# Best Soy Protein Powder

> The best soy protein powder picks, ranked on protein per scoop, testing and cost per 25 g, plus what the research really says about soy and testosterone.

- **URL**: https://www.best-proteinpowder.com/best-soy-protein-powder/
- **Updated**: 2026-11-08
- **Author**: [Best Protein Powder Editorial Team](https://www.best-proteinpowder.com/authors/editorial-team/)
- **Publisher**: [Best Protein Powder](https://www.best-proteinpowder.com/)

---

Soy is the cheapest complete plant protein you can buy and one of the most studied. Here are the isolates worth buying, ranked on what the label actually delivers, plus a straight answer on the testosterone question.

**How we rank:** We judge every powder on the label, not the marketing: protein per scoop, cost per 25 g of protein, the full ingredient list and third-party testing. No paid placements, and commission never sets a ranking. [Read our methodology](https://www.best-proteinpowder.com/how-we-test/)

Soy protein isolate is the most underrated powder on the shelf: a complete protein, roughly 90 percent protein by dry weight, cheaper per gram than most pea and nearly all whey, and backed by trials showing muscle gains on par with whey when the dose is right. The worry that keeps people away from it, that soy lowers testosterone, has been tested in dozens of clinical studies and does not hold up at normal intakes.

## Our Top Picks

| Award | Product | Why |
| --- | --- | --- |
| Best Overall | [NOW Sports Soy Protein Isolate (Unflavored)](https://www.best-proteinpowder.com/best-soy-protein-powder/#now-sports-soy-protein-isolate) | The soy isolate to buy if you want independent testing without paying a premium. |
| Best Budget | [BulkSupplements Soy Protein Isolate Powder](https://www.best-proteinpowder.com/best-soy-protein-powder/#bulksupplements-soy-protein-isolate) | The cheapest honest gram of soy protein, as long as you are happy blending it. |
| Best Flavored Isolate | [Nutricost Soy Protein Isolate Powder](https://www.best-proteinpowder.com/best-soy-protein-powder/#nutricost-soy-protein-isolate) | The pick if you want a flavored soy isolate, as long as you count scoops. |
| Best Big Bag | [Horbaach Soy Protein Isolate Powder (Unflavored)](https://www.best-proteinpowder.com/best-soy-protein-powder/#horbaach-soy-protein-isolate) | A solid bulk alternative when it is cheaper per gram than the other plain isolates. |
| Best Flavored Shake | [Soylent Complete Protein Powder](https://www.best-proteinpowder.com/best-soy-protein-powder/#soylent-complete-protein-powder) | The easiest-drinking soy shake here, if you will pay for convenience. |

## 1. How We Ranked These Soy Powders

Soy protein powder is a small category on Amazon, and a lot of what shows up for the search is not soy at all. Many of the top sellers for the phrase are pea and rice blends from plant brands, and a few are whey. We started by throwing out anything where soy was not the main protein source on the ingredient list, which left a short list of unflavored isolates, a handful of flavored isolates and a couple of soy-based meal shakes.

From there we ranked on four things. First, protein per serving and protein per gram of powder, because a 10 g scoop and a 27 g scoop are not comparable until you put them on the same basis. Second, independent testing: a sport certification such as Informed Sport carries more weight than a brand saying its own batches are tested. Third, the ingredient list, where shorter is generally better. Fourth, cost per 25 g of protein at typical prices.

We did not hand out points for marketing claims about isoflavones, hormones or weight loss, and we did not use star ratings, because they say more about flavor and shipping than about what is in the tub. Where a figure differed between listings, as it does for some bulk brands, we give the range and tell you to check the label. Where we could not confirm a figure at all, we left it out rather than guess.

One practical note: almost every pure soy isolate here is unflavored. That is not an oversight. Flavored soy shakes largely disappeared from the US market as whey and pea took over, so if you want chocolate or vanilla, your realistic options are a smaller flavored isolate, a soy-based meal shake, or an unflavored isolate blended with fruit, cocoa or a spoon of nut butter.

| Criterion | What we checked | Why it matters |
| --- | --- | --- |
| Protein density | Grams of protein per gram of powder | Separates real isolates from filler-heavy blends |
| Testing | Informed Sport, NSF, or brand lab claims | Plant proteins carry more heavy-metal risk |
| Ingredients | Soy first, few additives | Fewer sweeteners and gums, fewer surprises |
| Cost per 25 g | Typical price divided by total protein | The only fair way to compare tub sizes |

## 2. Does Soy Build Muscle as Well as Whey?

Mostly, yes, once the dose is matched. Soy is a complete protein with all nine essential amino acids, and soy protein isolate scores the maximum 1.0 on the older PDCAAS scale used for food labels. On the newer DIAAS scale it lands a little below whey, mainly because whey is unusually rich in leucine, the amino acid that does the most to switch on muscle protein synthesis after a meal.

