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Reference topic 22: Cancer in a New Environment

Why is cancer increasing in young adults? The evidence

ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.60 sources10 graded recommendationsTopic hub: Women's healthTopic hub: Aging

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Every night for two years in the late 1980s, the anthropologist Beverly Strassmann noted which women slept at the two menstrual huts of Sangui, a Dogon village in Mali.1 Custom sent women there during their periods, and nobody used contraception.2 Women averaged more than eight live births.1

Her counts upended the modern idea of a normal cycle. On an average day, only a quarter of the women were having menstrual cycles. The rest were pregnant, nursing without periods, or past menopause. Women in their twenties and early thirties averaged fewer than four periods in two years; the only two sterile women that age had 23 and 29. "Thus regular menses were a sign of sterility, not fecundity," Strassmann wrote.1

Mismatch card

The system

Cell division and the defenses that keep cells cooperating (tumor suppressors, DNA repair, cell suicide).

What it expects

A body of ancestral size and lifespan; menarche in the mid-teens, many pregnancies, years of breastfeeding; no tobacco and little alcohol.

What it gets

Longer lives; earlier menarche, fewer and later births and shorter breastfeeding (three to four times the Dogon's menstrual cycles); excess body fat; tobacco and alcohol.

What happens in the gap

More hormone-driven, obesity-related, tobacco and alcohol cancers; much of the remaining rise reflects age and closer looking.

The handrail

No tobacco, less alcohol, limit adult weight gain, move, breastfeed where possible, HPV vaccination and timely screening (●●●)

Strength of evidence

●●○ Large, consistent studies tie reproductive history, body fat, tobacco and alcohol to cancer; the evolutionary account of why is inference.

What this topic covers

  • ·Older Than Our Species
  • ·Evolution Inside the Body
  • ·The Elephant Problem
  • ·Fewer Births, More Cycles
  • ·Smoke, Fat and Drink
  • ·Cancer Arriving Early
  • ·The Night Shift Question

The practical version is in the book: Chapter 16, Aging Well.

The handrail

Topics 40 to 42 hold the full programs. For cancer the list is short and unglamorous.

Each recommendation carries the strength of the evidence behind it. How the grades work.

  1. Don't smoke; quit at any age

    StrongEvidence grade: Strong. Multiple randomized controlled trials, or consistent large cohort studies with a well-understood mechanism.

    Smoking causes about one US cancer in five and more than a quarter of cancer deaths.28 Ask about yearly low-dose CT screening if you are 50 to 80 with a 20 pack-year history and smoke now or quit within 15 years. (A pack-year is a pack a day for a year.)55

  2. Drink less

    StrongEvidence grade: Strong. Multiple randomized controlled trials, or consistent large cohort studies with a well-understood mechanism.

    "It is best not to drink alcohol," says the American Cancer Society. For those who do: "no more than 1 drink per day for women or 2 drinks per day for men."56

  3. Limit adult weight gain

    StrongEvidence grade: Strong. Multiple randomized controlled trials, or consistent large cohort studies with a well-understood mechanism.

    Evidence grade: Strong. Multiple randomized controlled trials, or consistent large cohort studies with a well-understood mechanism. for the link·Evidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials. for the benefit of losing weight

    Stay in a healthy range and avoid gaining through adult life.56 Skip crash diets, get clinical support if you have an eating disorder, and treat weight-loss drugs as a decision for you and a clinician (Topic 13).

  4. Move

    ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.

    Evidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials. for prevention·Evidence grade: Strong. Multiple randomized controlled trials, or consistent large cohort studies with a well-understood mechanism. after colon cancer

    Aim for 150 to 300 minutes of moderate activity a week, or 75 to 150 of vigorous.56 The most active adults had lower risk of 13 cancers.57 In 889 colon cancer survivors, three years of structured exercise lengthened disease-free survival, though with more muscle and joint complaints. Plan it with your care team.58

  5. Breastfeed if you can and want to

    StrongEvidence grade: Strong. Multiple randomized controlled trials, or consistent large cohort studies with a well-understood mechanism.

    Each 12 months of breastfeeding, over a lifetime, lowers the relative risk of breast cancer by about 4.3%. That is modest for any one woman, and never a reason for guilt (Topic 33).19

  6. Vaccinate against HPV

    StrongEvidence grade: Strong. Multiple randomized controlled trials, or consistent large cohort studies with a well-understood mechanism.

    Swedish girls vaccinated against HPV (human papillomavirus) before 17 had 88% less invasive cervical cancer.59

  7. Screen on time

    StrongEvidence grade: Strong. Multiple randomized controlled trials, or consistent large cohort studies with a well-understood mechanism.

