Lifestyle
and Obesity as Cancer Risks
By Nancy Cotugna, DrPH, RD
Today’s Dietitian
Vol. 7 No. 9 P. 14
Anyone can develop cancer. While overall cancer
incidence and mortality rates have declined slightly over the
past decade, the disease remains the second-leading cause of
death in the U.S. population at large, and it has surpassed
heart disease as the leading cause of death in Americans younger
than the age of 85.
Cancer is a group of more than 100 different
diseases characterized by uncontrolled growth and spread of
abnormal cells. The causes of various cancers are still unknown,
but consensus accepts both internal and external causation,
and research suggests that approximately 90% of cancer incidence
is due to lifestyle and environmental factors.1 The American
Cancer Society (ACS) has reported that roughly one third of
the 564,830 cancer deaths expected in 2006 will be related to
nutrition, physical inactivity, and overweight and obesity—and
therefore could have been prevented.2
Although everyone is at risk of developing cancer,
the risk increases with age and most cancers are diagnosed in
adults aged 55 and older. The risk or probability of developing
or dying from cancer is estimated at a little less than one
in two for U.S. males and a little more than one in three for
females. Nearly 1.4 million new cancer cases are expected to
be diagnosed in 2006, with the five-year survival rate from
all cancers being approximately 65%.2
The economic costs are high. The National Institutes
of Health has estimated that the annual cost for cancer is roughly
$210 billion, including direct medical costs and indirect costs
due to lost productivity.3 Clearly, prevention is preferable
to treatment—and nutritional risk factors are among those
that are changeable.
Protection and Promotion
Many nutritional factors have been associated with cancer risk.
Some have been identified as protective against the disease
(such as fruits and vegetables), some are thought to have a
promoting effect (alcohol), and others remain under investigation
(such as selenium).
Some have changed roles. For example, the early
hypothesis that dietary fat affected many cancers, including
breast and colon, has not been supported by more current research
showing little or no relationship between fat consumption and
these cancers.4
Diet-related factors such as obesity and physical
inactivity have also been associated with increased cancer risk.
According to the Institute of Medicine’s report “Fulfilling
the Potential of Cancer Prevention and Early Detection,”
obesity is more strongly associated with cancer incidence and
mortality than any other lifestyle factor except smoking.5 The
report also indicated that if current obesity trends continue,
the adverse effects will be seen on breast and colon cancers
by 2015.
Walter Willett, MD, MPH, DrPH, chairman of Harvard
University’s nutrition department, has stated that besides
avoiding tobacco, staying lean and active provides the greatest
potential for minimizing cancer risk.6 The ACS agrees that for
nonsmoking Americans, increasing physical activity and maintaining
a healthy weight are the most important approaches to reducing
the risk of developing cancer.7
An Obesity Epidemic
Obesity is a worldwide problem. The World Health Organization
(WHO) uses the term globesity to describe the global epidemic,
estimating that there will be 300 million people who are obese
by 2025.8,9 Overweight and obesity have reached epidemic proportions
in the United States, with 33.5% of adults overweight (body
mass index [BMI] between 25 and 29.9) and 30.5% obese (BMI of
30 or above).10 More disturbing is that the number of overweight
children has tripled over the last two decades, making obesity
one of the biggest public health challenges of our time.11 Among
children aged 6 to 19, 31% are at risk for or are already overweight.12
Since overweight in adolescence tends to continue throughout
life, increased incidence of future cancers seems predictable
if this trend continues.
The problem of obesity cuts across all ages,
educational levels, ethnicities, and genders—and few seem
aware of it. A survey by the American Institute for Cancer Research
(AICR) indicated that Americans are deeply concerned about both
obesity and cancer, but fewer than one half are aware of a link
between the two. When asked specifically whether they believe
obesity causes cancer, 44% of the respondents say yes.13
Risk for Specific
Cancers
There is firm evidence that overweight and obesity are related
to increased risks of six of the most prevalent forms of cancer:
colon, endometrial, kidney, esophageal, prostate, and postmenopausal
breast cancers. Associations have also been shown with gallbladder
and pancreas cancers.
A report issued by the WHO International Agency
for Research on Cancer estimated that being overweight and inactive
accounts for one quarter to one third of worldwide cases of
breast, colon, endometrial, kidney, and esophageal cancers.14
The AICR has noted that 10% of all U.S. cancer
deaths among nonsmokers are caused by overweight and obesity.
