Market researches, based on the data put together in health maps, show that communities in South Wales take eight of the first 10 unhealthiest places.
Valley areas have held such undesirable titles since long; the researchers suggest inner-city areas will soon overtake them.
Poor diet and a growing obesity crisis in South Wales has become more problematic than the post-industrial traumas seen there, and this may put more pressure on the NHS.
Based on a combination of official data, the census and surveys, the market researchers CACI and TNS, put local authority areas into four groups.
Merthyr Tydfil currently tops the 'group one' list, closely followed by Rhondda Cynon Taf and Neath Port Talbot.
Inner-city areas of London, head a second group with severely unhealthy lifestyles prone to cause serious illness.
They warn the health service department that, they can expect these extensive unfit and unhealthy lifestyles to become a major area of concern.
The Assembly Government said it was very much interested in helping people lead healthier lives, and this could only be done by concentrating on causes of ill- health in deprived areas.
Ian Thurman of CACI said, 'The danger for the NHS is that this [second] group could be presenting itself with poor health at a relatively young age.'
He said these deprived urban areas would soon replace communities of South Wales.
'It looks like we have got a set of attitudes to food that look fairly entrenched in terms of fast food and high levels of fat,' he said.
'Group one isn't surprising. It's the classic ex-mining areas, steel works, ship building - all those post-industrial areas. Age will get to those group-one areas in that they are old, retired, unhealthy.'
Nottingham and Manchester are among other areas predicted to witness more health problems.
The City of London and areas such as the Isles of Scilly are the healthiest places.
Mr Thurman added, 'The NHS is already overburdened, but this is nothing compared to the time bomb that is set to explode if people don't make major changes to their current lifestyles.'