People living in England enjoy better health than Americans, despite less investment in healthcare, research published in the US has revealed.
Across all ages, US residents tend to fare worse in terms of diabetes, high cholesterol and heart disease markers, data on over 100,000 people show.
The reason remains a mystery, says the US team, and challenges the idea that resources necessarily improve health.
It may be due to the UK's bigger drive on disease prevention, they say.
Transatlantic health gap
Despite the greater use of health care technology in the US, Americans receive less preventive health care than their English counterparts.
They have fewer physician consultations per year.
Acute hospital visits are also shorter in the US, potentially resulting in missed opportunities for follow-up, say the report authors in the American Journal of Epidemiology.
It is also possible that differences in social or environmental conditions or lifestyle play a role.
But despite looking, the researchers did not find any real evidence that differences in obesity, alcohol consumption or physical activity were to blame.
Smoking may be a factor, but Dr Melissa Martinson and colleagues doubt it because even younger Americans who have not yet been exposed to decades of tobacco smoke appear to be in worse health than English counterparts.
And although a larger share of Americans are uninsured or under insured compared to populations in England or other European countries, even groups with good access to health insurance experienced worse health than people in England.
The researchers say: "Why health status differs so dramatically in these two countries, which share much in terms of history and culture, is an unresolved puzzle.
"Given our finding of health differences between the US and England at young ages, a promising focus of future research - one that could help to elucidate the causes of poor health across the life course - is on health differences between countries at the earliest ages."
Public health experts suggested more generous holiday entitlements and more favourable working conditions in the UK might also play a part.
A spokesperson from the Department of Health said: "The NHS offers care free to all at the point of use and based on need.
"Whilst in some areas our outcomes may be favourable compared with those in the US, we are still clear that we have a long way to go before we achieve outcomes comparable with the best performing health systems.
"That is exactly why we are modernising the NHS."