About Soy Protein Isolate
Soy protein is one of the most-studied plant proteins. Tang 2009 PMID 19589961 directly compared whey, casein, and soy isolate post-exercise — whey produced the largest MPS response, with soy intermediate and casein slowest. Hartman 2007 PMID 17684205 (n=56, 12 weeks resistance training) showed lean mass gains were equivalent between soy and milk groups, though milk produced slightly more lean mass at trend level. The 1999 FDA-approved heart health claim (25 g/day reduces CHD risk via LDL reduction) was proposed for revocation in 2017 — current status is a 'qualified' health claim, not unqualified. Concerns about isoflavones suppressing testosterone in men are largely unsupported in healthy adults — Messina 2010 meta-analysis (PMID 19524224) found no significant effect at typical supplemental doses. Isolate forms (~90% protein) contain minimal isoflavones; concentrate forms retain more. Use caution with thyroid medication if dosed simultaneously (absorption interference). CONTRAINDICATIONS: soy allergy.
What Soy Protein Isolate supports
- Intermediate MPS response between whey (fastest) and casein (slowest)
- Complete plant-based protein with adequate leucine for MPS at typical doses
- FDA-qualified heart health claim — 25 g/day for LDL reduction
How much Soy Protein Isolate to take
The RDA prevents deficiency. The effective range is what clinical trials used to actually move the outcome.
Effective
20–30
g
20-30 g per serving. Tang 2009 (PMID 19589961) compared whey, casein, and soy isolate for muscle protein synthesis. Hartman 2007 (PMID 17684205) showed equivalent lean mass gains vs milk protein in young men over 12 weeks. Plant-based 'complete' protein — adequate leucine (~8%) when dosed at the upper end.
Clinical evidence
Moderate clinical evidence. Tang 2009 PMID 19589961 (MPS head-to-head with whey/casein); Hartman 2007 PMID 17684205 (12-week lean mass parity vs milk); FDA heart-claim downgraded from unqualified to qualified in 2017
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