The leucine gap is real but small. Whey protein runs around 10 to 11 percent leucine, while soy runs closer to 8 percent. In practice that means a 25 g scoop of whey delivers roughly the leucine of 30 to 32 g of soy protein. If you are using soy as your main post-workout shake, take a slightly bigger serving, or pair it with a meal, and the difference largely disappears.

The long-term trials back that up. A 2018 meta-analysis by Messina and colleagues in the International Journal of Sport Nutrition and Exercise Metabolism pooled resistance-training studies that compared soy supplements with animal protein supplements and found no significant difference in gains in lean mass or strength. The authors did note the trials were small and mostly short, which is typical for this kind of research.

The most cited head-to-head is a 2021 study by Hevia-Larraín and colleagues in Sports Medicine. Previously untrained vegans and omnivores ate about 1.6 g of protein per kilogram of body weight per day for 12 weeks while lifting, with the vegans topping up using soy protein isolate and the omnivores using whey. Both groups gained similar amounts of leg lean mass, muscle size and strength.

Where soy falls short is not muscle but taste and texture. Soy isolate is thicker and beanier than whey in plain water, and most buyers end up blending it. If that is acceptable, soy is one of the best value ways to hit a daily protein target, whether you are plant-based or simply trying to spend less. For a wider plant comparison, see our [best plant-based protein powder](https://www.best-proteinpowder.com/best-plant-based-protein-powder/) guide and our [whey vs plant protein](https://www.best-proteinpowder.com/whey-vs-plant-protein/) breakdown.

## 3. The Best Soy Protein Powders

Five soy powders from five brands, all sold on Amazon US at the time of writing. Label figures come from the brand or retailer listing and can change between batches and flavors, so check the panel on the tub you actually buy. We have not tasted these ourselves; anything about flavor or mixing is buyer feedback.

### 1. NOW Sports Soy Protein Isolate (Unflavored)

**Award:** Best Overall | **Type:** Unflavored soy protein isolate

- **Protein per serving:** 20 g per 24 g scoop (label)
- **Calories:** About 90
- **Third-party testing:** Informed Sport (label)
- **Best for:** Tested athletes, smoothies

NOW's unflavored soy isolate lists one ingredient, non-GMO soy protein isolate, and delivers 20 g of protein in a 24 g scoop with about 90 calories, 0 g of carbohydrate and 170 mg of sodium, per the label at the time of writing. It is Informed Sport certified and Non-GMO Project Verified per the label, which is rare for a soy powder at this price. A 2 lb tub holds about 38 servings.

**Bottom line:** The soy isolate to buy if you want independent testing without paying a premium.

**Pros:**

- Informed Sport certified, per the label
- Single ingredient, no sweeteners or gums
- Non-GMO Project Verified
- Budget-tier cost per 25 g

**Cons:**

- 20 g per scoop, so you need about one and a half scoops for a full post-workout dose
- Unflavored only, and buyers describe it as thick in plain water

[Check price on Amazon](https://www.amazon.com/dp/B0013OSS4M?tag=sktiger-20)

### 2. BulkSupplements Soy Protein Isolate Powder

**Award:** Best Budget | **Type:** Unflavored soy protein isolate

- **Protein per serving:** About 25-27 g per 30 g (listings vary)
- **Calories:** About 110-120
- **Third-party testing:** No sport certification
- **Best for:** Lowest cost per gram, baking

BulkSupplements sells plain soy protein isolate in a 1 kg pouch, with smaller sizes available. Listings give roughly 25 to 27 g of protein in a 30 g serving, at about 110 to 120 calories, so check the panel on your bag. It has no flavor, sweetener or additives, and at typical Amazon prices it is one of the cheapest sources of protein in powder form, cheaper still on Subscribe and Save.

