‘So he took notice of what they said and put them to the test for ten days. And at the end of ten days their faces appeared fresher, and they were fatter in the flesh, than all the youths who ate of the king's food.'
So he did what they asked. The result of the test was that they gave a better overall impression facially than those who ate the king's food. They looked fresher and more full-faced than the others. By observing God's law given in the Torah they had demonstrated its truth. Given the effects of overindulgence we can quite appreciate how this might be, but it is possible that we are intended to see this whole affair as being the result of a revelation from God to Daniel by means of a dream or vision (Daniel 1:17).