(CNN) -- Babies who died from sudden infant death syndrome, or SIDS, show lower amounts of the brain chemical serotonin, says a study published Wednesday in the Journal of the American Medical Association.
Levels of serotonin, which regulate a baby's sleep, heart rate and breathing, were 26 percent lower in the brainstems of babies who died of SIDS than in those of babies who died of other causes, the study says.
Researchers believe a low level of serotonin may be the "fundamental abnormality" in babies who die from SIDS. In the brainstem, serotonin acts as an alarm system, waking up a baby when breathing is disrupted. Babies without it can be unaware that they are not breathing enough oxygen, causing them to die in their sleep, according to the study.
The study also found that infants with insufficient serotonin also had low levels of the enzyme tryptophan hydroxylase, which is needed to make serotonin. Levels of that were 22 percent lower in SIDS babies.
The next phase of the study, which was funded by the National Institutes of Health in Bethesda, Maryland, will focus on why some babies lack enough tryptophan hydroxylase to produce serotonin, said the study's author, Dr. Hannah Kinney, who is affiliated with Children's Hospital Boston in Massachusetts.
There is no test to determine which infants may be at risk for SIDS. Kinney told CNN that researchers believe they are decades away from discovering a physical marker of a brainstem problem that could be identified in the blood.
To avoid the risk of SIDS, she and other doctors recommend that parents make sure that babies can breathe uninhibited during sleep: Place babies on their backs to avoid their rolling over, don't over-bundle the baby, and don't leave loose bedding or stuffed toys in the crib.