world cancer report - iarc

OVERWEIGHT, OBESITY AND
PHYSICAL ACTIVITY
The body mass is most usefully measured
as the body mass index (BMI), calculated
by dividing the body weight in kilograms
by the height in metres squared. The normal
range is 18.5 to 25; overweight corresponds
to BMI > 25 and obesity to a value
greater than 30. In many developed countries,
as much as half of the adult population
may be overweight and more than
25% obese. Epidemiological studies have
shown with varying degrees of consistency
that excess body mass is associated
with an increased cancer risk.
The strongest and most consistent association
with body mass has so far been
seen for endometrial cancer, the risk of
which is increased two- to six-fold in
obese compared to lean women, both
before and after menopause. A possible
biological explanation for this association
is that adipose tissue is rich in aromatase,
which converts androstenedione to
estrone, thus increasing estrogenic stimulation
of the endometrial mucosa.
Several studies have investigated markers
of fat distribution such as waist-to-hip
ratio or subscapular-to-tight-skinfold ratio
Caloric intake and other dietary-related
factors
The results of animal experiments in
which dietary restriction decreases the
risk of cancer at some sites are not readily
extrapolated to humans. While caloric
intake can be employed as a single parameter
of diet, caloric intake considered in
isolation is an inadequate basis upon
which to address a broad spectrum of
studies concerning cancer risk. These
studies indicate inter-relationships
between caloric intake, body mass and
physical activity. Thus it is argued that
high energy intake per se is not a risk factor
for cancer, but positive energy balance
(energy balance being the difference
66 The causes of cancer
in relation to endometrial cancer risk, with
inconsistent results. Some studies found
increased risk for markers of abdominal or
android obesity (high waist-to-hip ratio or
subscapular-to-tight-skinfold ratio) after
adjustment for body mass index, while others
did not.
The relationship between body mass index
and breast cancer is even more complex.
The majority of case-control and prospective
studies found that high body mass
index increased breast cancer risk in postmenopausal
women, while it may slightly
reduce risk in premenopausal women. A
possible explanation for this apparent paradox
is that overweight before menopause
could be related to anovulatory cycles and
fewer ovulatory cycles (as determined by
pregnancy and lactation) are generally
associated with lower breast cancer risk.
After menopause, obesity may act as for
endometrial cancer by enhancing the
peripheral (as opposed to gonadal and
adrenal) production of estrogens.
There is growing evidence that metabolic
factors related to diet, nutritional status,
anthropometry and physical activity have
an influence on the development and clinical
manifestation of various forms of can-
between caloric intake and caloric expenditure)
leading to obesity is a cancer risk
factor [17]. Data have accumulated suggesting
that some metabolic factors related
to nutritional status, such as obesity
and physical activity, may also play a role
by increasing the risk of certain cancers
(Box: Overweight, obesity and physical
activity, above).
Recently, several prospective studies have
lent strong support to the hypothesis formulated
decades ago regarding the prominent
role of endogenous hormone levels in
determining risk of cancer of the breast. It
is also proposed that the insulin-resistance
syndrome may underlie the relationship
between obesity and hormone-
Fig. 2.59 Regular physical exercise appears to be
correlated with decreased risk of cancer.
cer (Weight Control and Physical Activity,
IARC Handbooks of Cancer Prevention,
Vol. 6, 2001). Epidemiological studies
suggest certain different dietary patterns
may be specifically related to higher risk
of particular types of cancer. The Western
diet and lifestyle are generally associated
with high incidence of cancers of the colorectum,
breast, prostate and endometrium,
but with low incidence of cancers of
the stomach, oesophagus, liver and cervix
uteri (see Reproductive factors and hormones,
p76).
dependent cancers. Variations in the pattern
of estrogens, androgens, insulin-like
growth factor and their binding proteins
are probably determined by both environmental
and lifestyle factors, as well as by
inherited genetic characteristics, as suggested
by recent studies on polymorphisms
of genes encoding for enzymes
regulating steroid hormone metabolism
and hormone receptors (Reproductive factors
and hormones, p76)
Accordingly, the relationship between diet
and cancer is proving to be more complex
than was previously thought. Research
based on a combination of laboratory
investigations on human subjects and
sound epidemiological projects of a