Does soy protein affect circulating levels of unbound IGF-1?

Area of Science:

• Nutrition Science
• Endocrinology
• Dietary Interventions

Background:

• The precise mechanisms of soy's physiological effects remain unclear despite extensive research.
• Soy intake does not appear to alter reproductive hormone levels in humans.
• Emerging evidence suggests soy protein, distinct from isoflavones, may influence insulin-like growth factor I (IGF-1).

Purpose of the Study:

• To investigate the impact of soy protein on insulin-like growth factor I (IGF-1) levels.
• To examine the effect of soy protein on IGF-1's primary binding protein, IGFBP-3.
• To synthesize findings from existing clinical studies comparing soy protein to control proteins.

Main Methods:

• A systematic review of six clinical trials comparing soy protein with control proteins.
• Analysis focused on studies measuring IGF-1 and insulin-like growth factor-binding protein 3 (IGFBP-3).
• Acknowledged limitations due to variations in study designs and outcome measures.

Main Results:

• Some evidence indicates that high soy protein intake (over 25 g/day) may modestly elevate IGF-1 levels compared to control proteins.
• The effect on IGFBP-3 levels was less clear due to limited data.
• Interpreting results was challenging due to heterogeneity in study methodologies.

Conclusions:

• The current clinical data do not support using potential effects on IGF-1 as a basis for dietary soy inclusion.
• The physiological impact of soy protein on IGF-1 warrants further investigation with standardized methodologies.
• Dietary decisions regarding soy should consider a broader range of health outcomes.