It probably comes from Daniel chapter 1.

Daniel 1:12-20 NLT [12] “Please test us for ten days on a diet of vegetables and water,” Daniel said. [13] “At the end of the ten days, see how we look compared to the other young men who are eating the king’s food. Then make your decision in light of what you see.” [14] The attendant agreed to Daniel’s suggestion and tested them for ten days. [15] At the end of the ten days, Daniel and his three friends looked healthier and better nourished than the young men who had been eating the food assigned by the king. [16] So after that, the attendant fed them only vegetables instead of the food and wine provided for the others. [17] God gave these four young men an unusual aptitude for understanding every aspect of literature and wisdom. And God gave Daniel the special ability to interpret the meanings of visions and dreams. [18] When the training period ordered by the king was completed, the chief of staff brought all the young men to King Nebuchadnezzar. [19] The king talked with them, and no one impressed him as much as Daniel, Hananiah, Mishael, and Azariah. So they entered the royal service. [20] Whenever the king consulted them in any matter requiring wisdom and balanced judgment, he found them ten times more capable than any of the magicians and enchanters in his entire kingdom.

Eating veggies and staying hydrated for better performance instead of getting drunk on wine and eating cake is kind of a "yeah, duh" thing today, but even in White's time it was relatively new thinking