What causes cancer?

What Causes Cancer

Cancer develops through the accumulation of genetic and epigenetic changes in cells, driven predominantly by modifiable environmental and lifestyle factors rather than inherited genetics—with tobacco use, obesity, poor diet, physical inactivity, alcohol consumption, infections, and environmental carcinogens accounting for 90-95% of all cancer cases. 1

Fundamental Mechanism

Cancer arises from disturbed cell function due to accumulated chromosomal or molecular aberrations leading to genetic instability. 2 The cancerous process results from multiple genetic and epigenetic steps, with repeated exposure to carcinogens and tumor promoters frequently necessary for cancer formation. 3

Primary Causative Factors

Tobacco Use - The Single Most Important Cause

• Tobacco use is the single most significant cause of cancer, responsible for 30% of all cancer deaths and 25-30% of all cancer-related deaths. 4, 1
• Cigarette smoking is the predominant cause of lung cancer and the leading worldwide cause of cancer death, with almost all lung cancer deaths caused by cigarette smoking. 4
• There is convincing evidence that smoking causes large increases in risk (relative risk ≥2.0) for lung, oral, pharyngeal, laryngeal, esophageal, bladder, kidney, and pancreatic cancers. 4, 5
• The risk follows a clear dose-response relationship, increasing with number of cigarettes smoked, deepness of inhalation, and duration of smoking. 4, 5
• Exposure to secondhand tobacco smoke (passive smoking), cigars, pipes, and smokeless tobacco also contributes to increased cancer risk. 4

Diet and Nutrition

• As many as 30-35% of cancer-related deaths are linked to diet. 1
• Diets low in fruits and vegetables are established risk factors for multiple cancer types. 6
• Diets high in red meat and fried foods increase cancer risk, particularly for colorectal cancer. 4, 1
• For colorectal cancer specifically, diets high in vegetables and fruits decrease risk, while diets high in red meat increase risk. 4
• Excessive alcohol consumption is a significant risk factor, with up to 75% of oral cancer cases in the United States attributable to tobacco and alcohol use combined. 5

Obesity and Physical Inactivity

• Obesity is a major modifiable risk factor, with convincing evidence linking excessive weight to renal cell cancer, endometrial cancer, and breast cancer. 4
• Physical inactivity contributes significantly to cancer risk, with 15-20% of cancer-related deaths attributed to lack of exercise. 1
• Regular moderate physical activity lowers colon cancer risk, with more vigorous activity providing even greater benefit. 4
• For breast cancer, longer duration and greater intensity of physical activity are associated with reduced risk. 4

Occupational and Environmental Exposures

• Established environmental risk factors include occupational exposure to asbestos, nickel, chromium, arsenic, and radiation (including radon gas). 4
• Indoor and outdoor air pollution contribute to cancer risk. 4
• Exposure to carcinogens such as toxins, chemicals, and radiation increases cancer risk. 7
• According to current knowledge, 80-90% of malignant tumors are caused by external environmental factors (carcinogens). 2

Infectious Agents

• Infections account for approximately 15-20% of cancer-related deaths. 1
• Ongoing research is investigating the potential role of HIV in lung carcinogenesis. 4

Other Contributing Factors

• Sun exposure is a significant risk factor, particularly for cutaneous melanoma. 6
• Stress contributes to cancer development. 1
• Radiation exposure increases cancer risk. 1

The Limited Role of Genetics

• Only 5-10% of all cancer cases can be attributed to genetic defects, whereas the remaining 90-95% have their roots in environment and lifestyle. 1
• While genetic inheritance influences cancer risk and cancer arises from genetic mutations in cells, most variation in cancer risk across populations and individuals is due to non-inherited factors. 4
• For specific cancers like ovarian cancer, approximately 20% or more are genetically inherited, but this is the exception rather than the rule. 4

Clinical Implications for Prevention

The evidence overwhelmingly demonstrates that cancer is a preventable disease requiring major lifestyle changes. 1 Cancer prevention requires smoking cessation, increased ingestion of fruits and vegetables, moderate use of alcohol, caloric restriction, exercise, avoidance of direct sun exposure, minimal meat consumption, use of whole grains, vaccinations, and regular check-ups. 1

Key Prevention Strategies

• Avoid all tobacco products and secondhand smoke exposure. 4
• Maintain healthy weight through diet and regular physical activity. 4
• Consume at least five servings of vegetables and fruits daily. 4
• Limit red meat consumption and avoid fried foods. 4, 1
• Limit alcohol to no more than one drink per day. 4
• Engage in vigorous physical activity for at least four hours per week. 4
• Minimize exposure to occupational carcinogens and environmental pollutants. 4
• Avoid excessive sun exposure. 1, 6

Important Caveat

While these modifiable factors account for the vast majority of cancer cases, individual risk assessment is complex due to both inherited predispositions and the interaction of multiple exposures over time. 3 However, a very large proportion of cancer's impact could be ameliorated if more people avoided tobacco and alcohol use, obesity, diets low in fruits and vegetables, lack of physical activity, and sun exposure. 6