sugar for him. >> jack loves cheese. >> reporter: why would any parent do this to a sick child? they hope tell trick his brain into a system reset. here's how it works. normally our brains are fueled by glucose sugar. but when we eat only fats, we don't get enough glucose so the body makes a new brain fuel from the fat. something called ketones. seizures are like an electrical storm in the brain. mistearsly, a big dose of these ketones minimize or even eliminate seizures. one thing i worry about is giving false hope to people. how effective is this treatment? >> about one-third of children who go on this diet become completely seizure-free. >> reporter: and it's working. jack's seizures are down 90%. but it's strict. >> epilepsy took him away from us. the diet is -- is -- it's a way to get him back. >> reporter: remember little evan? >> are you ready to come out? >> reporter: this is him now. after four days on this diet, evan's seizures stopped. after six months he was off all his medication. he's even off the diet. >> he was cured, absolutely.