The number of under-50s worldwide being diagnosed with cancer has risen by nearly 80% in three decades, according to the largest study of its kind.

Global cases of early onset cancer increased from 1.82 million in 1990 to 3.26 million in 2019, while cancer deaths of adults in their 40s, 30s or younger grew by 27%. More than a million under-50s a year are now dying of cancer, the research reveals.

Experts are still in the early stages of understanding the reasons behind the rise in cases. The authors of the study, published in BMJ Oncology, say poor diets, alcohol and tobacco use, physical inactivity and obesity are likely to be among the factors. Breast cancer also had the highest number of cases in this age group in 2019, but windpipe (nasopharynx) and prostate cancers saw the fastest increase since 1990. On the other hand, early-onset liver cancer diagnoses declined. "In the case of liver cancer, large numbers are caused by hepatitis B Virus for which there is effective vaccination.

 Whereas for nasopharyngeal cancer, large numbers are caused by the Epstein-Barr virus, for which we do not have effective vaccination." While genes play a role in some cancer cases, even in protecting against cancer, the researchers note that there are many preventable factors to consider.

Based on the observed trends for the past three decades, the researchers estimate that the global number of new early onset cancer cases and associated deaths will rise by a further 31% and 21% respectively by 2030, with people in their 40s the most at risk.

Genetic factors are likely to have a role, the researchers said. But diets high in red meat and salt and low in fruit and milk, along with alcohol and tobacco use, are the main risk factors underlying the most common cancers among under-50s, with physical inactivity, excess weight and high blood sugar contributory factors, the data indicates.