Milk and Cancer Risk?: PCRM's IGF-I Study
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;90:876-879.
2. Chan JM, Stampfer MJ, Giovannucci E, et al. Plasma insulin-like growth factor-I and prostate cancer risk: a prospective study. Science. 1998;279:563-566.
3. Hankinson SE, Willett WC, Colditz GA, et al. Circulating concentrations of insulin-like growth factor-I and risk of breast cancer. Lancet. 1998;351:1393-1396.
4. Heaney RP, McCarron DA, Dawson-Hughes B, et al. Dietary changes favorably affect bone remodeling in older adults. J Am Dietetic Asso. 1999;99:1228-1233.
5. Cadogan J, Eastell R, Jones N, Barker ME. Milk intake and bone mineral acquisition in adolescent girls: randomised, controlled intervention trial. BMJ. 1997;315:1255-1260.
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;249:875-884.