In the shadow of the sugar factory, belching steam behind its high wire fences and patrolled gates, are the bungalow homes of Clewiston's black community. African American men are sweating under their hats in the Florida morning sun, pushing lawnmowers across bumpy grass. Outside the Harlem community centre, the last building before the plant, Joel is selling soul food from a van.

This is Hendry County, deep in the rural south of the state, known only to most visitors as the gateway to the tranquil fishing pleasures of Lake Okeechobee. You might think it offers a healthy, outdoor, rural life, and that's what most here people will tell you. But it has some of the lowest life expectancy in Florida.

Joel, like everybody else, says it doesn't concern him. "I'm Jamaican. I've only been here 10 years. I'm young – all that's about old people," he says.

But it's not. It's about the African Americans who get seasonal work at the plant belonging to "Big Sugar" – the US Sugar Corporation. It's about jobless Hispanics hanging around in the trading center arcade opposite the courthouse waiting for casual work in Hendry's county town, LaBelle. It's about the Caucasian women and girls whose girth is almost the same as their height. And it's about the tobacco shop in Clewiston where they sell raw tobacco and empty tubes. "You rent a rolling machine and save lots of money!" declares the signboard outside. It's busy.

Boys born in 2009 in Hendry can expect to live, on average, 72.6 years and girls 77.7 years. It's on a par with Uruguay, Syria and Vietnam, according to the latest data from the Institute of Health Metrics and Evaluation in Seattle, Washington.

Drive south and it's a dramatically different story. Women in Hendry's neighbouring county, Collier, enjoy the longest average life expectancy in the US and almost in the world, close to that of Japan. Average life expectancy for girls born in 2009 is 85.8 years, and 80.7 years for boys. That means Hendry's population will have an average eight years less of life than Collier's.

Collier's county town, Naples, is full of seniors in shorts, with white hair but healthy complexions. In the evening, except for the average age, this could be an Italian city. Expensively dressed women and men with well-fitted shirts and heavy gold watches amble down Fifth Street, glancing into boutiques and galleries. There is an Italian bistro, among the many restaurants with tables on the pavement under the strings of elegant tiny white lights wound up the trunks of palm trees, which has a Dali-esque clock on the wall and stained glass portraits of each of the Beatles, mixing up the European cultures.

'They have everything in Naples. We have Walmart'

It's less than a hundred miles from Clewiston to Naples and just 66 from LaBelle, but in terms of the social and cultural gap, they are a million miles apart. At Hendry County's only hospital, a nurse snorted when she heard of the life expectancy gap. "Have you been to Naples? You go to Naples and then you'll see why. They have everything in Naples. Five hospitals. We have Walmart."

In the next week or so, the US supreme court will deliver its ruling on Barack Obama's healthcare law. But whether the justices in Washington decide to strike down the Affordable Care Act or allow it to stand, America will still have to deal with the steadlily growing chasm between the healthcare haves and have-nots.

The US spends more on healthcare than any other country in the world – around 18% of GDP – but there are huge and growing disparities in life expectancy. In Florida, between 1989 and 2009, the gap between best and worst grew faster than anywhere else, according to the IHME.

Their recently published data suggests that the health of the average American woman, in particular, is not improving over time – in fact, in hundreds of counties like Hendry, girls today are likely to live shorter lives than their mothers did. They blame the tsunami of preventable chronic diseases linked to lifestyle, such as diabetes, heart problems and some cancers, which is sweeping the US.

Naples has extraordinary wealth. A house on the beach went for $48m a few weeks ago, and last winter another sold for $42m. These are huge estates with their own boat docks and every possible luxury, but houses in the single figure millions are almost the norm.

Healthy millionaires are not the whole story, according to Deb Millsap, Collier County's director of nutrition and health education. "Palm Beach County should be healthier if it was just about wealth," she said. Collier has its share of the needy and 26% of its children are considered to be in poverty. A lot of people from other counties and from Mexico arrive looking for work and a better life and that population has been hit by economic recession, which has drastically slowed the once-booming construction industry in the area.

