can someone explain to me why it's okay to give mice a human-amount of sweetener, and not a mice-amount?
This means that the mice got a dose per unit of body weight like what humans would get if they ate that FDA-defined maximum. That's what equivalent doses are taken to mean in animal models of human nutrition or medicine. When there is known to be a different bioavailability or digestive response in animals from humans, then the dose is adjusted with that in mind before the experiment begins.
So, no, the tiny bodies of mice were not subjected to the large servings that much bigger human beings eat. They got a dose adjusted for the body weight of mice.
it's more than just a "per kg" dosing as well. Rats/mice have a very different level of metabolism than humans do just because of scale.
Basically there's a fudge factor of 0.75. Of course, the following article goes on to explain that it's actually drug mechanism dependent as well. Very complicated stuff!
"The value of the exponent for whole body metabolic rate was originally calculated by Max Kleiber in 1932 to be 0.74 (Kleiber, 1932). A few years later, Brody et al. published their famous mouse to elephant curve and calculated the exponent to be 0.734 (Brody, 1945). A value of 0.75 is now accepted because it is easier to use, and the difference from 0.734 is considered to be statistically negligible (Schmidt-Nielsen, 1984). However, it should be noted that exponents in the range 0.6–0.8 have been reported for metabolic rate (Agutter and Wheatley, 2004). A value of 0.75 means that the whole body metabolic rate increases as body weight increases, but to a lesser extent than would be expected of a simple proportional relationship. It follows on from this that the specific metabolic rate (the metabolic rate per unit mass) decreases as animals get larger (the exponent is −0.25); the metabolic rate of 1 g blue whale tissue is 1000 times less than that of 1 g shrew tissue (Kirkwood, 1983)."
This means that the mice got a dose per unit of body weight like what humans would get if they ate that FDA-defined maximum. That's what equivalent doses are taken to mean in animal models of human nutrition or medicine. When there is known to be a different bioavailability or digestive response in animals from humans, then the dose is adjusted with that in mind before the experiment begins.
So, no, the tiny bodies of mice were not subjected to the large servings that much bigger human beings eat. They got a dose adjusted for the body weight of mice.