Re: Plasma Insulin-Like Growth Factor-I, Insulin-Like Growth Factor-Binding Proteins, and Prostate Cancer Risk: a Prospective Study

Extract

The important article by Stattin et al. (1) provides further evidence that insulin-like growth factor-I (IGF-I) may play a key role in the development of prostate cancer. Prediagnostic levels of serum IGF-I were, on average, 7% higher in men who subsequently developed prostate cancer compared with healthy individuals, a result similar to that found in the Physicians' Health Study (2).

Circulating IGF-I levels have been shown to be sensitive to nutritional factors: A diet severely restricted in energy and/or protein intake substantially reduces serum IGF-I concentrations in both animals and humans (3). Stattin et al. (1) suggest that elevated IGF-I levels could be a potential mechanism through which a Western high-energy diet may increase prostate cancer risk. However, they acknowledge that obesity is not related to an increase in IGF-I concentration and that a high body mass index is not a strong risk factor for prostate cancer.

An alternative explanation is that protein intake may be the critical determinant of IGF-I serum concentrations that ultimately influence prostate cancer risk. The rationale is that a diet low in protein and essential amino acids will reduce IGF-I production and thereby reduce the growth of prostate cancer from a latent to a clinically manifest form. One test of this hypothesis is to examine IGF-I levels among men who consume a vegan diet, which contains only plant protein and, therefore, fewer essential amino acids than diets containing primarily animal protein (4). We have recently shown that the mean serum IGF-I concentration was 9% (95% confidence interval = 6% to 13%) lower among 233 vegan men who consumed no animal products than it was among either 237 lacto-ovo-vegetarians, who consumed no meat or fish but did consume either dairy and/or egg products, or 226 meat eaters (5). Among the 696 men studied, consumption of a diet rich in essential amino acids (i.e., containing protein derived from animal and soy products) was positively correlated with serum IGF-I concentrations. Serum IGF-I concentration increased 20% from 17.4 nmol/L in the lowest decile of intake (<9.7 g/day of animal and soy protein) to 20.8 nmol/L in the highest decile of intake (≥74.5 g/day of animal and soy protein after adjustment for age, body mass index, smoking, vigorous exercise, and factors related to blood collection (test for linear trend; P = .007). These results suggest that the low essential amino acid intake among vegan men may explain their low serum IGF-I concentrations. It is important to note that total protein, total energy intake, and body mass index were not statistically significantly associated with IGF-I concentration in this population.