A third of US teenagers and 14% of adults under the age of 50 are fundamentally unfit, scientists have found.
Obesity is a major problem in the US
Over 5,000 Americans were assessed for the Journal of the American Medical Association research.
Those with poor fitness also had a higher chance of risk factors for heart disease, such as high blood pressure and cholesterol levels.
A UK obesity expert said the study was further proof of people taking too little exercise, and warned a similar picture was likely to already be emerging in the UK.
In the UK, around two thirds of men and over half of women are overweight or obese.
'Prevalent health problem'
In the US study, researchers at the Feinberg School of Medicine at Northwestern University in Chicago analysed data on 3,110 adolescents aged 12 to 19, and 2,205 adults aged 20 to 49 who took part in a major US health survey.
None had previously being diagnosed with cardiovascular disease.
The volunteers underwent a strenuous treadmill test designed to push their heart rates to at least 75% to 90% of the maximum for their age.
Oxygen consumption was estimated by measuring the heart rate response to
reference levels of energy-expending work.
The test showed that 19.2% of those surveyed were in the "low fitness" category.
If extrapolated to the whole US population, that would equate to around 16m people, the researchers said.
A third of the teenagers studied had low fitness, which would translate to around 7.5m, they added.
The same proportion of males and female adolescents were found to be unfit.
But among adults, women were significantly more likely to be unfit than men.
Total cholesterol levels and maximum blood pressure were higher in unfit than
highly fit participants.
And levels of 'good' high-density lipoprotein cholesterol were reduced in those with low fitness.
Writing in JAMA, the researchers led by Dr Mercedes Carnethon, said: "This report indicates that low fitness is a prevalent and important public health problem in the US population.
"The consequences of declines in physical activity over time are now evident by the large proportion of society with low levels of fitness."
They said the correlations between low fitness and heart disease risk factors suggested a potential trend of "increasing morbidity (illness) and mortality from chronic diseases", the first sign of which was the "burgeoning obesity epidemic" now being seen.
The researchers suggested a campaign like that seen to reduce the numbers of people smoking should be considered in a bid to address poor fitness levels in the population.
UK obesity expert Dr Ian Campbell, who practices in Nottingham, said he was not surprised by the US study.
"The writing has been on the wall for some time. From infancy right up to adulthood, people are less active than they could be and less active than they should be.
"And we're going to have to pay the price for that."
Dr Campbell said healthy eating habits had to be combined with increased activity levels.
And he added: "We can reasonably assume the UK is in the same position as the US already, as we know levels of activity are too low in both children and adults."