Exploring the Link Between Obesity and Cancer
By Kari Bohlke, ScD
“For the great majority of Americans who do not use tobacco, weight control, dietary choices, and levels of physical activity are the most important modifiable determinants of cancer risk.”
—From the 2006 American Cancer Society Guidelines on Nutrition and Physical Activity for Cancer Prevention1
The issue of body weight is a loaded one. For many of us, it consumes far too much of our energy and attention. In recent years the media have given a lot of attention to the psychological effects that weight and body image issues can have, particularly on women. But despite the unhealthy effect that too much emphasis on an ideal body can have, some attention to body weight—and the associated health issues—is warranted. An unhealthy weight, whether it’s too little weight or too much, can in fact have dramatic effects on health. In addition to increasing the risk of chronic conditions such as diabetes and heart disease, there is evidence that obesity increases the risk of developing certain types of cancer and can decrease survival with cancer.
The link between obesity and chronic disease is clearly a concern, but it also offers hope. Body weight is modifiable, and weight management may provide an important approach to reducing our cancer risk and improving our overall health as we age. We can’t change our family history, but we can take control of what we eat and how active we are.
“I think that an important message,” says Dr. Rachel Ballard-Barbash of the National Cancer Institute, “is that it’s very clear that being physically active, maintaining a healthy weight, and eating a healthy diet—a diet that allows one not to gain weight, that’s high in fruit and vegetables, low in saturated fat, and consistent with dietary guidelines—may be helpful for people from the standpoint of a number of chronic diseases, cancer being one of them.”
What Does It Mean to Be “Overweight”?
Body weight is often classified using the body mass index (BMI; see sidebar). A BMI between 18.5 and 24.9 is generally considered healthy, a BMI between 25 and 29.9 is considered overweight, and a BMI of 30 or higher is considered obese.2
Most of us are aware that the prevalence of overweight and obesity has increased dramatically in the United States in recent years. And the statistics exist to back up what we’re seeing for ourselves: since the late 1970s, the prevalence of obesity among adults has doubled, rising from 15 percent to 31 percent.3 An estimated 65 percent of U.S. adults are overweight or obese. Why this recent increase in our national waist size? The answer is likely to involve a combination of factors, including changes in diet, reductions in physical activity, and other behavioral changes.
A wide range of health problems has been linked with excess body weight. These problems include high blood pressure, abnormal blood lipid levels (such as high cholesterol), Type II diabetes, heart disease, stroke, gallbladder disease, osteoarthritis, sleep apnea and respiratory problems, and certain types of cancer.4 And the severity of obesity itself as a health concern should not be underestimated: obesity at midlife has been linked with a two- to threefold increased risk of death.5
Obesity and Cancer Risk
Although most people are aware that being overweight carries health risks, the link between obesity and cancer is sometimes overlooked. When a recent study asked women with colorectal or breast cancer to identify risk factors for their type of cancer, common responses included smoking, genetic factors or family history, environmental pollutants, and stress. Less than half of the women believed that their type of cancer could be caused by obesity.6
The reality may be quite different from what many people believe. Although there is little evidence linking environmental pollutants or stress to breast or colorectal cancer, the evidence linking obesity to these cancers is strong and growing. And the risk of other cancers—particularly endometrial (uterine) cancer—appears to be linked with body size as well.
Most studies of body size and colon cancer tell us that risk is increased among those with a high body mass index or large waist measurement. In a comprehensive European study, for example, women with the largest waist sizes were almost 50 percent more likely to develop colon cancer than women with the smallest waist sizes.7 Though this study did not find a link between BMI and colon cancer risk in women, a BMI of 27 or greater was linked with an increased risk of colon cancer in a large U.S. study.8 Most studies have not found a link between body size and risk of rectal cancer.
Obesity has been linked with a decreased risk of breast cancer in premenopausal women, possibly as a result of disrupted menstrual cycles and altered hormone levels. In postmenopausal women, however, obesity has consistently been linked with an increased risk of breast cancer. The link between obesity and postmenopausal breast cancer appears to be strongest among women who have never used postmenopausal hormone therapy1,9 and may be explained by the higher estrogen levels in obese postmenopausal women.
A study of more than 87,000 postmenopausal women showed a strong link between adult weight gain and breast cancer risk.10 Among women who had never used postmenopausal hormones, those who had gained 55 pounds or more during adulthood were almost twice as likely to develop breast cancer as those who had maintained a stable weight. Fortunately, the results of this study also suggest that it may never be too late to reduce your risk of breast cancer. Women who had never used postmenopausal hormones and who had lost 22 pounds or more after menopause—and kept it off—had roughly half the breast cancer risk of women who neither gained nor lost weight after menopause.
