Do People With Obesity
Have a Greater Risk of Cancer?

What Is Obesity?

Obesity is a disease in which a person’s weight is higher than what is considered healthy given his or her height. Compared with people of healthy weight, obese people are at a greater risk for many diseases, including cardiovascular disease, high blood pressure, stroke, diabetes and at least 13 types of cancer. They also have a higher risk of death from all causes.

To make an obesity diagnosis, doctors consider a measure known as body mass index. To calculate BMI, doctors will divide a person’s weight (in kilograms) by his or her height (in meters) and square the dividend. The equation looks something like this:

• kg/m²

Weight alone is not a measure of obesity, which is why doctors rely on BMI. For most people, BMI is a strong measurement of one’s level of body fat.

What Is Considered a Healthy BMI?

Global health organizations maintain ranges for BMI that help health providers ascertain which category a person falls into, with categories ranging from “underweight” to “severely obese.” If these ranges vary from organization to organization, they do ever so slightly. The general standard ranges for healthy adults over 20 years of age are as follows:

• Underweight = Below 18.5
• Healthy = 5 to 24.9
• Overweight = 0 to 29.9
• Obese = 0 to 39.9
• Severely Obese = 0 or higher

For children, the weight categories are the same. However, doctors calculate BMI based on age and sex, and represent the figures in terms of percentiles. For instance, a child who is below the sex-specific 5^th percentile is considered underweight. A child who falls within the 5^th and 85^th sex-specific percentile is considered healthy, while a child who is at or above the 95^th sex-specific percentile is obese.

BMI measurements are considered reliable because they give healthcare providers a good idea of how much fat is on the body and whereabouts it may be distributed. However, for a more accurate understanding of how fat is distributed on one’s person, doctors may also use waist-to-hip ratio, waist-to-height ratio, waist circumference and fat distribution as measured by dual-energy X-ray absorptiometry. More doctors are taking these measurements as well as research increasingly shows that fat distribution is a strong indicator of disease risk. For instance, visceral fat — or fat that surrounds the internal organs — is more dangerous than subcutaneous fat (fat that sits beneath the skin), as it puts significant pressure on the organs.

Is Obesity a Problem in South Africa?

Obesity is a growing problem across the globe, with South Africa just as affected by this epidemic as Western countries. According to a recent report, half of all South Africans are overweight (23%) or obese (27%). Studies suggest that these figures will rise by as much as 10% by 2030, with obesity rates soaring to as high as 37% in adults.

Already, approximately 12 million South African adults receive care from public healthcare providers for weight-related illnesses. Those include hypertension, diabetes, cardiovascular disease and some cancers.

What Cancer Types Are Linked to Obesity?

After extensive research and tracking, the International Agency for Research on Cancer Working Group concluded that higher amounts of body fat are strongly associated with a heightened risk for at least 14 types of cancers. Below is a list of each of these cancers, along with the estimated increased risk when compared to people of a healthy weight:

• Endometrial = 2 to 4 times more likely in people who are obese or overweight; 7 times more likely in individuals who are severely obese
• Oesophageal adenocarcinoma = 5 times more likely in people who are overweight; 2.4 to 2.7 times more likely in people with obesity; 4.8 times more likely in people with severe obesity
• Gastric cardia = 2 times more likely in people with obesity
• Liver = 2 times more likely in people who are overweight or obese
• Kidney = 2 times more likely in people who are overweight or obese
• Multiple myeloma = 1 to 1.2 times as likely in people who are overweight or obese
• Meningioma = 2 times as likely in people who are overweight; 1.5 times as likely in people with obesity
• Pancreatic = 5 times more likely in people who are overweight or obese
• Colorectal = 3 times more likely in people with obesity
• Gallbladder = 2 times more likely in people who are overweight; 1.6 times more likely in people with obesity
• Breast (postmenopausal) = 2 to 1.4 times more likely for people who are overweight or obese; 1.2 times more likely for every woman who has a 5-unit increase in BMI above the obese range
  • (Premenopausal) = 0.8 times more likely in women who are overweight or obese
• Ovarian = 1 times more likely for every 5-unit increase in BMI over the normal range
• Thyroid = 26. times more likely in people who are overweight; 1.3 times more likely in people with obesity

Individuals who have survived cancer or had a high BMI at the time of their first cancer diagnosis are more likely to develop a second, unrelated cancer in the future.

What Is the Connection Between Cancer and Obesity?

It is common for individuals to question the connection between cancer and obesity or, more specifically, what about obesity increases one’s risk of cancer. Researcher has identified several links between high body mass indexes and increased cancer rates:

• People with obesity often experience chronic inflammation, which contributes to the development of conditions such as non-alcoholic fatty liver disease and gallstones. Moreover, inflammation causes oxidative stress, which leads to DNA damage and, therefore, increases one’s risk of certain types of cancer.
• Fat tissue triggers the production of an excessive amount of oestrogen, high levels of which have been linked to increased rates of endometrial, breast, ovarian and other cancers.
• Fat cells also produce hormones called adipokines, which can either inhibit or stimulate cell growth. Both outcomes contribute to increased rates of cancer.
• Fat stifles tissue cells, thereby causing them to die earlier than they normally would.
• People with obesity often have increased levels of insulin and insulin-like growth factor-1 (IGF-1) in their blood. High insulin levels, if left unchecked, lead to type 2 diabetes, a well-known risk factor for cancer. High levels of IGF-1 are associated with increased risks of prostate, colon, kidney and endometrial cancers.

These are just a few ways in which obesity and excess body fat contribute to individuals’ cancer risks. Other possible effects obesity can have on cancer risk include hindered tumour immunity and changes in the properties of scaffolding tissue that surround developing tumours.

Not only can obesity have biological effects on people but also, it can interfere with cancer screenings. High levels of body fat may make it difficult for providers to detect tumours in the early stages with typical diagnostic screening measures. By the time a tumour does become detectable, the cancer may have become malignant and spread to nearby tissues.

Does Obesity Affect Cancer Outcomes?

Unfortunately, research does suggest that people with obesity who have also been diagnosed with cancer tend to have worse outcomes than people who are within a normal weight range. Aspects that may be worsened by a high BMI include cancer recurrence, quality of life, cancer progression, cancer prognosis (survival) and risk of developing a second primary cancer.