Soy foods
Pooled trials found soy protein lowered LDL cholesterol about 3% to 4% against other proteins, and did not change testosterone or estrogen in men. Hot flash results disagree across reviews. Cohorts link soy to less breast cancer in Asian women, less recurrence and less prostate cancer. Trials found a slight TSH rise, and the levothyroxine label warns about soybean flour.
Add to Bio HackChecked 2026-11-13
- Status
- A food, not a regulated health product. FDA regulations authorize a food label health claim about soy protein and reduced risk of coronary heart disease. The FDA proposed revoking it in 2017, and stated in 2024 that the regulation remains in effect until that rule is finalized [1]. The Dietary Guidelines for Americans, 2025-2030 list soy among plant-sourced protein foods [2]. Soybeans are one of the nine major food allergens under US law [3].
- Also called
- soy, soya, soybeans, tofu, soy milk, soymilk, edamame, tempeh, miso, natto, soy protein, soy protein isolate, textured vegetable protein, soy isoflavones, phytoestrogens
- Studied for
- cholesterol, heart health, protein, menopause, hot flashes, breast cancer, prostate cancer, thyroid concerns, testosterone concerns, estrogen concerns in men
Strong several reviews agree · Moderate one review or consistent trials · Limited one or a few small studies · Untested no study in people, listed with what it rests on
Shown to help4
- StrongShown to lower LDL cholesterol by about 4.8 mg/dL, 3% to 4%, at a median 25 g of soy protein a day against non-soy protein, in pooled trials.
- ModerateLinked to 41% lower odds of breast cancer with higher soy isoflavone intake in studies of Asian women, with no clear link in Western women.
- ModerateLinked to 25% lower breast cancer recurrence at 10 mg or more of soy isoflavones a day after diagnosis, in pooled cohorts of US and Chinese women.
- ModerateLinked to 29% lower prostate cancer with higher total soy food intake in pooled studies of men, 10% lower in cohort studies alone, with no link to advanced disease.
Reported risks4
- LimitedA national survey estimated convincing soy allergy in 0.6% of US adults.Uncommon
- ModerateShown to raise TSH slightly, by 0.25 mIU/L, in pooled trials, with no change in free T3 or free T4.How often is not known
- LimitedThe levothyroxine label warns soybean flour may decrease its absorption. A case report agrees, while a crossover study of a soy isoflavone supplement found no change.How often is not known
- LimitedCase reports: breast growth in a man drinking 3 quarts of soy milk a day and in an 8-year-old boy eating a lot of soy, resolving after soy was stopped.How often is not known
What it was studied for, and what was found
Shown to help4
LDL cholesterol
Shown to lower LDL cholesterol by about 4.8 mg/dL, 3% to 4%, at a median 25 g of soy protein a day against non-soy protein, in pooled trials.
Compared with non-soy protein [4]; control groups the abstract does not describe [5]
The numbers and sources
- Effect
- Across 41 trials identified by the FDA, soy protein at a median 25 g a day for a median 6 weeks lowered LDL cholesterol by 4.76 mg/dL (95% CI -6.71 to -2.80) and total cholesterol by 6.41 mg/dL (95% CI -9.30 to -3.52), with no dose-response or publication bias. About 75% of trials showed a fall, though few were individually significant [4]. In 23 trials of soy protein with its isoflavones intact, total cholesterol fell 3.77%, LDL cholesterol 5.25% and triglycerides 7.27%, and HDL cholesterol rose 3.03%. Falls were larger in men and in people with high cholesterol. Tablets of extracted isoflavones did not lower total cholesterol [5].
- Population
- Adults with baseline LDL cholesterol of 110 to 201 mg/dL in 43 trials [4]. 23 randomized trials published 1995 to 2002 [5].
- Sources
- [4], [5]
- Caveats
- Heterogeneity was moderate (I² 55% for LDL) [4]. Most trials lasted weeks. The FDA has proposed revoking the soy protein heart claim [1], which the review authors attribute to a perceived lack of consistent LDL reduction in trials [4].
