Daniel then said...

Please test your servants for ten days:

Give us nothing but vegetables to eat and water to drink.

 He agreed to this and he tested them for ten days.

At the end of the ten days they looked healthier and better nourished

So the guard took away their choice of food and wine they were to drink and gave them vegetables and water instead.

To these four young men God gave knowledge and understanding of all kinds of literature and learning.

And Daniel could understand visions and dreams of all kinds.

...he found none equal to Daniel, Hananiah, Mishael and Azariah; in every matter and question, he found them ten times better than all the magicians and enchanters in his whole kingdom.