Relationship of Dietary Protein and Soy Isoflavones to Serum IGF-1 and IGF Binding Proteins in the Prostate Cancer Lifestyle Trial

Abstract:

High levels of insulin-like growth factor 1 (IGF-1) are associated with increased risk of prostate cancer, whereas increased levels of some of its binding proteins (IGFBPs) seem to be protective. High intakes of dietary protein, especially animal and soy protein, appear to increase IGF-1. However, soy isoflavones have demonstrated anti-proliferative and apoptotic effects both in vitro and in vivo. We evaluated dietary intakes of total protein and soy isoflavones in relation to the IGF axis in prostate cancer patients making comprehensive lifestyle changes including a very low-fat vegan diet supplemented with soy protein (58 g/day). After one year, intervention group patients reported significantly higher intakes of dietary protein and soy isoflavones compared to usual-care controls (P < 0.001). IGF-1 increased significantly in both groups, whereas IGFBP-1 rose in the experimental group only (P < 0.01). Increases in vegetable protein over one year were associated with increases in IGFBP-1 among intervention group patients (P < 0.05). These results suggest that dietary protein and soy isoflavones, in the context of comprehensive lifestyle changes, may not significantly alter IGF-1. However, given the recent literature indicating that high intake of protein rich in essential amino acids (animal or soy protein) may increase IGF-1, it may be prudent for men with early stage prostate cancer not to exceed dietary protein recommendations.

Acknowledgments and Notes

Supported by the Department of Defense Uniformed Services University Grant MDA905-99-1-0003 via the Henry M. Jackson Foundation Grant 600-06971000-236 Department of the Army (U.S. Army Medical Research Acquisition Activity W81XWH-05-1-0375-P0001), the Department of Health and Human Services (Health Resources and Services Administration #4 C76HF00803-01-01), The Prostate Cancer Foundation, National Institutes of Health 5P50CA089520-02 University of California–San Francisco Prostate Cancer Specialized Program of Research Excellence, the Safeway Foundation, and the Walton Family Foundation. Representatives Nancy Pelosi and John Murtha, and Senators Arlen Specter and Ted Stevens provided support. Insulin and IGF proteins analyses were done by Dr. Rusty Nicar at Diagnostic System Laboratory. We thank Colleen Kemp for her helpful contribution to this study, and The Solae Company, formerly DuPont Technologies, for providing the soy protein powdered beverage.

Notes

∗ P < 0.05.

∗∗ P < 0.01.

∗∗∗ P < 0.001.