But now there's a treatment that could change their lives.
A Novato-based company called BioMarin has developed a new drug as part of its treatments for rare diseases.
A special low protein formula is a key part of Hannah's diet. She was born with phenylketonuria or PKU, an enzyme missing from her liver means her body can't properly process protein.
"Their liver can no longer control phenylalanine and all the protein they intake is now converted to this phenylalanine level that goes very high and is very neuron toxic to them," said Bio Marin Chief Medical Officer Dr. Emil Kakkis.
That's why newborns are screened for PKU at birth. It affects about one in every 15,000 babies.
"If she were on no diet for an extended period of time, it would be brain damage," said Hannah's mom, Marie Coleman.
"Diet has saved a lot of children from mental retardation, but it's an imperfect treatment," said Dr. Kakkis.
The pharmaceutical company BioMarin from Novato is dedicated to finding treatments for rare diseases.
Kuvan is their drug for PKU and recently received FDA approval.
"What it does is activate the little bit of enzyme they have left in their bodies. We activate that enzyme in their liver, causing its activity to increase and allows it to help manage phenylalanine levels in a very natural way, that is by activating that enzyme," said Dr. Kakkis.
"This is a tremendous breakthrough, as a physician taking care of these kids, we haven't had anything new for, well forever," said Dr. Gregory Enns from Lucile Packard Children's Hospital.
Dr. Enns has already seen dramatic improvement for some patients.
"Kuvan works best for most mild cases of PKU. And the kids who have the most severe cases, the classical PKU types that have the highest level of blood phenalaline, those are the cases where Kuvan has less of a chance of working," said Dr. Enns.
Biomarin researchers are working on developing a second drug for patients with PKU, patients who don't respond well to Kuvan.
"The potential is that it would be a complete treatment, that is, it would bring your phenalaline levels all the way down and potentially allow a normal diet," said Dr. Kakkis.
Human trials will get underway this year, and it is research Hannah knows is critical.
She and her parents didn't expect Kuvan to make a difference for her, but took part in the clinical trials with the idea it would help others. They were shocked to learn it worked.
"I was really surprised and I was very excited to know that I could eat more food and maybe even try some bread," said Hannah McGary.
"The cheeses are huge for her, she's always wanted to try cheddar, parmesan and all these things, so it's changed her life," said Hannah's dad Jim McGary.
She's gone from relying on specially formulated products and medical food to keep her protein intake at six grams or below, to being able to eat up to 30 grams of protein a day. It is a new found freedom with food that's also created a sense of well being for Hannah.
"I don't know. I just feel more happy, I guess," said Hannah McGary.
The breakthrough comes with a hefty price tag, more than $100,000 a year. Hannah's insurance covers it, even though it's not considered a medical necessity.
BioMarin has created a support program to make sure every patient who would benefit from Kuvan receives the drug, regardless of financial status or insurance coverage.