Charles Spurgeon Commentary Daniel 1:15

"And at the end of ten days their countenances appeared fairer, and they were fatter in flesh, than all the youths that did eat of the king`s dainties." — Daniel 1:15 (ASV)

I do not doubt that the satisfaction of heart which they had in keeping themselves undefiled tended to give them a good digestion, and thus they were more likely to be healthy than the others were.