Bowel Cancer Causes

Overview

Bowel cancer is a type of cancer that affects the large bowel (colon and rectum). Sometimes, it is also referred to as colorectal cancer. It is one of the most common types of cancer and is estimated to be on the rise.¹ The exact cause of bowel cancer is still unknown. You are more likely to be diagnosed with bowel cancer after the age of 45 and significantly more likely over the age of 80.

However, a variety of factors are linked to the development of tumours in the bowel. These include genetics, other diseases and personal dietary and lifestyle choices.² If you would like to find out what could potentially cause and contribute to the development of bowel cancer, this article may be helpful and provide some clarity on the topic.

What is bowel cancer?

Bowel cancer affects the colon and rectum (Figure 1). It arises from cells within the large intestine that suddenly start to multiply more than usual (hyperplasia) due to genetic mutations within them. This can lead to non-cancerous masses (adenomas), which can progress to cancerous growths (carcinomas).³

Types of bowel cancer

There are 3 main types of bowel cancer:

• Sporadic (random)
• Hereditary (inherited)
• Colitis-associated

Most bowel cancers are sporadic type and usually progress from hyperplasia (excess growth) into the formation of small growths from mucosal membranes referred to as polyps or straight into adenomas (bowel cancer stage 0). These can then further grow into bigger masses and eventually become cancerous adenomas (bowel cancer stage 1).^1,2,3

Symptoms of bowel cancer

Bowel cancer is typically asymptomatic hence its early stages may be overlooked. Once the cancer progresses, the symptoms include:

• Presence of blood in the stool
• Pain in the abdomen
• Feeling of fullness
• Feeling nauseous and vomiting
• A mass in the abdomen that can be felt through palpations

Otherwise, only when the cancer is advanced some other symptoms may manifest, such as being underweight and fatigued or acquiring anaemia. These symptoms may vary depending on the location of your tumour within the large intestine.²

Causes and risk factors

While the direct cause of bowel cancer remains unknown, there is a variety of well-studied risk factors that may increase your chances of developing this type of tumour.¹

Genetics

If you have a family history of colorectal cancer, then you are significantly more likely to develop it yourself. The risk does depend on a variety of other aspects, such as:

• How closely related you are to affected relatives
• The age of the relatives when bowel cancer appeared
• How many relatives have bowel cancer
• History of other cancers in the family
• Your cancer history¹

Adenomatous polyposis coli and Lynch syndrome 

Two inherited genetic syndromes are most commonly a significant risk factor for developing bowel cancer, and these are Lynch syndrome and familial adenomatous polyposis coli (FAP).

FAP is caused by a mutation in a tumour suppressor gene, which causes the patient to develop multiple polyps within their large intestine, which often eventually turn cancerous. Similarly, Lynch syndrome is caused by gene mutations that fix errors during cell replication. These are most often genes MLH1 and MSH2, but other genes are also affected.

Due to the accumulated errors, cells frequently turn cancerous, and hence, a patient suffering from this condition is more likely to develop cancer, especially the colorectal type. 

Both FAP and Lynch syndrome are inherited in an autosomal dominant manner, meaning if one of your biological parents has the mutation, there’s a 50% chance you may also have it.^1,3 

Medical conditions (non-cancer-related)

Polyps within the colon

Polyps can be either non-cancerous or cancerous. Polyps that have the potential to become cancerous are adenomatous polyps and it is estimated that 95% of colorectal cancers develop from these growths. However, colon polyps are generally harmless; about 5% of them will progress into the cancerous stage (adenocarcinoma). If you have polyps of size larger than 1 or 2cm, then the chance of getting bowel cancer is higher, and ageing is also another factor that elevates this risk.¹

Inflammatory bowel diseases

Two main inflammatory bowel diseases (IBDs) linked to the development of bowel cancer are Crohn’s disease and Ulcerative colitis. The exact cause of IBDs is unknown, but they are characterised by continuous inflammation of mucosal membranes that cover the insides of the gastrointestinal tract as well as higher cell replication and sporadic mutation rates. These abnormalities lead to increased tumour growth and spread hence the risk of colorectal cancer is significantly higher in individuals suffering from these IBD conditions.^1,3

