I wrote a book a few years ago on profitable Instagram Niches. It was "Project 1" in my journey thread here:
http://www.blackhatworld.com/seo/my-journey-to-completing-a-bunch-of-unfinished-projects-sitting-on-my-desk.683733/
It has 24 niches, 59 sub-niches, and 216 micro-niches to choose from. I update it every year. The micro-niches is where the money is at.
You said you were in the weight loss niche. Weight loss is definitely one of the niches in the book. However, "weight loss" is not the correct term. The correct term is "fitness". Here is the fitness section of the book:
Niche:
Fitness
Sub-‐Niche:
Workouts
Micro-‐Niche:
Weight lifting
Weight loss before and after pics
Workout routines
Leg workouts
Women weight lifters
You can choose the niche or the sub-niche, but the micro-niches are extremely targeted, and they are not as saturated.
If you chose the micro-niche "women weight lifters" and started posting photos of women lifting on Instagram, you would only have people on your page who are interested in that specific topic. Which is what you want.
But you have to place fitness photos where all the fitness people hang out at in order to get traffic. They hang out where they can see photos of hard bodies and become inspired: Instagram and Pinterest.
Once you get traffic on your Instagram/Pinterest photos, tell them to go to your website to see more micro-niches, such as "women's lifting before and after photos".
When they are on your site, you can have something like Amazon affiliate links that lead to women's lifting workout DVD's, women's lifting books, workout clothing, and even music to workout to, in order to monetize the site.
PM me if you want the whole book so you can see what other niches besides fitness are profitable.