**Bottom line:** The cheapest honest gram of soy protein, as long as you are happy blending it.

**Pros:**

- Among the lowest costs per 25 g of any protein powder
- Single ingredient, unflavored
- Large pouch suits smoothies and baking

**Cons:**

- No sport certification
- Published protein figure varies slightly between listings
- Pouch and scoop are basic compared with a tub

[Check price on Amazon](https://www.amazon.com/dp/B01K2UDXBK?tag=sktiger-20)

### 3. Nutricost Soy Protein Isolate Powder

**Award:** Best Flavored Isolate | **Type:** Soy protein isolate, unflavored, chocolate or vanilla

- **Protein per serving:** 10 g per scoop (label)
- **Calories:** About 40 unflavored, 60 vanilla
- **Third-party testing:** Independent lab batch testing (brand)
- **Best for:** Flavored soy, small top-ups

Nutricost is one of the few brands still selling a flavored soy isolate, in chocolate and vanilla alongside unflavored. The scoop is small: 10 g of protein per serving, in an 11 g unflavored scoop of about 40 calories, or a 17 g vanilla scoop of about 60 calories with stevia and a few grams of added sugar, per the label. Nutricost says each batch is tested by independent ISO-accredited labs.

**Bottom line:** The pick if you want a flavored soy isolate, as long as you count scoops.

**Pros:**

- Chocolate and vanilla options, rare for a soy isolate
- Very high protein per gram in the unflavored version
- Batch tested by independent labs, per the brand

**Cons:**

- Only 10 g of protein per scoop, so three scoops for a full dose
- Carries a California Prop 65 lead warning
- Flavored versions add sugar and stevia

[Check price on Amazon](https://www.amazon.com/dp/B0DJWST3ZN?tag=sktiger-20)

### 4. Horbaach Soy Protein Isolate Powder (Unflavored)

**Award:** Best Big Bag | **Type:** Unflavored soy protein isolate

- **Protein per serving:** About 25 g (listing)
- **Calories:** About 110 per 30 g
- **Third-party testing:** No sport certification
- **Best for:** Bulk buyers, cooking

Horbaach sells an unflavored, vegan, non-GMO and gluten-free soy isolate in a 3 lb bag on Amazon. Product listings describe about 25 g of protein per serving, and nutrition databases list a rounded 30 g scoop at about 110 calories, which is consistent with a plain isolate. Treat it as a large-bag commodity option and check the per-ounce price against BulkSupplements before you buy.

**Bottom line:** A solid bulk alternative when it is cheaper per gram than the other plain isolates.

**Pros:**

- About 25 g of protein per serving, per listings
- Large 3 lb bag
- Unflavored, vegan and non-GMO, per the brand

**Cons:**

- No sport certification
- Label detail is thinner than NOW's, so check the panel on arrival

[Check price on Amazon](https://www.amazon.com/s?k=Horbaach%20Soy%20Protein%20Isolate%20Powder%20(Unflavored)&tag=sktiger-20)

### 5. Soylent Complete Protein Powder

**Award:** Best Flavored Shake | **Type:** Soy-based protein shake powder with added nutrients

- **Protein per serving:** 30 g (brand)
- **Calories:** Check the label for your flavor
- **Third-party testing:** None listed by the brand
- **Best for:** Flavored shakes, light meals

Soylent's powder is built on soy protein, which Soylent says is the base of every product it makes. Per the brand, a serving delivers 30 g of protein with 0 g of sugar, 5 g of BCAAs, MCT oil, DHA and 28 vitamins and minerals, sweetened with allulose, in chocolate or vanilla. A tub holds only 10 servings, so it is a premium buy, and it was out of stock on Soylent's own site when we checked.

**Bottom line:** The easiest-drinking soy shake here, if you will pay for convenience.

**Pros:**

- 30 g of protein per serving, a full dose in one shake
- Zero sugar, sweetened with allulose
- Flavored and ready to drink, unlike most soy isolates

**Cons:**

- Only 10 servings per tub, premium cost per 25 g
- Carries a California Prop 65 lead warning
- Stock has been patchy at the time of writing

[Check price on Amazon](https://www.amazon.com/dp/B0CZ57PXRD?tag=sktiger-20)

## 4. Soy and Testosterone: What the Research Actually Shows

This is the question that keeps most men away from soy, so it deserves a straight answer. Soy contains isoflavones, mainly genistein and daidzein, which are plant compounds that can bind to estrogen receptors. That is where the phytoestrogen label comes from, and it is a reasonable thing to wonder about. But binding to a receptor in a lab dish is not the same as changing hormone levels in a person eating normal amounts.

