Colon health

The colon is a muscular tube that runs from the end of the small intestine to the rectum. It absorbs water and salts, stores waste until it is passed, and hosts the densest community of microbes in the body. These bacteria ferment dietary fibre into short-chain fatty acids, which feed the cells of the bowel lining. A diet high in fibre changes which microbes thrive and what they produce; in a 17-week randomised trial of 36 healthy adults, a high-fermented-food diet increased microbiome diversity and lowered 19 of 93 measured inflammatory proteins in the blood, including interleukin-6, while a high-fibre diet changed microbial enzymes without the same fall in inflammation [7]. Chronic low-grade inflammation is described under inflammation.

Colorectal cancer usually arises through the adenoma-carcinoma sequence: a benign adenoma accumulates genetic changes over years until it becomes an invasive cancer. Screening works by interrupting this sequence, either by finding and removing adenomas or by detecting cancers at an early, treatable stage.

The disease is shifting in pattern. In US registry data the share of new cases diagnosed at an advanced stage rose to 60% in 2019, from 52% in the mid-2000s, and the proportion of cancers in the rectum rose from 27% in 1995 to 31% in 2019. Incidence in people under 55 has risen by 1 to 2% a year since the mid-1990s, and the reasons for the increase in generations born since 1950 are not known [1]. Symptoms that lead to diagnosis include blood in the stool, a lasting change in bowel habits, abdominal pain and unexplained anaemia (a low red blood cell count).

Two broad types of test are used. Stool tests, such as the faecal immunochemical test (FIT), detect traces of blood and are repeated regularly, with a positive result followed by colonoscopy. Colonoscopy examines the whole colon with a camera and allows polyps to be removed during the same procedure.

The US Preventive Services Task Force recommends screening for all adults aged 50 to 75, with high certainty of substantial net benefit, and for adults aged 45 to 49, with moderate certainty of moderate benefit; it lowered the starting age from 50 to 45 in 2021. Screening from 76 to 85 is offered selectively [8]. The same review noted that 26% of eligible US adults had never been screened in 2016.

Evidence on the size of the benefit comes from two kinds of study. The National Polyp Study compared colorectal cancer deaths among 2,602 patients who had adenomas removed with the number expected from national cancer registry rates: 12 deaths were observed against 25.4 expected, a standardised mortality ratio (observed deaths divided by expected deaths) of 0.47 [2]. The NordICC randomised trial (a study that assigns participants to groups by chance) invited 28,220 adults aged 55 to 64 in Poland, Norway and Sweden to a single screening colonoscopy and compared them with 56,365 who were not invited. Only 42% of those invited attended. After 10 years, colorectal cancer risk was 18% lower in the invited group (0.98% against 1.20%), while the reduction in colorectal cancer deaths (0.28% against 0.31%) was not statistically significant, and all-cause mortality was identical [9].

Dietary fibre has the most consistent protective association. A series of systematic reviews covering 185 prospective studies and 58 clinical trials found 15 to 30% lower all-cause and cardiovascular mortality and lower incidence of several diseases, including colorectal cancer, in people eating the most fibre compared with the least. Risk reductions were greatest at 25 to 29 g of fibre a day, and the certainty of evidence for fibre was graded moderate [3]. Fibre intake is covered in detail under dietary fibre.

Processed meat was classified in 2015 by a Working Group of 22 experts convened by the International Agency for Research on Cancer as carcinogenic to humans (Group 1), and red meat as probably carcinogenic (Group 2A), mainly on the basis of colorectal cancer. The group estimated that each 50 g portion of processed meat eaten daily raises colorectal cancer risk by about 18% [4]. Group 1 describes the strength of the evidence that a substance causes cancer, not the size of the risk.

Alcohol raises risk in a graded way. A meta-analysis (a study that pools the results of earlier studies) of 27 cohort and 34 case-control studies found a relative risk (the risk in one group relative to another; 1.21 means 21% higher) of 1.21 for moderate drinking and 1.52 for heavy drinking of 4 or more drinks a day. Modelled across intake, 10 g of alcohol a day (under one drink) carried a relative risk of 1.07, 50 g 1.38 and 100 g 1.82 [5].

Excess body fat is also associated with the disease. Across 41 studies of general obesity and 13 of waist size, including about 9 million participants, obesity measured by body mass index carried a relative risk of 1.33, and the largest waist size 1.46, compared with the lowest categories [10]. Fermented dairy has weaker evidence: among 32,606 men who had lower endoscopy, eating two or more servings of yogurt a week was associated with 19% lower odds of conventional adenoma (odds ratio 0.81), but no association was found among 55,743 women [11].

Physical activity is one of the most consistently reported protective factors. A meta-analysis of 52 studies found a relative risk of colon cancer of 0.76 in the most compared with the least active people, with similar estimates in men and women; the association was stronger in case-control studies (0.69) than in cohort studies (0.83) [6]. Proposed mechanisms include faster passage of stool through the bowel, lower insulin levels and reduced inflammation.

Smoking is associated with both incidence and death. In a meta-analysis of 106 observational studies, ever smokers had a pooled relative risk of colorectal cancer of 1.18 compared with never smokers, and 1.25 for death from the disease across 17 cohorts. The association was statistically significant only after 30 years of smoking, rose with pack-years and cigarettes per day, and was stronger for rectal than colon cancer [12]. Tobacco use is described under smoking.

Full deep dive: colon health.

Most evidence on diet, alcohol, body weight and activity comes from observational studies, which track people rather than assigning them to a behaviour. People who eat more fibre or exercise more also tend to smoke less and to attend screening more often, and statistical adjustment cannot remove all of these differences. The meat classification by the International Agency for Research on Cancer rests largely on such studies [4].

The two main screening studies measure different things. The National Polyp Study compared patients with a registry-based expectation rather than with a randomised control group [2]. The NordICC trial measured the effect of being invited, and because fewer than half of those invited attended, its intention-to-screen estimates understate the effect of actually having a colonoscopy; its 10-year follow-up may also be too short to show the full effect on deaths [9]. The yogurt finding comes from one pair of US health-professional studies and was confined to men [11].