Evaluation by the International Agency for Research on Cancer

An International Agency for Research on Cancer (IARC) Working Group on the Evaluation of Cancer-Preventive Strategies published a comprehensive evaluation of the available literature on weight and cancer that considered epidemiological, clinical, and experimental data (18). Their 2002 report concluded that there is "sufficient evidence" in humans for a cancer-preventive effect of avoidance of weight gain for cancers of the endometrium, female breast (postmenopausal), colon, kidney (renal cell), and esophagus (adenocarcinoma) (18). Regarding premenopausal breast cancer, the report concluded that available evidence on the avoidance of weight gain "suggests lack of a cancer-preventive effect." For all other sites, IARC characterized the evidence for a cancer-preventive effect of avoidance of weight as "inadequate" in humans.

The conclusions regarding the evidence in humans are based on epidemiological studies of overweight and/or obese individuals compared with leaner individuals, not on studies of individuals who have lost weight. Unfortunately, few individuals lose and maintain significant amounts of weight, making it extremely difficult to examine cancer outcomes in large populations of weight losers. Consequently, the IARC report concluded that there is "inadequate evidence" in humans for a cancer-preventive effect of intentional weight loss for any cancer site. Of note, though, three very recent studies of the impact of weight loss on breast cancer (25,26) and endometrial cancer (27) suggest that weight loss over the course of adult life may substantially reduce the risk for these cancers.

The IARC also concluded that in experimental animals, there is "sufficient or limited evidence" for a cancer-preventive effect of avoidance of weight gain by calorie restriction, based on studies of spontaneous and chemically induced cancers of the mammary gland, liver, pituitary gland (adenoma), and pancreas, for chemically induced cancers of the colon, skin (nonmelanoma), and prostate, and for spontaneous and genetically induced lymphoma. An association between overweight and obesity and cancer at many sites is consistent with animal studies showing that caloric restriction dramatically decreases spontaneous and carcinogen-induced tumor incidence, multiplicity, size, and growth (28-32). Possible mechanisms for these observations include altered carcinogen metabolism, decreased oxidative DNA damage, greater DNA repair capacity (18), and a reduction of IGF-I levels in calorie-restricted animals (30).

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