Poor diet is a factor in one in five deaths around the world, according to the most comprehensive study ever carried out on the subject.

Millions of people are eating the wrong sorts of food for good health. Eating a diet that is low in whole grains, fruit, nuts and seeds and fish oils and high in salt raises the risk of an early death, according to the huge and ongoing study Global Burden of Disease.

The study, based at the Institute of Health Metrics and Evaluation at the University of Washington, compiles data from every country in the world and makes informed estimates where there are gaps. Five papers on life expectancy and the causes and risk factors of death and ill health have been published by the Lancet medical journal.

It finds that people are living longer. Life expectancy in 2016 worldwide was 75.3 years for women and 69.8 for men. Japan has the highest life expectancy at 84 years and the Central African Republic has the lowest at just over 50. In the UK, life expectancy for a man born in 2016 is 79, and for a woman 82.9.

Diet is the second highest risk factor for early death after smoking. Other high risks are high blood glucose which can lead to diabetes, high blood pressure, high body mass index (BMI) which is a measure of obesity, and high total cholesterol. All of these can be related to eating the wrong foods, although there are also other causes.

“This is really large,” Dr Christopher Murray, IHME’s director, told the Guardian. “It is amongst the really big problems in the world. It is a cluster that is getting worse.” While obesity gets attention, he was not sure policymakers were as focused on the area of diet and health as they needed to be. “That constellation is a really, really big challenge for health and health systems,” he said.

The problem is often seen as the spread of western diets, taking over from traditional foods in the developing world. But it is not that simple, says Murray. “Take fruit. It has lots of health benefits but only very wealthy people eat a lot of fruit, with some exceptions.”

Sugary drinks are harmful to health but eating a lot of red meat, the study finds, is not as big a risk to health as failing to eat whole grains. “We need to look really carefully at what are the healthy compounds in diets that provide protection,” he said.

Prof John Newton, director of health improvement at Public Health England, said the studies show how quickly diet and obesity-related disease is spreading around the world. “I don’t think people realise how quickly the focus is shifting towards non-communicable disease [such as cancer, heart disease and stroke] and diseases that come with development, in particular related to poor diet. The numbers are quite shocking in my view,” he said.

The UK tracks childhood obesity through the school measurement programme and has brought in measures to try to tackle it. “But no country in the world has been able to solve the problem and it is a concern that we really need to think about tackling globally,” he said.

Today, 72% of deaths are from non-communicable diseases for which obesity and diet are among the risk factors, with ischaemic heart disease as the leading cause worldwide of early deaths, including in the UK. Lung cancer, stroke, lung disease (chronic obstructive pulmonary disorder) and Alzheimer’s are the other main causes in the UK.

The success story is children under five. In 2016, for the first time in modern history, fewer than 5 million children under five died in one year – a significant fall compared with 1990, when 11 million died. Increased education for women, less poverty, having fewer children, vaccinations, anti-malaria bed-nets, improved water and sanitation are among the changes in low-income countries that have brought the death rate down, thanks to development aid.

People are living longer but spending more years in ill health. Obesity is one of the major reasons. More than a billion people worldwide are living with mental health and substance misuse disorders. Depression features in the top 10 causes of ill health in all but four countries.

“Our findings indicate people are living longer and, over the past decade, we identified substantial progress in driving down death rates from some of the world’s most pernicious diseases and conditions, such as under age-five mortality and malaria,” said Murray “Yet, despite this progress, we are facing a triad of trouble holding back many nations and communities – obesity, conflict, and mental illness, including substance use disorders.”

In the UK, the concern is particularly about the increase in ill-health that prevents people from working or having a fulfilling life, said Newton. A man in the UK born in 2016 can expect only 69 years in good health and a woman 71 years.

“This is yet another reminder that while we’re living longer, much of that extra time is spent in ill-health. It underlines the importance of preventing the conditions that keep people out of work and put their long term health in jeopardy, like musculoskeletal problems, poor hearing and mental ill health. Our priority is to help people, including during the crucial early years of life and in middle age, to give them the best chance of a long and healthy later life,” he said.