BOSTON, MASSACHUSETTS. IGF-1 (insulin-like growth factor 1) is responsible for human growth in particular the synthesis of lean muscle mass. Human IGF-1 levels vary with age; they are highest during puberty and decline to about half the value of a young adult by the age of 60 years. About 95 per cent of the IGF-1 circulating in the blood is bound to a large protein complex called IGFBP-3 (IGF-binding protein-3). IGF-1 is normally synthesized in the liver, but external sources such as supplements containing growth hormone enhancers and possibly milk from growth hormone-treated cows can increase blood plasma levels significantly. High blood plasma levels of IGF-1 have been linked to an increased risk of prostate cancer and premenopausal breast cancer. Now researchers at the Harvard Medical School report that a high blood level of IGF-1 combined with a low level of IGFBP-3 is a most potent risk factor for the development of cancer of the colon and rectum (colorectal cancer). The study involved 14,916 participants in the Physicians' Health Study. All participants had blood samples taken in 1982. After 14 years of follow-up, 193 of the men (1.3 per cent) had developed colorectal cancer. The 1982 blood levels of IGF-1 and IGFBP-3 among the cancer patients were compared to the levels among 318 age- and smoking-matched controls. Results showed that men with similar IGFBP-3 levels, but high in IGF-1 levels had a 2.51 higher risk of colorectal cancer than did men with low IGF-1 levels. Men with similar IGF-1 levels and high IGFBP-3 levels had a 72 per cent lower risk of colorectal cancer than did men with low IGFBP-3 levels. Men with high IGF-1 levels and low IGFBP-3 levels were most at risk. The researchers caution against artificially increasing IGF-1 levels in older men to delay the effects of aging as this may significantly increase the risk of cancer. Researchers at the National Institute of Environmental Health Sciences echo this concern and suggest that "attempts to improve quality of life by modulating plasma IGF-1 or IGFBP-3 must be approached with caution."
Ma, Jing, et al. Prospective study of colorectal cancer risk in men and plasma levels of insulin-like growth factor (IGF)-I and IGF-binding protein-3. Journal of the National Cancer Institute, Vol. 91, April 7, 1999, pp. 620-25
Burroughs, Kevin D., et al. Insulin-like growth factor-I: a key regulator of human cancer risk? Journal of the National Cancer Institute, Vol. 91, April 7, 1999, pp. 579- 81 (editorial)