But money makes it easier to solve problems. Every year, Naples holds its winter wine festival – the largest wine auction in the world. It is an extraordinarily glitzy event which in a little more than a decade has raised $100m. Tickets start at $8,500 a couple.

That is Naples' safety net. "The money goes to children's services in Collier county," says Millsap. "People fly in from all over the world in nice jets. They bring in top chefs and on Friday night there are dinners in people's homes to raise money and there are very expensive wine lots. At the last auction in January there was a lot which sold for $150,000."

The money allows Collier to help fund daycare and kids' clubs and other programmes to keep children healthy. The county has an infant mortality rate of six per 1,000 live births, which is good for the US as a whole. And the charitable Women's Health Foundation takes care of many women from the uninsured and indigent population, who account for about 50% of births.

Medical care is excellent. "We tend to attract outstanding practitioners because of where we are – people who have spent the first part of their career up north," says Millsap. As well as numerous private hospitals, there are two community hospitals, linking with a network of physicians, which take care of everybody, including the uninsured and those on Medicaid.

Collier County's public health department promotes activity through encouraging walking and cycling. "We have a lot of gated communities in Collier which are not conducive to walking," says Millsap. "Kids can't always walk to their friend's house because there are lakes in between. We started a new development checklist." It includes sidewalks and siting houses near schools and designing homes for a mixture of age groups, so people don't have to move away from friends and family as they get older.

It has an active anti-smoking initiative in schools. "When other counties lost funding, our health director decided to fund it from revenue from another programme," said Millsap. In Collier County, that's the sort of thing you can do.

Brenda Barnes, who does the same job in Hendry, is dealing with a population of about 35,000 – a tenth of the number in Collier – but has much the tougher job. There are few millionaire wine buyers and philanthropic organisations to top up what has become a shrinking pot of money for preventive health efforts.

"We have had cutbacks in the state of Florida as far as public health is concerned. The governor is revamping the whole state of Florida including other state agencies. He has more of a mindset of private industry rather than state-run programmes," she says.

'Prevention is always a hard sell'

Hendry's population is scattered across a wide rural area and there are just two towns, Clewiston and LaBelle, 35 miles apart. "Because our population is so spread out, it is a bigger challenge to the health department to get the message out, but I think we do a good job," she says, citing an aggressive tobacco prevention programme.

But they have their work cut out. "We have a very large Hispanic population. They are not as highly educated as some Hispanic populations. Prevention is always a hard sell. People tend to be more reactive rather than proactive, unfortunately. But we only have one hospital between the two cities."

There is a family health centre which charges people on a sliding scale, according to income, but people do not go to a doctor until they really need one. "People tend to use the emergency rooms because they know they can't be turned away."

It is a low income area, she says. And there is high unemployment. Many people are on Medicaid. The work on the large sugar cane and citrus plantations, owned by old established families, and in the sugar factory is seasonal. Otherwise, there are cattle and some tourism on the lake and the stores and fast food restaurants in the towns – where Naples has bistros, Clewiston and LaBelle have burgers. According to the county health rankings survey carried out US-wide annually by the University of Wisconsin, 48% of Hendry's restaurants offer fast food and 35% of the population is obese. In Collier, that is 31% of restaurants and 21% obesity.

You don't see cyclists in Clewiston and LaBelle, just cars and truck driving down highways, most of them on the way to somewhere else. Where Naples has spas and pools and gyms, Clewiston has beauty parlours and nail bars for its teenage girls, who drive 45 miles for a night out at the movies.

Clewiston likes to call itself America's sweetest town, in honour of its main employer, the US Sugar Corporation. Ironically, the long-term forecast for an area like Hendry, dependent through lack of prospects, income and education on fast food and tobacco, is anything but sweet. It is chronic disease on an epic scale. Poverty may not in itself kill, but obesity-related diabetes, heart disease, strokes and cancer surely do.