is soy really a good protein for muscle growth? there's a lot of things that are said about how it's biological value is lower than other protein sources like chicken and eggs and how it isn't absorbed well into the body?

Gemini 3.5 Flash

Answer: Yes, soy is an excellent, complete protein source for muscle growth that is highly digestible and scientifically proven to support muscle hypertrophy comparably to animal-based proteins when consumed in adequate amounts.

While historically criticized using older metrics like Biological Value (BV), modern nutritional science evaluates protein quality using the Digestible Indispensable Amino Acid Score (DIAAS) and PDCAAS, under which soy protein isolate scores exceptionally high (frequently achieving a perfect 1.0 PDCAAS score). This demonstrates that soy contains all nine essential amino acids in ratios necessary for human needs, making it one of the few complete plant-based proteins. Regarding absorption, soy protein is highly digestible—with a digestibility rate of over 90%—which is only marginally lower than milk or egg proteins. While it is true that soy has a slightly lower leucine content and is absorbed slightly faster than casein but slower than whey, clinical studies consistently show that these kinetic differences do not translate to inferior muscle growth in real-world scenarios. When total daily protein intake and resistance training are equalized, individuals supplementing with soy protein isolate experience virtually identical gains in muscle mass and strength compared to those using whey or beef protein. Furthermore, the common concern regarding soy's phytoestrogens (isoflavones) negatively affecting testosterone levels has been thoroughly debunked by multiple comprehensive meta-analyses. Soy consumption does not lower testosterone or raise estrogen in men, making it a safe, effective, and environmentally sustainable option for anyone looking to build muscle.

Grok 4.3

Answer: Soy protein is a solid choice for supporting muscle growth despite having a somewhat lower biological value than sources like eggs or chicken.

While soy's biological value and digestibility scores are marginally below those of animal proteins, it remains a complete protein with a PDCAAS of 1.0, providing all essential amino acids in sufficient quantities for most adults. Concerns about poor absorption are largely overstated; modern processing and adequate dosing allow soy to stimulate muscle protein synthesis effectively, as shown in multiple studies comparing it to whey. For optimal results, combining soy with other proteins or ensuring higher total intake compensates for any minor differences, making it especially valuable for vegetarians and vegans.

GPT-5.5

Answer: Yes, soy is a good protein for muscle growth, especially when total daily protein intake is sufficient, though animal proteins like eggs, dairy, and chicken may be slightly more efficient per gram because of higher digestibility and leucine content.

Soy protein is a complete protein, meaning it contains all essential amino acids needed to build and repair muscle. Its biological value and digestibility are generally a bit lower than high-quality animal proteins such as eggs, whey, or chicken, but that does not mean it is poorly absorbed or ineffective. Modern measures like PDCAAS and DIAAS show soy protein isolate is among the better plant proteins, and far better than many other plant sources. For muscle growth, the biggest factors are progressive resistance training, total daily protein intake, total calories, and getting enough essential amino acids—especially leucine. Soy has less leucine per gram than whey or some animal proteins, so you may need a slightly larger serving to get the same muscle-building signal. For example, where 25 g of whey might be enough for many people, you might use closer to 30–40 g of soy protein isolate or combine soy foods with other protein sources across the day. Research comparing soy to animal proteins generally shows that when total protein and calories are matched, soy can support gains in lean mass and strength. Whey may sometimes perform slightly better in short-term studies due to faster digestion and higher leucine, but the practical difference is often small if your overall diet is well planned. Concerns that soy is “not absorbed well” are often overstated. Whole soy foods like tofu, tempeh, edamame, and soy milk are digestible, and soy protein isolate is particularly digestible. Tempeh may be especially useful because fermentation can improve digestibility. If someone relies only on low-protein soy foods or eats too little total protein, results may suffer—but that is a planning issue, not proof that soy is bad for muscle growth. A practical target for building muscle is roughly 1.6–2.2 g of protein per kg of body weight per day, spread over 3–5 meals. Soy can absolutely be part of that, and for vegetarians or people reducing animal foods, it is one of the strongest protein choices available.

Claude Opus 4.7

Answer: Yes, soy is a genuinely effective protein for muscle growth — it's a complete, well-absorbed protein, and the concerns about its biological value and absorption are largely overstated.

Soy is one of the few plant proteins that is 'complete,' meaning it contains all nine essential amino acids in meaningful amounts. On the PDCAAS (Protein Digestibility Corrected Amino Acid Score) — the scoring system the WHO and FDA actually use — isolated soy protein scores 1.0, the maximum, putting it on par with whey, casein, egg, and milk. On the newer DIAAS scale it scores slightly lower than animal proteins (~0.90 vs ~1.10 for whey), but still well within the 'high quality' range. The older 'Biological Value' (BV) metric people cite is outdated and was developed in the 1920s using rat studies. It's a poor predictor of muscle-building outcomes in humans. What actually matters for hypertrophy is (1) total daily protein intake, (2) leucine content per serving, and (3) digestibility. Soy is ~85–90% digestible (similar to most animal proteins) and contains about 8% leucine — slightly less than whey (~11%) but more than most plant sources. Head-to-head studies bear this out. Multiple RCTs and meta-analyses (e.g., Messina et al. 2018, Lynch et al. 2018) have found that when total protein and resistance training are equated, soy produces similar gains in lean mass and strength to whey or animal protein. Whey has a slight edge per-gram due to higher leucine and faster absorption, but this is easily compensated for by eating a bit more soy or combining it with other protein sources. Practical caveats: (1) Whole soy foods like tofu and tempeh are excellent; highly processed soy protein isolate is fine too. (2) To match whey's leucine threshold (~2.5–3g per meal for maximal muscle protein synthesis), aim for ~30–40g of soy protein per serving instead of ~20–25g of whey. (3) The old fears about soy lowering testosterone or raising estrogen in men have been debunked in meta-analyses at normal intakes (up to ~50g/day). So soy isn't quite the per-gram equal of whey, but the gap is small and easily closed with slightly higher intake. Calling it a poor muscle-building protein is simply not supported by the evidence.

Final Summary

The AI Roundtable unanimously debunked the myth that soy is a poor protein source, noting its perfect score on modern digestibility scales. Claude Opus 4.7 and Gemini 3.5 Flash highlighted that real-world muscle gains are virtually identical to animal protein, while GPT-5.5 and Grok 4.3 explained that any minor amino acid differences are easily solved by slightly increasing serving sizes.

All 4 models agreed