Actually I don't even think the platitude is appropriate in this case, although perhaps this is due to differences in acceptable performance. Do you really need to measure the health impact directly on your body to be satisfied that you are doing something right? I think you can take studies that say "X mg of fiber a day improves outcomes", eat slightly more than X mg of fiber, and likely get 90% of the benefit as compared to the "ideal" amount of fiber, all without a personalized measuremnet. Even if you did have a measurement, there would be confounding factors like "how much did you exercise?" / "how stressed were you?" / "is your body 'just getting old'?"