A large study reported in 2004 that excess body mass accounted
for 7.7% of all cancers in Canada, providing ongoing evidence
that obesity increases the risk of overall cancer.15 A 2005
study by Danaei and colleagues showed that in high-income countries,
overweight and obesity were among the most important causes
of cancer affecting the colon, uterus, gallbladder, kidney,
and postmenopausal breast sites.16
In the largest U.S. study to date, researchers
followed more than 900,000 men and women for 16 years and found
that as BMI increased, so did the risk of death from most types
of cancer.17 They estimated that being overweight or obese could
account for 14% of all cancer deaths in U.S. men and 20% in
U.S. women. This means that 90,000 cancer deaths annually could
be prevented if Americans maintain a healthy body weight.
While the above studies examined overall cancer
risks, there are also many studies on how obesity affects cancer
at specific sites. In women with a BMI of 34 or greater, the
risk of developing endometrial cancer is increased by more than
six times.18 Studies have consistently shown a positive association
between obesity and colorectal cancer in men and a twofold increased
risk among premenopausal women.19 Data from a Harvard growth
study showed a ninefold increase in cancer mortality for men
who were obese during adolescence.20 Possible reasons for the
increased risk of colon cancer are higher levels of circulating
insulin in obese individuals. This may lead to higher levels
of insulinlike growth factor, which has been linked to colon
cancer.
There is increasing evidence that women who
are overweight and obese, especially those who gain weight throughout
adulthood, are at an increased risk for developing breast cancer
after menopause.21,22 Data reported in 2006 from the Nurses’
Health Study suggested that weight gain of 55 pounds or more
during adult life increased the risk of postmenopausal breast
cancer by 45% and women who had gained 22 pounds or more since
menopause had an 18% increased risk.23
The good news from this study was that weight
loss after menopause was associated with a decreased risk of
breast cancer. Obese postmenopausal women may be more susceptible
to breast cancer because excess fat cells continue to produce
estrogen. The cumulative exposure to estrogens and progestins
seems to determine the lifetime risk of breast cancer. Weight
loss reduces levels of bioavailable estrogens that promote cancer
growth and inhibit cancer cell death, or apoptosis. Women who
are obese are also at high risk for developing metabolic syndrome,
which includes insulin resistance and hyperinsulinemia. Elevated
insulin levels are associated with increased cancer risk because
insulin stimulates cell proliferation and also interferes with
apoptosis.24
In two large prospective cohort studies (the
Health Professionals Follow-Up Study and the Nurses’ Health
Study), obesity was found to significantly increase the risk
of pancreatic cancer while physical activity appeared to decrease
the risk, especially in those who were overweight.25 Researchers
speculate that the increased risk may be due to the influence
of obesity on insulin resistance.
Chow and colleagues examined the health records
of a cohort of more than 363,000 Swedish men who had been followed
from at least one physical examination until the time of their
death.26 After adjusting for age, smoking status, BMI, and blood
pressure, the men with the highest levels of obesity had nearly
double the risk for renal-cell cancer.
Some ways that obesity is thought to influence
cancer risk are by raising levels of steroid hormones such as
estrogen and peptide hormones such as insulin and insulin-related
growth factors. Losing weight can improve insulin sensitivity
and decrease the level of sex hormones in the blood, so it is
reasonable to assume that losing weight will decrease cancer
risk. In addition to these mechanisms, obesity can cause acid
reflux, which is associated with esophageal cancer, and excess
weight is associated with gallstones, which can increase gallbladder
cancer risk.16
Obesity is an important modifiable, although
challenging, risk factor in the development of many cancers.
The increasing trend toward obesity also represents a serious
health concern for many other chronic diseases and is a problem
that must be addressed.
Physical Inactivity
Physical activity can be defined as any bodily movement that
increases energy expenditure and can include job-related activity,
household chores, and leisure time activity. Exercise, on the
other hand, is considered a subset of physical activity that
is planned, structured, and repetitive with the purpose of improving
physical fitness.5 Lifetime exposure, time, intensity, and amount
of exercise can also vary in methods of measurement. It is useful
to understand these distinctions and recognize that the way
physical activity is defined and measured varies widely and
thus has made it difficult to compare studies and summarize
the overall effect of this factor on cancer risk. Still, with
all this variation, experts have concluded that physical activity
is associated with decreased risks of some cancers.
