Cancer & Obesity : Understanding the Link in South Africa

When most people think about obesity, conditions such as diabetes, high blood pressure and heart disease usually come to mind. Cancer is mentioned far less often.

However, excess body fat is now recognised as an important risk factor for several cancers. This does not mean that every person living with obesity will develop cancer, or that body weight is the only reason cancer occurs. It means that, over time, excess body fat can create biological conditions that make certain cancers more likely to develop.

This matters greatly in South Africa, where obesity has become a serious public-health concern. It also needs to be discussed carefully. Obesity is a complex chronic disease, not simply the result of laziness or a lack of willpower. Genetics, medication, income, food prices, mental health, working conditions, neighbourhood safety and access to healthcare can all influence a person’s weight.

Why this issue matters in South Africa

More than half of South Africa’s adult population is living with overweight or obesity. Among women, the figure is more than two-thirds. WHO data separately estimate that approximately 30.8% of South African adults were living specifically with obesity in 2022. South Africa had one of the highest adult obesity levels in the African region.

The National Department of Health has described obesity as an urgent public-health crisis. Its national strategy recognises that the problem cannot be solved by telling people to “eat less and move more”. Food systems, advertising, poverty, limited access to healthy food, unsafe neighbourhoods and a lack of suitable opportunities for physical activity all affect the choices available to South Africans.

Cancer is also placing a substantial burden on the country. The International Agency for Research on Cancer estimated that South Africa had approximately 101,598 new cancer cases and 55,356 cancer deaths in 2024. Breast cancer, colorectal cancer and cancer of the uterus were among the significant cancers affecting South Africans.

Several cancers associated with excess body fat are already present in meaningful numbers. The 2024 estimates included:

• 13,338 breast cancer cases;
• 6,816 colorectal cancer cases;
• 2,263 pancreatic cancer cases;
• 1,779 cancers of the body of the uterus; and
• 1,143 kidney cancer cases.

These figures represent the estimated total number of cases and should not be interpreted as the number directly caused by obesity. Cancer usually develops through a combination of factors, including age, genetics, infections, tobacco use, alcohol, environmental exposures and chance cellular changes.

What do doctors mean by obesity?

Obesity is a medical condition involving an unhealthy amount or distribution of body fat.

Body mass index, usually called BMI, is commonly used as a screening tool. It is calculated by dividing a person’s weight in kilograms by the square of their height in metres:

BMI = weight in kilograms ÷ height² in metres

For adults, a BMI from 25 to 29.9 is commonly classified as overweight, while a BMI of 30 or above is classified as obesity. However, BMI does not directly measure body fat and cannot, on its own, provide a complete picture of an individual’s health. Muscle mass, waist circumference, fat distribution, age and metabolic health also matter.

The number on a scale is therefore only one part of the picture. Two people with the same BMI may have different amounts of muscle, different levels of abdominal fat and very different health risks.

Which cancers are linked to obesity?

Research reviews have linked excess body fat with an increased risk of at least 13 types of cancer.

These include:

• breast cancer after menopause;
• colorectal cancer;
• endometrial cancer, which affects the lining of the uterus;
• kidney cancer;
• pancreatic cancer;
• liver cancer;
• gallbladder cancer;
• ovarian cancer;
• thyroid cancer;
• adenocarcinoma of the oesophagus;
• cancer of the upper part of the stomach;
• multiple myeloma; and
• meningioma, a tumour arising in the membranes surrounding the brain and spinal cord.

The increase in risk is not equal for every cancer.

One of the strongest associations is seen with endometrial cancer. According to the United States National Cancer Institute, people with severe obesity have approximately seven times the risk of certain estrogen-related endometrial cancers compared with people in a healthy weight range. The relationship is also particularly strong for oesophageal adenocarcinoma.

For breast cancer, the relationship with body weight depends partly on age and hormonal status. The clearest established association is between excess body fat and breast cancer after menopause.

How can body fat influence cancer growth?

Body fat does more than store excess energy. Fat tissue is biologically active. It produces hormones and chemical signals that can affect inflammation, metabolism, immune function and cell growth.

There is no single pathway through which obesity causes every cancer. Different biological processes may be involved in different parts of the body.

Increased estrogen levels

After menopause, the ovaries produce much less estrogen. Fat tissue then becomes an important source of estrogen in the body.

Higher amounts of body fat can result in higher estrogen exposure. Estrogen can stimulate the growth of certain hormone-sensitive cells, helping to explain part of the association between obesity and postmenopausal breast, endometrial and ovarian cancers.

Insulin and growth signals

People living with obesity may develop insulin resistance. This means that the body does not respond to insulin as effectively as it should, so the pancreas produces more insulin to compensate.

