Confirmation of the hypothesis could have revolutionary implications for the prevention of certain types of autism.
“10-20% of cases of autism could be prevented by changes in infant nutrition,” wrote Arthur Beaudet, the Henry and Emma Meyer Chair and Professor of Molecular and Human Genetics at Baylor College of Medicine, Houston.
Results from animal studies and genetic factors observed in autistic patients led Beaudet to suggest that low brain carnitine levels might help trigger a mild form of autism, which predominantly affects males.
Previous research at Baylor College had identified that around 1 in 350 males are unable to produce their own carnitine due to an inactive form of the TMLHE (trimethyllysine hydroxylase, epsilon) gene.
Around 3% of males with the inactive TMLHE gene develop autism. Furthermore, TMLHE gene variation only accounts for 1% of autism cases. Consequently, Beaudet hypothesises that brain carnitine deficiency may underlie a much larger proportion of autism.
"We speculate that the individuals with a normal physical examination and normal brain imaging results in studies, which represents 10 to 20 percent of all cases of autism spectrum disorders, might have in common a mechanism that leads to a mild form of autism. This mechanism might involve brain carnitine deficiency," Beaudet said.
In many cases, infants are born without symptoms of autism and have normal physical examination and brain imaging results. However, "Sometimes behavioural regression occurs,” commented Beaudet.
“Onset of symptoms in autism often occurs in the 6–18 month age range. If an infant has a social smile and is very interactive at 5–7 months of age, but loses the social interactions by 12–18 months of age,” he explained.
Beaudet elaborated reasons for the timing of autism onset. "Carnitine is usually delivered across the placenta, and most infants are born with adequate carnitine stores.”
Additionally, breast milk, infant formulas and cow’s milk all contain abundant carnitine, protecting the infant from deficiency in its initial months.
"In many cultures, when the infant is introduced to new foods between 4 and 8 months of age, the first non-milk foods are fruits, juices, cereals and vegetables, all of which contain almost no carnitine, and meats are introduced later," Beaudet said.
“This reduction in carnitine might lead to brain carnitine deficiency and autism,” he continued.
Beaudet proposes “that this carnitine deficiency would evolve from a gene-environment interaction.”
“But diet is an equally important variable in our thinking,” he added. “Other non-genetic factors could be important including many medications and minor illnesses (especially gastrointestinal illnesses), which may deplete carnitine in the body.”
Existing data from the Baby Siblings Research Consortium shows a high recurrence risk for families who already have one child with a milder form of autism, especially if the new infant is male.
"Families such as these could be enrolled in a study to determine whether supplementation with carnitine will reduce the frequency of autism in the new siblings. This would be a very direct and powerful test of the hypothesis," Beaudet said.
If results meet his expectations, Beaudet also advocates reviewing the need for a recommended daily allowance (RDA) for carnitine, together with the question of fortification.
“If this hypothesis is correct, one option would be to fortify all baby foods such as fruits, cereals and vegetables with carnitine,” he proposed.
Published online, doi: 10.1002/bies.201700012
“Brain carnitine deficiency causes nonsyndromic autism with an extreme male bias: A hypothesis”
Author: Arthur L Beaudet