Although people of any age can benefit from
physical activity, more than 60% of U.S. adults do not get any
regular physical activity, and 25% are not active at all. Among
U.S. youths aged 12 to 21, it is estimated that approximately
one half are not vigorously active on a regular basis.27 By
the end of the high school years, rates for youths are comparable
to those of adults. These figures have changed little over the
past decade.7
In addition to providing a multitude of other
health benefits, physical activity may reduce the risk of several
types of cancer, including cancers of the breast, colon, pancreas,
and possibly other sites such as the kidney, endometrial, and
prostate.28-31
The mechanisms by which physical activity reduces
cancer risk are not fully understood. Besides helping maintain
a healthy body weight, physical activity may accelerate the
movement of food through the intestine (thus limiting the time
bowel lining is exposed to cancer-causing agents), decrease
exposure of breast tissue to circulating estrogen, and reduce
circulating levels of insulin and related growth factors. The
beneficial effect of physical activity on breast cancer risk
is thought to be independent of its effect on obesity.
Research has suggested that physical activity
may also play an indirect role in certain cancers because it
helps prevent type 2 diabetes, which is associated with increased
risk of colon and pancreas cancers.32,33
The benefits of physical activity in preventing
cancer accumulate over the course of a lifetime.34 Though exercise
needs to be promoted early in life, it can provide important
benefits at any age.
Proactive Prevention
A WHO report cites prevention (defined as eliminating or minimizing
exposure to the causes of cancer) as one of the four main approaches
to cancer control, others being early detection, diagnosis and
treatment, and palliative care. The report also states that
prevention offers the greatest public health potential and most
cost-effective, long-term method of cancer control.35 Cancer
prevention focuses on risks associated with the disease and
protective factors. A healthy diet, physical activity, and avoiding
obesity are among the cancer prevention activities WHO emphasizes.
WHO specifically recommends the following:
• maintaining a BMI of 18.5 to 25;
• avoiding weight gain in adulthood; and
• engaging in regular physical activity.
Appropriate nutrition education in schools and
public health education campaigns for adults are also suggested.
Food preferences and physical activity habits are set early
in life and Uauy and Solomons recommend that cancer prevention
efforts should begin in childhood and continue through life.36
The 2002 ACS guidelines on nutrition and physical
activity for cancer prevention place an increased emphasis on
the role of physical activity and weight control in reducing
cancer risk.37 These guidelines, which are due for review this
year, were developed by a national panel of cancer experts and
are based on the most current scientific evidence in the field.
The new guidelines are expected to be published in the September/October
CA: A Cancer Journal for Clinicians and should also be available
at the ACS Web site (www.cancer.org) once they have been published.
The new guidelines should not be significantly different, but
there may be more specific recommendations for obesity and activity.
As shown in Table
1, the guidelines include eating a variety of foods with
an emphasis on plant sources, engaging in at least moderate
activity for 30 minutes or more for a minimum of five days per
week, maintaining a healthy weight throughout life, and limiting
alcohol consumption to no more than two drinks per day for men
and one drink per day for women. Table
2 provides specific suggestions for increasing physical
activity.
Because individual behavioral choices occur
within a community environment that can either support or block
healthy behaviors, the ACS has for the first time included a
recommendation for community action to facilitate its four guidelines
for individuals. The organization should be commended for recognizing
how critical environmental-level support is to making healthy
behavior choices. An environment that promotes sedentary lifestyles
and overconsumption of high-calorie foods is certainly one of
the contributing factors to obesity in our society.
In 2004, the ACS, in collaboration with the
American Heart Association and the American Diabetes Association,
launched a prevention-focused initiative: Everyday Choices for
a Healthier Life. It addresses risk factors for all three diseases
and includes information to promote good nutrition, weight control,
and increased physical activity. Information is available at
www.everydaychoices.org.
Noted obesity expert James Hill, PhD, has helped
create an initiative called America on the Move (www.americaonthemove.org)
to promote greater physical activity through environmental changes.
This program stresses two behavioral changes: walking an extra
2,000 steps daily and reducing calorie intake by 100 kilocalories
per day to prevent weight gain.
If obesity and physical activity continue to
escalate, a rise in cancer cases seems likely. At a press conference
at the AICR/World Cancer Research Fund International Research
Conference on Food, Nutrition, and Cancer, W.P.T. James, MD,
said, “We are used to thinking about the obesity epidemic
on one hand and the cancer epidemic on the other. We need to
think of them as linked.”
As individuals, Americans need an increased
awareness that their excess pounds and physical inactivity increase
the risk of cancer. From a public health perspective, the government,
food industry, media, and communities need to work together
to modify the environment so it is less conducive to weight
gain and more favorable to physical activity.34 Education, public
health policy, and environmental support are all key in cancer
prevention and obesity control. As educators, advocates, and
community leaders, dietitians can assist their clients and the
public at large in reducing cancer risk.
