But as the family today celebrates the ``big milestone'' of the twins' first birthday, the girls' parents Sue and Joel Chapman said they had seen a ``huge difference'' in their health.

Mr Chapman attended a conference in America in September on glycogen storage disease, a condition so rare it afflicts just nine other Australians.

He returned to Yinnar with new knowledge about his daughters' disease and how to manage it better, and most importantly a sense of hope.

As his daughters crawled about the family's living room floor, Joel said changes to their diet had improved their quality of life.

He said American research showed a low carbohydrate diet was best for glycogen storage disease sufferers, whose livers did not release glycogen into the blood stream.

He said when he and Ms Chapman changed the girls' diet they became happier and healthier.

``Katelyn can crawl now, before she would just lay there. The high carb diet was making a zombie of her,'' Mr Chapman said.

Katelyn's abdomen was severely distended in September as her spleen was 11cm long when it should have been about 2.5cm.

Since the diet change her stomach is 6-7cms smaller and her daily hypoglycemic seizures have ceased.

Ms Chapman said the difference was they now added cornstarch, which is shipped from America every two months, to Katelyn and Emily's milk and protein formula to Kelly's soy milk.

Mr Chapman's parents Elaine and Kevin Chapman, who travelled to America with him, said the glycogen storage disease community there was ``like an extended family''.

``We took a very disillusioned and very unhappy boy to America but when he left he had a smile on his face, he had hope,'' Elaine said.

``We met other people with the disease and they were just normal people,'' Kevin said.

While the girls' blood sugar levels still require constant monitoring, which prevents Mr and Ms Chapman from working, Mr Chapman said the time between their meals had increased and they were much bubblier.

``There's definitely hope around the corner, it's promising anyway.''