If you've ever been interested in finding out what people search for when they Google for this, the most hot of hot topics for a personal trainer here's some data and research

Weight Loss (Locale: UK) had an average 1,500,000 searches
Fat Loss (Locale: UK) had an average 165,000 searches

So there you go - weight loss has almost ten times more search volume - which is going to lead to a no brainer of a statement. Use "weight loss" on your website, articles, blog posts and copy!