Written by the Spacemilk team · Last updated October 2026

Yeast protein and soy protein are both complete proteins with near-top quality scores: PDCAAS 1.0 for yeast protein and about 0.98 for soy. Per gram of protein, yeast protein carries somewhat more essential amino acids and leucine. The biggest practical difference is allergens: soy is one of the nine major food allergens, so anyone avoiding soy needs a soy-free protein powder.

What's the difference between yeast protein and soy protein?

Soy protein comes from soybeans, a legume grown in soil. Most soy protein powders use soy protein isolate, the concentrated protein from soybeans. Yeast protein is isolated from non-GMO baker's yeast (Saccharomyces cerevisiae), the single-celled organism behind bread, beer and wine. The yeast is grown in water through fermentation, then the protein is separated, filtered and dried. What ends up in the bag is protein isolate, with no live or active yeast.

That makes yeast protein a single-cell protein: not plant, and not animal. Spacemilk is the first protein made from baker's yeast, and each serving of Spacemilk Complete Protein delivers 20g of complete protein.

Which has better protein quality?

Both score very well. PDCAAS (Protein Digestibility Corrected Amino Acid Score) rates how well a protein covers your essential amino acid needs, adjusted for digestibility. 1.0 is the highest possible score. Our guide to PDCAAS explains how it works.

Protein	PDCAAS
Yeast protein (Spacemilk)	1.0
Whey	1.0
Soy protein isolate	0.98
Pea	0.89

The soy figure comes from a study that measured 14 protein sources in growing rats, where a soy protein isolate scored 0.979 (Rutherfurd et al., 2015, The Journal of Nutrition). In a separate comparison of six proteins, yeast protein measured a PDCAAS of 100%, ahead of soy, pea and wheat (Cao et al., 2025, Food Chemistry).

The gap is small. Soy is one of the highest-quality plant proteins, and 0.98 is close to the top of the scale.

How do the amino acids compare?

Per 20g of protein	Spacemilk yeast protein	Soy protein isolate
Essential amino acids	about 9.4g	about 8.0g
BCAAs (leucine, isoleucine, valine)	about 4.3g	about 3.4g
Leucine	about 1.8g	about 1.5g
Methionine + cysteine	about 0.5g	about 0.5g

Spacemilk figures are calculated from our amino acid profile per 100g of protein. Soy figures are calculated from published data: Cao et al., 2025 for essential amino acids, and the USDA FoodData Central entry for soy protein isolate for leucine, BCAAs and sulfur amino acids. Treat them as approximate. Soy isolates vary by product.

Measured side by side in one study, yeast protein contained 508mg of essential amino acids per gram of protein, against 398mg for soy protein isolate (Cao et al., 2025). Leucine matters because it's the amino acid most closely tied to muscle protein synthesis.

Which is better for building muscle?

Protein helps build and repair muscle, whatever the source. Both proteins have human evidence, though not against each other: we're not aware of a published trial comparing yeast protein and soy protein directly.

• Yeast protein. Yeast protein, the protein in Spacemilk, was shown in a 2024 double-blind randomized controlled trial to match whey for building lean muscle and strength in men over 40 doing resistance training. The placebo-controlled trial gave 79 men 40g a day for eight weeks (Briskey et al., 2024, Journal of Food and Nutrition Research).
• Soy protein. A 2018 meta-analysis pooled nine resistance-training studies with 266 participants. Soy protein produced similar gains in strength and lean body mass as whey and other animal proteins (Messina et al., 2018, International Journal of Sport Nutrition and Exercise Metabolism).

Short-term studies tell a slightly different story. In one, whey raised muscle protein synthesis about 31% more than soy after a workout, which the authors linked to digestion speed or leucine content (Tang et al., 2009, Journal of Applied Physiology). Over weeks of training, though, the soy studies above found no meaningful difference. Both yeast and soy were tested as ingredients, not as finished products.

Which is easier to digest?

The studies don't crown a winner. In one lab digestion model, yeast protein's digestibility (97.66%) was significantly higher than soy protein isolate's (Qiao et al., 2025, Food Chemistry). In a study in rats, yeast protein's true digestibility was lower than the other proteins tested, soy included (Cao et al., 2025).

What sets Spacemilk apart is what's left out. It has no dairy or lactose, no sugar alcohols and no gums like xanthan or guar, and it's formulated to be low in FODMAPs. That's why we describe it as gentle on digestion and light on your stomach.

Is soy a major allergen? Why choose a soy-free protein powder?

Yes. Soy is one of the nine major food allergens recognized by the FDA, which is why it must be listed on US food labels (FDA). Soy allergy is one of the most common food allergies in young children, and many outgrow it (ACAAI). If your household avoids soy, a soy protein powder is off the table.

Spacemilk is free from all 9 major allergens: dairy, soy, gluten, eggs, tree nuts, peanuts, fish, shellfish and sesame. Every batch is tested for gluten, dairy, soy and peanuts, with results on our testing page. The flavored versions use sunflower lecithin (hexane-free and cold-processed) to help the powder mix, and Unflavored is one ingredient: yeast protein isolate. More on choosing powders in Allergen-free protein powder.

Yeast protein has its own rare caveat: if you have a true baker's yeast allergy, check with your allergist first. Read Can you have yeast protein with a yeast allergy or candida?

Is soy protein bad for you? What about isoflavones?

For most people, no. Soy contains isoflavones, plant compounds similar in structure to the hormone estrogen. They're often called phytoestrogens, and they're the source of most worries about soy (NCCIH).

