Various types of neoplasms, historically more linked to older individuals, have been diagnosed with greater frequency in young adults across different nations. This phenomenon is notable in tumors such as colorectal and breast cancer, although it does not mean that all cancers are increasing in this age group.
The cause of this increase does not appear to reside in a single element. Researchers are investigating a combination of factors, including individual predisposition, metabolic conditions, lifestyle habits, environmental exposures, and elements that can influence from early life stages.
In the scientific field, the term 'early-onset cancer' refers to tumors detected before the age of 50, although studies use various age classifications. An extensive analysis on the subject was published in 2025 by scientists from the U.S. National Cancer Institute (NCI), who examined 33 categories of cancer.
This study revealed that the incidence of 14 types of cancer increased in at least one of the age groups below 50 between 2010 and 2019. Concurrently, 19 types showed a reduction among young people. When considering all cancers together, the overall incidence rate did not rise among people under 50, and mortality from all cancers also did not show growth in the young age groups analyzed.
In the United States, the largest absolute increases in early-onset cases were observed in female breast cancer, colorectal, renal, and uterine cancer. In 2019, researchers estimated approximately 4,834 extra cases of breast cancer, 2,099 of colorectal, 1,793 of kidney, and 1,209 of uterus, compared to the number predicted if 2010 rates remained constant.
The study also identified elevations in melanoma, cervical cancer, stomach cancer, myeloma, and bone and joint tumors in at least one young age group. Nine of the 14 types that grew among young people also showed an increase in some older age group.
Five others—melanoma, cervical cancer, stomach cancer, myeloma, and bone and joint cancer—showed growth in some young phase but not in the older groups studied. According to NCI researchers, the reasons are likely specific to each type of cancer, potentially involving risk factors more common in younger ages, changes in screening or detection, or alterations in diagnostic classification.
Among the highlighted tumors, colorectal cancer is considered the most consistent example of elevation in young adults. Research published in The Lancet Oncology analyzed data from 50 countries and territories and found an increase in incidence among individuals aged 25 to 49 in 27 of them.
In 20 of these locations, the youth growth was exclusive to this age group or occurred at a faster rate than in older adults. In 23 of the 50 countries and territories, the recent trend remained stable.
Maria Ignez Braghiroli, an oncologist specializing in colorectal cancer at Oncologia D’Or in São Paulo, comments that 'colorectal cancer is probably one of the best documented examples of this increase in cancer among young adults.' Historically, the disease is much more common after age 50, but she points to a change in the young curve: 'the absolute risk remains higher in older people, and the total number of cases is also higher in this population. What is striking in young people is the change in the curve and the speed of growth.'
The pattern across different generations also caught the researchers' attention. A study in JNCI Cancer Spectrum, using data from 35 countries, found signs of birth cohort effect in several populations. When people born in different decades exhibit varied risks upon reaching the same age, researchers can investigate whether any characteristic of those generations' lives—such as diet, metabolic conditions, or environmental exposures—affected the risk throughout life.
The cohort effect itself does not determine which exposure is responsible. It only signals that disparities between generations may be linked to something that changed in the life trajectory of these populations.
Factors associated with colorectal cancer include family history and hereditary syndromes, as well as obesity, metabolic imbalances, alcohol intake, and certain dietary patterns. Research also indicates links with sedentary lifestyle and other aspects of lifestyle.
Obesity is one of the associations examined. In a study with over 85,000 women, obese women showed almost twice the risk of developing colorectal cancer before age 50 compared to women of adequate weight. Although the study identified a correlation between excess weight and risk, it does not allow concluding that obesity is, in isolation, the cause of the population increase in early-onset colorectal cancer.
Nutrition is also featured in the research. Maria Ignez Braghiroli cites studies connecting diets low in fiber and high in ultra-processed foods, processed meats, and sugary drinks to the risk of colorectal cancer. She also mentions research finding a link between consuming sweetened beverages during adolescence and a higher future risk of the disease.
However, the doctor emphasizes that it is impossible to create a single explanation for these factors. 'These factors only explain part of the phenomenon,' she states. She adds that young patients who are physically active, not obese, without a known family history or genetic syndrome, also contract the disease. 'Therefore, we are probably facing a combination of exposures.'
Possibilities under investigation include metabolism, gut microbiome, use of certain drugs, environmental exposures, and factors that may begin to act very early in life.