Breast cancer
Linked to 41% lower odds of breast cancer with higher soy isoflavone intake in studies of Asian women, with no clear link in Western women.
Compared with the lowest isoflavone intake [9]; 10 mg a day less isoflavones, in a dose-response model [10]; the lowest quartile, 4.5 mg a day [10]
The numbers and sources
- Effect
- In Asian studies, higher soy isoflavone intake was linked to lower breast cancer in premenopausal (OR 0.59, 95% CI 0.48 to 0.69) and postmenopausal women (OR 0.59, 95% CI 0.44 to 0.74). In Western postmenopausal women the link was marginal (OR 0.92, 95% CI 0.83 to 1.00) and not significant by study design [9]. In 300,000 Chinese women, the top quartile (19.1 mg a day) against the bottom (4.5 mg) was not linked to breast cancer (RR 1.00, 95% CI 0.81 to 1.22), and a meta-analysis of prospective studies found 3% lower risk per extra 10 mg a day (95% CI 1% to 5%) [10].
- Population
- 35 studies of pre- and postmenopausal women [9]. Over 300,000 women aged 30 to 79 with 2,289 cases over 10 years, plus prospective cohorts [10].
- Sources
- [9], [10]
- Caveats
- Observational, and many studies in one review were case-control [9]. Intake in Western populations is much lower than in Asian ones.
Breast cancer recurrence and survival
Linked to 25% lower breast cancer recurrence at 10 mg or more of soy isoflavones a day after diagnosis, in pooled cohorts of US and Chinese women.
Compared with women eating under 10 mg of isoflavones a day [11]; the lowest soy and isoflavone intake [12]
The numbers and sources
- Effect
- In pooled US and Chinese cohorts, 10 mg or more of isoflavones a day after diagnosis was linked to lower recurrence (HR 0.75, 95% CI 0.61 to 0.92), and to death from breast cancer (HR 0.83, 95% CI 0.64 to 1.07) and from any cause (HR 0.87, 95% CI 0.70 to 1.10) without reaching significance [11]. A meta-analysis linked intake before diagnosis to better overall survival (HR 0.84, 95% CI 0.71 to 0.98), found no significant link for intake after diagnosis, and linked both to lower recurrence [12].
- Population
- 9,514 breast cancer survivors followed a mean 7.4 years [11]. 37,275 women with breast cancer in 12 articles [12].
- Sources
- [11], [12]
- Caveats
- Observational. Survivors who eat soy may differ in other habits and treatments.
Prostate cancer
Linked to 29% lower prostate cancer with higher total soy food intake in pooled studies of men, 10% lower in cohort studies alone, with no link to advanced disease.
Compared with the lowest soy intake [13]
The numbers and sources
- Effect
- Total soy food intake was linked to lower prostate cancer (RR 0.71, 95% CI 0.58 to 0.85), 0.61 in case-control studies and 0.90 (95% CI 0.82 to 0.99) in cohort studies. Tofu gave 0.73 and miso 1.01. Total isoflavone intake, fermented soy and blood isoflavone levels were not linked, nor was soy food with advanced prostate cancer (RR 0.87, 95% CI 0.74 to 1.06) [13].
- Population
- Men in 30 articles, 16 of them on total soy intake [13].
- Sources
- [13]
- Caveats
- Observational, and case-control studies gave the strongest results [13]. One review.
Mixed results1
Hot flashes in menopause
Mixed results: two reviews found soy isoflavones, mostly as extracts, cut hot flashes by about 20% or one a day, while a Cochrane review found no conclusive evidence in women.
