As a father of four sons, I will give my opinion and advice.
First, I believe it is important to never lie to your children. I am sure some will disagree with me on that. None of my sons were ever led to believe that Santa Claus was real. I have always told them how much money I made and always showed them the bills that had to be paid. I believe it is our job to raise up productive and honest adults. I have also tried to take any fear out of them telling me the truth. Its ok that you put the ball through the window, but now we are going to work on how you will fix it.
To the subject at hand. I also believe that we should tell them the truth in the context of their understanding. So if they are let's say 10, and they ask why you look like you do, I think a legitimate answer is that "I go to the gym more than most, and I pay attention to my diet and nutrition/supplements in a more advanced way than most."
As they get older, they must also learn that it is prudent to not discuss many things with those outside the small circle of family. It is not that we hide it, we simply don't discuss certain things when there is no good that come from it. As you grow in your trust of their judgment, you can grow in your disclosure to them.
My oldest and I share everything in the knowledge and use of supplements -- I will leave it at that (he is now 24, its been this way for a about 2 years now). He listens to my advice, and I believe that this is because he knows that I have never lied to him. As we approached the point of total enlightenment, I made it clear that there was to be no discussion whatsoever with anyone and certainly no help for any of his friends in procuring cutting edge supplements.
If they are younger, and the questions are more pointed, I still think that you could say something like "I am just more willing to seek out alternatives that others are not." Or, something along those lines.
If you handle this and the rest of life with honesty, I assure you that you will greatly enjoy your relationship with them as they mature.