The best evidence is a 2021 meta-analysis by Reed and colleagues in Reproductive Toxicology, which pooled dozens of clinical studies in men and found that neither soy protein nor isoflavone intake significantly affected total testosterone, free testosterone, estradiol or estrone, even at intakes well above what a typical Japanese diet provides. An earlier 2010 meta-analysis by Hamilton-Reeves and colleagues in Fertility and Sterility reached the same conclusion.

Two pieces of evidence get passed around as proof of harm, and both need context. One is a small 2013 study by Kraemer and colleagues in which ten resistance-trained men taking soy for 14 days showed a slightly blunted testosterone response after workouts compared with whey. It was short, tiny and measured acute post-exercise responses, and the longer muscle-growth trials above show no penalty in actual results.

The other is a handful of case reports, the best known a 2008 report by Martinez and Lewi in Endocrine Practice of a 60-year-old man who developed breast tissue growth while drinking about three quarts of soy milk a day. His symptoms resolved after he stopped. That is an extreme, sustained intake, many times what one or two scoops of soy isolate provides, and it is the kind of exception that does not change the overall picture.

> **THE SHORT ANSWER**
>
> **One or two soy shakes a day will not lower your testosterone.**
>
> Pooled clinical trials in men show no meaningful change in testosterone or estrogen from soy protein or isoflavones at normal intakes. The case reports of feminizing effects involve extreme, sustained consumption. If you have a hormone-sensitive condition or take hormone therapy, ask your doctor; this is not medical advice.

Isoflavone content also varies by how the isolate is made. Water-washed isolates keep more of their isoflavones, while alcohol-washed isolates lose a large share in processing. Most brands do not say which method they use or list isoflavones per scoop, so do not expect to find it on the label. If you want to minimize isoflavones for any reason, a serving or two a day still keeps you in the range of ordinary soy-food intake.

## 5. Isolate, Concentrate and Textured Soy: What You Are Buying

Soy protein comes in three main forms. Soy flour is the least processed, around 50 percent protein, and is not sold as a protein powder. Soy protein concentrate is around 70 percent protein, with more carbohydrate and fiber left in. Soy protein isolate is the most refined, defined in the industry as at least 90 percent protein on a dry basis, and it is what every product on our list uses as its main ingredient.

You can check this yourself with simple division. NOW Sports lists 20 g of protein in a 24 g scoop, about 83 percent protein as sold. BulkSupplements lists roughly 25 to 27 g in a 30 g serving, and Nutricost lists 10 g in an 11 g unflavored scoop. Those ratios are exactly what an isolate with a bit of moisture should give. Flavored versions drop lower because cocoa, sweetener and flavoring take up weight.

If a soy powder claims more protein per gram than an isolate can physically contain, be suspicious. A flavored product listing 30 g of protein in a 31 g scoop, for example, does not add up. That kind of mismatch can point to rounding, a mislabeled scoop size or, in the worst case, added free amino acids that inflate the protein test. We cover that problem in detail in our [best cheap protein powder](https://www.best-proteinpowder.com/best-cheap-protein-powder/) guide.

Textured soy protein, sold as TVP, is a different product: a dried, extruded soy flour or concentrate used as a meat substitute. It is cheap and useful in cooking, but it is not a shake powder. Likewise, soy milk powders and soy-based meal shakes contain soy protein alongside sugar, fat and added nutrients, so their protein per gram is much lower than an isolate's, and they are priced accordingly.

## 6. How Much Soy Protein per Shake

For muscle building, aim for about 30 to 35 g of soy protein per shake if the shake is doing the job of a meal or a post-workout feed. That gets you to roughly 2.5 g of leucine, the amount most researchers treat as a practical threshold for a strong muscle-building response. If the shake sits alongside a protein-rich meal, a standard 20 to 25 g serving is fine.

Translated into scoops, that is about one and a half scoops of NOW Sports, a little over one serving of BulkSupplements or Horbaach, and three scoops of Nutricost, whose 10 g scoop is unusually small. Soylent's 30 g serving reaches the target in one go. Always weigh a scoop once on a kitchen scale; scoop sizes and powder density vary, and the label serving is measured by weight, not by how full the scoop looks.

