Despite extensive research it has proved difficult to establish the role of diet in the aetiology of common types of cancer. Obesity and alcohol definitely increase the risk for several types of cancer, but the importance of particular foods and nutrients is not clear. Part of the difficulty is our poor understanding of the physiological changes that might mediate the effect of diet on cancer risk. Recent research in prospective studies with biobanks of stored blood samples has shown that the serum concentration of insulin-like growth factor-1 (IGF-1) is positively associated with the risk for both breast cancer in women and prostate cancer in men. It is also known that circulating IGF-1 concentrations can change in response to nutritional changes including energy and protein restriction, and some studies suggest that, even within well-nourished western populations, men and women with relatively high intakes of protein from dairy products have higher blood levels of IGF-1. These observations have led to the hypothesis that high intakes of protein from dairy products might increase the risk for some cancers by increasing the endogenous production of IGF-1. Further evaluation of this hypothesis requires clinical nutritional studies of the effects of diet on IGF-1 metabolism, and large epidemiological studies of cancer risk incorporating reliable measures of diet and serum IGF-1 concentrations.