— Nancy Cotugna, DrPH, RD, is professor
of nutrition and dietetics at the University of Delaware. She
completed a postdoctoral fellowship at the National Cancer Institute.
References
1. Cancer Control. Objectives for the nation: 1985-2000. Division
of Prevention and Control. National Cancer Institute. NCI Monograph.
1986;2:1-93.
2. American Cancer Society: Cancer Facts &
Figures 2006. Atlanta: American Cancer Society; 2006.
3. National Institutes of Health. National Cancer
Institute. Costs
of Cancer. Accessed January 20, 2006.
4. Willet WC. Diet and cancer: An evolving picture.
JAMA. 2005;293(2):233-234.
5. Curry SJ, Byers T, Hewitt M, Eds. Fulfilling
the Potential for Cancer Prevention and Early Detection. National
Cancer Policy Board. Institute of Medicine. National Research
Council of the National Academies. Washington, D.C.: National
Academies Press, 2003.
6. Willett WC. Harvesting the fruits of research:
New guidelines on nutrition and physical activity. CA Cancer
J Clin. 2002;52(2):66-67.
7. American Cancer Society. Cancer Prevention
& Early Detection Facts & Figures 2006. Atlanta: American
Cancer Society, 2006.
8. Obesity: Preventing and managing the global
epidemic. Report of a WHO consultation. World Health Organ Tech
Rep Ser. 2000;894:i-xii,1-253.
9. Life in the 21st century: A vision for all.
The world health report. Geneva, Switzerland: World Health Organization,
1998.
10. Flegal KM, Carroll MD, Ogden CL, et al.
Prevalence and trends in obesity among U.S. adults, 1999-2000.
JAMA. 2002;288(14):1723-1727.
11. Daniels SR, Arnett DK, Eckel RH, et al.
Overweight in children and adolescents: Pathophysiology, consequences,
prevention, and treatment. Circulation. 2005;111(15):1999-2012.
12. Hedley AA, Ogden CL, Johnson CL, et al.
Prevalence of overweight and obesity among U.S. children, adolescents,
and adults, 1999-2002. JAMA. 2004;291(23):2847-2850.
13. American Institute for Cancer Research.
“Facts vs. Fears
Survey 2005.” Accessed July 31, 2006.
14. International Agency for Research on Cancer.
Handbooks of Cancer Prevention, volume 6: Weight Control and
Physical Activity. Washington, D.C.: IARC Press North America,
2002.
15. Pan SY, Johnson KC, Ugnat AM, et al. Association
of obesity and cancer risk in Canada. Am J Epidemiol. 2004;159(3):259-268.
16. Danaei G, Vander Hoorn S, Lopez AD, et al.
Causes of cancer in the world: Comparative risk assessment of
nine behavioural and environmental risk factors. Lancet. 2005;366(9499):1784-1793.
17. Calle EE, Rodriguez C, Walker-Thurmond K,
et al. Overweight, obesity and mortality from cancer in a prospectively
studied cohort of U.S. adults. N Engl J Med. 2003;348(17):1625-1638.
18. Weiderpass E, Persson I, Adami HO, et al.
Body size in different periods of life, diabetes mellitus, hypertension,
and risk of postmenopausal endometrial cancer. Cancer Causes
Control. 2000;11(2):185-192.
19. Terry PD, Miller AB, Rohan TE. Obesity and
colorectal cancer risk in women. Gut. 2002;51(2):191-194.
20. Dietz WH. Childhood weight affects adult
morbidity and mortality. J Nutr. 1998;128(2 Suppl):411S-414S.
21. Kuhl H. Breast cancer risk in the WHI study:
The problem of obesity. Maturitas. 2005;51(1):83-97.
22. Key TJ, Appleby PN, Reeves GK, et al. Body
mass index, serum sex hormones, and breast cancer risk in postmenopausal
women. J Natl Cancer Inst. 2003;95(16):1218-1226.
23. Eliassen AH, Colditz GA, Rosner B, et al.
Adult weight change and risk of postmenopausal breast cancer.
JAMA. 2006;296(2):193-201.
24. Bianchini F, Kaaks R, Vainio H. Overweight,
obesity, and cancer risk. Lancet Oncol. 2002;3(9):565-574.
25. Michaud DS, Giovannucci E, Willett WC, et
al. Physical activity, obesity, height and the risk of pancreatic
cancer. JAMA. 2001;286(8):921-929.
