The United Nations on Wednesday released its first-ever recommendations on physical activity for children under five, with disputed advice on subjects ranging from screen time to "tummy time".
The guidelines from the World Health Organization may read to some parents like common-sense practices, including not exposing babies under one-year-old to screens.
But several experts noted that WHO's broad recommendations were based on thin evidence and chastised the agency for adopting overly simplistic definitions of key terms, notably "sedentary screen time."
WHO said the guidelines "fill a gap" in the global effort to promote healthy living.
With obesity posing a rising public health threat and 80 percent of adolescents "not sufficiently physically active," WHO said it was necessary to outline best practices for children under five -- a crucial period for lifestyle development.
Despite acknowledging that its "strong recommendations" were based on "very low quality evidence," the UN health agency said its advice could apply to all young children, regardless of gender, cultural background or socio-economic status.
For infants under one, the WHO recommends at least 30 minutes of physically activity a day, including prone position -- or tummy time -- for those not yet mobile.
Babies under one should also not be restrained in a pram, highchair or strapped to someone's back for more than an hour at a time and should sleep between 12 and 17 hours per day, the agency said.
For infants between one and two years old, WHO recommends two hours of physical activity per day, with no more than an hour of "sedentary screen time" and at least 11 hours of sleep.
And for children aged three to four, two hours of daily physical activity should include at least an hour of "moderate to vigorous" movement, while screen time should be kept under and hour.
"I do rather wonder to what extent global guidelines on public health policy, affecting millions of families, should be based on 'very low quality evidence,'" Kevin McConway, emeritus professor of applied statistics at The Open University in Britain said in a statement.
He noted that it is often not possible to conduct experiments on young children, which forced WHO to rely on observational studies.
"What exactly is 'sedentary screen time' anyway?" McConway further asked.
WHO's "glossary says that it excludes 'active screen-based games where physical activity or movement is required', but that's not very clear in my view."
Andrew Przybylski, director of research at the Oxford Internet Institute at Oxford University, agreed that while restricting screen time among young children appeared to "make sense... in many ways the conclusions drawn about screens are out of step with scientific evidence of harm."
"Not all screen time is created equal," he added, urging WHO to call for "higher quality studies" to more broadly assess the various types of screen-based activities available to children and their impacts.
"The report represents a missed opportunity for the WHO," he said.
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