Have you seen Pedro Bastos fantastic AHS lecture? IGF-1 came up lots as I recall... http://www.slideshare.net/ancestralhealth/ahs-slidespedro-bastos-8827808
*edited to say - check out the slides from page 50
Protein restriction seems to work, at least in combination with a calorie restricted diet. "Reducing protein intake from an average of 1.67 g kg(-1) of body weight per day to 0.95 g kg(-1) of body weight per day for 3 weeks in six volunteers practicing CR [calorie restriction] resulted in a reduction in serum IGF-1 from 194 ng mL(-1) to 152 ng mL(-1)." http://www.ncbi.nlm.nih.gov/pubmed/18843793
I'm with H.T. I'm not sure why you would want to.
I blogged on a study demonstrating increased protein content in the diet increasing IGF-1 and improving glucose homeostasis: http://carbsanity.blogspot.com/2011/06/protein-insulin-like-growth-factor-1.html
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