    In the US, colorectal screening starts at 45 and mammography every two years from 40 to 74; elsewhere, follow your national program. Screening also finds some cancers that would never have caused harm, so decide with a clinician.5546

  8. Heed warning signs at any age

    ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.

    Rectal bleeding, iron deficiency anemia or abdominal pain deserve a doctor's visit. Three in four colorectal cancers under 50 are found late.33

  9. Night-shift workers: no proven protective step

    EmergingEvidence grade: Emerging. Early, small, animal-only, or actively contested.

    Don't count on melatonin; in one test of 31 supplements, contents ranged from 83% below to 478% above the label.60

  10. Hormone medicines

    StrongEvidence grade: Strong. Multiple randomized controlled trials, or consistent large cohort studies with a well-understood mechanism.

    Weigh contraception and menopausal hormone therapy with a clinician; never stop a prescribed medicine without advice.

The skeptic's corner

The case against this topic, from the research itself.

Some of the rise in cancer reflects living longer and looking harder. Three in five US diagnoses come at 65 or older. The US cancer death rate, adjusted for age, has fallen by about a third in three decades, "largely because of smoking reductions, earlier detection, and improved treatment."26 Screening can conjure epidemics. In South Korea, thyroid cancer diagnoses rose fifteenfold in under two decades while deaths held steady. Its analysts called this a sure sign of overdiagnosis: finding cancers that would never do harm.46 Deaths from the fastest-rising early-onset cancers have stayed flat overall, so Patel, Adamson and Welch argue that much of that rise appears to reflect closer checking and overdiagnosis.47 Young adults' colorectal cancer deaths, though, are climbing.

One review called the rarity of ancient cancer "undisputed."48 Yet CT scans of medieval skeletons from around Cambridge suggest that at least 9% to 14% of adults died with cancer, against four or five in ten in modern Britain.49 Peto's paradox is disputed too: two recent analyses found that bigger species do get somewhat more cancer, just far less than their cell counts predict.50

Strassmann's figures come from one farming village over two years, extrapolated. Eaton and colleagues' estimate that modern Western women face up to 100 times the breast cancer risk of women before farming, by age 60, is a model output, not a measurement.18 And pregnancy itself raises breast cancer risk for years, peaking about five years after a birth and turning protective only after about 24 years.51

Hormonal birth control cuts both ways. In Denmark it raised breast cancer risk by about 20% in current and recent users, roughly one extra case for every 7,690 women using it for a year.52 Yet the pill cuts ovarian cancer risk by up to 29% for every five years of use. It cuts the risk of cancer of the uterine lining by about a quarter, and the protection lasts three decades or more.5354 Finally, nobody has systematic cancer data for living hunter-gatherers; "hunter-gatherers don't get cancer" is a slogan, not a finding.

