Link between Obesity and Cancer

Obesity and cancer are more closely linked than is generally believed. Although a direct link has not been proven, a growing number of clinical studies worldwide show that excess body fat is associated with an increased risk of developing certain types of cancer. With these findings, the WHO (World Health Organization) has designated obesity as the second most preventable cause of cancer (after smoking).

Excess Weight: A Significant Cancer Risk Factor

Research coordinated by the IARC, the International Agency for Research on Cancer, and the University of Bristol involved over 100,000 adults of both sexes. 58,221 individuals had colorectal cancer, and 67,694 were control subjects (without the disease). Data were analyzed using Mendelian randomization, which uses genetic variants to investigate cause-and-effect relationships between modifiable risk factors (like weight) and disease (in this case, colorectal cancer) and, compared to more traditional investigative methods, presents a reduced risk of bias or error. The study was funded by the WCRF, the World Cancer Research Fund, Cancer Research UK, and Diabetes UK, among others. This study highlights the importance of focusing more on overweight and obesity in cancer prevention strategies worldwide. The mechanisms by which body fat causes colorectal cancer remain to be understood. However, it's crucial for people to understand how extra pounds and poor diet contribute to increased cancer risk.

Obesity and Cancer: The Numbers

Cancer seems to affect overweight or obese individuals the most. The number of obese people worldwide has doubled since 1980, according to WHO data: in 2014, over 1.9 billion adults were overweight, of whom 600 million were obese.

Mechanisms Linking Obesity and Cancer

The mechanisms that might explain how obesity can increase the risk of certain types of cancer are diverse. Some of them may be linked to inflammation and hormonal changes.

Overweight and obese individuals are more likely than normal-weight individuals to develop conditions or disorders related to – or causing – chronic local inflammation and constitute risk factors for some types of cancer.

Here are some examples to better explain these points:

• Chronic local inflammation induced by gastroesophageal reflux is a likely cause of esophageal adenocarcinoma;
• Obesity is a risk factor for gallstones, a condition characterized by chronic inflammation of the gallbladder, and a history of gallstones is a major risk factor for gallbladder cancer;
• Adipose tissue (also called fat tissue) also produces excessive amounts of estrogens. High levels of these hormones have been associated with an increased risk of developing tumors, such as breast, endometrial, and ovarian cancer;

Finally, obese individuals often have high blood levels of insulin and insulin-like growth factor, or IGF-1. This state, known as hyperinsulinemia or insulin resistance, precedes the development of type 2 diabetes and may promote the development of colon, kidney, prostate, and endometrial cancers.

Obesity and Cancer: The Dangers

Many scientific studies show that obese patients have a higher risk of developing more aggressive and difficult-to-treat forms of cancer, with a greater probability of complications during treatment and recurrences over time, compared to normal-weight individuals. Furthermore, drug treatments can be problematic in terms of dosage and may be locally weaker or higher than necessary due to uneven distribution of body fat.

According to the National Cancer Institute, greater amounts of body fat are associated with an increased risk of a number of cancers, including:

• Endometrial cancer;
• Esophageal adenocarcinoma;
• Gastric cardia cancer;
• Liver cancer;
• Kidney cancer;
• Meningioma;
• Pancreatic cancer;
• Gallbladder cancer;
• Colorectal cancer;

This non-exhaustive list of links between cancer and obesity highlights the importance of not underestimating the problem and, in case of significant overweight, consulting a doctor to lose weight.

Is Weight Loss Good for Health?

Limited weight gain in adulthood is thought to reduce the risk of developing colon, kidney, and, for postmenopausal women, breast, endometrial, and ovarian cancers. Weight loss or avoidance of excess weight appears to decrease cancer risk, although further studies on this subject are needed.

Increasing evidence points to a relationship between weight loss and decreased cancer risk in patients who have undergone bariatric surgery. Obese individuals appear to have a lower risk of developing obesity-related cancers after bariatric surgery than those who haven't pursued this weight-loss avenue.

What to Do and What to Avoid?

There is no one-size-fits-all strategy for those who want to avoid accumulating dangerous excess weight. However, science offers some general guidelines that everyone can follow to approach the goal of a normal weight. For example, the risk of obesity and overweight has been shown to be reduced if one adopts a lifestyle that includes physical activity, a high-fiber diet, including whole grains and vegetables, and, more generally, following the Mediterranean diet. The risk increases if you are more sedentary and spend more time in front of television and computer screens, and if you also have a diet rich in sugary drinks and fatty foods.