Childhood obesity is so entrenched that the perception of a healthy weight is shifting, with one in five NSW children qualifying as overweight or obese, a state government report says.

The NSW government's first annual report into overweight children will be used to set a benchmark to track the state's progress against the Premier's commitment last year to reduce childhood obesity by 5 per cent by 2025.

The proportion of overweight Australian children has doubled since the 1980s, their waistlines billowing in a new age of convenient fast food and sugar-loaded drinks, couch-based entertainment and growing anxiety about stranger danger and busy roads.

Children now obtain more than one-third of their daily kilojoules from junk food and drink, the report found, and only 5 per cent consume the recommended amount of vegetables.

Premier Mike Baird named childhood obesity a priority last year, along with other urgent social issues such as tackling domestic violence and reducing young homelessness.

But his stated target – to reduce obesity prevalence by 5 per cent by 2025 – is one that administrators privately concede will not be achieved without enacting dramatic changes across various sectors and probably require federal intervention.

Mr Baird said if the prevalence of childhood obesity was reduced to 16.5 per cent in line with his target, 62,000 fewer children would be overweight or obese in 2025.

"It is an ambitious target as no country has been successful in reversing the trend in child overweight and obesity," Mr Baird said.

"But NSW is a world-leader in many areas and childhood obesity is one of 12 Premier's Priorities because I firmly believe that improving the health and wellbeing for NSW kids and families is investing in our future.

"We have made some good headway with the programs in place but we will now step up new actions in order to gain further traction."

Body mass index (BMI), the ratio of weight in kilograms divided by height in metres squared, is used to measure overweight and obesity in children. BMI changes substantially with age and can differ between boys and girls, rising steeply in infancy, falling during the preschool years and increasing through to adolescence and into adulthood.

New research, published midweek in JAMA Internal Medicine, showed that Big Sugar may have done more than just advocate for favourable policies, distorting scientific research by dictating what questions get asked about sugar. The paper focuses on a debate that first popped up in the 1950s, when the rate of heart disease started to shoot up in the United States. Scientists began searching for answers, and zeroed in on dietary saturated fat as the leading contributor. (The energy we get from food comes in three kinds of nutrients: fats, carbohydrates, and protein.)

This may not have been an accident. Through an examination of archival documents, the JAMA paper shows how a sugar trade association helped boost the hypothesis that eating too much saturated fat was the major cause of the nation's heart problems, while creating doubt about the evidence showing that sugar could be a culprit too.

Excess weight and obesity are known causes of chronic diseases such as type two diabetes and gestational diabetes, and are estimated to reduce life expectancy by three years.

They are also very difficult to reverse, with more than 80 per cent of obese children going on to become obese adults, compared to just 15 per cent of healthy weight children.

The NSW Childhood Overweight and Obesity Premier's Priority annual data report found that childhood obesity had become "normalised".

NSW Centre for Population Health executive director Jo Mitchell said reducing its prevalence would take years, and involve education programs, clinical interventions and environmental changes.

"It really is a very complex issue. If it was a simple issue I don't think we as a society would find ourselves in this place," she said.

One of the programs would focus on educating parents about what constituted an appropriate body size for the age of their children, with around 70 per cent of parents who had an overweight child and 30 per cent of those with an obese child believing their child was healthy.

"We need to look at ways to help people recognise that there's an issue," Dr Mitchell said.

"Fifty per cent of adults are overweight and obese so our perception of what's healthy has changed over time."

Western Sydney Local Health District chief executive Danny O'Connor said he recently attended a meeting where three photographs of young children were presented and participants were asked to identify which child was healthy, which was underweight and which was overweight or obese.

Almost all of them wrongly identified an overweight child as being the healthy weight, and the one they thought was skinny turned out to be a healthy weight.

"My perception was wrong and most of the health professionals got it wrong," Dr O'Connor said.

"We have become used to what we see around us."

But University of Sydney professor of child and adolescent health Louise Baur said although childhood obesity appeared to have plateaued this disguised the fact that disadvantaged children were more at risk than ever.

Obesity was also increasing among adolescents and young adults, and the worst cases were more severe than they were 20 years ago, causing more medical conditions such as sleep apnoea, psycho-social problems and type 2 diabetes.

But while the NSW government had done good work in creating healthy food environments in early childhood centres and primary schools, these programs would not work in isolation, Professor Baur said.

"If we look at the upstream issues, many are beyond the capacity of the health system alone and many of them are beyond the capacity of the state government to develop alone. We have clearly seen a disinvestment in preventative health."

The federal government could contribute by better regulating food marketing directed at children and exploring a sugar tax, which has been introduced in Mexico, Britain and some US states, she said.

A NSW upper house parliamentary inquiry into childhood overweight and obesity has heard that there is a gap in programs targeted at adolescence, when more girls are dropping out of sport and both genders are spending more time in front of screens.

A quarter of adolescents drink five or more sugary drinks every week.

Committee chair Scott Farlow, a Liberal MLC, said it appeared that resources were being spread too thinly between various programs in schools, health and sport.

"What keeps coming up is there's no silver bullet solution and in order to achieve the Premier's goal we're going to need to do a lot of different things," Mr Farlow said.

"They're saying there's a lot of good programs. We just need to emphasise a few of them."

Meanwhile, new research has emerged to show the sugar industry paid scientists in the 1960s to play down the link between sugar and heart disease and promote saturated fat as the culprit instead, newly released historical documents show.

The internal sugar industry documents, recently discovered by a researcher at the University of California, San Francisco, and published Monday in JAMA Internal Medicine, suggested that five decades of research into the role of nutrition and heart disease, including many of today's dietary recommendations, may have been largely shaped by the sugar industry.

"They were able to derail the discussion about sugar for decades," said Stanton Glantz, a professor of medicine at UCSF and an author of the JAMA Internal Medicine paper.

The documents show that a trade group called the Sugar Research Foundation, known today as the Sugar Association, paid three Harvard scientists the equivalent of about $50,000 in today's dollars to publish a 1967 review of research on sugar, fat and heart disease. The studies used in the review were handpicked by the sugar group, and the article, which was published in the prestigious New England Journal of Medicine, minimised the link between sugar and heart health and cast aspersions on the role of saturated fat.