Brandel France de Bravo, MPH, Noy Birger, and Sandy Wang
There are many reasons why being overweight is bad for our health, but most people don’t realize that cancer is one of them. Of course, excess body weight can contribute to serious medical conditions such as heart disease and type II diabetes, but new research shows that the habits that lead to excess body fat might also increase the risk of developing certain types of cancer.
Dr. Andrew Renehan and his colleagues estimated that more than 481,000 of newly diagnosed cancer cases worldwide in 2012 were due to overweight or obesity.1 They estimated that North America had 111,000 cancer cases that were caused by being overweight or obese; this was calculated to be 23% of total global cancer cases (the highest of any region). Three cancers accounted for 73% of all obesity-related cancers among women globally: endometrial cancer, postmenopausal breast cancer, and colon cancer. For men, two cancers accounted for 66% of all obesity-related cancers: kidney and colon cancer. Other cancers that have been associated with overweight and obesity include prostate cancer, several gastrointestinal cancers, and non-Hodgkin’s lymphoma.
The good news is that a large 2014 study showed that with a healthy diet and regular exercise, post-menopausal women were able to significantly reduce their cancer risk.2 The researchers defined a healthy diet as one that limits red meat and processed meat, emphasizes whole grains over refined grains, and includes 2 and a half cups of vegetables and fruits every day; regular exercise meant at least 150 minutes of moderate intensity physical activity or 75 minutes of vigorous intensity physical activity every week. Those with the healthiest diets and most active lifestyles, saw their risk of breast cancer drop by 22%, their risk of colon cancer drop by 52%, and their overall risk of dying during the 12 years of the study drop by 27%! While all the women who ate better and exercised more lowered their chances of developing cancer; non-white women seemed to benefit the most.
How does obesity increase the risk of developing cancer?
What does excess body weight have to do with cancer, and why is being overweight even riskier for women? Researchers think that the danger of excess weight is partly due to the hormones secreted by fat tissues. One of those hormones is estrogen, which is known to increase the risk of breast cancer 3 and endometrial cancer (also called cancer of the uterus) in women.4 Fat cells also trigger a type of chronic immune response in the body because it recognizes that the extra fat is not supposed to be there, and so the immune system responds, causing inflammation in the body.5 Long-term inflammation can prevent the immune system from working properly, making the body less able to fight off disease. In addition, chronic inflammation may cause insulin resistance,6 which along with high blood sugar levels,7 has been proposed as another explanation for why obesity increases the risk of cancer. The higher the insulin level of a breast cancer patient,8 the greater the chance of death: insulin is believed to play a role in promoting tumor growth and in inhibiting cancer treatment.9
Having fat in certain places can also increase the risk for cancer. One study followed 3,086 men and women for up to seven years and used CT scans and physical exams to assess the fat deposits that accumulated in the abdomen region, liver, and tissues around the heart. Over the next seven years, the men and women in the study had 90 heart-related incidents, 141 cases of cancer, and 71 deaths (from various causes). After statistically controlling for the effect of age, exercise habits, BMI and self-reported eating habits, the researchers concluded that those with more abdominal fat were more likely to develop heart disease and cancer.10
It is not clear at what stage in life excess weight is most likely to increase the risk of cancer, pointed out by Dr. Marian L. Neuhouser, a nutritional epidemiologist at the Fred Hutchison Cancer Research Center in Seattle, and colleagues in a June 2015 study of obesity and breast cancer in 67,000 postmenopausal women. For example, during adolescence and pregnancy, breast cells undergo rapid division and differentiation. It is possible that excessive weight during those periods of this cell growth may be especially likely to increase the risk of breast cancer. Another susceptible time point may be the menopause, when breast tissue is also undergoing changes. 11
How new is this news?
Researchers have been documenting the link between obesity and cancer for many years. Even before this new study, the National Cancer Institute estimated that 20 to 30% of the most common cancers in the United States “may be related to being overweight and/or lack of physical activity” and might also “increase the risk of death from many cancers, accounting for up to 14% of cancer deaths in men and 20% of cancer deaths in women.”12 Every new study fills in another piece of the puzzle, helping to explain which cancers “feed” off fat, and how it is that fat fuels tumor growth. In fact, Renehan’s study is based on an earlier meta-analysis done by him and many of the same co-authors, in which they looked at over two hundred comparable datasets gathered from different countries around the world.13
The meta-analysis found that excess weight in men was most strongly associated with cancer of the esophagus (esophageal adenocarcinoma), the thyroid, colon cancer, and renal cancer (cancer of the kidneys). Being overweight did not appear to increase a man’s risk of prostate cancer (although a study in the U.S. has found that an overweight man with prostate cancer is more likely to die of it than a man with prostate cancer who is not overweight14). According to the meta-analysis, excess weight in women is most strongly associated with endometrial cancer, cancer of the gallbladder and the esophagus, and kidney cancer. A few other cancers were also associated with being overweight for both men and women, such as leukemia, multiple myeloma, and non-Hodgkin’s lymphoma, but the link was weaker. In men, rectal cancer and malignant melanoma also seemed to be related to weight, and women with higher BMI were slightly more likely to be diagnosed with post-menopausal breast cancer, cancers of the pancreas and thyroid, and colon cancer.