Terms this topic defines

Related topics

Sources cited above

  1. 1.Strassmann, B.I. (1997). "The biology of menstruation in Homo sapiens: total lifetime menses, fecundity, and nonsynchrony in a natural-fertility population." Current Anthropology, 38(1), 123-129. https://doi.org/10.1086/204592 (lifetime menses: median 109, mean 128, after dividing hut visits by 0.86 to allow for menses spent at home; the comparison figure of about 400 menses for American women is Strassmann's working assumption, pp. 124-125; proportions cycling, pregnant, in lactational amenorrhea and postmenopausal were 0.25, 0.16, 0.29 and 0.31) (hut records kept for 736 nights, July 1986 to July 1988; 8.6 live births per woman; women went to the hut for 86% of their menses; each of 25 monitored births came about nine months after the last hut visit; women aged 20 to 34 averaged 3.8 menses in two years; median age at menarche 16)
  2. 2.Strassmann, B.I., and Warner, J.H. (1998). "Predictors of fecundability and conception waits among the Dogon of Mali." American Journal of Physical Anthropology, 105(2), 167-184. https://doi.org/10.1002/(SICI)1096-8644(199802)105:2<167::AID-AJPA5>3.0.CO;2-S (village size and nights at the hut from the PubMed abstract summary) (Sangui had 460 people in January 1988; about five nights at the hut each period)
  3. 3.Strassmann, B.I. (1999). "Menstrual cycling and breast cancer: an evolutionary perspective." Journal of Women's Health, 8(2), 193-202. https://doi.org/10.1089/jwh.1999.8.193 (Dogon lifetime menses rounded to about 100, a third of the lifetime total for an American woman with three children)
  4. 26.Siegel, R.L., Kratzer, T.B., Wagle, N.S., Sung, H., et al. (2026). "Cancer statistics, 2026." CA: A Cancer Journal for Clinicians, 76(1), e70043. https://doi.org/10.3322/caac.70043 (breast cancer incidence rose 1% a year in 2013-2022, 1.4% a year under 50; share of US women aged 15-44 never pregnant, 54% in 2024, from US Census Bureau data) (60% of US diagnoses come at 65 or older; the age-adjusted cancer death rate fell 34% from 1991 to 2023)
  5. 46.Ahn, H.S., Kim, H.J., and Welch, H.G. (2014). "Korea's thyroid-cancer 'epidemic': screening and overdiagnosis." New England Journal of Medicine, 371(19), 1765-1767. https://doi.org/10.1056/NEJMp1409841 (national screening began in 1999; the published title is punctuated with a dash) (diagnoses rose 15-fold between 1993 and 2011 while deaths held at 300 to 400 a year, a pattern the analysts called "pathognomonic for overdiagnosis")
  6. 47.Patel, V.R., Adamson, A.S., and Welch, H.G. (2025). "The rise in early-onset cancer in the US population: more apparent than real." JAMA Internal Medicine, 185(11), 1370-1374. https://doi.org/10.1001/jamainternmed.2025.4917 (the published title is punctuated with a dash)
  7. 48.David, A.R., and Zimmerman, M.R. (2010). "Cancer: an old disease, a new disease or something in between?" Nature Reviews Cancer, 10(10), 728-733. https://doi.org/10.1038/nrc2914
  8. 49.Mitchell, P.D., Dittmar, J.M., Mulder, B., Inskip, S., et al. (2021). "The prevalence of cancer in Britain before industrialization." Cancer, 127(17), 3054-3059. https://doi.org/10.1002/cncr.33615 (six cemeteries, 6th-16th centuries) (143 skeletons)
  9. 50.Butler, G., Baker, J., Amend, S.R., Pienta, K.J., et al. (2025). "No evidence for Peto's paradox in terrestrial vertebrates." PNAS, 122(9), e2422861122. https://doi.org/10.1073/pnas.2422861122; Compton, Z.T., Mellon, W., Harris, V.K., Rupp, S., et al. (2025). "Cancer prevalence across vertebrates." Cancer Discovery, 15(1), 227-244. https://doi.org/10.1158/2159-8290.CD-24-0573 (16,049 necropsy records from 292 species) (both published in 2025)
  10. 18.Eaton, S.B., Pike, M.C., Short, R.V., Lee, N.C., et al. (1994). "Women's reproductive cancers in evolutionary context." Quarterly Review of Biology, 69(3), 353-367. https://doi.org/10.1086/418650
  11. 51.Nichols, H.B., Schoemaker, M.J., Cai, J., Xu, J., et al. (2019). "Breast cancer risk after recent childbirth: a pooled analysis of 15 prospective studies." Annals of Internal Medicine, 170(1), 22-30. https://doi.org/10.7326/M18-1323 (women under 55; peak HR 1.80, 95% CI 1.63-1.99)
  12. 52.Mørch, L.S., Skovlund, C.W., Hannaford, P.C., Iversen, L., et al. (2017). "Contemporary hormonal contraception and the risk of breast cancer." New England Journal of Medicine, 377(23), 2228-2239. https://doi.org/10.1056/NEJMoa1700732 (RR 1.20, 95% CI 1.14-1.26, current and recent users vs never-users) (1.8 million women)
  13. 53.Collaborative Group on Epidemiological Studies of Ovarian Cancer (2008). "Ovarian cancer and oral contraceptives: collaborative reanalysis of data from 45 epidemiological studies including 23,257 women with ovarian cancer and 87,303 controls." The Lancet, 371(9609), 303-314. https://doi.org/10.1016/S0140-6736(08)60167-1 (29% reduction per five years of use when use had ended less than 10 years earlier, smaller for use that ended longer ago)