Compared with placebo [6][8]; placebo or control groups in randomized trials [7]
The numbers and sources
- Effect
- Extracted or synthesized soy isoflavones (median 54 mg a day) for 6 weeks to 12 months reduced hot flash frequency by 20.6% (95% CI -28.38 to -12.86) and severity by 26.2% against placebo, with high heterogeneity [6]. Dietary and supplemental soy isoflavones were linked to fewer daily hot flashes (mean difference -0.79, 95% CI -1.35 to -0.23) and less vaginal dryness [7]. A Cochrane review of 43 trials found no conclusive evidence that phytoestrogens reduce hot flashes or night sweats, noted a placebo response of 1% to 59%, and found extracts high in genistein (over 30 mg a day) consistently reduced hot flashes in four trials [8].
- Population
- Perimenopausal and postmenopausal women in 17 trials [6], 62 studies of 6,653 women [7] and 43 trials of 4,364 women [8].
- Sources
- [6], [7], [8]
- Caveats
- Most trials tested extracts or supplements, not soy foods, and dietary soy trials could not be pooled [8]. In one review 74% of trials had a high risk of bias in three or more areas [7].
Not shown to help1
Testosterone and estrogen in men
Not shown to change total or free testosterone, estrogen or SHBG in men eating soy foods, soy protein or isoflavones, in two pooled analyses of clinical studies.
Compared with placebo groups [15]; values before intake in the same men, in simpler models [15]; control and baseline values the abstract does not detail [14]
The numbers and sources
- Effect
- Across 41 studies, soy protein or isoflavone intake had no significant effect on total testosterone, free testosterone, estradiol, estrone or SHBG, regardless of the statistical model, dose or study duration [14]. An earlier analysis of 15 placebo-controlled treatment groups, and 36 groups in simpler models, found no effect on testosterone, SHBG, free testosterone or free androgen index [15].
- Population
- Total testosterone measured in 1,753 men, free testosterone in 752 and estradiol in 1,000 [14]. Adult men in 32 reports [15].
- Sources
- [14], [15]
- Caveats
- Several authors of both analyses overlap. Studies included isoflavone extracts from soy or red clover as well as soy foods [14][15].
Amounts studied
Cholesterol trials gave a median 25 g of soy protein a day for a median 6 weeks [4], and in an earlier review isoflavone intakes above 80 mg a day showed larger lipid effects [5]. Hot flash trials used a median 54 mg of isoflavones a day for 6 weeks to 12 months [6], and Cochrane included trials of at least 12 weeks [8]. Breast cancer cohorts compared 10 mg or more of isoflavones a day with less [11], and Chinese women ate a mean 9.4 mg a day, 19.1 mg in the top quarter [10]. Case reports of breast growth involved 3 quarts of soy milk a day in a man [22] and marked soy intake in a boy [23].
Risks
Reported risks3
Soy allergy
A national survey estimated convincing soy allergy in 0.6% of US adults.
The numbers and sources
- How common
- Uncommon: between 1 in 1,000 and 1 in 100 people.
- Effect
- Convincing soy allergy was estimated at 0.6% (95% CI 0.5 to 0.7) of US adults, about 1.5 million people [21]. Soybeans are one of the nine major food allergens under US law [3].
- Population
- 40,443 US adults surveyed [21].
- Sources
- [21], [3]
- Caveats
- The estimate rests on reported symptoms, not food challenge testing [21].
Thyroid function
Shown to raise TSH slightly, by 0.25 mIU/L, in pooled trials, with no change in free T3 or free T4.
The numbers and sources
- How common
- How often is not known: the sources give no rate.
- Effect
- Soy raised thyroid stimulating hormone (WMD 0.248 mIU/L, 95% CI 0.001 to 0.494, I² 80.31%). Free T3 (0.027 pmol/L, 95% CI -0.052 to 0.107) and free T4 (-0.003 pmol/L, 95% CI -0.018 to 0.011) did not change. The authors call the clinical significance of the TSH rise unclear [16].
- Population
- Randomized trials in 18 articles [16].
- Sources
- [16]
- Caveats
- High heterogeneity, and the lower bound of the TSH interval is close to zero [16].
Breast growth in men and boys
Case reports: breast growth in a man drinking 3 quarts of soy milk a day and in an 8-year-old boy eating a lot of soy, resolving after soy was stopped.
