Days later, Hefford was pronounced dead. Only after her death did her family learn that Hefford, the mother of a 7-year-old girl and a 5-year-old boy, had a rare genetic disorder that prevented her body from properly metabolizing her high-protein diet.
Urea cycle disorder, which causes a deficiency of one enzyme in the urea cycle, stops the body from breaking down protein, according to the nonprofit National Urea Cycle Disorders Foundation. Normally, the body can remove nitrogen, a waste product of protein metabolism, from the blood. However, a urea cycle disorder would prohibit this. Therefore, nitrogen, in the form of toxic ammonia, would accumulate in the blood and eventually reach the brain, where it can cause irreversible damage, coma and death.
"The enzyme deficiency can be mild enough so that the person is able to detoxify ammonia adequately -- until there's a trigger," said Cynthia Le Mons, executive director of the foundation. The trigger could be a viral illness, stress or a high-protein diet, she added.
"There was just no way of knowing she had it because they don't routinely test for it," said Michelle White, Hefford's mother and a resident of Perth. "She started to feel unwell, and she collapsed."
White blames protein shakes for her daughter's death.
Since 2014, Hefford, who worked at Princess Margaret Hospital for Children and studied paramedicine, had been competing as a bodybuilder.
It was only after Hefford's death that White discovered containers of protein supplements in her daughter's kitchen, along with a strict food plan. White understood then that her daughter, who had been preparing for another bodybuilding competition, had also been consuming an unbalanced diet.
Hefford was eating "way too much protein," said White, which triggered her daughter's unknown urea cycle condition. (For most healthy people, a high-protein diet, when followed for a short time, generally isn't harmful, according to the Mayo Clinic.) Hefford's diet included protein-rich foods, such as lean meat and egg white, in addition to protein shakes and supplements, her mother said.
"There's medical advice on the back of all the supplements to seek out a doctor, but how many young people actually do?" White asked.
Le Mons said, "typically, there are nuanced symptoms that just go unrecognized" with mild cases of urea cycle disorder. Symptoms include episodes of a lack of concentration, being very tired and vomiting.
"Sometimes, people think it's the flu and might even go to the ER thinking they have a really bad flu," Le Mons said, adding that a simple serum ammonia level test, which can detect the condition, is not routinely done in ERs.
It's unclear whether Hefford suffered symptoms of her condition. White, who hopes her daughter's story will serve as a warning to help save lives, believes protein supplements need more regulation.
The Australian Medical Association says there's no real health benefit to such supplements. And, while they may not be necessary for most people, they're not dangerous to most, either.
The estimated incidence of urea cycle disorders is 1 in 8,500 births. Since many cases remain undiagnosed, the exact incidence is unknown and believed to be underestimated.
"There's a myth that this disorder only affects children," Le Mons said, noting that one patient reached age 85 before diagnosis.
Regarding Hefford, Le Mons said that "this is not the first time this has happened." Other athletes, who like Hefford were unaware of their condition, have died when a high-protein diet triggered their condition.
Though there is no cure for urea cycle disorder, a balanced diet is all that is needed for some patients, according to the National Urea Cycle Disorders Foundation. Treatment may include supplementation with special amino acid formulas, while in some more severe cases, one of two forms of an FDA-approved drug may be prescribed. When these therapies fail, liver transplant may become necessary.