Show the proof
"Daniel made up his mind not to let himself become ritually unclean by eating the food and drinking the wine of the royal court, so he asked Ashpenaz to help him, and God made Ashpenaz sympathetic to Daniel. Ashpenaz, however, was afraid of the king, so he said to Daniel, “The king has decided what you are to eat and drink, and if you don't look as fit as the other young men, he may kill me.” So Daniel went to the guard whom Ashpenaz had placed in charge of him and his three friends.
“Test us for ten days,” he said. “Give us vegetables to eat and water to drink. Then compare us with the young men who are eating the food of the royal court, and base your decision on how we look.” He agreed to let them try it for ten days. When the time was up, they looked healthier and stronger than all those who had been eating the royal food.
So from then on the guard let them continue to eat vegetables instead of what the king provided. God gave the four young men knowledge and skill in literature and philosophy. In addition, he gave Daniel skill in interpreting visions and dreams.
At the end of the three years set by the king, Ashpenaz took all the young men to Nebuchadnezzar. The king talked with them all, and Daniel, Hananiah, Mishael, and Azariah impressed him more than any of the others. So they became members of the king's court. No matter what question the king asked, or what problem he raised, these four knew ten times more than any fortune teller or magician in his whole kingdom."
~ Daniel 1.8-20 GNB
Was that a secret diet that only they knew? I am inclined to think it was more than what they ate. Eating exactly what the king ate implied privilege and probably came with excess and partying.
The four Jews chose to practice clean eating and clean living. This helped them study hard and be the best.
Daniel was confident that if they ate healthy and studied hard, while the others partied hard, they would have enough proof in ten days.
Am I confident that I have the result to prove that I am a child of God?