June 2019 Issue

Obesity-Related Cancers in Young Adults
By Karen Collins, MS, RDN, CDN, FAND
Today’s Dietitian
Vol. 21, No. 6, P. 36

An Exploration of the Latest Research and Why Incidence Is on the Rise, Plus Strategies for Counseling Patients

You’ve probably learned that cancer is a disease of aging. Except for a few relatively uncommon forms, most cancer cases do occur in older adults. What’s new—and generating concern—is an analysis of US cancer registries that reports rising rates of several cancers among young adults compared with past generations of the same age. Incidence rates for eight different cancers have increased more in adults younger than 50 than in those aged 50-plus in recent decades. Notably, most of the cancers with this increased incidence are obesity-related cancers.

This article examines the research behind this reported increase in young adults developing these cancers and implications for how dietitians address weight and wellness.

The Changing Face of Cancer’s Demographics
Advancing age is an important factor in risk of cancer overall and of many specific cancers. However, cancer can occur at any age, and a few forms, such as bone cancer, are more likely to occur before age 35. Yet, the most recent National Cancer Institute statistics identify the median age at the time of a cancer diagnosis as 66 years, and the most common cancer types in the United States increase with age. Median age at diagnosis is 62 years for breast cancer, 67 years for colorectal cancer, 70 years for lung cancer, and 66 years for prostate cancer.1

Because of the strong association of most cancers with age, cancer incidence statistics (usually expressed as number of new cancer cases diagnosed per 100,000 people in a population) often are stated as age-adjusted rates. Most US data provided by the National Cancer Institute is adjusted to the distribution of ages in the 2000 US population. This means that cancer rates can be compared without reflecting changes in the age distribution in the population over time or different age distributions among ethnic groups within the population.

For the new analysis published in Lancet Public Health, researchers analyzed 25 state cancer registries for incidence trends from 1995 through 2014. The absolute number of cancer cases is smaller in younger adults than older adults, so a small increase could result in a disproportionately large annual percent increase in incidence. However, authors of the study statistically adjusted these incidence rates for age and birth time period. This allows comparison of adjusted rates between different age groups, and differences don’t reflect a general trend of more or fewer cancers diagnosed in 1995–1999 compared with 2010–2014, for example. The findings show incidence rates of eight cancers particularly increasing in adults younger than 50, and the fastest increase is in the youngest age group in the study (aged 25–34).2

Is It Obesity?
Obesity has a well-established link to risk of at least 12 cancers.3,4 Of these obesity-related cancers, one-half of them—colorectal, pancreatic, gallbladder, kidney, and uterine cancers, and multiple myeloma—are among those reported as increasing in successively younger-aged US adults. Of the 18 other cancers analyzed, including those associated with smoking or HIV infection, rates of only two (leukemia and noncardia stomach cancer) increased among younger adults, and eight decreased.

Increased prevalence of obesity in the United States began about 1980, and more than doubled by 2014.5,6 And those who were young adults (aged 25–34) in 2014 are among those in the population who showed the greatest increase in obesity prevalence throughout these years. So the trends identified for increased incidence of these cancers in young adults is compatible with the timing of obesity’s increased prevalence. Other research, including the following, is consistent with such a link:

 • Obesity issues can result in several physiological effects that promote cancer development (see table on page 38). A meta-analysis of cross-sectional observational studies shows that even in adolescence, obesity often is accompanied by greater insulin resistance and elevated insulin levels—changes that can promote development of colorectal, pancreatic, and other cancers.7

• Incidence (new diagnoses) of diabetes in the United States climbed steadily from the early 1990s until 2008. And among people younger than 20, incidence of diabetes continued to climb through 2012.8 Among adults, type 2 diabetes is associated with greater risk of several of the same cancers linked to obesity.9,10 Chronic inflammation and the elevated levels of insulin and insulin-related growth factors resulting from insulin resistance that accompany type 2 diabetes are consistent with this connection to cancer risk.