> **By the Numbers**
>
> - **~8%**: Leucine in soy protein
> - **30-35 g**: Soy protein to match 25 g whey
> - **1.0**: Soy isolate PDCAAS
> - **1.6 g/kg**: Daily protein in the vegan vs omnivore trial

Daily totals matter more than any single shake. Most active adults do well between 1.4 and 2.0 g of protein per kilogram of body weight per day, per the International Society of Sports Nutrition position stand, and powder should fill gaps rather than replace food. Our [how much protein do I need](https://www.best-proteinpowder.com/how-much-protein-do-i-need/) guide walks through the maths for your body weight and goals.

Timing is flexible. A soy shake after training is convenient, but total protein spread across three or four meals does more than squeezing everything into a narrow window. If you have kidney disease, are pregnant, or have been told to limit protein, agree your daily target with a doctor or registered dietitian before adding shakes.

## 7. Cost per 25 g: The Soy Price Ladder

Soy is usually the cheapest complete plant protein per gram, and the unflavored bulk isolates are among the cheapest protein powders of any kind. The fair way to compare them is cost per 25 g of protein: divide the price by the total grams of protein in the container, then multiply by 25. Price per tub and price per scoop both mislead, because tub sizes and scoop sizes vary so much in this category.

Take NOW Sports as a worked example. A 2 lb tub holds about 38 servings of 20 g, or 760 g of protein. If the tub costs around $30, that is about $0.99 per 25 g; at $36 it is about $1.18. Nutricost lists its 2 lb unflavored tub on its own site at about $40 for roughly 81 servings of 10 g, which works out to about $1.23 per 25 g before any discount.

| Powder | Approx. cost per 25 g | Notes |
| --- | --- | --- |
| BulkSupplements Soy Protein Isolate | About $0.75-0.90 | 1 kg pouch; Subscribe and Save brings it lower |
| NOW Sports Soy Protein Isolate | About $0.95-1.20 | 2 lb tub; Informed Sport |
| Horbaach Soy Protein Isolate | Check the listing | 3 lb bag; price varies by seller |
| Nutricost Soy Protein Isolate | About $1.10-1.30 | Small 10 g scoops; flavored options |
| Soylent Complete Protein | Premium | 10-serving tub with added nutrients |

These are approximate ranges based on typical prices at the time of writing, and protein powder prices move constantly, so treat them as a guide to relative value rather than a quote. Amazon shows a price per ounce on most listings; that is a quick shortcut, but it ignores how much of each ounce is protein, which matters when you compare an isolate with a meal shake.

The cheapest way to buy soy is the same as for any powder: the largest unflavored size, on Subscribe and Save, during a sale. Unflavored isolates keep well, so a bigger bag is low risk if you already know you can live with the taste in a smoothie. For whey and plant options across every budget, see our [cheap protein powder rankings](https://www.best-proteinpowder.com/best-cheap-protein-powder/).

## 8. Allergies, Thyroid, Heavy Metals and When to Ask a Doctor

Soy is one of the nine major food allergens in the United States, and soy allergy is common in young children, though many outgrow it. If you react to soy foods, soy protein isolate is not a safe workaround, because the protein is the allergen. People with a peanut allergy sometimes ask about soy too; cross-reactivity is possible but not the rule, so ask an allergist rather than testing it yourself.

Thyroid questions come up often. Research reviews have generally found that soy does not cause hypothyroidism in people with normal thyroid function and adequate iodine intake. The more practical issue is medication: soy can interfere with the absorption of levothyroxine, so people on thyroid medication are usually advised to separate the two. Ask your doctor or pharmacist how far apart to take them; this is not medical advice.

Breast cancer is the other common worry. The American Cancer Society and the American Institute for Cancer Research both say that eating traditional soy foods is safe for breast cancer survivors and may even be beneficial. Evidence on concentrated isoflavone supplements is thinner, and a protein isolate sits somewhere in between, so anyone with a hormone-sensitive cancer should discuss soy powders with their oncology team.

Heavy metals are a plant-protein issue generally, not a soy-specific one. Crops take up lead, cadmium and arsenic from soil, and independent testing has found plant powders tend to carry more lead on average than whey. Nutricost's soy isolate and Soylent's powder both carry California Proposition 65 lead warnings on their listings, which reflect a very low threshold rather than proof of harm, but are worth knowing about.

If metals concern you, a sport certification like NOW Sports' Informed Sport status adds a layer of independent screening, though it focuses on banned substances rather than metals. Our [is protein powder safe](https://www.best-proteinpowder.com/is-protein-powder-safe/) guide covers what each certification does and does not test for, and how to request batch results from a brand.

