Obesity and Cancer – Cancer 1

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We’ve talked extensively over the last few years about obesity, metabolic syndrome, fasting, and type 2 diabetes. These are extremely important in the development of cardiovascular disease (heart attacks and strokes). This addresses the number 1 killer of Americans, but we have not yet touched upon the very close #2 killer of Americans – the big C – Cancer. 

You can see that heart disease, stroke and cancer really dwarf the other causes of death. It is important to realize that many of the other major causes of death also have a metabolic base. This includes diabetes, Alzheimer’s disease, liver disease, and kidney disease. This means that fully 6 of the top 10 causes of death are metabolic in nature. The other major causes are infectious, smoking related (lung disease) and suicide/ accidents.

The links between obesity and cancer are quite preliminary because the obesity epidemic really only get started in 1977 or so. Prior to that, obesity was stable in prevalence so there was nothing to compare to. The other problem is that there is a prevailing view of cancer as a genetic disease that permeates current scientific thinking. For various reasons which we will get into later, Cancer, as a whole, is almost certainly NOT a genetic disease.

First of all, what do I mean by ‘Cancer’? Cancer is not a single disease. There are multiple types of cancer, all of which are different. For example, there are the common cancers, such as breast, colorectal, prostate, skin, pancreatic, liver etc. There are blood cancers such as the various leukemias and lymphomas. They are all different, but they also share some common characteristics. It is these common characteristics that I want to discuss.

It is like discussing professional sports. You can say that football, soccer, hockey, fencing and baseball are all different but they can all be considered together in general. All involve some sort of competition and physical skill despite all the differences. Similarly, cancer shares many commonalities. One of the most cited article in oncology (the study of cancer) is the classic Weinberg paper detailing 8 common characteristics. We will cover that in detail later, too.

The links between cancer and obesity was really solidified in 2003 with a large scale epidemiological study published in the NEJM. This was a huge prospective cohort study called the Cancer Prevention Study II. This means that participants were identified and enrolled as healthy people and then followed to see what happened to them. It started in 1982 and require 77,000 volunteers simply to enroll all the participants who numbered over 1 million. In 1984, 1986 and 1988, volunteers would personally call these million participants to see who died and why. That is truly mindboggling. After 1988, national databases made collecting this data much easier. The variable of interest was death from cancer.

Overall, the increased risk of cancer does not appear with modest degrees of increased body fat. There is no increase in risk in the ‘overweight’ group (BMI 25-30). From there it gradually rises and the relative risk for all cancers is 1.52 for those in the ‘morbidly obese’ (BMI >40) group. In plain English, that means that morbid obesity is associated with a disastrous 52% increased risk of dying from cancer compared to if you are merely overweight or normal weight. Certain cancers are worse than others, including liver cancer where your risk is increased by 452%!

The news is almost certainly worse than that, though. Lung cancer, shows an inverse relationship. The relative risk is 0.67 meaning that obese people have 33% LESS lung cancer. But that is highly likely due to the well known effect of weight loss and cigarette smoking. Since lung cancer is among the biggest cancer killers, this means that the 52% increased risk is almost certainly an underestimate. If you remove all the smokers from the cohort, then you start to see a positive association with weight and cancer even in the ‘overweight’ category. In non smoking women the relative risk went up to 1.88 in the BMI>40 or an 88% increased risk of cancer.

Similarly, due to the well known phenomenon of cancer cachexia (the tendency for advanced cancer patients to lose their appetite and weight), there is a tendency towards weight loss, which will similarly obscure the true link between obesity and cancer. This effect will, once again, lead to an underestimation of the risk.

From this data, researchers calculate something called the population attributable fraction (PAF), based upon how strong the association and the prevalence of obesity. This gives an estimate of how much obesity contributes to cancer in general. In men, the estimate is 4.2-14.2% and women it is 14.3-19.8%. In other words, about 15% of cancer overall can be directly attributed to obesity. Keep in mind that this is an estimate that only includes up to 1998 data. Since obesity has continued to surge upwards in the last 19 years, this fraction is certainly too low at present obesity rates. For some cancers, the risk is even higher. For endometrial cancer, for example, the PAF is an estimated 56.8%.

So, which cancers are the most linked to obesity? Breast cancer was one of the first cancers to be linked. Epidemiologic studies since the 1970s have consistently found this link, both with cancer occurrence and prognosis. In postmenopausal women, rates of breast cancer increase by 30-50% with increase body weight. Some studies have found that central adiposity is an additional risk factor but others have not. Morbidly obese women have breast cancer death rates 3 times higher than those that are very lean. The reasons for this link is not entirely known, but one strong hypothesis is that adipose tissue may increase estrogen effects. Endometrial cancer may show a similar link for the same reason.

However, other cancers are also strongly linked despite estrogen playing little or no role in cancer development. For example, adenocarcinoma (a type of cancer type) of the esophagus also shows a massive 52.4% PAF in the US, for reasons that are not yet clear. Kidney cancer is also massively increased in the obese. Pancreatic cancer, colorectal cancer, liver and gallbladder cancer all show lower degrees of association, but still significant nonetheless.

Certain cancers are not related to obesity at all. Lung cancer, shows little association with obesity, which makes sense since smoking plays a dominant role. Cervical cancer similarly shows no association. Again this makes sense since Human Papilloma Virus is thought to play the starring role. But ovarian cancer, and prostate cancer also show no evidence that obesity has any influence.