26. Chow WH, Gridley G, Fraumeni JF Jr, et al.
Obesity, hypertension, and the risk of kidney cancer in men.
N Engl J Med. 2000;343(18):1305-1311.
27. U.S. Department of Health and Human Services.
Physical Activity and health: A Report of the Surgeon General.
Atlanta, Ga.: U.S. Department of Health and Human Services,
Centers for Disease Control and Prevention, National Center
for Chronic Disease Prevention and Health Promotion, 1996.
28. Bernstein L, Patel AV, Ursin G, et al. Lifetime
recreational exercise activity and breast cancer risk among
black women and white women. J Natl Cancer Inst. 2005;97(22):1671-1679.
29. Friedenreich CM, McGregor SE, Courneya KS,
et al. Case-control study of lifetime total physical activity
and prostate cancer risk. Am J Epidemiol. 2004:159(8):740-749.
30. Schouten LJ, Goldbohm RA, van den Brandt
PA. Anthropometry, physical activity, and endometrial cancer
risk: Results from the Netherlands Cohort Study. J Natl Cancer
Inst. 2004;96(21):1635-1638.
31. McTiernan A, Ulrich C, Slate S, et al. Physical
activity and cancer etiology: Associations and mechanisms. Cancer
Causes Control. 1998;9:487-509.
32. Calle EE, Murphy TK, Rodriquez C, et al.
Diabetes mellitus and pancreatic cancer mortality in a prospective
cohort of United States adults. Cancer Causes Control. 1998;9(5):403-410.
33. Will JC, Galuska DA, Vinicor F, et al. Colorectal
cancer: Another complication of diabetes mellitus? Am J Epidemiol.
1998;147(9):816-825.
34. Freidenreich CM. Physical activity and cancer
prevention: From observational to intervention research. Cancer
Epidemiol Biomarkers Prev. 2001;10(4):287-301.
35. World Health Organization. Approaches to
Cancer Control. In: National Cancer Control Programmes: Policies
and Managerial Guidelines, 2nd Ed. Geneva: World Health Organization,
2002.
36. Uauy R, Solomons N. Diet, nutrition and
the life-course approach to cancer prevention. J Nutr. 2005;135(12
Suppl):2934S-2945S.
37. Byers T, Nestle M, McTiernan A, et al. American
Cancer Society guidelines on nutrition and physical activity
for cancer prevention: Reducing the risk of cancer with healthy
food choices and physical activity. CA Cancer J Clin. 2002;52(2):92-119.
Examination
1. The proportion of cancer deaths attributable to nutrition,
physical inactivity, overweight, and obesity is:
a. one quarter.
b. one third.
c. one half.
d. two thirds.
e. three quarters.
2. Physical activity may decrease the risk of
colon cancer by:
a. altering intestinal villi.
b. increasing bowel motility.
c. lowering colon pH level.
d. decreasing bile acid production.
e. increasing lean muscle tissue.
3. Women who are obese are at increased risk
for breast cancer:
a. before menopause.
b. after menopause.
c. throughout the life cycle.
d. regardless of menstrual status.
e. only during pregnancy.
4. The mechanism by which physical activity
may reduce breast cancer is:
a. reducing insulin levels.
b. reducing growth hormone factors.
c. reducing oxidation.
d. reducing exposure to estrogen.
e. increasing aerobic fitness.
5. The probability of developing or dying from
cancer is:
a. greater for men than women.
b. greater for women than men.
c. equal in both sexes.
6. The minimum amount of activity recommended
by the American Cancer Society is:
a. 30 minutes daily.
b. 30 minutes three times weekly.
c. 30 minutes five times weekly.
d. 45 minutes daily.
e. based on weight and body mass index.
7. Children and adolescents should engage in
the following amount of activity per day:
a. 15 minutes
b. 30 minutes
c. 45 minutes
d. 60 minutes
e. 90 minutes
8. What action regarding meat consumption is
suggested to reduce cancer risk?
a. Reduce red meat consumption
b. Omit meat from the diet
c. Limit fish due to potential mercury contamination
d. Reduce consumption of all meats
e. Eliminate processed and cured meats
9. Which of the following dietary factors is
no longer considered to have a major impact on breast and colon
cancers?
a. Dietary fat
b. Fruits and vegetables
c. Alcohol
d. Excess calories
e. All are still considered to have a major impact
10. What is the number of cancer deaths that
could be prevented annually by maintaining a healthy body weight?
a. 25,000
b. 35,000
c. 50,000
d. 64,000
e. 90,000
