# Is soy protein safe? Does it affect hormones in South Africa?

Source: https://www.healthnews24.co.za/answers/is-soy-protein-safe-hormones-south-africa/
Publisher: Healthnews24 (South Africa's supplement answer desk)
Updated: 2026-10-27

> Soy protein is safe for most healthy adults in South Africa and does not lower testosterone or raise oestrogen at normal intakes. A 2021 meta-analysis of 41 clinical studies found no effect on total or free testosterone, or on oestrogen, in men. Avoid it if you are allergic to soy, and ask your doctor first if you take thyroid medication.

Key facts:
- Reed et al. 2021 pooled 41 clinical studies: soy protein and isoflavones did not change total testosterone, free testosterone or oestrogen in men.
- Total testosterone was measured in 1 753 men across the studies in that meta-analysis.
- Soy isolate ranks above pea protein concentrate on DIAAS, but below whey (Mathai et al. 2017).
- Consumer Reports (October 2025) found plant-based protein powders averaged about 9 times the lead of dairy-based ones.

## Does soy protein lower testosterone or raise oestrogen?

No. The strongest evidence is a 2021 meta-analysis by Reed and colleagues in *Reproductive Toxicology*, which pooled 41 clinical studies. Total testosterone was measured in 1 753 men, free testosterone in 752 and oestradiol in 1 000. Neither soy protein nor isoflavone intake changed any of them, whatever the dose or study length.

The worry comes from isoflavones, plant compounds that bind weakly to oestrogen receptors, and from animal studies that used doses far above what anyone eats or shakes. In people, the effect simply has not appeared. Evidence level: **strong** for male reproductive hormones.

## Is soy a good protein for building muscle?

It is the best single plant protein for amino-acid completeness. In the DIAAS ranking (Mathai, Liu & Stein 2017), whey and milk protein scored highest, soy isolate came next and pea protein concentrate lower. Because plant proteins carry less leucine than whey, aim for roughly 30-40 g of soy protein per serving instead of the 20-25 g typical for whey. The ISSN position stand puts the target for training adults at 1.4-2.0 g of protein per kg of body weight per day, whatever the source.

For a head-to-head view, see [plant protein vs whey for muscle](/answers/plant-protein-vs-whey-for-muscle-south-africa/) and [pea vs rice vs soy](/answers/pea-vs-rice-vs-soy-protein-south-africa/).

## Who should be careful with soy protein?

- **Soy allergy:** a firm contraindication for soy isolate.
- **Thyroid medication:** soy taken close to a levothyroxine dose may reduce how well the tablet is absorbed. Speak to your doctor or pharmacist about timing rather than adjusting doses yourself.
- **Hormone-sensitive conditions** (for example a history of hormone-receptor-positive breast cancer): follow your oncologist's advice on soy foods and concentrated isoflavone products.
- **Under-18s:** meet protein needs through food first.

## What about contaminants in plant protein powders?

This is the more practical concern. Consumer Reports' October 2025 test of 23 protein powders found plant-based products averaged about nine times the lead of dairy-based ones. No South African brand has published comparable testing that we could verify, so favour brands that publish certificates of analysis, and rotate protein sources rather than relying on one powder every day.

## Can you buy soy protein powder in South Africa?

In our October 2026 catalogue, the plant powders whose ingredient lists we captured are built on pea, rice, pumpkin seed, chickpea or hemp, not soy isolate. For example, Faithful to Nature Pea Protein Isolate costs R185 for 650 g (regular price, October 2026) and the USN BlueLab Plant Based Protein costs R399 to R474.99 for 900 g, depending on retailer. Whole soy foods (tofu, tempeh, edamame, soya mince and soy milk) are easy to find in South African supermarkets and are a cheap way to get soy protein. See the [best plant protein in South Africa](/best/best-plant-protein-south-africa/) for the full comparison.

## The bottom line

For healthy adults, one or two servings of soy foods or soy protein a day is safe and does not interfere with testosterone. The sensible exceptions are allergy, thyroid medication and hormone-sensitive disease, where a doctor or registered dietitian should guide you.

## Products mentioned

| Product | Size | Price (source, date) | Servings | R per 30 g protein | R per 100 g powder | Sweeteners |
|---|---|---|---|---|---|---|
| Faithful to Nature (FtN) Pea Protein Isolate Powder (650 g) | 650 g | R148-R185 (Faithful to Nature, 2026-10) | — | R11.38 | R28.46 | none |
| USN BlueLab 100% Plant Based Protein (900 g) | 900 g | R399-R474.99 (Dis-Chem Living Fit, 2026-10) | — | R24.54 | R52.78 | sucralose, acesulfame-K, steviol glycosides |
| Biogen Plant Based Protein (700 g) | 700 g | R400 (Biogen shop (biogen.co.za), 2026-10) | — | R34.28 | R57.14 | sucralose and steviol glycosides |

## Sources

- [Reed et al. 2021, Neither soy nor isoflavone intake affects male reproductive hormones: an expanded and updated meta-analysis of clinical studies (Reprod Toxicol 100:60-67)](https://pubmed.ncbi.nlm.nih.gov/33383165/)
- [Mathai, Liu & Stein 2017, DIAAS values for dairy and plant proteins vs PDCAAS (Br J Nutr), AGRIS record](https://agris.fao.org/search/en/records/65de3cca0f3e94b9e5cc4b98)
- [Jäger et al. 2017, ISSN position stand: protein and exercise (JISSN)](https://pubmed.ncbi.nlm.nih.gov/28642676/)
- [OPB/NPR (Oct 2025): Consumer Reports finds lead in popular protein powders](https://www.opb.org/article/2025/10/16/consumer-reports-finds-lead-in-popular-protein-powders/)