Trends in the Big Picture
It’s tempting to look for a simple answer to the increased incidence of these cancers in young adults. But cancer isn’t a simple disease, and researchers still have much to learn about how genetic, environmental, and lifestyle factors interact to affect risk. Although it’s logical that rising obesity could lead to the observed increase in the six obesity-related cancers, several questions remain, including the following:

Lifestyle vs obesity. What other changes in diet and lifestyle from 1980 to 2014, during which this rise in obesity occurred, might have enhanced cancer development? Examples could include decreased physical activity associated with the workplace or transportation, increased sedentary time with television and computers, increased prevalence of short sleep duration, increased binge drinking, and changes in dietary choices and eating patterns.11-17 Each of these could influence cancer risk by promoting weight gain and obesity. But it’s also possible that each could influence one or more hormonal, metabolic, or cell-signaling pathways promoting cancer development more directly.

Age. Obesity increased in older adults during the same period, too, so why was the increase in incidence especially in younger adults? Perhaps the smaller increase in obesity in older adults produced smaller changes in the pathways through which obesity poses risk. Or perhaps, at least for the six cancers, youth and young adulthood is a window of time with increased vulnerability to these harmful influences. Limited evidence from animal studies suggests that expanded adipose cells could promote some stages of cancer development and reduce its latency period.2

Constellation of influences. Why did some, but not all, obesity-related cancers increase in young adults? It’s possible that other obesity-related cancers have a longer latency period during which they develop, and differences in risk will unfold in this group of young adults when they’re another decade or more older. Obesity’s association with greater breast cancer risk is clearly documented to relate to only postmenopausal cancers.3 Whatever obesity’s role, however, it’s among one of multiple influences. For example, oral contraceptives, which have become widely used and decrease risk of ovarian cancer, may have countered a potential obesity-related increase in risk of this cancer.2

Complexities of Breast Cancer Risk
Evidence is rated as convincing that excess adiposity beyond early adulthood (after age 30) increases risk of postmenopausal breast cancer—whether adiposity is estimated by greater BMI, waist circumference, or adult weight gain.3 However, excess adiposity in childhood, adolescence, and early adulthood (aged 18–30) is associated with a lower risk of both premenopausal and postmenopausal breast cancer.3

Childhood and adolescence is hypothesized to be a critical window for carcinogenesis in breast cells. Puberty, especially, is a period of rapid development of breast tissue and higher rates of cell proliferation, which may increase susceptibility to cell damage. Scientists say hormonal and other influences at this time stemming from diet, activity, body composition, and other exposures may particularly influence breast cancer risk later in life.18 Greater velocity of adolescent growth in height is associated with increased risk of breast cancer. Greater adiposity during childhood has been associated with slower adolescent growth and development, and women who had higher body fat during childhood and adolescence may have lower circulating levels of insulinlike growth factor-1, a strong influence on growth hormone activity and an established risk factor for breast cancer.3,19

Body fat’s influence on sex hormones also may contribute to the inverse relationship between early-life adiposity and risk of breast cancer later in life. However, mechanisms underlying the association of body fatness in early life with lower risk of breast cancer are complex and not clearly delineated.3 Animal models suggest that estrogen formed in adipose tissue in adolescents may induce early differentiation of breast cells, making them less susceptible to carcinogenesis. In human studies, young women with obesity are more likely to experience anovulation and lower levels of progesterone and estradiol, which are likely to be protective; yet higher insulin and androgen levels would be expected to increase breast cancer risk.3,20,21

Putting all of these findings in the big picture, dietitians can share the following with clients when asked about breast cancer’s relationship to overweight and obesity in youth and young adults:

• Although several obesity-related cancers seem to be increasing in young adults, breast cancer wasn’t one of them in the Lancet Public Health study.2 This is consistent with other research, which shows a complex relationship between adiposity at different ages and breast cancer risk.

• Despite a lower risk of breast cancer with greater body fat in youth and up to age 30, greater body fat through the rest of the adult years is associated with greater risk of postmenopausal breast cancer, the most common form of breast cancer in the United States.3 In addition, excess body fat at any age increases risk of type 2 diabetes and CVD.10,22-24 And high BMI in young adult years (aged 18–25) isn’t without cancer risk; each five-unit increase (eg, a BMI of 24 compared with a BMI of 29) is associated with a 42% increased risk of endometrial cancer.3

• Obesity and overweight established at a younger age can be reduced later in life, but studies suggest there may be both behavioral and metabolic or physiological factors that make it more difficult than maintaining a healthy weight throughout life.25 Regardless of whether BMI is elevated in youth and young adulthood, weight gain after age 18 increases risk of postmenopausal breast cancer.3,19 Therefore, aiming for a healthful lifestyle to maintain a healthy weight throughout life makes the most sense for overall health.

