Over the past several years, we’ve seen more studies and increasing media coverage about a rise in cancer, especially among younger adults. These studies and claims can be difficult to parse. Is cancer becoming more common, or is diagnosis just improving? How large are these increases actually? And, most importantly, do we know anything about why this is happening?
The data on this has been dominated by a few recent studies that look at cancer rates over a long time frame. This includes a 2023 paper in JAMA on cancer in young people, specifically, and a 2024 paper in The Lancet, which looked across all ages for a very long period. I’ll focus on these two papers in the discussion below.
What cancers are increasing?
Both of these papers make use of large datasets that cover cancer cases in the U.S. One is the National Cancer Institute’s SEER program, a database with information on cancer cases covering about 26.5% of the U.S. population. The second dataset is the North American Association of Central Cancer Registries, which covers over 90% of the U.S. Both of these are enormous datasets, which makes it possible to estimate changes even in very rare cancers.
These papers look at changes in cancer incidence over time, both overall and in people under 50. It is worth noting that incidence (the number of people being diagnosed with cancer) and mortality are not the same. Cancer mortality has gone down over time, for nearly all cancers, as treatments have improved. Earlier detection has also helped. This is good news! But even when treatable, cancer is bad. Cancer incidence is important to pay attention to.
Big picture: Based on either analysis, some types of cancer have gone up in younger people, and some have gone down. The 2023 JAMA paper allows us to look most directly at changes in early-onset cancers. The table below, which is based on that paper, shows the types of cancers with significant increases and decreases over this period.
The second paper highlights many of these same cancers and adds a few others that have increased across birth cohorts (e.g., cardia gastric, gallbladder).
From this, we get a good sense of the overall trends. Breast, uterine, colon, kidney, and pancreatic cancers are increasing. Acute myeloid leukemia and prostate and liver cancers are going down.
It isn’t just the trend that matters for how we think about these changes. Many of the cancers on this list are extremely rare. Intrahepatic bile duct cancer (for example) has increased by an estimated 8% per year. This is a large increase, but the overall number in 2019 is an estimated 0.2 cases per 100,000 people under 50. That’s about 400 cases per year, meaning the 10-year increase is about 200 cases per year. This is a very small number given the U.S. population.
In the end, what is clear in the data is that the overall increase in cancer in young people is being driven by breast cancer (for women specifically) and colon cancer (for everyone). If we are looking to better understand the overall increase, then we need to focus on these two types of cancers.
Why has breast cancer increased?
It is clear from these papers, but also from more specific analyses, that breast cancer rates have increased globally in younger women. To understand why, we need to go deeper than looking at overall counts. A 2022 paper provides a much more comprehensive analysis of breast cancer in particular, digging more deeply into both the overall patterns of increased diagnosis and a bit into why these are occurring.
One obvious explanation is changes in screening practices. With more use of mammograms, more cancer cases will be detected. That’s a good thing, of course, since early detection leads to better outcomes. However, the 2022 paper notes that while this probably does explain some of the increase in the 1980s and 1990s, screening rates in the past decade haven’t changed much. The paper ultimately concludes that at least some of the increase in cancer over the past decade is not due to screening practices.
If we put aside screening, there are many possible explanations for a true rise. These include changes in diet, changes in obesity rates, changes in reproductive behaviors, and changes in chemical exposures.
The authors of this paper isolate a particular kind of tumor — “HR-positive” tumors — as driving much of the increase over time. This points to one possible explanation, since HR-positive tumors are more strongly associated with a later first birth (or having no children). In one study covering a large population in Europe, having children at all, and having them earlier in life, was associated with a lower risk of HR-positive tumors (but not HR-negative). This suggests that the overall decline in fertility in the population and the fact that people are having babies later could play a role in rising cancer rates. However, these factors are not large enough to explain the entire change.
Another likely important factor is changes in obesity rates over time; obesity is associated with higher rates of cancer. Again, unlikely to explain this entire increase, but this may be an important part of it. Ultimately, there is more work to be done to figure out what is going on here.
Why has colorectal cancer increased?
Colorectal cancer is less common than breast cancer, but has increased proportionally more in younger adults. It’s important to put this in context. Overall, colon cancer cases have been declining about 1% per year over the past two decades.
By far the most important determinant of colon cancer risk is age. In data from 2020, people in their early 80s were more than 12 times as likely to be diagnosed as people in their early 40s. For these older people, though, colon cancer has declined over time, likely due to better detection and a reduction in smoking. The decline in this high-risk group has led to the overall population decline in colon cancer rates. However, this overall reduction masks an increase in cases in people under 50, as seen in the data above. This risk in absolute terms is still, however, very small; colon cancer is much less common, for example, than breast cancer.
It is unclear why colon cancer rates have increased over time. There are some risk factors, including obesity, lack of fiber, or a high-fat diet, that have increased over time and may play a role. A recent finding that use of GLP-1 medications lowers the colon cancer risk rate does suggest that something about food and metabolism matters here. Some research has suggested that changes to the microbiome could matter.
Similar to breast cancer, there is no smoking gun in the data that would really explain this increase.
What can you do?
Although the risk of these types of cancers is still small, the increase naturally leads to the question: Is there anything you can do to decrease your risk?
One answer is that improvements in diet and exercise could reduce the risk of cancer. The causality here is not as strong as we would like, but there is enough data to suggest that these baseline healthy behaviors might help (and would have other possible benefits).
In addition, and more directly relevant, is cancer screening. Screening for both breast and colon cancer can catch cancers early, before they spread. For colon cancer, a colonoscopy can remove polyps that may grow into cancers before they do. Mammography can detect tumors when they can be removed. Depending on your risk factors, these may be recommended starting at 40, 45, or 50. Talk to your doctor about when and what kind of screening is right for you.
The bottom line
- Among people under 50, cancer incidence is rising, primarily due to increases in breast and colon cancer.
- The rise in breast cancer diagnoses can be attributed to better screening practices, but beyond that, it is possibly driven by a mix of factors, especially later-in-life childbearing and higher obesity rates.
- It is unclear why colon cancer rates have increased over time. There are some diet-related risk factors that may play a role, but there is nothing clear in the data to point to one cause.
- Improving your diet and exercising could lower your risk (and at least can’t hurt). But the most important thing to do is make sure you are getting screened as early and as often as your doctor recommends for breast and colon cancers.