The authors analyzed the relation between lactation history, organochlorine serum
levels-in particular, 2,2-bis(p-chlorophenyl)-1,1,1-trichloroethane (DDT) and 1,1-dichloro-2,2-bis(p-chlorophenyl)ethylene
(DDE)-and the risk of breast cancer within a subsampe from a larger breast cancer
case-control study conducted among women living in Mexico City, Mexico, between 1990
and 1995. From the original study, they selected a random sample of 260 subjects (1:1
case/control ratio). Analysis was restricted to 120 cases and 126 controls who had
given birth to at least one child and had complete information on all key variables.
Serum DDE levels were higher among cases (mean = 3.84 microg/g lipids, standard deviation
= 5.98) than among controls (mean = 2.51 microg/g lipids, standard deviation = 1.97).
After adjustment for age, age at menarche, duration of lactation, Quetelet index,
and serum DDT levels, serum DDE levels were positively related to the risk of breast
cancer (adjusted odds ratio (OR)Q1-Q2 = 1.24, 95% confidence interval (CI): 0.50,
3.06; ORQ1-Q3 = 2.31, 95% CI: 0.92, 5.86; ORQ1-Q4 = 3.81, 95% CI: 1.14, 12.80; test
of trend, p = 0.02). The increased risk associated with higher serum DDE levels was
more apparent among postmenopausal women (ORQ1-Q4 = 5.26, 95% CI: 0.80, 34.30; test
of trend p = 0.03). A longer period of lactation was associated with a slightly decreased
risk of breast cancer independently of serum DDE levels (OR = 0.91, 95% CI: 0.85,
0.99 change in risk per 10 months of lactation). Serum DDT level was not related to
the risk of breast cancer. The data suggest that high levels of exposure to DDE may
increase women's risk of breast cancer, particularly among postmenopausal women.