Higher levels of insulin and insulin-like growth factor 1, known as IGF-1, can encourage some cells to grow and divide. Increased insulin and IGF-1 levels have been associated with colorectal, breast, ovarian, thyroid and endometrial cancers, among others.

This does not mean that insulin itself suddenly creates a tumour. Rather, persistently altered metabolic signals may contribute to an environment in which abnormal cells are more likely to survive or multiply.

Long-term inflammation

Obesity can be accompanied by chronic, low-level inflammation.

Inflammation is normally part of the body’s healing and defence system. When it continues over a long period, however, it can damage cells and alter the signals that control cell growth. Chronic inflammation may therefore contribute to the development and progression of some tumours.

Changes in hormones released by fat cells

Fat cells release signalling substances called adipokines.

One of these, leptin, generally rises as body fat increases and may encourage abnormal cell growth under certain conditions. Another, adiponectin, may have protective effects against uncontrolled cell growth but is often present at lower levels in people living with obesity.

Researchers are continuing to investigate how these hormones interact with the immune system, metabolism and tumour cells.

Does obesity mean that cancer is inevitable?

No.

Obesity increases risk, but it does not make cancer unavoidable. Many people living with obesity never develop cancer, while people in a healthy weight range can still be diagnosed with cancer.

Cancer is a complex group of diseases. Age, inherited genetic variants, tobacco, alcohol, infections, radiation, environmental exposures and random changes during cell division can all contribute to its development. WHO estimates that a substantial proportion of cancers can be prevented by reducing known risks and implementing evidence-based prevention strategies, but no lifestyle can offer complete protection.

The purpose of discussing obesity and cancer should therefore be prevention and support, not blame.

Can losing weight reduce cancer risk?

Research suggests that maintaining a healthy weight and avoiding excessive weight gain may reduce the risk of several cancers.

Some observational studies have found lower rates of breast, endometrial, colorectal and prostate cancers among people with obesity who lost weight. For example, intentional weight loss of more than 5% was associated with a lower risk of obesity-related cancers in one study of postmenopausal women.

A 5% reduction does not need to be dramatic. For a person weighing 100 kilograms:

100 kg × 5% = 5 kg

Their weight after losing 5% would therefore be 95 kilograms.

However, the evidence must be interpreted carefully. Much of it comes from observational studies, which cannot prove that weight loss alone caused the reduction in cancer risk. Randomised trials have not yet provided a precise answer about how much weight a person must lose to reduce the risk of each cancer.

The realistic goal is not rapid or extreme weight loss. It is a sustainable improvement in eating patterns, physical activity, metabolic health and overall wellbeing.

Practical steps that can help

Cancer prevention does not require expensive imported foods, a gym membership or an extreme diet. Small, repeatable habits are more useful than a strict programme that lasts only a few weeks.

Build meals around affordable whole foods

South African households can make healthier meals using familiar foods such as dry beans, lentils, split peas, vegetables, fruit, oats, sorghum, wholegrain bread, brown rice, mealie meal and samp.

Beans, lentils and other pulses are useful because they provide fibre and can replace part of the meat in stews, soups and curries. Seasonal fruit and vegetables are often more affordable than out-of-season produce.

CANSA recommends meals that include fruit, non-starchy vegetables, wholegrains and legumes, while limiting highly processed foods and drinks that are high in sugar, salt and fat.

A practical meal could be as simple as samp and beans with vegetables, lentil curry with brown rice, or pap served with beans, spinach and a moderate portion of lean protein.

Drink more water and fewer sugary drinks

Sugar does not directly “feed cancer” in the simplistic way often claimed online. Every cell in the body uses glucose.

The more relevant concern is that sugary drinks can provide large amounts of energy without creating much fullness, which can contribute to weight gain over time. South Africa introduced its Health Promotion Levy on sugar-sweetened beverages in 2018 as part of broader attempts to improve the food environment.

Water should be the everyday drink wherever clean drinking water is safely available.

Increase fibre

Foods containing fibre include beans, lentils, wholegrains, vegetables, fruit, nuts and seeds.

Fibre supports digestive health and is particularly relevant to colorectal-cancer prevention. CANSA advises South Africans to increase fibre intake and reduce processed meat consumption as part of lowering colorectal-cancer risk.

Changes should be gradual, especially for someone who currently eats little fibre. Drinking sufficient water is also important when increasing fibre intake.

Move in ways that are realistic

Physical activity does not need to mean running or joining a gym. Brisk walking, dancing, gardening, active housework, cycling and community sport can all contribute.

