Many of the connections between diet and cancer are not yet understood. One link may be insulin-like growth factor, or IGF-I, a peptide that circulates in the blood, influencing growth and other biological functions. While a certain amount of IGF-I is normally found in the blood, at higher levels it appears to stimulate the growth of cancer cells.

Studies in diverse populations have shown that the more IGF-I there is in a man's blood, the higher his prostate cancer risk.¹ The Physicians Health Study, using serum collected from 152 men with prostate cancer and an equal number of controls, found that serum IGF-I concentrations were strongly linked to prostate cancer risk.² Increased serum IGF-I concentrations have also been found in women with breast cancer compared to healthy women.³

The foods you eat can influence how much IGF-I circulates in the blood. Diets higher in overall calories or in animal proteins tend to boost IGF-I, and there seems to be an especially worrisome role played by milk. In a recent study, the addition of three daily eight-ounce servings of nonfat or 1 percent milk for 12 weeks caused a 10 percent rise in IGF-I levels.⁴ An earlier study had shown much the same result in adolescent girls.⁵ If milk increases the amount of IGF-I in the blood, it may increase cancer risk.

Cancer researchers have been looking into links between milk drinking and cancer risk for many years. A review published by the World Cancer Research Fund and the American Institute for Cancer Research in 1997 found that cancer risk paralleled milk consumption in numerous studies.⁶

There are several reasons why milk may be suspect. Aside from the load of fat in whole or 2 percent milk and its content of animal protein and concentrated calories, milk also contains preformed IGF-I which is identical to that of humans. Its concentration varies depending on how many calves the cow has had and her current stage of lactation.⁷

Is it the calories or the protein in milk that increases IGF-I in a milk-drinker's body—or is it the preformed IGF-I itself? In a pilot study, PCRM is now testing the effect of milk on IGF-I. A group of adult men agreed to drink a quart of cow's milk each day for four weeks. Then, after a washout period of four weeks, the men switched to soymilk for four weeks. A second group of men had the two types of milk in the opposite order. If cow's milk and soymilk increase IGF-I similarly, it suggests that the effect may be due to overall calories and protein, rather than to IGF-I content.

The study is being conducted by PCRM president Neal Barnard, M.D.; Anthony Scialli, M.D., of Georgetown University; and PCRM's research coordinator Matthew Fritts. First results are expected early in the year 2000.

1. Cohen P. Serum insulin-like growth factor-I levels and prostate cancer risk—interpreting the evidence. J Natl Cancer Inst 1998;90:876-9.
2. Chan JM, Stampfer MJ, Giovannucci E, et al. Plasma insulin-like growth factor-I and prostate cancer risk: a prospective study. Science 1998;279:563-6.
3. Hankinson SE, Willett WC, Colditz GA, et al. Circulating concentrations of insulin-like growth factor-I and risk of breast cancer. Lancet 1998;351:1393-6.
4. Heaney RP, McCarron DA, Dawson-Hughes B, et al. Dietary changes favorably affect bone remodeling in older adults. J Am Dietetic Asso 1999;99:1228-33.
5. Cadogan J, Eastell R, Jones N, Barker ME. Milk intake and bone mineral acquisition in adolescent girls: randomised, controlled intervention trial. BMJ 1997;315:1255-60.
6. World Cancer Research Fund/American Institute for Cancer Research. Food, Nutrition, and the Prevention of Cancer: A Global Perspective. American Institute for Cancer Research, Washington, D.C., 1997, p. 461.
7. Juskevich JC, Guyer CG. Bovine growth hormone: human food safety evaluation. Science 1990;249:875-84.

© 2000, PHOTODISC