Is Soy Protein Bad for You? Hormones, Thyroid and the Trials

On the trial evidence, no. Across 41 clinical studies, soya protein and isoflavones did not change testosterone or oestrogen in men, whatever the dose or duration; across 18 randomised trials soya left thyroid hormones unchanged, with a borderline rise in TSH; and soya protein sits in the same high-quality DIAAS band as whey in the 17-protein compilation and matched milk protein for muscle across 17 trials.^{1, 2, 6, 8} Soya is also one of the UK's 14 labelled allergens.¹¹ This is the soya profile in the Wellgard guide to vegan protein sources.

What is soya protein?

The protein of the soya bean, eaten as a food or extracted as an ingredient. The foods are tofu, tempeh, edamame, cooked soya beans, soya drinks and yoghurts, and textured vegetable protein, which is defatted soya flour (see what is textured vegetable protein). The extracted ingredient, isolate or concentrate, goes into powders, bars and meat alternatives. The UK figures below are grams of protein as eaten from the national composition dataset; they say nothing about digestibility, which is scored further down.¹⁴

Does soya protein raise oestrogen or lower testosterone in men?

Not in the clinical studies. Soya contains isoflavones, plant compounds whose chemical structure resembles oestrogen. The 2021 meta-analysis pooled 41 clinical studies in which adult men ate soya foods, soya protein or isoflavone extracts: total testosterone was measured in 1,753 men, free testosterone in 752, oestradiol in 1,000, oestrone in 239 and sex hormone binding globulin in 967. Soya had no significant effect on any of them, at any isoflavone dose or duration.¹ One author, who also appears on the muscle and cholesterol meta-analyses below, is affiliated with a soya nutrition institute. Isoflavones are a soya-bean compound, so the question does not arise for pea, rice or hemp protein.

Does soya affect the thyroid?

Across 18 randomised trials pooled in 2019 by a UK-led group, soya did not change free T3 or free T4, while TSH rose by 0.248 mIU/L (95% CI 0.001 to 0.494), a rise the authors call very modest and of unclear clinical significance.² An earlier review of 14 trials in presumably healthy adults found no effect or only very modest changes in all but one, and concluded there is little evidence that soya affects thyroid function in people with normal thyroids and adequate iodine, so soya consumers should make sure their iodine intake is adequate; the NHS lists iodine among the nutrients a vegan diet may need from fortified foods or a supplement.^{3, 13} It also notes that soya foods may inhibit absorption of thyroid hormone tablets and that people with an underactive thyroid need not avoid soya.³ The NHS asks for levothyroxine on an empty stomach 30 to 60 minutes before the first meal of the day; soya is food, so it sits after that gap, the same clock as on can I take collagen with thyroid medication, and a pharmacist settles the timing.¹⁵

Does soya lower cholesterol?

In the trials, modestly. A 2019 meta-analysis of 46 controlled trials found that soya protein at a median 25 g a day for a median six weeks lowered LDL cholesterol by 4.76 mg/dL, about 3 to 4%; most trials showed a reduction, but only a minority were individually significant.⁴ That is a trial finding, not a label claim: the GB register lists the "25 g of soya protein a day" cholesterol and heart wordings among its non-authorised entries, so no UK food or supplement may make them.¹⁶ Anyone prescribed a statin keeps taking it.

How good is soya protein? Quality and the muscle trials

By composition, soya is a complete protein, all nine essential amino acids in adequate amounts, a fact about amino acids rather than a benefit (is soy a complete protein explains the scores). Two laboratories scoring soya isolates and a concentrate on the PDCAAS scale returned 1.00 and 0.95 to 1.00 (authors employed by a soya processor); a DIAAS compilation of 17 proteins placed whey and soya at or above 75, "high quality", and pork, casein, egg and potato above 100 (authors employed by a potato-protein producer).^{5, 6}

In 2018, nine trials in 266 people compared soya protein with whey, beef, milk or dairy protein during resistance training: strength and lean mass rose in every group, with no difference between soya and the animal proteins.⁷ In 2025, a pooling of 30 randomised trials found no difference between soya and milk protein for muscle mass across 17 trials; plant protein as a whole gave slightly less muscle mass (standardised mean difference -0.20), seen under 60 and not at 60 and over, and carried by five trials of non-soya single sources and seven of plant-based diets; strength (14 trials) and physical performance (5 trials) did not differ.⁸ In one diet trial, 19 vegans on soya isolate and 19 omnivores on whey, both at 1.6 g of protein per kilogram a day, trained for 12 weeks: leg lean mass rose 1.2 kg in both groups, with no difference in muscle size or leg-press strength.⁹ Powder labels are on is vegan protein powder healthy; the food side is on how much protein vegans need.

Who should be careful with soya?

