First, let me clear up one thing right away. THERE IS ABSOLUTELY NO "ONE TRICK" TO FITNESS! Fat loss and muscle growth are the result of many different practices, methods, and good habits that are carried out with great effort and consistency over the course of time. There is no way to avoid that fact.
That is not the point of this blog, however. I'm here to caution against any person that promises a "key" or "trick" to fitness. Someone making such a promise is most surely selling something and hoping you'll be a customer. There is, of course, absolutely nothing wrong with selling something. It's what these people are selling, and the way they sell it, that is cause for concern.
The danger comes from the fact that these people are only offering this one "thing" that is supposed to change your life apparently. They are promising a magic bullet, which simply does not exist, rather than providing an effective, holistic approach. So even if their "trick" will help you build muscle or lose fat to a small degree, it is surely not the end-all-be-all tactic they are promising.
Besides the fact of exaggerated claims, the science of fat loss and muscle growth is ever advancing. Many times what was once thought to be an effective method turns out to be proven false or less than optimal. If this occurs, do you think this person will tell you that their "one trick" turned out to be useless? Absolutely not! They have based their entire business and reputation around this. If they admit it's ineffectiveness they'll be ruined because they have absolutely nothing else to offer. They'll fight and lie to protect it. I guarantee it!
Over the course of my career there have been instances where a method I was using turned out to be less than optimal. I was able to then approach my clients and tell them to change the way we did things. Do you know why? It's because it was not the only thing I was offering them. It was merely a piece of a larger puzzle. I was able to then easily remove the broken piece and plug in a better piece.
So be very careful before placing any trust in those offering the one key to fat loss and muscle growth. They are either looking to make a quick buck or simply not knowledgeable enough to know any better. Either way, you'll be the one suffering the consequences in the end.