^ There's a few issues there. One is that you may not necessarily lose that sale, as long as the prices are comparable between the two products. Sometimes you'll have to throw in an extra something to get the customer to go with you. I don't know the situation in which you're doing this so I don't know if this applies, but the customer is there with already, why woudl htey go through the hassle of finding another person with the same quality as yours for the same or similar price?
The other thing is, it sounds like you're operating in essentially a commodity market (supplements are hard to differentiate). You can either find some way to add some more value, differentiate your product, market more or get out of it.
Don't compromise your values, you'd be surprised at how many people can and do appreciate the truth. A simple, "I want to be honest with you about this, even if it loses me this sale: Our products are very similar. However, they're over there and you're right here, right now." might help. I'm sure you can think of other stuff.
Excuse me if I completely misunderstood your post, but I hope this helps.