Soybeans can be eaten whole or made into a variety of products, including tofu, tempeh, soy milk and other dairy and meat alternatives.
It can also be turned into soy protein powder.
For vegetarians, vegans and those who avoid or are allergic to dairy foods, soy protein often serve as a major source of this important nutrient.
However, soy is a somewhat controversial food.
While some think of it as a nutritional powerhouse, others see it as an enemy to health.
This article takes a look at the evidence to tell you whether soy protein is good or bad for you.
Soy protein isolate powder is made from defatted soybean flakes that have been washed in either alcohol or water to remove the sugars and dietary fiber. They’re then dehydrated and turned into a powder.
This product contains very little fat and no cholesterol.
Soy protein powder is used to make infant soy formula, as well as a variety of meat and dairy alternatives.
Here’s the nutrient content of one ounce (28 grams) of soy protein isolate powder (1):
- Calories: 95
- Fat: 1 gram
- Carbs: 2 grams
- Fiber: 1.6 grams
- Protein: 23 grams
- Iron: 25% of the Daily Value (DV)
- Phosphorus: 22% of the DV
- Copper: 22% of the DV
- Manganese: 21% of the DV
Though it’s a concentrated source of protein, soy protein isolate powder also contains phytates, which can decrease mineral absorption.
Summary While a good source of plant-based protein and rich in nutrients, soy protein and its powder contain phytates, which decrease mineral absorption.
Unlike most other plant-based proteins, soy protein is a complete protein.
This means it contains all of the essential amino acids your body cannot make and needs to obtain from food.
While each amino acid plays a role in muscle protein synthesis, branched-chain amino acids (BCAAs) are the most important when it comes to muscle building (, ).
One study showed that people who drank 5.6 grams of BCAAs after a resistance workout had a 22% greater increase in muscle protein synthesis than those given a placebo ().
Specifically, the BCAA leucine activates a particular pathway that prompts muscle protein synthesis and helps build muscle (, ).
Compared to whey and casein proteins, soy protein sits somewhere in the middle as far as muscle protein synthesis goes.
One study showed that soy was inferior to whey protein in regards to synthesizing protein for muscle but performed better than casein. Researchers concluded that this could be due to digestion rate or leucine content ().
Similarly, a review study found that whey protein supports muscle protein synthesis better than soy protein in young people and older adults ().
Interestingly, soy may benefit you best when combined with other proteins.
Some research suggests that combining dairy and soy proteins may result in greater muscle protein synthesis than whey, casein or soy alone ().
Summary Though soy protein contains the BCAA leucine and enhances muscle protein synthesis to some degree, it appears to be inferior to whey protein for building muscle.
Studies indicate that high-protein diets can result in weight loss, even without limiting calories or nutrients (, , ).
However, the evidence is mixed regarding the relationship between soy protein and weight loss.
Some studies show that soy protein can boost weight loss as effectively as animal-based proteins.
In one study, 20 men with obesity participated in both a soy-based high-protein diet, as well as a meat-based high-protein diet. Real food was used rather than soy-based meal replacements ().
Appetite control and weight loss were similar in both groups. The researchers concluded that soy-based high-protein diets were as effective for weight loss as animal-based high-protein diets.
Another 12-week weight loss study found similar results with soy protein powder. Participants received soy-based or non-soy-based meal replacements. Both resulted in an average weight loss of 17.2 pounds (7.8 kg) by the end of the study ().
What’s more, one study in people with diabetes and obesity showed that soy protein-based meal replacements, such as shakes, may be superior to standard weight loss diets ().
Those who consumed the soy-protein-based meal replacement lost an average of 4.4 pounds (2 kg) more than those following standard diets.
However, while some studies observe weight loss benefits, a review of 40 studies evaluating soy protein’s effect on weight, waist circumference and fat mass found no significant positive effects ().
Overall, the evidence for consuming soy protein for weight loss is not as strong as it is for other proteins like whey and casein (, ).
Summary Some research indicates that soy can be effective for weight loss, but the evidence is mixed and doesn’t show it to be more effective than other proteins.
Some research suggests that adding soy protein to your diet may provide a variety of health benefits.
Another review showed that replacing animal protein with 25 grams or more of soy protein resulted in decreased total cholesterol, “bad” LDL cholesterol and triglyceride levels ().
In regards to cancer, the evidence appears mixed.
Many observational studies have seen a protective effect of a high-soy diet.
However, they note that it remains unknown whether this applies to soy protein isolate powder or other textured vegetable protein made from soybeans.
Some observational and case-controlled studies link soy intake to a reduced risk of breast cancer (, , ).
Yet others show no protective benefit of consuming soy for this type of cancer. One study even connected soy intake to stimulated rapid cell production in the breasts of premenopausal women, possibly increasing their potential breast cancer risk (, ).
When discussing soy’s role in men’s health, some observational studies indicate that consuming soy foods may reduce the risk of prostate cancer in older men (, 27).
Though results of observational studies are encouraging, human clinical trials on the potential cancer-protective effects of soy are inconclusive at this point.
Additionally, many studies are based on soy foods rather than soy protein powder specifically.
Nonetheless, soy protein can serve as a good source of plant-based protein for people who don’t consume animal proteins, including vegetarians and vegans, allowing them to reap the important benefits of this nutrient ().
Summary Soy foods may provide potential health benefits like reducing cholesterol and possibly reducing cancer risk, but more research is needed.
Some people have concerns about soy.
As mentioned, soy protein contains phytates, also known as antinutrients. These reduce the availability of iron and zinc in soy protein (, ).
However, phytates do not adversely affect your health unless your diet is severely imbalanced and you rely on soy protein as a source of iron and zinc.
There is also some concern that soy intake may affect a person’s thyroid function.
The isoflavones in soy function as goitrogens that can interfere with thyroid function and the production of hormones (, ).
Furthermore, many people stay clear of soy protein due to its phytoestrogen content, as they fear that phytoestrogens can disrupt natural hormone levels in the body.
Phytoestrogens are chemical compounds that occur naturally in plants and have estrogen-like properties that bind to estrogen receptors in your body. Soy is a notable source of these ().
Yet soy protein powder is made from soybeans rinsed in alcohol and water, which removes a good portion of the phytoestrogen content (, ).
Similarly, many men worry that soy protein could decrease their testosterone levels, but research does not support this claim.
An extensive review study indicated that neither soy foods nor soy isoflavone supplements alter measures of testosterone in men ().
Finally, soy products are controversial because they are frequently genetically modified (GMO). There is currently no good evidence that eating genetically modified soybeans has any adverse health effects compared to non-GMO varieties.
Many of the potential drawbacks of soy are attributed to eating soy in general, not specifically soy protein powder. More research needs to be conducted on soy protein powder specifically to see how it affects health.
Summary While there are some potential drawbacks to eating soy, the evidence is fairly weak and suggests that most people can consume soy with no issues.