Is colorectal cancer related to obesity?

Colorectal cancer (CRC) is a disease with a high incidence and mortality rate, both in Europe and worldwide. In Portugal, it is the most common cancer, the second leading cause of cancer death and the sixth leading cause of death in general.

CRC has multiple risk factors, some of which are non-modifiable, such as gender, family history of colorectal cancer and/or polyps, or the presence of inflammatory bowel disease. However, other risk factors are modifiable and are estimated to account for 70% to 90% of CRC cases. These factors are related to diet and lifestyle, including being overweight, a sedentary lifestyle, smoking and drinking, as well as a diet high in processed or red meat and low in vegetables and fruits – a dietary pattern also associated with obesity.

According to the World Health Organization, there are currently 650 million overweight adults (a body mass index (BMI) greater than 25 kg/m2) in the world. In Portugal, 67.6% of the population is overweight and 28.7% of adults aged 25 to 74 are obese (BMI ≥ 30 kg/m2).

Obesity is recognized as an important risk factor for CRC. Compared with individuals of normal weight, obese individuals have a 7% to 60% increased risk of developing CRC, with this association being stronger for colon cancer than for rectal cancer. The relationship is direct and independent: for every 2 kg/m² increase in BMI, the risk of CRC increases by 7%. This risk is more pronounced in males: for every 5 kg/m² increase in BMI, the risk of CRC increases by 24% in men and 9% in women. It is important to note that this increased risk begins early, being present in obese children at 7 years of age and in young adults.

The risk of CRC is particularly associated with visceral fat, assessed mainly by intra-abdominal fat: each 2 cm increase in waist circumference translates into a 4% increase in the risk of CRC.

The pathophysiological relationship between obesity and cancer is complex and multifactorial, involving metabolic dysregulation (such as insulin resistance), chronic low-grade systemic inflammation and hormonal changes. It is also believed that the intestinal microbiota plays a relevant role in this association.

The treatment of CRC in obese individuals presents additional technical challenges, particularly in the surgical approach. In addition to increasing the incidence of the disease, obesity is also associated with higher rates of recurrence and mortality. Individuals with higher BMI and greater amounts of visceral fat have lower overall survival. Furthermore, mortality from CRC increases progressively with the number of metabolic syndrome factors present, such as diabetes mellitus, arterial hypertension or dyslipidemia, frequently associated with obesity.

Sarcopenic obesity also stands out, characterized by the reduction of skeletal muscle mass in obese individuals, a particularly serious condition in the oncological context, associated with a worse prognosis and requiring early diagnosis.

Fortunately, recent studies have shown that bariatric surgery with sustained weight loss reduces the risk of CRC, showing that this risk relationship can be modified.

Therefore, it is imperative to act early, promoting healthy lifestyles and eating habits from childhood, as a fundamental strategy to reduce the incidence and mortality not only from CRC, but also from all diseases associated with obesity.