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Media Headline Hype
Reports of the apparent increasing rates of obesity-related cancers in young adults tend to generate headlines that may not convey the findings in accurate context.

For example, headlines such as “Millennials Are Getting More Cancer Than Baby Boomers” are misleading. The truth is, older age groups still get more cancer than young adults, and most of the obesity-related cancers increased in all adult age groups from 1995 to 2014. However, the pace with which rates increased was faster among adults under age 50.

Other headlines such as “Cancer Rates Are Exploding in Younger Generations” also are deceiving. Incidence rates of many cancers are stable or decreasing in young adults. Of the cancers increasing faster among young adults, most are obesity related (whether directly because of obesity or the various factors that have led to the rise in obesity). Cancers that are particularly linked to smoking or HIV infection decreased in younger generations, which are attributed to the success of preventive health education and actions.

Key Talking Points
Create healthful eating and lifestyle habits that help reach and maintain a healthy weight. Excess body fat and adult weight gain, and the hormonal and metabolic changes that can ensue, raise risk of many common cancers. These same changes also can pave the way for other chronic diseases, including type 2 diabetes, CVD, and liver disease.

Focus on the package of habits that promote health, not just a target weight. Excess weight and weight gain can be an indicator of a range of unhealthful habits. Emphasize the set of American Institute for Cancer Research Recommendations for Cancer Prevention.26 This collection of steps for eating and activity habits prioritizes choices that current evidence supports for potential to lower cancer risk both directly and through promoting a healthy weight. And this package of habits is consistent with choices to boost overall health.

Know that risk doesn’t begin with obesity. Risk of obesity-related cancers is greatest at highest BMI. But many of these cancers also are associated with BMI classified as overweight or elevated waist circumference below that usually associated with an obese BMI.3 Even with a normal BMI, a significant portion of adults meet criteria for metabolic syndrome, which reflects abnormalities related to excess body fat that may increase cancer risk. So avoiding or limiting weight gain across adulthood is a worthwhile target.

Emphasize that it’s never too early to target a healthful lifestyle. Absolute risk of cancer is still small in young adults. But risk inherently rises with age, and small increases in risk at a young age become ever more concerning as years go by. The link between increased BMI and many of the obesity-related cancers is in a range similar to the 20% to 40% increased risk of lung cancer associated with secondhand smoke or high exposure to outdoor air pollution.3,27 Groundwork for chronic disease risk is laid many years before disease appears—both in unseen physiological effects and establishment of habits that, once created, can be challenging to change.

— Karen Collins, MS, RDN, CDN, FAND, is a nutrition consultant specializing in cancer prevention and cardiometabolic health, and nutrition advisor to the American Institute for Cancer Research.


1. SEER Cancer Statistics Review, 1975¬–2015. National Cancer Institute Surveillance, Epidemiology, and End Results Program website. https://seer.cancer.gov/archive/csr/1975_2015/. Updated September 10, 2018.

2. Sung H, Siegel RL, Rosenberg PS, Jemal A. Emerging cancer trends among young adults in the USA: analysis of a population-based cancer registry. Lancet Public Health. 2019;4(3):E137-E147.

3. World Cancer Research Fund/American Institute of Cancer Research. Continuous Update Project expert report 2018: body fatness and weight gain and the risk of cancer. https://www.wcrf.org/dietandcancer/exposures/body-fatness

4. Lauby-Secretan B, Scoccianti C, Loomis D, Grosse Y, Bianchini F, Straif K. Body fatness and cancer — viewpoint of the IARC Working Group. New Engl J Med. 2016;375(8):794-798.

5. Fryar CD, Carroll MD, Ogden CL. Prevalence of overweight, obesity, and severe obesity among adults aged 20 and over: United States, 1960–1962 through 2015–2016. Centers for Disease Control and Prevention website. https://www.cdc.gov/nchs/data/hestat/obesity_adult_15_16/obesity_adult_15_16.htm. Updated September 5, 2018.

6. Fryar CD, Carroll MD, Ogden CL. Prevalence of overweight, obesity, and severe obesity among children and adolescents aged 2–19 years: United States, 1963–1965 through 2015–2016. Centers for Disease Control and Prevention website. https://www.cdc.gov/nchs/data/hestat/obesity_child_15_16/obesity_child_15_16.htm. Updated September 5, 2018.

