Dr. Fung: Why Obesity and Diabetes Increase Cancer Risk

Dr. Fung: Why Obesity and Diabetes Increase Cancer Risk

Dr. Jason Fung is widely known as an advocate for the technique of intermittent fasting in patients hoping to reverse or prevent diabetes and obesity. He has written several books on dieting and metabolic health, including The Diabetes Code.

Speaking at the online Metabolix 2021 conference, Dr. Fung outlined the surprisingly strong connections between diet and cancer. It’s a relationship that’s been long known, but until recently was very poorly understood, and therefore not much talked about.


Cancer and Environmental Factors

“For many years we thought of cancer predominantly through the genetic paradigm. We thought about mutations in genes that lead to cancer… with less emphasis on the environment.” But the truth is that “genetics contribute the minority of causation towards cancer. Almost all of cancer is a result of the environment.”

That environmental factors can contribute to cancer shouldn’t be surprising, of course. The association between tobacco and lung cancer is enough to make this principle abundantly clear. Experts have also known, for decades, that diet and lifestyle factors contribute to other types of cancer. By studying genetically homogeneous populations that have undergone major lifestyle shifts—such as by comparing migrants to family members that stayed home, or the development of cancer in twins—researchers can tease out which cancers are caused mostly by genetic factors, and which mostly by environmental.

“It’s not all cancers, only certain cancers” that are highly determined by environmental factors. Experts have determined that the factors causing stomach cancer, to take just one example, are something like 28% heritable, and 72% environmental. And many cancers seem especially related to diet, notably breast cancer, colorectal cancer, and certain cancers of the vital organs, such as the liver, pancreas and stomach.


A Range of Theories

Unfortunately, it took researchers decades to get close to an understanding of how diet actually increases cancer risks. Dr. Fung walked his audience through a short history of popular hypotheses that more or less had to be cast aside after further scrutiny:

  • Lack of dietary fiber. However, studies showed no real difference between low- and high-fiber diets.
  • Vitamin deficiency. This was an encouraging theory, given how easily vitamin deficiencies can be solved. Unfortunately, several experiments have found that study participants receiving a placebo develop less cancer than those that receive supplements of vitamins such as beta carotene and vitamins E and B12 (in various combinations). Other vitamins showed no effect.
  • Too much dietary fat. “This was popular in the 90s, when fat was blamed for everything,” Dr. Fung says. This connection was the subject of several major studies, but low-fat diets generally proved to have no benefit for cancer.
  • Too much red or processed meat. This connection has been accepted enough for the WHO to officially label red and processed meats as a carcinogen. Dr. Fung says that the blame for red meat is “tenuous,” arguing that diet-related cancer continues to grow in the United States, despite a long and consistent decline in red meat consumption. Processed meat may be a real but minor contributor to some cancers.

“This is the conclusion of 50 years of dietary research and hundreds of millions of dollars… all these hypotheses that we had—they didn’t pan out…”


Cancer, Obesity and Insulin

Experts didn’t know what we were eating that increased cancer rates, but we did know at least one thing: obesity is highly correlated with cancer. As body mass index rises, so does the risk of cancer; for some diet-related cancers, such as liver cancer, the rise can be very steep. Unfortunately, this didn’t help much, because the mainstream medical establishment has notably struggled to explain why obesity occurs in the first place.

“It’s really only in the last 10 years that we started to shed some light on what part of the diet increases your risk of cancer.”

Dr. Fung has an explanation: “Obesity is probably the dominant factor in our diet that needs to be treated…but it might be not just about the obesity, but maybe more specifically about hyperinsulinemia.”

Obesity is almost invariably accompanied by hyperinsulinemia, an abnormal amount of insulin in the blood—in fact, there is some question about which condition causes the other. The chemical link between hyperinsulinemia and obesity has been known for many decades, but has only recently been thrust into mainstream discussion by a new generation of doctors. Could it be too much insulin that’s causing cancer?

There is some good data to support the contention. Type 2 diabetes—a disease of hyperinsulinemia—also shows very strong correlations to cancer risk. And high insulin levels are associated with increased cancer risk and cancer severity even in adults of healthy weight. A study of colon cancer sufferers showed that those consuming a high-insulin diet (“a lot of sugar, refined carbohydrates”) had twice the risk of death as those consuming a lower insulin diet.

According to Dr. Fung, the mechanism by which this may work is easy to understand. Insulin is a powerful growth factor, and cancer is, in essence, a disease of uncontrolled growth. “Cancer cells are six to ten times more receptive to insulin.” The hyperinsulinemic body is an especially fertile environment for cancer.

There is some amazing evidence on the other side of the coin, too. There is very rare genetic condition known as Laron syndrome, in which the body does not respond to the growth hormone function of insulin and insulin-like growth factors. While the syndrome entails major disabilities, it also seems to confer immunity to both cancer and diabetes.


The Prescription

So, how can we reduce our risk of lifestyle-related cancers?

“Clearly stopping smoking is the most important thing you can do. But right behind it, and really just as important, is diet. 30% of total cancer deaths are attributable to diet. And these two factors dwarf everything else—occupational factors and family history are [closer to] 5%.”

“We’re doing a good job. Most cancers are decreasing in prevalence. The only ones that are increasing are these obesity-related cancers.”

Dr. Fung made his point simply, and for readers that are already familiar with his work, his conclusion won’t be surprising:

“This is where a low-carbohydrate diet may help, and where intermittent fasting might help.”

A diet heavy in carbohydrates—especially highly refined carbohydrates—prompts the body to overproduce insulin, initiating the cycle of insulin resistance that leads to obesity, Type 2 diabetes, and perhaps to a much higher risk of cancer.

“Presumably, if you’re going to lower the risk of Type 2 diabetes and obesity, you’re going to lower your risk of cancer overall.”

Want to learn more about the diet-cancer connection? Pre-order Ravenous, the extraordinary story of the Nazi-era scientific genius who discovered how cancer cells eat—and what it means for how we should.

Ross Wollen
Ross Wollen

Ross Wollen is a chef and writer based in Maine's Midcoast region. Before moving East, Ross was a veteran of the Bay Area restaurant and artisanal food scenes; he has also worked as a food safety consultant. As executive chef of Belcampo Meat Co., Ross helped launch the bone broth craze. Since his diagnosis with Type 1 diabetes in 2017, he has focused on exploring the potential of naturally low-carb cooking. Follow Ross on Twitter: @RossWollen

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