Anyone allergic to it. Soybeans are one of the 14 allergens UK law requires to be emphasised in every ingredients list; peas, chickpeas and lentils are not on that list, although the NHS now names "peanuts, soybeans, peas and chickpeas" together among the most common allergic foods, and pea protein isolates have caused reactions of their own.^{11, 12, 17} In 146 legume-allergic patients, co-allergy to other legumes was uncommon in people allergic to soya or peanut (16.7% or less), while people allergic to green pea, lupin, lentil or bean were often co-allergic to soya (50 to 64.7%).¹⁰ A 2024 review reports anaphylaxis after eating a legume and then exercising for soya, lentils and chickpea.¹⁷ The NHS route is a GP if you think you have a food allergy, 999 for sudden swelling of the lips, mouth, throat or tongue or struggling to breathe.¹² Babies: the NHS says soya formula is used only under medical supervision, when a health visitor or GP has recommended it.¹⁸

Is soya protein isolate different from soya foods?

In what it leaves behind. An isolate is the bean's protein with most of the fat, carbohydrate and fibre removed; the whole foods bring fibre, and the fortified drinks calcium, vitamin D and B12.¹³ "Ultra-processed" is the word people now search beside it, and the trial record answers it: the hormone meta-analysis pooled soya foods, soya protein and isoflavone extracts alike, the cholesterol trials used soya protein, and the vegan training trial used soya protein isolate.^{1, 4, 9} Which form fits depends on whether the meal needs 12 g of protein with fibre or 20 g in a glass.

Frequently asked questions

Is soya bad for men?

Not on the clinical evidence: across 41 studies, testosterone, oestradiol, oestrone and SHBG did not change with soya protein or isoflavones at any dose or duration.¹

Does soya protein powder contain oestrogen?

No. It contains isoflavones, plant compounds whose structure resembles oestrogen; in men they did not raise oestradiol or oestrone across the pooled studies.¹

Is soya a complete protein?

Yes, by composition: all nine essential amino acids, PDCAAS 0.95 to 1.00 for isolates, DIAAS at or above 75.^{5, 6}

Is soya protein isolate bad for you?

The same trial record applies to the isolate as to the foods, including equal muscle gains to whey at a matched 1.6 g/kg; it is a labelled allergen.^{1, 4, 9, 11}

Can you have soya with levothyroxine?

The thyroid review says an underactive thyroid is no reason to avoid soya foods, though soya may inhibit absorption of the tablet; the NHS asks for levothyroxine on an empty stomach 30 to 60 minutes before the first meal.^{3, 15} A pharmacist settles the timing.

Sources

1. Reed KE, et al. Neither soy nor isoflavone intake affects male reproductive hormones: an expanded and updated meta-analysis of clinical studies. Reproductive Toxicology, 2021. PubMed 33383165.
2. Otun J, et al. Systematic review and meta-analysis on the effect of soy on thyroid function. Scientific Reports, 2019. PubMed 30850697.
3. Messina M, Redmond G. Effects of soy protein and soybean isoflavones on thyroid function in healthy adults and hypothyroid patients. Thyroid, 2006. PubMed 16571087.
4. Blanco Mejia S, et al. A meta-analysis of 46 studies identified by the FDA demonstrates that soy protein decreases circulating LDL and total cholesterol concentrations in adults. Journal of Nutrition, 2019. PubMed 31006811.
5. Hughes GJ, et al. Protein digestibility-corrected amino acid scores (PDCAAS) for soy protein isolates and concentrate. Journal of Agricultural and Food Chemistry, 2011. PubMed 22017752.
6. Herreman L, et al. Comprehensive overview of the quality of plant- and animal-sourced proteins based on the digestible indispensable amino acid score. Food Science and Nutrition, 2020. PubMed 33133540.
7. 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. International Journal of Sport Nutrition and Exercise Metabolism, 2018. PubMed 29722584.
8. Reid-McCann RJ, et al. Effect of plant versus animal protein on muscle mass, strength, physical performance, and sarcopenia: a systematic review and meta-analysis. Nutrition Reviews, 2025. PubMed 39813010.
9. Hevia-Larraín V, et al. High-protein plant-based diet versus a protein-matched omnivorous diet to support resistance training adaptations. Sports Medicine, 2021. PubMed 33599941.
10. Smits M, et al. Co-sensitization between legumes is frequently seen, but variable and not always clinically relevant. Frontiers in Allergy, 2023. PubMed 37007648.
11. Food Standards Agency. Allergen labelling for food manufacturers, 14 December 2017. gov.uk.
12. NHS. Food allergy. Page last reviewed 20 August 2026. nhs.uk.
13. NHS. The vegan diet. Page last reviewed 5 June 2026. nhs.uk.
14. Public Health England. McCance and Widdowson's Composition of Foods Integrated Dataset (CoFID), 2021. gov.uk.
15. NHS. Levothyroxine. Page last reviewed 6 July 2026. nhs.uk.
16. Department of Health and Social Care. Great Britain nutrition and health claims (NHC) register, spreadsheet of 19 May 2026. gov.uk.
17. Abu Risha M, et al. Legume allergens pea, chickpea, lentil, lupine and beyond. Current Allergy and Asthma Reports, 2024. PubMed 38990406.
18. NHS. Types of formula milk. Page last reviewed 6 April 2023. nhs.uk.

General information, not medical advice: anyone with a food allergy, a thyroid condition or a levothyroxine prescription talks to their GP or pharmacist before changing what they eat.