The bottom line is that obesity is an important factor in cancer in general, although not all cancers. Clearly, cancer is a multi-factorial disease, meaning that many different factors determine its course. This is similar to cardiovascular disease, where it is well known that there is no single cause of disease. Smoking, genetics, sex, menopausal state, inflammation, diet, exercise, stress, obesity, diabetes, high blood pressure, metabolic syndrome all play a role in its development. This does not minimize the importance of any of these factors, but simply means we must accept that multiple pathways are important. In heart disease, this is well established dogma.

However, in cancer, there has been a pervasive consensus that it is caused by a single problem – mutations, and that everything that causes cancer does so through genetic mutations. This is certainly true for some things such as ionizing radiation causing cancer. However, this so-called somatic mutation theory (SMT) is almost certainly wrong, for reasons that we will go into later in detail. The strong association with the obesity is also a great example. This is rarely discussed given that obesity accounts for 20-30% of common cancers (PAF). Becoming obese does not cause genetic mutations. Fat cells are not mutagenic. But it certainly does open the door to considering the hormonal/ metabolic side of cancer. Because if metabolic disease plays a key role in certain cancers, then prevention of those disease will depend upon reversing those metabolic defects. Again, a new hope arises.


2017-10-12T17:24:58+00:00 15 Comments

About the Author:

Dr. Fung is a Toronto based kidney specialist, having graduated from the University of Toronto and finishing his medical specialty at the University of California, Los Angeles in 2001. He is the author of the bestsellers ‘The Obesity Code’ and ‘The Complete Guide to Fasting’. He has pioneered the use of therapeutic fasting for weight loss and type 2 diabetes reversal in his IDM clinic.

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15 Comments on "Obesity and Cancer – Cancer 1"

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Dr. Fung is my number 1 resource for anything fasting. Everything in his book (the complete guide to fasting) was an eye opener for me. I don’t have diabetes and only have had a small (stage 1a) case of breast cancer a couple of years ago. I have been a life long runner, weight lifter and generally active person. I am 53 years old, 5′ and started out fasting at a weight of 135. I am down to 120 now and would like to lose another 5. I will never stop fasting and have adopted a keto WOE. I feel… Read more »

Or possibly those of us with a BMI>40 are not diagnosed until a more advanced stage because primary care physicians are more than happy to put any and all symptoms down to our being fat. Just a thought.

Roger Bird

Makes sense to me.

Roger Bird

I actually ran into people who hated Dr. Fung with a passion. I was in shock. Really. How could anyone not love Dr. Fung. He is brilliant, adorable, funny, witty, a freaking genius with a heart and a sense of humor. I guess some people are just freaked out about change.

And, oh, I loved this article. Thank you again, Dr. Fung.

Stephen T
Whatever the accepted treatment, a whole range of vested interests build up behind it. If a good diet and fasting can deal with diabetes (and much else), pharma needs to oppose that at every opportunity. Also, the medical world is hierarchical and those at the top will do nearly anything to avoid admitting that they’ve been getting it wrong for their whole career. It’s a sad indictment of human nature that most people continue to do harm rather than think again. Dr Fung is effectively saying that current treatments are wrong and hurting patients. This is professionally embarrassing; and likely… Read more »

Dr. Jason Fung is doing the same thing for obesity and diabetes as Dr. John E. Sarno has done for back pain and other body pain.


I agree wholeheartedly! And I am a heart patient benefiting from the good doctor’s advice!

Jerome Kahn
Hopefully I am on the same trail as Dr. Fung In 1983 after reading Cancer Biology, I embraced the 6-7 mutation theory of cancer until about 2006, when I read some articles then and in 1999 that were favoring the role of stem cells. I later modified the cause to favor the role of pluripotent cells. However about 3 years ago I came across the Warburg Hypothesis (https://en.wikipedia.org/wiki/Warburg_hypothesis), concerning defective metabolism, which opened the door to staring cancer with fasting and ketogenic diet. Cancer metabolize mainly glucose anaerobically (and some glutamine). Last year I came across a seminal article outline… Read more »
sten bjorsell
I just finished the book “tripping over the truth” that provides a decent overview of all cancer research. I agree that the few simple experiments Thomas Seyfried pointed out are satisfactory to prove that the problem of cancer is in the mitochondria, not in a mutated cell nucleus: Moving cancerous cell nucleus to healthy cells disabled the nuclear mutations and cancer was stopped! Yet, moving defect mitochondria from cancer cells to healthy cells did not stop it, it propagated cancer! Healthy cells were becoming cancerous when exposed to a defect mitochondria. Origin solved! That repeated such tests by different researchers… Read more »
Craig E

Hi Sten I have read Seyfried’s cancer as a metabolic disease and think it’s a compelling case that defective mitochondria are causative of cancer. I read an article back in March about researchers in Sydney trying to prevent glutamine from entering cancer cells. This along with restricting dietary glucose sources might help starve cancer cells of their fuel sources.

A quick correction on your post I think you meant to say lactate not lactose in your response.

Ron Hunter

I’m very much looking forward to this series. As a lifelong stuggler with obesity and type 2 diabetes, I as fortunate to catch a small 1 mm colon cancer through routine colonoscopy at 50.

alejandro heredia



[…] is a strong relationship between cancer and obesity as discussed in our last post. Since I’ve spent several years arguing why hyperinsulinemia is the root cause of obesity and […]


So far, so relative risk.


This is so great Dr. Fung, I found your blog and you know how can I escape this common issue of a person? Well I read about your blog and I love those ways you mention. Obesity problem is so ridiculous. You’ll be inspired by this and try to check out this blog and feel free to ask Nurse Audrey who happens to have great recommendations on http://www.justbecauseicare.org/