While this insider knowledge can be advantageous to investors, it’s not the type of “knowing” Lynch intended for you to base your investment decisions on. What Lynch meant and Buffett actually said was that you should only invest in companies whose business models you understand. Knowing Planet Fitness (PLNT) has the hottest instructors is great, but it doesn’t necessarily make it a good investment.
The right way to invest in what you know is to use your knowledge about gyms and fitness as a launching off point to researching investment opportunities. From there you need to ask the right questions about the company and its industry to determine if it’s worth investing in.