Valid point, but your starting mass also increases. Then there's also that if the measurement changes, the results can become pretty bizarre. Say, you are on a

Valid point, but your starting mass also increases. Then there's also that if the measurement changes, the results can become pretty bizarre. Say, you are on a diet for one month. At the start of the month, the kilo prototype weights 1 skg and at the end 1 ekg = 0.9 skg. You measure yourself at the start of the diet, and get that you weight 100 skg. You use some very precise diet-fu to draft a program that should see you losing 5 skg during the month. And lo and behold, you manage to stick to the program and loose exactly 5 skg. But at the end of the month, when you measure yourself against the now-lighter prototype, you get that you now weight 106 ekg. So, you actually achieved your target and lost 5 kilos, but meanwhile ended up with 6 kilos more body mass than before the diet! (Disclaimer: real changes in the prototype mass are of course way, way smaller.)