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Head-to-head evidence comparison — which supplement is right for you?
Calcium wins 2 of 3 categories. Both are solid choices — the best pick depends on your specific goals.
Verdict
Likely helps
18 of 20 studies with measurable effects showed benefit.
Top outcomes
Verdict
Likely helps
11 of 13 studies with measurable effects showed benefit.
Top outcomes
Shared outcomes (2)
Outcomes where both Calcium and Soy Isoflavones have evidence — compare verdict strength side-by-side.
1000–1200mg daily (split doses)
Split doses with meals, 500mg with breakfast, 500mg with dinner
Calcium Citrate
40-80mg isoflavones daily
Divided doses with meals, Consistent daily timing
Standardized soy isoflavone extract
Meta-analyses show soy isoflavone supplementation combined with calcium supplementation significantly reduces bone loss in postmenopausal women compared to calcium alone, particularly at the lumbar spine.
Soy isoflavones (40–80 mg/day) may be combined with calcium to enhance bone protection, particularly in estrogen-deficient states. Note: soy proteins contain phytate which may modestly reduce calcium absorption if taken as a single large meal dose.
6-24 months
12-36 months
Throughout pregnancy
Within days of starting
4-12 weeks
6-12 months
8-12 weeks
Initial use
Effects of combined calcium and vitamin D supplementation on osteoporosis in postmenopausal women: a systematic review and meta-analysis of randomized controlled trials.
Food & Function (2020) · Meta analysis · n=12000
Combined calcium and vitamin D significantly improved lumbar spine BMD in postmenopausal women
Association Between Calcium or Vitamin D Supplementation and Fracture Incidence in Community-Dwelling Older Adults: A Systematic Review and Meta-analysis.
JAMA (2017) · Meta analysis · n=51145
33 trials involving 51,145 participants were analyzed
Vitamin D and Calcium for the Prevention of Fracture: A Systematic Review and Meta-analysis.
JAMA Network Open (2019) · Meta analysis · n=51419
Meta-analysis of 11 randomized clinical trials with 51,419 participants
Soy isoflavones prevent bone resorption and loss, a systematic review and meta-analysis of randomized controlled trials
Critical reviews in food science and nutrition (2020) · Meta analysis · n=5313
Soy isoflavones prevent osteoporosis-related bone loss in any weight status or treatment duration.
Effect of Soy Isoflavones on Measures of Estrogenicity: A Systematic Review and Meta-Analysis of Randomized Controlled Trials
Advances in nutrition (Bethesda, Md.) (2025) · Meta analysis · n=3285
Addressing public health concerns may promote soy foods as high-quality plant protein sources with low environmental impact and cost, particularly benefiting postmenopausal women and aligning with sustainable dietary patterns and guidelines.
Effects of soy isoflavones on menopausal symptoms in perimenopausal women: a systematic review and meta-analysis
PeerJ (2025) · Meta analysis · n=533
In October 2024, we screened 2,099 articles, of which 12 were eligible for meta-analysis, and found that soy isoflavones were effective for treating menopausal symptoms (seven studies, 533 participants, Hedges' g = -0.25, 95% CI [-0.42 to -0.08], p = 0.00).
Based on meta-analysis of 59 RCTs showing 0.6-1.8% BMD increases. Requires vitamin D co-supplementation for optimal effect. Effectiveness plateaus beyond 1200mg daily.
Meta-analysis of 533 participants showed effectiveness for menopausal symptoms. Effect size appears modest and heterogeneity was noted across studies. Individual response varies significantly.
AI-estimated from published studies. Interpret as directional guidance.
Calcium has a higher evidence score (8/10 vs 6/10) and wins in 2 of 3 categories.
For bone health, Calcium has a higher relevance score (95 vs 75).
Calcium and Soy Isoflavones may work well together: Meta-analyses show soy isoflavone supplementation combined with calcium supplementation significantly reduces bone loss in postmenopausal women compared to calcium alone, particularly at the lumbar spine. Soy isoflavones (40–80 mg/day) may be combined with calcium to enhance bone protection, particularly in estrogen-deficient states. Note: soy proteins contain phytate which may modestly reduce calcium absorption if taken as a single large meal dose.