WHO recommends that adults aim for at least 150 minutes of moderate-intensity activity each week, or 75 minutes of vigorous activity. Muscle-strengthening activity is recommended on at least two days per week.

One way to reach the basic target is:

30 minutes × 5 days = 150 minutes per week

Someone who is currently inactive can begin with shorter periods, such as ten minutes at a time, and increase gradually. Some movement is better than none.

A person with heart disease, severe obesity, mobility limitations or another medical condition should obtain professional advice before beginning strenuous exercise.

Avoid crash diets

Very restrictive diets may produce quick results, but they can be difficult to maintain and may lead to repeated weight loss and regain.

A healthier approach is to make changes that can continue in ordinary life: cooking at home more often, reducing portion sizes gradually, increasing vegetables and legumes, replacing sugary drinks with water and finding a form of movement that is enjoyable and accessible. The World Cancer Research Fund recommends small, achievable changes rather than fad diets.

Ask for medical support when needed

Obesity is a chronic disease and may require professional treatment.

A doctor can assess blood pressure, blood glucose, cholesterol, liver health, medication use and other factors that may be contributing to weight gain. Depending on a person’s circumstances, appropriate care may include support from a registered dietitian, psychological support, structured lifestyle treatment, medication or metabolic surgery.

South Africa’s obesity response increasingly recognises the need for care to be available through community and primary-health services, rather than relying only on individual willpower.

Weight management does not replace cancer screening

Living healthily can reduce risk, but it cannot guarantee that cancer will not develop.

People should still attend appropriate cancer screening and seek medical attention for persistent or unusual symptoms. These may include a new lump, unexplained bleeding, lasting changes in bowel habits, difficulty swallowing or unexplained weight loss.

South Africans should speak to a healthcare professional about screening appropriate to their age, sex, family history and personal risk factors. Early detection can improve treatment options and outcomes for many cancers.

What about people who already have cancer?

A person receiving cancer treatment should not begin a strict weight-loss programme without speaking to their oncology team.

Cancer and its treatment can cause weight gain in some people and dangerous loss of weight or muscle in others. Nutritional needs may also change during chemotherapy, radiation therapy or surgery.

Anyone having difficulty with weight during treatment should ask their cancer-care team or a registered dietitian for personalised guidance.

A conversation without shame

Weight is a sensitive subject, especially in a country where many people face food insecurity while obesity is increasing at the same time.

Some families may have access to enough calories but not consistently to affordable, nutritious food. Others live in areas where outdoor exercise is unsafe or where long commuting hours leave little time for meal preparation and physical activity.

This is why obesity cannot be treated only as an individual failure. South Africa’s national strategy places personal behaviour within a wider system that includes family, schools, workplaces, food companies, healthcare, community safety, policy and socioeconomic conditions.

Shame rarely helps someone improve their health. Respectful information, realistic options and access to proper care are far more useful.

Final thoughts

The connection between obesity and cancer is real, but it is not simple.

Excess body fat can affect estrogen, insulin, inflammation and other biological signals that influence how cells grow. This helps explain why obesity is associated with cancers of the breast after menopause, bowel, uterus, kidney, pancreas and several other organs.

For South Africa, the challenge is particularly important because the country faces high levels of obesity alongside a growing burden of noncommunicable diseases.

The answer is not a perfect body or a crash diet. It is a long-term effort to make nutritious food, safe physical activity and evidence-based healthcare more accessible to everyone.

At an individual level, regular movement, balanced meals, less sugary and highly processed food, and professional support where necessary can help lower cancer risk and improve health more broadly. These steps reduce risk; they do not offer a guarantee.

References

1. World Health Organization. South Africa: Applying the Lessons Learned from Tackling HIV to Accelerate Action on Obesity. 4 March 2025.
2. World Health Organization. South Africa Country Health Data: Adult Obesity. 2022 estimate.
3. South African National Department of Health. Strategy for the Prevention and Management of Obesity in South Africa 2023–2028.
4. International Agency for Research on Cancer. Global Cancer Observatory: South Africa Fact Sheet, GLOBOCAN 2024. Published July 2026.
5. National Cancer Institute. Obesity and Cancer Fact Sheet. Updated January 2025.
6. World Cancer Research Fund International. Weight, Obesity and Cancer.
7. World Health Organization. Physical Activity Recommendations.
8. Cancer Association of South Africa. THINK Before You Eat & Drink. Updated September 2025.
9. Cancer Association of South Africa. Colorectal Cancer: The Power of Balance, Fibre and Gut Health. 2025.

This article is intended for general education and does not replace medical advice, diagnosis or treatment from a qualified healthcare professional.