One hypothesis gaining traction is that certain exposures crucial for cancer development occur during childhood or adolescence, long before the first symptoms appear.
In colorectal cancer, studies have linked excess weight in late adolescence or early adulthood to a later elevated risk. Connections have also been found between higher consumption of sugary drinks during adolescence and colorectal cancer before age 50.
Such findings do not prove, in isolation, that a specific exposure is responsible for the increase in cases. They do, however, help point to periods in life that may be crucial for risk formation.
The gut microbiome is one of the newest areas of this research. Certain strains of Escherichia coli present in the intestine produce colibactin, a substance capable of causing DNA damage.
In a study published in Nature in 2025, researchers analyzed 981 genomes of colorectal tumors from 11 countries, including Brazil. Mutational markers linked to colibactin were 3.3 times more frequent in tumors diagnosed before age 40 than in those diagnosed after age 70.
This result offers a particularly relevant clue because it is not limited to a statistical correlation between behavior and cancer. Scientists located in the tumor genome a marker compatible with a specific mutagenic process.
The work suggests that exposure to colibactin-producing bacteria early in life may contribute to the incidence of early-onset colorectal cancer, but it does not prove that this exposure is, alone, responsible for the increase in cases.
There is an important distinction: the study analyzed tumors from people who already had cancer and identified markers compatible with specific mutagenic processes. It did not follow individuals from childhood to prove when exposure to colibactin-producing bacteria occurred.
Thus, the theory of early exposure is supported by molecular evidence but does not yet constitute a complete explanation for the epidemiological trend. For Braghiroli, this possibility raises another question still under investigation: 'One of the big current questions is precisely whether the cancer we diagnose at 35 or 40 years old began biologically much earlier than we imagined.'
Research still needs to define which exposures could generate such a change, at what point in life they would be most impactful, and how the various factors might interact.
Female breast cancer is another tumor listed among those with increased incidence in certain young age groups. In the NCI study, it was the cancer that registered the largest estimated absolute increase in early-onset cases in the United States, with about 4,834 additional cases in 2019, compared to the expected number based on 2010 rates.
There is also Brazilian evidence. A study published in 2026 in the Brazilian Journal of Cancerology analyzed cancer incidence among people aged 20 to 39 in two areas of the state of São Paulo: the city of São Paulo and the Barretos region. The research evaluated incidence data from 2002 to 2018 and mortality data from 2000 to 2020.
The results differed between the two locations. In Barretos, there was a significant increase in the incidence of breast cancer in women aged 20 to 39, with an annual percentage variation of 5.91%, and cervical cancer, of 8.96%. Among men, the incidence of colorectal cancer grew by 5.75% annually. In São Paulo, the incidence of breast cancer rose by 0.5% per year, while colorectal cancer remained stable among men and women.
The data shows that one cannot claim a generalized increase in cancer in young adults across all of Brazil. The study was conducted in two specific locations and revealed distinct patterns between them. The results suggest trends for certain tumors that warrant investigation but cannot be directly applied to the entire country.
In the case of breast cancer, an explanation based solely on increased screening also has limitations. Laura Testa, a breast cancer specialist at Oncologia D’Or in São Paulo, states: 'We already have several data showing the increase in incidence in young women.' Studies from various Brazilian centers have identified increases in both women aged 40 to 50 and those under 40.
The age group under 40 is not covered by population mammography screening. In Brazil, the Ministry of Health recommends mammography every two years for women aged 50 to 74. Women between 40 and 49 can undergo the exam upon request, after receiving guidance on benefits and risks, but this group is not part of the systematic population screening program.
Therefore, the expansion of access to mammography for women aged 40 to 49 does not seem sufficient on its own to justify past trends or the increase seen in women under 40.
Breast cancer has known risk factors, such as reproductive characteristics, obesity, sedentary lifestyle, and alcohol consumption. However, the strength of these associations varies by age. Testa clarifies that obesity and sedentary lifestyle are well-established risk factors for breast cancer, especially after menopause. For younger patients, however, there is less clarity on how much these factors specifically contribute to the increase observed in this age group. Alcohol, on the other hand, presents a firmer link.
Alcohol consumption increases the risk of breast cancer at all ages; the higher the consumption, the greater the danger.