Sudden infant death syndrome (SIDS) may be caused by a defective gene which prevents babies digesting milk, a breakthrough study has found.
Scientists said some of the roughly 200 UK babies who die unexpectedly from the condition each year could have suffered fatal cardiac arrests because they cannot break down fat.
SIDS, also known as “cot death”, is particularly devastating for parents because often experts are unable to determine a cause of death.
Published in Nature Communications, the laboratory study is the first time a plausible causal link between a genetic anomaly and some instances of the syndrome has been drawn.
It opens the door for scientists to one day test babies in the womb for the deficiency and possibly even treat it.
The researchers said a mutation of the HADHA gene leaves babies unable to metabolize lipids, molecules that include fats, cholesterol and fatty acids, meaning they can die suddenly at just a few weeks old.
According to the NHS, other causes of SIDS may be low birthweight, breathing obstruction, or environmental stresses such as tobacco smoke, although health leaders recognise that the evidence is far from clear.
Dr Jason Miklas, who led the study at Stanford University, said: “If a child has a mutation, depending on the mutation the first few months of life can be very scary as the child may die suddenly.
"An autopsy wouldn't necessarily pick up why the child passed but we think it might be due to the infant's heart stopping to beat."
The US team used stem cells in a laboratory to see how they behaved with various genetic abnormalities.
They found the breakdown occurring when enzymes fail to complete a process known as fatty acid oxidation.
It is currently possible to screen for the genetic markers of the deficiency, but effective treatments are still a ways off.
In 2017 a study by the University of Adelaide suggested that babies with the lack of a brain protein named Substance P may have difficulty with head movements, which could also be a contributing factor for SIDS.
Experts believe another cause could be dangerous methods of co-sleeping.
Previous research by The Lullaby Trust has indicated that 40 per cent of parents are not co-sleeping safely.
An adult falling asleep on a sofa or armchair with a baby increases the risk of SIDS by up to 50 times, the charity says.
Referring to HADHA abnormality, Hannele Ruohola-Baker, who also worked on the new study from the University of Washington, said: “There is no cure for this.
"But there is now hope, because we've found a new aspect of this disease that will innovate generations of novel small molecules and designed proteins, which might help these patients in the future."
She added: It's very exciting to think that our work may contribute to future treatments, and help for the heartbreak for the parents who find their children have these mutations."
Nearly half - 44 per cent - of babies who die within a month of being born in the UK do so due to complications arising from a premature birth, and around a third due to medical conditions arising in the womb or very shortly after birth.
According to the Office for National Statistics, the unexplained infant death rate in England and Wales has fallen to its lowest level on record.