub-niches ? with long tail keywords

anyone tried this? if so any info on traffic or monopolization would be great!

It's really specific to the actual keyword.

Even though they are long tail, some are still more competitive than othes.

You would need to give more info

-Red

Most fitness/fatloss stuff is extremely popular and competitive. People are fat, they want to lose weight.

It depends entirely on the top 10 competitors, just because it's a long tail doesn't mean it's going to be easy to rank for, it just means it should be easier to rank for than the broader keyword.