It's ok to say that sometimes, and it's something I think I've had to come to terms with as a personal trainer and physical therapist. I don't know. I often get asked questions by family members and friends about health and fitness. I don't mind really. I like being able to answer most questions that come my way. However, sometimes my family will ask me questions that I honestly just can't answer.
Now I'm not talking about those questions that have nothing to with my field of expertise. Some of my family doesn't really understand what physical therapy is and assume I'm a medical doctor, so I sometimes get questions like, "my friend has thiscrazydiseaseitis, what is that?" or, "my doctor says I have acute diverticulitis, what can I do to help that?" Questions like that I have no problem saying I don't know. What I'm talking about is questions that pertain to fitness or physical therapy that I just can't answer. The reason why I can't answer them is because everyone is individual. Everybody reacts differently to a given treatment, heals at somewhat different rates, builds muscle, losses weight, all that jazz different that their neighbour.
What I have done in the past, I'm afraid to admit, is give an answer that seems pleasing, such as, "Oh, well if you have a net of negative 3500cal in a week then you'll lose one pound, so if you want to loss 5 pounds then it will take you 5 weeks if you can just get rid of 500 calories everyday." It sounds logical. yes, 3500cal does equate to one pound, but that's in such a pure sense. Like I said everyone is different. A person's genes, lifestyle, particular metabolic conditions, yada, yada, all come in to play. One time I used to think it was as simple as eliminating 3500 calories and that was it, you were bound to lose weight. oh, the naivety. So now when someone asks me, "how long will it take me to lose 5 pounds" my answer is, "I don't know, but I can help you get there."
The same thing goes for question like:
"Yesterday at the gym I wasn't able to lift as much weight as I could last week, why not?" I don't know.
"How come my friend who never works out and eats worse than me is skinnier than I am." I don't know.
"How long before I can fit into that pair of jeans again." I don't know.
Sometimes we need to realize as fitness professionals that we don't have all the answers. Sometimes things just are. I don't know why.