Diabetes

Diabetes is a condition when your blood sugar levels are chronically elevated, and your insulin (a hormone that processes sugar) is not working as it should. Especially in diabetes type 2, high blood sugar and high insulin cause an inflammatory reaction in the body that, stimulates the large intestine cells to multiply more and in an error-prone manner. Hence, there is a higher chance of developing cancerous growth and bowel cancer in diabetes patients.^1,3

Other conditions

Other conditions and procedures have also been found to be associated with a higher risk of developing bowel cancer. Among them are:

• Cystic fibrosis
• Androgen deprivation therapy in patients suffering from prostate cancer
• Gallbladder removal surgery (it is suspected that exposure of the intestines to the bile is the main contributing factor)
• Abdominal radiotherapy (the more radiation the patient receives, the higher the chance of colorectal cancer manifestation)⁴

Lifestyle 

Additionally, various lifestyle factors have been linked to contribute to the development of bowel cancer.

Dietary choices

Consumption of red meat or processed meat has also been proposed to have cancerogenic (cancer-inducing) effects on humans due to certain chemicals released during meat cooking and heat processing. Additionally, heme iron present in red meat has also been found to induce oxidative stress on the cells, leading to mutations and damage to genetic material. Consequently, these abnormalities may evolve into bowel cancer.¹

Obesity/being overweight

Research shows that for every five kilograms gained, the risk of developing bowel cancer increases by 3%. A large amount of adipose (fat) tissue releases inflammatory factors that affect the body’s biological process, such as:

• Increase in the rate of cell division
• Suppression of the cells’s programmed death (process that removes cells that no longer work) 
• Increase in oxidation-induced stress
• Reduction of immune responses

This abnormal activity in the epithelial cells of the large intestine has been implicated in an increased risk of developing colorectal cancer.¹

Sedentary lifestyle

Studies show that a sedentary lifestyle is linked to a 50% higher chance of developing bowel cancer, while being physically active regularly can reduce the risk of this cancer by 25%. This is because physical inactivity often leads to obesity, which affects the bowel cells, as explained above.⁴

Cigarette smoking

In general, cigarette smoking is associated with a greater risk of different cancers, especially lung cancer. However, the vast chemicals inhaled in cigarette smoke have carcinogenic effects on the cells within the colon and rectum. As it was mentioned earlier, these can cause genetic damage and mutations that lead to the development of colorectal cancer.^1,4

Alcohol

Drinking alcohol in moderate to large volumes has been linked to an increased risk of bowel cancer of between 20% and 54%, depending on the amount of alcohol consumed. There is a variety of carcinogens (cancer-inducing chemicals) related to the metabolism of alcohol that induce stress on the cells and their processes. The risk is higher in individuals assigned male at birth (AMAB) due to the difference in hormones involved in alcohol metabolism and also because AMAB people tend to drink more alcohol.^1,4

Other factors

Other factors have been linked to higher chances of getting bowel cancer, such as:

• Imbalances and issues in gut microbiome
• Age - being over 50 years old
• Race - being of non-Hispanic black race
• Having lower socioeconomic status
• Sex - AMAB individuals are more likely to develop bowel cancer¹

Summary

Bowel (colorectal/large intestine) cancer is one of the most common cancers that affects the colon and rectum and is usually diagnosed in individuals over the age of 45. The main symptoms are blood in the stool, abdominal pain, fatigue and, anaemia in later stages. The exact cause of this cancer is unknown, but several factors have been implicated in its development, such as genetics, medical conditions and lifestyle choices.

Family history of colorectal cancer, genetic conditions such as Lynch syndrome and familial adenomatous polyposis coli (FAP), inflammatory bowel disease, and diabetes have all been linked to increased risk of developing this cancer. Lifestyle and environmental factors such as diet, obesity, sedentary lifestyle, smoking, drinking alcohol, poor socioeconomic background, age, race and sex have also been implicated in elevated risk of developing colorectal cancer.