Cancer Risk Factors: Smoking, Family History and Diet

The main cancer risk factors are tobacco, alcohol, viral infection, a sedentary lifestyle, obesity and a high-calorie Western diet, with smoking the most important of all. Family history matters far less than most patients believe: only 5 to 10 per cent of cancers are caused by an inherited gene mutation, and most patients have no strong family history of the disease.

This is the risk picture set out by Dr. Mohit Agarwal, Principal Director & Head Medical Oncology, Fortis Hospital, Shalimar Bagh, New Delhi, India, for doctors who are usually the first to hear a patient's fears about cancer. His framework separates the risks a person can change from the one most people worry about and cannot change, and it ends where risk turns into care: what a patient with cancer should eat. The wider framework of warning signs and screening is in the series pillar, Diagnosis and Detection of Cancer: A Complete Guide.

Is cancer hereditary?

Most cancer is not hereditary. In Dr. Agarwal's experience, the question commonly arises after a breast cancer diagnosis. The patient is usually an older woman, and her first worry is whether her daughter will now develop the same disease because her mother has. His answer is that this is not at all certain.

The numbers explain why. Even in the cancers most associated with families, such as breast and ovarian cancer, only 5 to 10 per cent of cases arise because of an inherited tendency. The majority are sporadic, meaning they arise from changes acquired during a person's life rather than a gene passed down a generation. For a doctor, this cuts both ways. A family history raises suspicion, but its absence never lowers it: most patients with cancer have no strong family history of it.

When is genetic testing for cancer recommended?

Genetic testing for an inherited cancer risk is not recommended for every patient with cancer. Patients increasingly ask for it, and Dr. Agarwal's position is that it should follow specific guidelines. The most common reasons to order it are:

• Two or more generations of a family affected by cancer.
• A young patient with breast, ovarian or colon cancer.
• A history of male breast cancer in the family.
• Two cancers occurring in the same person.

These criteria pick out the minority of families in whom an inherited mutation is likely, which is where testing changes decisions. Testing and the counselling around it belong to medical genetics.

Why test a patient who already has cancer?

A doctor in the session asked why anyone would order a genetic test for a patient already diagnosed, and whether it changes the prognosis. Dr. Agarwal gave three reasons. First, a positive result can open specific therapies that are available only for patients with that finding. Second, it tells the family whether other members are at risk and should be assessed. Third, it shows whether the patient has a tendency to develop cancer in another part of the body, which shapes how that person is followed up.

Smoking: the most important cancer risk factor

Smoking is the most common cancer risk factor across the entire globe, across both genders, and across every race and ethnicity. Dr. Agarwal illustrated it with three images: a normal lung, a smoker's lung with black deposits of tar inside it, and the lung of a cancer patient in which tar-damaged tissue has turned into a tumour, visible as the whitest spot on the image.

He gives any smoker, patient or not, three messages:

1. The risk is large. Half of smokers will die before their time, and one in six will die of lung cancer.
2. The deaths come in middle age, when people are at their most productive.
3. Stopping at any point in life helps. Quitting lowers the risk of cancer and early death, whatever the person's age.

Nicotine addiction can be stronger than fear of the disease. Dr. Agarwal described a patient he was treating for head and neck cancer, whom he found sitting outside the hospital trying to smoke through the opening in his neck that the team had created so that he could breathe.

Passive smoking and the risk to the family

Some smokers tell their doctor that they are not attached to a long life, so an early death is their own business. Dr. Agarwal's reply is that the decision is not theirs alone. Family members who live with a smoker have a 30 per cent increased risk of lung cancer. Second-hand smoke is a cause of death in its own right: the World Health Organization attributes about 1.3 million deaths a year to second-hand smoke among people who do not smoke, out of more than 8 million deaths a year caused by tobacco overall.

For a primary physician, this reframes the conversation. A patient who discounts his own risk may still act to protect his spouse and children.

Other cancer risk factors

Beyond tobacco, Dr. Agarwal listed the risk factors that apply across many cancers:

• Alcohol.
• Viral infection. Human papillomavirus and hepatitis B are the best-known examples, and both are preventable by vaccination.
• A sedentary lifestyle.
• Obesity.
• A high-calorie Western diet.

Many other risk factors apply only to particular cancers. The common thread in the list is that most of it is modifiable, which is why his prevention advice is so consistent: regular exercise, no tobacco, a minimum of alcohol and a balanced diet.

Diet, nutrition and cancer prevention

Diet is one of the most important daily factors in cancer risk, apart from alcohol and tobacco, which are well-known risks. Dr. Agarwal's emphasis is on both the amount and the quality of food: whether what a person eats is adding value to the body.

For prevention, his standard is concrete. A balanced diet is one calibrated to the individual's age, height and weight. The body weight should be ideal, and the person should exercise regularly. When these conditions are met, the benefit extends beyond cancer to the prevention of many other diseases.

Nutrition in cancer care

Nutrition is a core part of every cancer treatment plan, not an afterthought. The point came up when a doctor described a patient with an advanced soft-tissue sarcoma whose nutrition had been difficult to sustain despite every effort, and asked how nutrition care should be built into cancer management.

Dr. Agarwal began with the physiology. Cancer is a catabolic state: the body breaks down more than it builds up. The aim of nutritional support is to keep that breakdown to a minimum so that the body, and especially its muscle, is maintained. In practice he recommends:

• A good protein diet, with protein supplements for patients who are undernourished, to raise their nutritional intake.
• Calcium and vitamin D supplements where the patient is deficient.
• B vitamins, including folate, where the patient is deficient.
• Regular meals at regular intervals.
• Less sugar.
• Complete avoidance of alcohol and tobacco.

His conclusion was organisational as much as clinical. A nutritionist, in his view, is a must in every cancer care team: someone who assesses each patient individually and adjusts the plan alongside the treating oncologist.

From risk to action

Risk factors matter to a primary physician for two reasons. They point to the conversations worth having with every patient, about tobacco, alcohol, weight and diet. And they help identify who needs earlier or more frequent screening. Neither replaces the warning signs: a patient with no risk factors and no family history can still develop cancer, and the first step remains a biopsy and proper staging, set out in Can Tumour Markers Screen for Cancer? Biopsy and Staging.

The full exchange on nutrition in advanced cancer, and the other questions doctors raised, are on the Live Discussion page.