The latest findings about the rise in cancers in people under 50, from the World Cancer Research Fund (WCRF), are extremely concerning. While a cancer diagnosis is bad news at any time, it is a particular blow for those in younger age groups where most people are in good health. Overlapping trends observed in England, the Netherlands and the US include a troubling rise in the incidence of colorectal (or bowel) cancer. Other diseases that have increased in England include uterine and kidney cancers.

These diseases are still rare in young people: 10 times more adults in their 50s, 60s and 70s are diagnosed than those in their 20s, 30s and 40s. Earlier detection is thought to be one factor behind the rise in younger patients. Diagnosing more cancers sooner is an explicit policy goal in England, where late diagnosis and lower uptake of screening are two reasons for mortality rates that are higher than elsewhere in Europe.

But diagnostic improvements are unfortunately not the main reason for increased cases. While research is ongoing and many questions remain, evidence points to obesity and poor diet causing cancers as well as heart disease and type 2 diabetes. The Prospect-UK trial is currently seeking 250,000 participants for a longitudinal study into early-onset (under-50) bowel cancer. It will scrutinise information about nutrition in more detail than has been done previously, including what people are eating less of.

The success of international efforts to cut lung cancer through targeting smoking set a powerful precedent for public health campaigning. This took longer than it should have done, due to tobacco industry lobbying. But it remains the gold standard of regulation combined with messaging, to bring about behaviour change. The WCRF data shows that lung cancer rates in under-50s have continued falling.

So far, efforts to cut obesity have had nothing like the same impact. The patterns are not the same everywhere; one recent study found that in some countries a rapid rise had been replaced by a levelling-off. But English-speaking countries have some of the worst records, and the lack of strong action in the UK is among the big disappointments of health policy in the last decade.

Politicians from Boris Johnson to Wes Streeting have committed to obesity action plans, only to water them down in the face of opposition from the food and drink industries. Last year, scientists wrote to Mr Streeting to protest that the government had done “virtually nothing” to alert consumers to the carcinogenic properties of the nitrites added to bacon and ham.

There is a grim irony in dramatic advances in cancer research and treatment coinciding with the rise in early-onset cancers. For many doctors and patients, this a period of hope. But politicians and health bosses must guard against a situation in which some groups have improved prospects, while others face increased risks linked to socioeconomic disadvantages.

In relation to obesity and diet, the close link with poverty points to the need for ministers to view public spending holistically, and to recognise that the NHS is not the only institution with the capacity to improve health. While more research into early-onset cancers is needed, existing data tells us a great deal. Governments should be as bold in tackling the causes of obesity as they once were in confronting big tobacco.