Energy balance and cancer: the role of insulin and insulin-like growth factor-I
2001
Recent theories propose that a Western lifestyle may increase cancer risk through alterations in the metabolism of insulin and insulin-like growth factors (IGF; McKeown-Eyssen, 1994; Giovannucci, 1995; Kaaks, 1996; Werner & LeRoith, 1996). Insulin regulates energy metabolism, and increases the bioactivity of IGF-I, by enhancing its synthesis, and by decreasing several of its binding proteins (IGFBP; IGFBP-1 and -2). Insulin and IGF-I both stimulate anabolic processes as a function of available energy and elementary substrates (e.g. amino acids). The anabolic signals by insulin or IGF-I can promote tumour development by inhibiting apoptosis, and by stimulating cell proliferation. Furthermore, both insulin and IGF-I stimulate the synthesis of sex steroids, and inhibit the synthesis of sex hormone-binding globulin (SHBG), a binding protein that regulates the bioavailability of circulating sex steroids to tissues. The present paper reviews epidemiological findings relating the risk of cancers of the colorectum, pancreas, breast, endometrium and prostate to body size (obesity, height) and physical activity, and discusses the relationships between obesity and physical activity and plasma levels of insulin, IGF-I and IGFBP. Subsequent sections review epidemiological findings relating cancer risk to indices of chronic hyperinsulinaemia, and to plasma levels of IGF-I and IGFBP. Conclusions are that chronic hyperinsulinaemia may be a cause of cancers of the colon, pancreas and endometrium, and also possibly of the breast. On the other hand, elevated plasma IGF-I, as total concentrations or relative to levels of IGFBP-3, appears to be related to an increased risk of prostate cancer, breast cancer in young women, and possibly colorectal cancer. For cancers of the endometrium, breast and prostate, these findings are discussed in the context of relationships between insulin and IGF-I and levels of bioavailable sex steroids.
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