## 9. GMO, Hexane and Organic: Label Worries Sorted

The large majority of soybeans grown in the US are genetically engineered, which is why so many soy powders advertise non-GMO status. NOW Sports carries Non-GMO Project Verification on its soy isolate, and several other brands describe their soy as non-GMO or identity preserved, meaning the beans were kept separate from conventional supply. Major scientific bodies regard approved GM crops as safe to eat, so this is a preference rather than a safety rule.

Hexane is the other label worry. Conventional soybean processing uses the solvent hexane to extract oil, and protein isolates are made from the defatted flakes left over. The finished isolate goes through further washing and drying, and residues are generally considered trace. If you would rather avoid the issue altogether, USDA organic rules do not allow hexane-extracted organic ingredients, but certified organic soy isolate is hard to find in shake form.

Gums, lecithin and sweeteners are worth a glance too. Unflavored isolates usually list nothing but soy protein isolate. Flavored versions add cocoa, natural flavors, salt and a sweetener; Nutricost's vanilla, for example, lists stevia and a few grams of added sugar per scoop. Soylent sweetens with allulose and adds MCT oil, vitamins and minerals, which makes it closer to a light meal than a plain protein shake.

None of these issues should outweigh the basics. A soy powder that delivers the protein it claims, comes from a brand that tests its batches, and fits your budget is a good buy whether or not it carries every badge. Pay for organic or non-GMO if those matter to you, but do not pay a premium for them believing they make the protein itself work better.

## 10. Who Should Buy Soy Protein, and Who Should Skip It

Soy makes the most sense for three groups. Vegans and vegetarians who want a complete protein with no blending tricks; people on a tight budget who want the cheapest honest gram of protein in powder form; and anyone who cannot tolerate dairy but finds pea protein too earthy or too gritty. For all three, an unflavored isolate in a smoothie is about as efficient as protein gets.

Soy also suits people who already eat tofu, tempeh and soy milk and like the taste. If you are new to it, buy the smallest size first. Soy isolate in water is an acquired taste for many buyers, who report a thick, slightly beany shake; blended with frozen fruit, oats or cocoa, most of those complaints go away. If you want a pea option instead, see our [best pea protein powder](https://www.best-proteinpowder.com/best-pea-protein-powder/) rankings.

Skip soy if you have a soy allergy, obviously, and talk to your doctor first if you take thyroid medication, have a hormone-sensitive cancer, or are pregnant. Skip it too if you will only drink a shake that tastes like a dessert; the flavored soy options are limited, and you will likely be happier with a well-reviewed whey or pea blend. Hemp is another plant option worth a look in our [best hemp protein powder](https://www.best-proteinpowder.com/best-hemp-protein-powder/) guide.

For most buyers, our recommendation is simple. Start with NOW Sports Soy Protein Isolate if you value independent testing, or BulkSupplements if cost per gram is the priority. Blend it, take a slightly bigger serving than you would of whey, and ignore the hormone scare stories. The evidence on soy is better than its reputation, and the price is hard to beat.

## 11. Common Mistakes to Avoid

### Dosing soy like whey

A 25 g scoop of soy has a little less leucine than the same amount of whey. If the shake is your main post-workout feed, take 30 to 35 g of soy protein, or pair a smaller scoop with a protein-rich meal, so you are not shortchanging the muscle-building signal.

### Buying a meal shake by mistake

Many products that appear in a soy protein search are pea blends, whey, or soy-based meal replacements with sugar and only 13 to 15 g of protein. Read the ingredient list and the protein per serving before you buy, not the product photo or the word soy in the title.

### Comparing tubs instead of protein

A cheap tub of 10 g scoops can cost more per gram than a pricier tub of 25 g scoops. Divide the price by the total grams of protein in the container, then multiply by 25, and you will see which one is actually the better deal.

### Drinking unflavored isolate in water

Plain soy isolate in water is thick and beany, and buyers who try it that way often give up. Blend it with frozen fruit, oats, cocoa or nut butter, or stir it into yogurt or oatmeal, where the taste largely disappears.

### Ignoring medication interactions

Soy can interfere with the absorption of levothyroxine and may matter for hormone-sensitive conditions. If you take thyroid medication, are being treated for cancer, or are pregnant, ask your doctor before adding a daily soy shake rather than relying on what you read online.