7. Thota P, Perez-Lopez FR, Benites-Zapata VA, Pasupuleti V, Hernandez AV. Obesity-related insulin resistance in adolescents: a systematic review and meta-analysis of observational studies. Gynecol Endocrinol. 2017;33(3):179-184.

8. Centers for Disease Control and Prevention. Diabetes report card 2017. https://www.cdc.gov/diabetes/pdfs/library/diabetesreportcard2017-508.pdf. Published 2018.

9. Shlomai G, Neel B, LeRoith D, Gallagher EJ. Type 2 diabetes mellitus and cancer: the role of pharmacotherapy. J Clin Oncol. 2016;34(35):4261-4269.

10. Gallagher EJ, LeRoith D. Obesity and diabetes: the increased risk of cancer and cancer-related mortality. Physiol Rev. 2015;95(3):727-748.

11. Nelson MC, Neumark-Stzainer D, Hannan PJ, Sirard JR, Story M. Longitudinal and secular trends in physical activity and sedentary behavior during adolescence. Pediatrics. 2006;118(6):e1627-e1634.

12. Institute of Medicine of the National Academies, Transportation Research Board. Does the build environment influence physical activity?: examining the evidence — special report 282. https://www.nap.edu/read/11203/chapter/1. Published 2005.

13. Ford ES, Cunningham TJ, Croft JB. Trends in self-reported sleep duration among US adults from 1985 to 2012. Sleep. 2015;38(5):829-832.

14. Centers for Disease Control and Prevention. Short sleep duration among workers — United States, 2010. MMWR Morb Mortal Wkly Rep. 2012;61(16):281-285.

15. Kanny D, Liu Y, Brewer RD, Lu H; Centers for Disease Control and Prevention. Binge drinking — United States, 2011. MMWR Suppl. 2013;62(3):77-80.

16. White MC, Shoemaker ML, Park S, et al. Prevalence of modifiable cancer risk factors among U.S. adults aged 18-44 years. Am J Prev Med. 2017;53(3S1):S14-S20.

17. Rehm CD, Peñalvo JL, Afshin A, Mozaffarian D. Dietary intake among US adults, 1999-2012. JAMA. 2016;315(23):2542-2553.

18. World Cancer Research Fund/American Institute for Cancer Research. Continuous Update Project expert report 2018: diet, nutrition, physical activity and breast cancer. https://www.wcrf.org/dietandcancer/breast-cancer

19. Rosner B, Eliassen AH, Toriola AT, et al. Weight and weight changes in early adulthood and later breast cancer risk. Int J Cancer. 2017;140(9):2003-2014.

20. Jungheim ES, Travieso JL, Carson KR, Moley KH. Obesity and reproductive function. Obstet Gynecol Clin North Am. 2012;39(4):479-493.

21. Kang C, LeRoith D, Gallagher EJ. Diabetes, obesity, and breast cancer. Endocrinology. 2018;159(11):3801-3812.

22. Llewellyn A, Simmonds M, Owen CG, Woolacott N. Childhood obesity as a predictor of morbidity in adulthood: a systematic review and meta-analysis. Obes Rev. 2016;17(1):56-67.

23. Ortega FB, Lavie CJ, Blair SN. Obesity and cardiovascular disease. Circ Res. 2016;118(11):1752-1770.

24. Arnett DK, Blumenthal RS, Albert MA, et al. 2019 ACC/AHA guideline on the primary prevention of cardiovascular disease [published online March 17, 2019]. Circulation. doi: 10.1161/CIR.0000000000000678.

25. Simmonds M, Llewellyn A, Owen CG, Woolacott N. Predicting adult obesity from childhood obesity: a systematic review and meta-analysis. Obes Rev. 2016;17(2):95-107.

26. World Cancer Research Fund/American Institute of Cancer Research. Continuous Update Project expert report 2018: recommendations and public health and policy implications. https://www.wcrf.org/dietandcancer/recommendations/policy-public-health-implications

27. Lung cancer prevention (PDQ®)–health professional version. National Cancer Institute website. https://www.cancer.gov/types/lung/hp/lung-prevention-pdq. Updated April 11, 2019.