Obesity and colon cancer: Systematic review of 75 studies confirms strong link

A team of researchers led by Shelby Ziller of the University of Iowa published a large systematic review in the August issue of Obesity that assessed the relationship between obesity and colorectal cancer (CRC) risk in adults. The work pooled data from 75 studies (32 cohorts and 43 case-control studies) with a total of more than 10 million participants and nearly 150,000 cases of CRC.

Key Results

• BMI and CRC risk: Each 5 kg/m² increase in body mass index (BMI) was associated with an average 18% increase in the risk of colorectal cancer (RR 1.18; 95% CI 1.14–1.22).
• Abdominal fat. Each additional 10 cm of waist circumference was associated with a 13% increase in risk (RR 1.13; 95% CI 1.08–1.19), and each 0.1–0.2 unit increase in waist-to-hip ratio was associated with a 20% increase (RR 1.20; 95% CI 1.12–1.29).
• Fat mass by bioimpedance. A 5% increase in body fat percentage was associated with a 14% increase in CRC risk (RR 1.14; 95% CI 1.07–1.21).
• Tumor localization. The association of obesity was particularly strong for right colon and rectal cancer, while the effect of BMI was slightly weaker for left colon cancer.

Why is this important?

Colorectal cancer remains the third most common malignancy worldwide and the second leading cause of cancer-related death. Obesity is an easily measurable and potentially modifiable risk factor. Combining data from multiple studies, the authors showed that excess and abdominal fat mass consistently increased the likelihood of developing CRC.

Communication mechanisms

Experts emphasize that excess abdominal fat stimulates chronic metabolic inflammation, increases insulin and insulin-like growth factor 1 (IGF-1) levels, and disrupts the intestinal microbiota—all of which contribute to the growth of tumor cells in the colon and rectum.

Authors' recommendations

• Weight control. Maintaining a BMI between 18.5–24.9 kg/m² and reducing waist circumference below the critical 88 cm in women and 102 cm in men can significantly reduce the risk of CRC.
• Screening: It is important for obese people to start colonoscopy earlier than the standard age (45 years) and to have it more often.
• Prevention Programs: Bring together gastroenterologists, nutritionists, and exercise specialists to develop integrated weight loss and cancer prevention programs.

Limitations and Prospects

The authors note high heterogeneity in the methods of measuring obesity and strong differences between populations. Further clinical studies are needed to determine to what extent weight loss actually reduces the risk of CRC, and which approaches to weight loss are most effective in cancer prevention.

In the discussion, the authors highlight the following key points:

• Clinical Relevance
  “Our analysis demonstrates that obesity – and particularly abdominal obesity – is not just associated with, but likely directly contributes to, the development of colorectal cancer,” says Dr. Ziller. “This means that reducing BMI and waist circumference may be as important an intervention as other preventive measures.”

• The need for screening
  “Overweight patients should start colonoscopy earlier and have it more frequently,” says co-author Prof Johnson. “We hope these data will encourage clinicians and patients to reconsider current CRC screening guidelines.”

• Future research directions
  “It remains to be seen whether weight management reduces the existing risk of CRC,” explains Dr Lee. “Randomised controlled trials will be needed to confirm that weight loss programmes do reduce bowel cancer incidence.”

This review reaffirms that obesity control is a key strategy in preventing colorectal cancer and calls for active weight management measures at the public health level.