Americans have for the first time in decades cut their calorie intake, but are still eating a diet that is largely made up of junk food and doing very serious damage to their health, say experts.

The downturn in calorie intake is not so much a watershed in the fight against obesity as a first step down a long and hard road to better nutrition, according to Barry Popkin, professor of public health at the University of North Carolina at Chapel Hill, whose latest research paper flagged up the shift.

The drop in calories is largely attributable to the high level of public attention to sugar-sweetened beverages, such as colas and lemonades, says Popkin. That’s a good thing but, he says: “Americans are still eating a really bad diet. We haven’t increased whole grain. Still over 50% or 60% of the calories in kids and adults are from refined carbohydrates, desserts, fast food and savoury snacks.”

That means, he says, “more than half the calories for kids and adults in America are from junk food”. The food industry worldwide argues that calories are just calories, whatever foods they come from. While that may be true in terms of the impact on obesity, which looks as though it is starting to stabilise in the United States and even come down slightly among younger children, there is now a consensus among health experts that what you eat matters just as much as how much of it you eat.

“The diets we are eating are the antithesis of what we want for weight, for diabetes, heart disease and it’s the same for cancer,” Popkin said. “So we’re just consuming so much refined carbohydrate, so much added sugar that even if we cut the soft drinks, unless we cut the junk food and the desserts down – the desserts and fast food – it’s not going to help.

“Over half our diet is from food that we term really very much obesogenic, cardiovascular-genic, really the kind of food that feeds our insulin and our risk of diabetes and fatty liver disease and cancer.”

“It is better to say the cup is half-full but the half empty part is really dangerous.”

His new research paper in the American Journal of Clinical Nutrition shows a small but steady decline in the calorie content of the food and drinks Americans buy and consume from 2003 to 2011, which they showed was not caused by families cutting back on food spending in the economic recession.

The drop in calories is greater for children than for adults. In 2003, data from the major US nutrition survey called NHANES shows children between ages two and 18 were consuming a mean of 2,118 calories per day. That had dropped by 2009 to 1,908 calories. The mean decrease was a drop of 35 calories per day, but there were big variations, with children from the lowest income families and those where the head of the household had better education eating fewer. Nothing changed for adolescents, aged 12 to 18, who carried on eating and drinking as before.

But children in every sort of family cut down on calorific drinks, from a mean of 474 to 382 calories per day. Among adults in contrast to children, says the paper, Mexican Americans, women and people with a high school education cut their calories from food and drink – but not the rest of the population.

Now that everybody knows about the dangers of sugar-sweetened drinks, the focus needs to change, Popkin says. He is concerned that people may switch to smoothies and fruit juice, which are just as high in sugar. And he is very worried about the pervasive junk food diet and absence of cooking in the United States and other countries, such as the UK.

In the US and the UK, people spend only about 25 minutes on average per day in food preparation. For many, that means taking a packet out of the fridge and putting it in a microwave. A separate study from Popkin and colleagues published by the same journal in May revealed that most of the US diet is ready to eat or ready to heat.

“Desserts and snacks – they are the next target,” said Popkin. Children are eating more desserts. Over half their calorie intake comes from desserts, fast foods, savoury snacks and sugary beverages. “That is more than half the calories for kids and adults in America from junk food,” he said.

Ready to eat and ready to heat packaged food is often highly processed and tends to be loaded with sugar, salt and saturated fat. Eggs, fruit and vegetables, milk, brown rice and whole grains are unprocessed or minimally processed. Chocolate milk, sweetened or flavoured fruit or vegetable juice, potato chips, cured bacon, flavoured pasta, salted butter and cheese are among the foods Popkin and his team categorise as moderately processed.

But then there are the highly processed foods: soda, sports drinks, alcohol, restructured potato chips, instant mashed potatoes, hot dogs, meatloaf, crab cakes, chicken nuggets, tortillas, bagels, cookies, frozen pizza, ice cream, cheesecake, candy and any number of other packaged products. In their survey, they found that the moderately and highly processed foods, with high saturated fat, sugar and salt, provided more than three-quarters of people’s energy intake and there was little change in that between 2000 and 2012.

It’s the same story in similar countries – moderately and highly processed foods dominated purchasing in Canada (61.7%) and the UK (63.4%). While Mexico has brought in a tax on junk food as well as sugary drinks and Chile is about to ban their advertising, the most developed nations have not yet shown an interest in regulating junk food.

Even the apparent stabilisation of obesity in the United States, which the cut in calories has probably helped bring about, hides some difficult issues. At the extreme end, weight is going up in the US and the UK. The most obese are getting heavier. “That’s partly the diet and partly lack of exercise,” said Popkin.

Dr Tim Lobstein, director of policy at the World Obesity Federation, said that people’s reluctance to get moving might offset the reduction in calories. “Trends suggest no decline in physical activity and not much increase either, at least according to self-reported data during a decade when a lot of attention has been paid to this subject, so might affect people’s responses,” he said. The data shows, he added, that it is the richer households who are taking more exercise and eating more fruit and vegetables.

Derek Yach, senior vice-president of the Vitality Group and executive director of the Vitality Institute for Health Promotion, says that the reduction in the nation’s calorie intake owes much to the efforts of the food industry in reformulating foods.

“Sixteen major food and beverage companies reduced calories sold by 6.4 trillion from 2007 to 2012 as part of the Healthy Weight Commitment Foundation. This translates to 78 calories per day per person in the US, a significant amount of the required 220 calories cited needed to reduce obesity,” he said.

“Many are skeptical of industry efforts to make healthier products. However, shifting portfolios to healthier products is good for consumer health and supports the financial health of industry, as shown by the Hudson Institute.

The fact that Americans are finally eating less is worth celebrating even as a small victory on the way to a larger goal. Industry contribution has been significant and should be applauded.”