Renehans’s conclusions are quite consistent with the findings of other recent studies not included in the meta analysis.15 Similarly, the American Institute for Cancer Research (AICR) estimated that excess body fat is responsible for 49% of endometrial cancers; 35% of esophageal cancers; 28% of pancreatic cancers; 24% of kidney cancers; 21% of gallbladder cancers; 17% of breast cancers; and 9% of colon cancers.16 AICR estimates that will result in over 100,000 new cases of cancer each year due to excess body fat.17
Renehan did not find any significant link between obesity and ovarian cancer, but a more recent U.S. study did.18 Neuhouser’s study, conducted at 40 U.S. clinical centers, of women ages 50 to 79 followed for about 13 years, showed that women who gained more than 5% of their baseline weight during the study’s follow-up period had a modest increased risk of breast cancer. The risk was greatest for women with a body mass index over 35 — they were 60% more likely to develop breast cancer than women of normal weight.
A 2016 study found that the link between obesity and cancer is stronger in some countries than others. Of the seven countries they studied, they found a pattern of increasing BMI in the countries of the Middle East compared to Asian countries. The high average BMI in the Middle East is associated with higher percentages of people with cancer. Likewise, North America and Europe has a large proportion of obesity-related cancers, while countries in sub-Saharan Africa and Asia had a smaller proportion of obesity-related cancer. 19
Does losing weight reduce your risk of cancer?
Obesity is caused by too many calories in proportion to physical activity but where you get your calories (the kinds of foods you eat) and where you store your fat may also affect your cancer risk. Several studies show that high dietary fat intake increases the risk of post-menopausal breast cancer,20 prostate cancer,21 and pancreatic cancer.22
Other studies have indicated that high fat diets increase the risk of death from cancer and that low fat diets reduce the chances of cancer recurrence.23 A diet high in fat is, of course, also likely to cause obesity, so it is very difficult to determine whether weight or diet is a stronger predictor of increased cancer risk. In other words, is it the fat you eat or the fat on your body that puts you at the greatest risk of cancer? And not all body fat is equal: abdominal fat, for instance, is associated with colon cancer.24 Can losing weight prevent you from getting cancer? The evidence is clear for some cancers but not for others. Losing weight may reduce the risk of prostate cancer and definitely reduces the risk of reflux, an important risk factor for esophageal cancer.25
A 2009 study showed there were fewer cancer diagnoses in a group of formerly obese patients who had undergone bariatric surgery to reduce the size of their stomachs and lose weight compared to obese patients who hadn’t had this surgery. 26
Several studies have found that intentional weight loss can reduce the risk of breast cancer, particularly when the weight loss is after menopause.27,28,29 However, Neuhouser’s study found that already overweight or obese women who lost weight during the study did not lower their risk of breast cancer. 30
Studies since Rehehan indicate that cancer risk drops after bariatric surgery—provided patients keep the weight off 31—because of reduced hormone levels and inflammation, which are linked to cancer 32,33
What we know and don’t know
All of these studies provide preliminary evidence that the cancer risk from being overweight or obese may be reversible and may not only be related to the types of food eaten or exercise habits, but also to the excess fat tissue. In some of these studies, however, the people participating changed several habits at once, such as diet and exercise, so it is not always possible to determine if the reduction in cancer risk was related primarily to the physical weight loss, or to the change in diet or exercise, or both.
Excess body weight doesn’t only increase the risk of developing cancer and cause tumors to grow more quickly,34,35 it can also make detecting and treating cancer more difficult. If a person is very overweight, mammograms may be less accurate, tumors can be hard to feel, and veins for intravenous medication are not as easy to find. More importantly, doctors have difficulty determining the correct dose of medicine, and as a result, obese patients may not receive sufficient chemotherapy. Given these complications and the likelihood of other chronic health problems such as type 2 diabetes and heart disease, it is not surprising that cancer death rates are usually higher for people who are very overweight.
Overall, the research indicates that when weight goes up, so does the risk of many cancers. Similarly, when weight goes down, or is at least maintained throughout adulthood,24the risk of certain cancers also goes down. These are studies of large numbers of people–looking at patterns and associations — so the findings do not mean that every obese person will get cancer or that slender people are protected from it. The exact relationship between cancer and excess body weight has not been clearly established. We still do not understand how an individual person’s diet, level of physical activity, and genes can all be factored together to calculate his or her cancer risk. What we do know is that there are many health benefits to achieving and maintaining a healthy body weight, including lowering the risk of cancer and improving cancer treatment outcomes.
After giving up tobacco, watching your weight and staying active are your best forms of health insurance. For guidelines and tips on living a healthy lifestyle, read Eating Habits to Improve Health and Lower Body Mass Index.
All articles on our website have been approved by Dr. Diana Zuckerman and other senior staff.