• Men's hormones. A 2021 meta-analysis of 41 clinical studies found no significant effect of soy protein or isoflavones on testosterone or estrogen levels in men, regardless of dose or study length (Reed et al., 2021, Reproductive Toxicology).
• Women after menopause. The European Food Safety Authority reviewed isolated isoflavone supplements and concluded the evidence does not suggest harmful effects on the breast, uterus or thyroid in post-menopausal women. It noted it had no data to judge women with a personal or family history of cancer (EFSA, 2015).
• Pregnancy. The NIH's NCCIH says soy may be unsafe during pregnancy in amounts greater than those commonly found in foods (NCCIH).

If you have a hormone-sensitive condition, are pregnant, or take medication, ask your doctor which proteins suit you. Spacemilk contains no soy, so it contains no soy isoflavones. That's a fact about the ingredients. It isn't a reason to fear soy.

How do they taste and mix?

Taste is personal, and soy powders vary a lot by brand. Spacemilk is made to be smooth and creamy, not chalky. The flavored versions get their texture from konjac fiber and are sweetened with Reb M, a sweetener fermented from rice, with 0g sugar.

Is yeast protein more sustainable than soy protein?

We don't have a like-for-like footprint comparison with soy, so we won't claim one. What we can say: Spacemilk is grown in water, not farmland, and has about a 95% lower carbon footprint than beef protein.

Where does soy protein win?

• Price and availability. Soy protein is sold almost everywhere and is usually less expensive per serving.
• Track record. Soy protein has been studied for decades, with multiple training trials pooled in meta-analyses. Yeast protein's human evidence base is newer and smaller.
• Whole-food options. You can get soy protein from everyday foods like tofu, tempeh, edamame and soy milk. Yeast protein comes as a powder.

Yeast protein vs soy protein: the side-by-side

	Yeast protein (Spacemilk)	Soy protein isolate
Source	Isolated from non-GMO baker's yeast, grown in water through fermentation	Soybeans (legume)
PDCAAS	1.0	About 0.98
Essential amino acids per gram of protein	Higher	Slightly lower
Muscle research	Shown to match whey in a 2024 RCT (men over 40)	Similar to whey and animal protein in a 2018 meta-analysis
Digestibility (lab studies)	Mixed results	Mixed results
Major allergen	No. Free from all 9 major allergens	Yes. Soy is one of the 9
Whole-food form	No	Yes: tofu, tempeh, edamame, soy milk
Price and availability	Newer, fewer options	Widely available, usually cheaper

Who it's for, and who should check with a doctor

Yeast protein may suit you if you need a soy-free protein, you're avoiding major allergens, or you want a vegan protein at PDCAAS 1.0 with a bit more leucine per scoop.

Soy protein may suit you if you don't avoid soy, price matters most, or you like getting protein from whole foods like tofu.

Check with your doctor or allergist first if you have a known soy or yeast allergy, a hormone-sensitive condition, a medical condition that affects how much protein you should eat, you're pregnant or nursing, or you're choosing a protein for a child. For more on legume proteins, see Yeast protein vs pea protein.

FAQ

Is yeast protein better than soy protein?

On protein quality, the two are close: PDCAAS 1.0 versus about 0.98, with yeast protein carrying more essential amino acids and leucine per gram. Soy is cheaper, easier to find, has a longer research record and comes in whole-food forms. The clearest difference is that soy is a major allergen.

Does soy protein lower testosterone?

A 2021 meta-analysis of 41 clinical studies found no significant effect of soy protein or isoflavones on testosterone or estrogen in men.

Is Spacemilk soy-free?

Yes. Spacemilk contains no soy and is free from all 9 major allergens. Every batch is tested for gluten, dairy, soy and peanuts.

Is yeast protein plant-based?

No. Yeast is a single-celled fungus, not a plant, so yeast protein is a single-cell protein. It is vegan and 100% animal-free.

Sources

• Rutherfurd SM et al. Protein digestibility-corrected amino acid scores and digestible indispensable amino acid scores differentially describe protein quality in growing male rats. J Nutr. 2015;145(2):372–379. doi:10.3945/jn.114.195438
• Cao X et al. Evaluation of the nutritional quality of yeast protein in comparison to animal and plant proteins using growing rats and INFOGEST model. Food Chem. 2025;463:141178. doi:10.1016/j.foodchem.2024.141178
• USDA FoodData Central. Soy protein isolate (SR Legacy, FDC ID 174276). fdc.nal.usda.gov
• Briskey D et al. Journal of Food and Nutrition Research. 2024;12(5):292–300. doi:10.12691/jfnr-12-5-9
• Messina M et al. No difference between the effects of supplementing with soy protein versus animal protein on gains in muscle mass and strength in response to resistance exercise. Int J Sport Nutr Exerc Metab. 2018;28(6):674–685. doi:10.1123/ijsnem.2018-0071
• Tang JE et al. Ingestion of whey hydrolysate, casein, or soy protein isolate. J Appl Physiol. 2009;107(3):987–992. doi:10.1152/japplphysiol.00076.2009
• Qiao K et al. Research on the release and absorption regularities of free amino acids and peptides in vitro digestion of yeast protein. Food Chem. 2025;482:144176. doi:10.1016/j.foodchem.2025.144176
• Reed KE et al. Neither soy nor isoflavone intake affects male reproductive hormones. Reprod Toxicol. 2021;100:60–67. doi:10.1016/j.reprotox.2020.12.019
• EFSA Panel on Food Additives and Nutrient Sources. Risk assessment for peri- and post-menopausal women taking food supplements containing isolated isoflavones. EFSA Journal. 2015;13(10):4246. doi:10.2903/j.efsa.2015.4246
• NIH National Center for Complementary and Integrative Health. Soy. nccih.nih.gov
• US Food and Drug Administration. Food allergies. fda.gov
• American College of Allergy, Asthma and Immunology. Soy allergy. acaai.org