In the original show, Barry had a natural impulse to consume mass quantities of food right after he first manifested his powers, due to his increased metabolism.
Tina McGee explains this to him soon after they meet, since it's the most obvious conclusion.
In the Pilot for this new show, he suffers no ill effects after expending tremendous amounts of energy throughout the episode.
In the second episode, he starts having fainting spells, and the Star Labs crew has to run extensive tests, and the very last bit of data, which had been completely overlooked, was Barry's glucose levels.
Caitlin's response? "Ohhh, of course!"
Gee, they kinda shoe-horned that in, didn't they!
I just can't believe that Barry had to be told that he would have to increase his caloric intake.