## 12. Frequently Asked Questions

### Does soy protein lower testosterone?

Not at normal intakes. A 2021 meta-analysis in Reproductive Toxicology pooled dozens of clinical studies in men and found no significant effect of soy protein or isoflavones on testosterone or estrogen. Case reports of feminizing effects involve extreme intakes, such as about three quarts of soy milk a day. One or two soy shakes daily is well within the studied range.

### Is soy protein as good as whey for building muscle?

Close to it, once the dose is matched. Soy has slightly less leucine, so 30 to 35 g of soy protein roughly matches a 25 g whey scoop. Meta-analyses of resistance-training trials have found no significant difference in lean mass or strength gains, and a 2021 trial found vegans using soy gained as much as omnivores using whey.

### Is soy protein a complete protein?

Yes. Soy contains all nine essential amino acids in useful amounts, and soy protein isolate scores the maximum 1.0 on the PDCAAS scale used for food labeling. Its limiting amino acid is methionine, which is lower than in dairy or eggs, but a varied diet covers that easily. It is one of very few plant proteins that is complete on its own.

### What is the difference between soy protein isolate and concentrate?

Isolate is the most refined form, at least 90 percent protein on a dry basis, with most carbohydrate and fat removed. Concentrate is around 70 percent protein and keeps more carbohydrate and fiber. Isolate gives more protein per calorie and mixes more smoothly, which is why every powder on this list uses it as the main ingredient.

### Is soy protein safe for women?

For most women, yes. Soy foods are a staple in many diets, and major cancer organizations say traditional soy foods are safe for breast cancer survivors. Evidence on concentrated isoflavone supplements is thinner, so women with hormone-sensitive cancers, those who are pregnant, and anyone on hormone therapy should discuss soy powders with their doctor first.

### Can soy protein affect my thyroid?

Research reviews generally find soy does not cause thyroid problems in people with normal thyroid function and enough iodine. The practical concern is medication: soy can reduce the absorption of levothyroxine. If you take thyroid medication, ask your doctor or pharmacist how far apart to take your dose and your soy shake. This is not medical advice.

### What is the cheapest soy protein powder?

Unflavored bulk isolates are the cheapest. At typical prices, BulkSupplements Soy Protein Isolate works out to roughly $0.75 to $0.90 per 25 g of protein, with NOW Sports close behind and adding Informed Sport testing. Prices change often, so compare cost per 25 g of protein, not price per tub, before you buy.

## Related Plant Protein Guides

- [Best Plant-Based Protein Powder](https://www.best-proteinpowder.com/best-plant-based-protein-powder/): Pea, soy and blends ranked on dose, testing and taste.
- [Best Pea Protein Powder](https://www.best-proteinpowder.com/best-pea-protein-powder/): The other big plant isolate, ranked on leucine and cost.
- [Whey vs Plant Protein](https://www.best-proteinpowder.com/whey-vs-plant-protein/): How plant proteins stack up against whey for muscle.

## The Bottom Line

Soy protein isolate is one of the best value powders you can buy: complete, protein-dense, cheap per gram, and backed by trials showing muscle gains comparable to whey when you take a slightly bigger dose. The testosterone scare does not survive contact with the clinical evidence at normal intakes.

Buy NOW Sports Soy Protein Isolate if you want independent testing, BulkSupplements if you want the lowest cost per gram, and Nutricost or Soylent if you need flavor. Blend it, dose it at 30 to 35 g when it matters, and talk to your doctor if you have a soy allergy, take thyroid medication or have a hormone-sensitive condition.

## More in Plant Protein

Guides in this category that pair with what you just read.

- [Best Protein Powder for Menopause](https://www.best-proteinpowder.com/best-protein-powder-for-menopause/)
- [Best Plant-Based Protein Powder](https://www.best-proteinpowder.com/best-plant-based-protein-powder/)
- [Best Pea Protein Powder](https://www.best-proteinpowder.com/best-pea-protein-powder/)
- [Best Hemp Protein Powder](https://www.best-proteinpowder.com/best-hemp-protein-powder/)

About the Author

[Best Protein Powder Editorial Team](https://www.best-proteinpowder.com/authors/editorial-team/), Supplements Desk

The editorial team reads the label, the certificate and the price per gram of protein before a powder gets near a ranking -- and says plainly when a cheaper tub does the same job.

[Full profile](https://www.best-proteinpowder.com/authors/editorial-team/) | [How we test](https://www.best-proteinpowder.com/how-we-test/)