Daniel did worship the one and only true God and he also followed His laws. Daniel thought about the kings food and knowing the king worshiped idols and did not follow God or His laws, decided not to eat it. Now God had brought Daniel into favour and tender love with the prince of the eunuchs. If we do that, God will bring us into favour with the people over us. The prince of the eunuchs fears that if Daniel and his friends do not eat the kings food then they will look thin and the king would kill him.
Pulse is vegetable food.
Daniel is not doing this because he is a vegetarian but to avoid eating the king's idol tainted and unclean meat. So he consented to them in this matter, and proved them ten days. Daniel is telling him to compare how he and his friends who don't eat the king's meat look to the ones that do. Daniel already knew what the out come would be, that they look better than the others. And at the end of ten days their countenances appeared fairer and fatter in flesh than all the children which did eat the portion of the king's meat.