Though less common than cancer of the colon or breast, cancer of the uterine lining appears to have the strongest link with obesity. Studies have reported that obese women are between three and five times more likely to develop endometrial cancer than women with a BMI of less than 23.2,12 Some researchers have estimated that roughly 40 percent of all endometrial cancers may be related to obesity.13
According to the results of a large study conducted by the American Cancer Society, obesity increases the risk of death from several other cancers as well. Women with the highest BMIs were more likely than women with a healthy BMI to die of cancers of the gallbladder, pancreas, kidney, cervix, and ovary as well as from non-Hodgkin’s lymphoma. The researchers estimated that 90,000 cancer deaths per year could be prevented if Americans maintained a healthy weight.14
Obesity and Survival with Cancer
Not only does this growing body of research tell us that obesity can increase your risk of developing several types of cancer, there is also evidence that it can decrease survival rates. Obesity has been linked with a later stage at diagnosis for some cancers and may increase the risk of cancer recurrence or progression.
Among patients with colon cancer, studies have reported worse survival among those with a high BMI2,15 or a large waist circumference.16 In a study of more than 600 female, postmenopausal colorectal cancer patients, obese women were roughly twice as likely as normal-weight women to die of colon cancer.17 This study also reported an increased risk of death among underweight women.
Obesity has been linked with a higher risk of cancer recurrence and worse survival among both pre- and postmenopausal breast cancer patients.18,19 In a recent study of young breast cancer patients, obese women had a roughly 50 percent increased risk of death during the eight to 10 years of follow-up.20 And in addition to high body weight at the time of breast cancer diagnosis, there is some evidence that weight gain after diagnosis increases the risk of death in nonsmoking women.21
Taking Steps at the Individual Level
To maximize your health, the best approach to weight management is to maintain a healthy body weight throughout your adult life. But if you are already overweight, the modest goal of avoiding additional weight gain is likely to improve your health compared with the common path of continued weight gain. And a modest weight loss—say, 5 to 10 percent of your current weight—is likely to bring additional health benefits.22 Though the effects of weight loss on cancer risk and cancer survival are still uncertain, weight loss has been shown to improve blood pressure, blood lipid levels, and glucose tolerance.
Talk with your doctor about the approach to—and the goals of—weight management that are right for you. Approaches and goals may vary depending on your age, health history, current weight, and previous exercise experience.
Effective approaches to weight management generally include a combination of physical activity and a healthy diet. Exercise guidelines recommend at least 30 minutes per day of moderate-intensity physical activity at least five days per week, or at least 20 minutes per day of vigorous physical activity at least three days per week.23 Moderate-intensity activity includes brisk walking and cycling on level terrain. Vigorous activity includes cycling or walking up hills and jogging.
Eating a healthy diet involves watching the total number of calories you consume as well as making calories count by eating foods rich in important nutrients. Reducing your total caloric intake by 50 to 100 calories per day may prevent gradual weight gain; a reduction of 500 calories per day is a common approach to weight loss.21 To reduce the number of calories in your diet while getting the nutrients you need, try eating smaller portions and reducing your intake of added sugars, saturated and trans fats, and alcohol. Replace these foods with fruits, vegetables, and whole grains.
For certain subsets of patients, prescription weight-loss medications or surgery may be appropriate if other approaches to weight loss have failed.24
Taking Steps at the National Level
Issues related to weight management, physical activity, and diet are not only in the minds of women—they are in the forefront of the national scientific agenda. “There’s research trying to understand how these factors are related to cancer incidence and survival,” says Dr. Ballard-Barbash. By better understanding the mechanisms that tie weight, physical activity, and diet to cancer, it may be possible to design interventions that will have the greatest health benefits. Among cancer survivors, a particularly active area of research is exploring the link between physical activity and quality of life.
Researchers are also exploring the factors that influence our ability to comply with health recommendations. One emerging area of research is to characterize people who are successful at maintaining a healthy lifestyle. “What factors are characteristic of people who are physically active, who avoid gaining weight as they get older, and who are eating a healthy diet?” asks Dr. Ballard-Barbash, noting that the answer may provide clues to helping the rest of the population. Similarly, it’s important to identify those people who have the greatest struggles with weight, physical activity, and diet. “Those may be segments of the population on which we particularly need to focus our intervention in terms of trying to improve their health.”
Living Longer and Living Better
“The good news,” says Dr. Ballard-Barbash, “is that as our ability to diagnose cancer is improving and our ability to treat cancer is improving, people are living a very long time with cancer. Increasingly, cancer survivors are wanting to know what they can do beyond treatment to improve their health. This is probably one important area where they can, at a personal level, improve their overall health.”
Understanding the Body Mass Index
The body mass index (BMI) is commonly used to classify body weight. It involves a comparison of weight to height (weight in kilograms divided by height in meters squared).
The following table (converted for American weights and measures) shows the BMI for a range of heights and weights and is applicable to both men and women.
|Height (inches)||Body weight
Adapted from The Practical Guide: Identification, Evaluation, and Treatment of Overweight and Obesity in Adults25
For example, a woman who is 64 inches (5 feet 4 inches) tall and 128 pounds would have a BMI of 22. This is considered healthy. On the other hand, if this same woman weighed 174 pounds, she would have a BMI of 30, which is considered obese.
It’s important to note that BMI is not a perfect measure of body fat. It is, however, a useful screening tool and may help your physician determine whether additional evaluation of your weight or your health is necessary.
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