  14. 54.Collaborative Group on Epidemiological Studies on Endometrial Cancer (2015). "Endometrial cancer and oral contraceptives: an individual participant meta-analysis of 27 276 women with endometrial cancer from 36 epidemiological studies." The Lancet Oncology, 16(9), 1061-1070. https://doi.org/10.1016/S1470-2045(15)00212-0 (RR 0.76, 95% CI 0.73-0.78, per five years of use)
  15. 28.Islami, F., Marlow, E.C., Thomson, B., McCullough, M.L., et al. (2024). "Proportion and number of cancer cases and deaths attributable to potentially modifiable risk factors in the United States, 2019." CA: A Cancer Journal for Clinicians, 74(5), 405-432. https://doi.org/10.3322/caac.21858 (all evaluated factors together: 40.0% of cases and 44.0% of deaths) (in 2019, smoking accounted for 19.3% of cancer cases and 28.5% of cancer deaths among US adults 30 and older; excess body weight 7.6% of cases; alcohol 5.4%)
  16. 55.US Preventive Services Task Force (Davidson, K.W., Barry, M.J., Mangione, C.M., et al.) (2021). "Screening for colorectal cancer: US Preventive Services Task Force recommendation statement." JAMA, 325(19), 1965-1977. https://doi.org/10.1001/jama.2021.6238; US Preventive Services Task Force (Nicholson, W.K., Silverstein, M., Wong, J.B., et al.) (2024). "Screening for breast cancer: US Preventive Services Task Force recommendation statement." JAMA, 331(22), 1918-1930. https://doi.org/10.1001/jama.2024.5534; US Preventive Services Task Force (Krist, A.H., Davidson, K.W., Mangione, C.M., et al.) (2021). "Screening for lung cancer: US Preventive Services Task Force recommendation statement." JAMA, 325(10), 962-970. https://doi.org/10.1001/jama.2021.1117
  17. 56.Rock, C.L., Thomson, C., Gansler, T., Gapstur, S.M., et al. (2020). "American Cancer Society guideline for diet and physical activity for cancer prevention." CA: A Cancer Journal for Clinicians, 70(4), 245-271. https://doi.org/10.3322/caac.21591; quotations from American Cancer Society (n.d.). ACS Guidelines for Nutrition and Physical Activity for Cancer Prevention. https://www.cancer.org/cancer/risk-prevention/diet-physical-activity/acs-guidelines-nutrition-physical-activity-cancer-prevention/guidelines.html
  18. 57.Moore, S.C., Lee, I.M., Weiderpass, E., Campbell, P.T., et al. (2016). "Association of leisure-time physical activity with risk of 26 types of cancer in 1.44 million adults." JAMA Internal Medicine, 176(6), 816-825. https://doi.org/10.1001/jamainternmed.2016.1548 (90th vs 10th percentile of activity; activity was also associated with higher risk of melanoma and prostate cancer)
  19. 58.Courneya, K.S., Vardy, J.L., O'Callaghan, C.J., Gill, S., et al. (2025). "Structured exercise after adjuvant chemotherapy for colon cancer." New England Journal of Medicine, 393(1), 13-25. https://doi.org/10.1056/NEJMoa2502760 (disease-free survival HR 0.72, 95% CI 0.55-0.94; musculoskeletal adverse events 18.5% vs 11.5%)
  20. 19.Collaborative Group on Hormonal Factors in Breast Cancer (2002). "Breast cancer and breastfeeding: collaborative reanalysis of individual data from 47 epidemiological studies in 30 countries, including 50302 women with breast cancer and 96973 women without the disease." The Lancet, 360(9328), 187-195. https://doi.org/10.1016/S0140-6736(02)09454-0 (4.3% per 12 months of breastfeeding, 95% CI 2.9-5.8; 7.0% per birth, 95% CI 5.0-9.0) (breast cancer by age 70 estimated to fall from 6.3 to 2.7 cases per 100 women)
  21. 59.Lei, J., Ploner, A., Elfström, K.M., Wang, J., et al. (2020). "HPV vaccination and the risk of invasive cervical cancer." New England Journal of Medicine, 383(14), 1340-1348. https://doi.org/10.1056/NEJMoa1917338 (1.67 million girls and women aged 10-30 followed 2006-2017)
  22. 33.Siegel, R.L., Wagle, N.S., Star, J., Kratzer, T.B., et al. (2026). "Colorectal cancer statistics, 2026." CA: A Cancer Journal for Clinicians, 76(2), e70067. https://doi.org/10.3322/caac.70067 (incidence rose 3% a year at ages 20 to 49 from 2013 to 2022 and fell 2.5% a year at 65 and older; death rates under 50 rose 1% a year since 2004; the rise is seen in at least 14 countries; the increase in underlying risk is in people born after about 1950)
  23. 60.Erland, L.A., and Saxena, P.K. (2017). "Melatonin natural health products and supplements: presence of serotonin and significant variability of melatonin content." Journal of Clinical Sleep Medicine, 13(2), 275-281. https://doi.org/10.5664/jcsm.6462

This topic carries 60 notes in the research manuscript; the 23 above are those behind the material on this page. Full source register.