A recent report by the World Health Organisation (WHO) has linked 13 cancers to excess body fat. WHO says people who are obese or have excess fat are at risk of getting breast, colon, stomach, liver and endometrial cancers, among other cancers.
Dr Andrew Odhiambo, a medical oncologist at Nairobi Hospital Cancer Centre, says there have been many studies showing the link between obesity and cancer.
“In fact, we in the cancer community now call it the new smoking. It's almost as bad as smoking or being sedentary and not exercising.”
How is obesity linked to cancer?
Dr Odhiambo explains that the main culprit is visceral fat, the fat stored deep around the abdominal area.
“This visceral fat promotes inflammation throughout the body by producing insulin, insulin growth factors, and many cytokines that promote cell growth,” he says. “Those are the same mechanisms that lead to cancer.”
Dr Victor Oria, a chief scientist at the Integrated Cancer Research Foundation of Kenya (ICRF), adds that as body fat increases, many people develop insulin resistance, forcing the body to produce increasingly higher levels of insulin to keep blood sugar under control. This process increases the risk of pancreatic cancer because the pancreas produces insulin, but its effects extend far beyond one organ.
He explains that fat tissue is biologically active and functions almost like an organ, releasing hormones and inflammatory chemicals that can promote abnormal cell growth throughout the body, potentially leading to cancer.
Under normal circumstances, however, the immune system and a process known as programmed cell death eliminate these cells before they can become dangerous.
“Chronic inflammation can gradually weaken these protective mechanisms, allowing damaged cells to survive and multiply rather than be destroyed. If they accumulate repeatedly, this causes neoplastic growth, also known as tumour formation,” he says.
On whether abdominal fat is biologically different from fat stored elsewhere, Dr Odhiambo points to hormonal balance.
“If you read about adiponectin and leptin, they’re supposed to be balanced. So when the bad part is more dominant, that causes your whole body to become inflamed.”
“You can look thin on the outside, but be metabolically fat on the inside,” he says.
“Some people are heavy but relatively healthy. If doctors could look inside their abdomen, they'd find very little visceral fat.”
“But there are others who look thin yet have a lot of visceral fat. Even if you check their cholesterol, it’s high, and if you check their liver, they have a fatty liver.”
Which cancers show the strongest link?
Dr Odhiambo says the strongest association is seen in cancers that do not directly interfere with eating or digestion.
“If you look at breast, endometrium and ovary cancers, especially those three, you'll see that patients are overweight, heavy or obese,” he says.
“Endometrial cancer is probably one of the strongest examples. I can’t remember the last time I saw a patient of normal weight unless they had lived with the cancer for a long time.”
“For colon cancer, it's mostly people over the age of 50. Among those below 50, obesity is not yet as significant a factor, but it’s emerging as a reason.”
Kidney, pancreatic and breast cancers [which come after age 45] also show strong associations with obesity.
“In our own hospital, we study patients with breast cancer. For example, almost a quarter to a third of them are overweight or obese when they are diagnosed. In my own practice, most of my patients, even if I look at all cancers together, are overweight or obese,” he says.
However, Dr Odhiambo notes that there is still no full understanding of why obesity is strongly linked to some cancers but not others.
“If you look at breast, colon, endometrium, kidney, myeloma and even some brain tumours, the common thread is high inflammatory activity and a disturbed immune system,” he says, echoing Dr Oria’s explanation that the immune system plays a crucial role in preventing cancer from developing.
Obesity also complicates cancer treatment
Dr Odhiambo says that, beyond increasing the risk of developing cancer, obesity makes treatment considerably more challenging.
“When a person is overweight, it’s harder to treat. They require higher doses of chemotherapy. Their wounds [after surgery] don’t heal as fast. They have more infections. Cholesterol is a big issue,” he says.
Since chemotherapy doses are calculated using body size, heavier patients often require larger doses, thereby increasing the risk of toxicity. He says that surgery can also become more difficult.
“Those who are obese and require abdominal surgery or radiotherapy have much more difficulty healing. There are far more surgical complications among overweight and obese patients than among those of normal weight,” says Dr Odhiambo.
“I have a patient who is morbidly obese. She has a large stomach and a colostomy bag [a small plastic bag worn to collect stool]. Managing it is problematic.”
“With cancer treatment, you’re much better off if you’re lean. It’s not that we can’t treat you if you’re overweight, but the risk of complications is much higher.”
Over the years, more Kenyans in both urban and rural areas have accumulated excess weight, particularly around the abdomen. In Kenya, breast cancer remains the most commonly diagnosed cancer, followed by cervical, prostate, oesophagus, colon and rectal cancers.
For decades, obesity has been strongly associated with heart disease, high blood pressure, diabetes and stroke, but Dr Odhiambo says the cancer risk mirrors a broader global shift.
“If you look at photographs of tourists on a Florida beach in the 1960s or 1970s, everyone is lean, and cancers were rare back then. Now, if you take the same picture today, three-quarters of them are overweight or obese.”
He attributes this increase to two converging forces: increasingly unhealthy diets and declining physical activity.
“We eat processed food such as crackers, popcorn, crisps and chips. Nowadays, people just order online. Previously, you would go to the supermarket, walk around, pick things up, pay, and come home, but now you just use an app and everything is delivered.”
He says that the problem is no longer confined to cities, pointing out that obesity is also increasing in rural Kenya.
“People are getting sick earlier with conditions like high cholesterol, diabetes and cancer,” he says.
However, his greatest concern is childhood obesity.
“There is nowhere for children to play, and the estates are congested. There are no free parks or safe spaces to run around and burn energy,” he says.
“If we millennials already have this problem, you can imagine what our children are facing.”
For Dr Odhiambo, the biggest barrier to tackling obesity is cultural, not medical.
“People think being fat is a sign of wealth, a sign that you are doing well,” he says.
“Many people never realise that obesity can actually cause cancer. If you lose weight or maintain a normal body mass index (BMI), your risk decreases," says Dr Odhiambo, adding that exercise is now recognised as part of cancer treatment because patients who remain physically active and maintain a healthy weight after diagnosis generally have better treatment outcomes and lower recurrence rates.
