Across high-income countries, from Britain to Denmark to the US, cancers in people under the age of 50 are becoming more common. This is quite an unusual pattern given that, for decades, cancer was seen as an affliction of old age. In fact, the high rates of cancer in high-income countries in much older people – above the age of 80 – was a sign of humans overcoming infectious diseases to live a long life before chronic diseases such as cancer would set in.

So the increase in cancer rates in younger people has come as a surprise. The numbers are stark. Data from the G20 group of industrialised countries indicates that between 1990 and 2019, cancer rates increased by 22% in the age group 25-29. Rates of cancer in the next age group, 30-34, are at the highest level ever. And given cancer screening classically isn’t routine in younger age groups, these cancers tend to go undiagnosed for longer with the tumours being more aggressive.

And cancer incidence is projected to keep going up in young people. Prof Shuji Ogino of Harvard University argues that a “birth cohort effect” is occurring: simply put, the numbers show that each group of people born at a later time, such as a decade, have a higher risk of developing cancer later in life. He links this to early-life exposure to risk factors related to diet, lifestyle, weight and environmental exposures – factors that weren’t around for older age cohorts.

To understand exactly why this is happening, we need to look underneath the broad umbrella of “cancer”, to the specific type of cancer and linked risk factors. The increase in early-onset cancers since 1990 has mainly been in breast, colon, oesophagus, kidney, liver and pancreas cancer. Among the 14 cancer types on the rise, eight are related to the digestive system. Colorectal cancer, for example, increased 70% among those aged 15-39 in G20 countries between 1990 and 2019. This means there is clearly a specific set of organs and tissues where these cancers are occurring, so we can look at environmental factors that could be affecting all them.

While cancer itself is a complex group of diseases, the risk factors are generally the same: smoking, drinking alcohol, obesity, physical inactivity and poor diet. Ogino points to diet as being a key factor in the rise of cancers in young people (although this is still an educated guess rather than conclusive evidence). Ogino is not alone in pointing to diet: many experts have made this link after looking at the types of cancers on the rise, and their connection to the digestive system. Along with a colleague, Ogino is now investigating the links between what we eat when we’re young, how this changes the overall balance of bacteria in the digestive system (the microbiome) and the link to early onset cancer.

The microbiome is the roughly 100tn microbes that live inside us, largely in the gut, and play a key role in overall health such as digestion, regulation of the immune system and protecting against disease-causing bacteria. Increasing evidence indicates that eating ultra-processed foods, particularly those high in saturated fat and sugar, alters the composition of the microbiome in negative ways.

The basic problem is that the food we’re exposed to is often heavily processed but has the benefit of being affordable, easy, long-lasting and appealing to taste buds. But it is almost certainly quite bad for our health. And this is increasingly the case for children. Highly processed foods have become regular meals, including processed breakfast cereals, processed supermarket bread, ready meals, frozen pizzas, processed supermarket sandwiches, and processed biscuits, doughnuts and chocolates. As Dr Chris van Tulleken has written, we are becoming ultra-processed people. More than 80% of the processed food sold in Britain is considered unsafe for marketing to children by the World Health Organization.

This will only get worse with food prices having increased 19.1% in the past year. This means that for many families buying ultra-processed products is the only affordable way to eat, given how expensive fruit, vegetables, meat, fish and dairy products are in comparison. Figures from the Office for National Statistics in May showed eggs increased 37% in price, milk 33%, chicken 23% and, another survey showed, fruit and vegetables by 30%. Limiting access to ultra-processed foods is only possible if policies are put in place to subsidise healthier options, especially for low-income families.

This increase in cancer incidence in young people won’t stop without clear action and leadership focused on populations living longer, healthier lives. This should include healthier food options that are affordable and accessible, and decent salaries to be able to buy these.

Investing in prevention is essential for a productive workforce and lower healthcare costs. Why treat what can be prevented? Better cancer survival outcomes are important, but equally important is preventing cancer developing in the first place. And currently, a childhood diet of ultra-processed foods is already contributing to health problems. With evidence increasingly linking it to the shocking rise of cancers in younger people too, governments can’t wait another decade to take it on.

• Prof Devi Sridhar is chair of global public health at the University of Edinburgh