The numbers and sources
- How common
- How often is not known: the sources give no rate.
- Effect
- A 60-year-old man with breast enlargement, erectile dysfunction and estradiol and estrone about 4 times the upper reference limit drank 3 quarts of soy milk a day. His breast tenderness resolved and estradiol slowly returned to normal after he stopped [22]. An 8-year-old boy developed one-sided breast growth with marked dietary soy intake, which resolved completely when high-soy foods were excluded [23].
- Population
- One adult man [22]. One prepubertal boy [23].
- Sources
- [22], [23]
- Caveats
- Case reports cannot give a rate. Pooled clinical studies found no change in men's estrogen or testosterone (see Outcomes).
Mixed results1
Lower absorption of levothyroxine
The levothyroxine label warns soybean flour may decrease its absorption. A case report agrees, while a crossover study of a soy isoflavone supplement found no change.
The numbers and sources
- How common
- How often is not known: the sources give no rate.
- Effect
- The Synthroid label states that soybean flour, cottonseed meal, walnuts and dietary fiber may bind and decrease levothyroxine absorption, and its instructions for crushed tablets exclude soybean-based infant formula as a vehicle [17]. A woman taking a soy protein drink right after levothyroxine needed unusually high doses, and lower doses sufficed once the two were taken apart [18]. In a crossover study, 60 mg of soy isoflavones taken with levothyroxine did not change its absorption (ratio 0.99 for total exposure) [19]. A review found the data mainly case reports and the clinical impact, if any, small [20].
- Population
- People taking levothyroxine [17]. One 45-year-old woman [18]. 12 postmenopausal women on levothyroxine [19].
- Sources
- [17], [18], [19], [20]
- Caveats
- The crossover study tested an isoflavone supplement, not soy protein or soy flour [19]. Levothyroxine is on its own page.
Checked, no increase found1
Stimulation of the womb lining
Not shown to stimulate the womb lining or vagina in women using phytoestrogen products for up to two years, in a Cochrane review.
The numbers and sources
- How common
- How often is not known: the sources give no rate.
- Effect
- The Cochrane review found no evidence that phytoestrogen treatments, including dietary soy and soy extracts, caused estrogen-like stimulation of the endometrium or vagina or other adverse effects when used for up to two years [8].
- Population
- Perimenopausal and postmenopausal women in 43 trials [8].
- Sources
- [8]
- Caveats
- Most trials were of supplements and lasted under two years [8].
Claims tested and not found
- That soy lowers testosterone or raises estrogen in men. Two pooled analyses of clinical studies found no effect on total or free testosterone, estradiol, estrone or SHBG [14][15].
- That soy changes thyroid hormone levels. Pooled trials found no change in free T3 or free T4 [16].
- That soy raises breast cancer recurrence. Pooled cohorts of US and Chinese survivors linked 10 mg or more of isoflavones a day to lower recurrence [11].
- That soy isoflavone supplements block levothyroxine. A crossover study found absorption unchanged when 60 mg of isoflavones were taken with levothyroxine [19].
Related
Sources
- US Food and Drug Administration. Agency Information Collection Activities; Submission for Office of Management and Budget Review; Comment Request; Record Retention Requirements for the Soy Protein and Reduced Risk of Coronary Heart Disease Health Claim. Federal Register 89 FR 48434. 2024. https://www.federalregister.gov/documents/2024/06/06/2024-12359/agency-information-collection-activities-submission-for-office-of-management-and-budget-review
- US Department of Agriculture and US Department of Health and Human Services. Dietary Guidelines for Americans, 2025-2030. realfood.gov. 2026. https://cdn.realfood.gov/DGA.pdf
- US Food and Drug Administration. Food Allergies. FDA. Content current as of 2026-03-11. https://www.fda.gov/food/nutrition-food-labeling-and-critical-foods/food-allergies
- Blanco Mejia S, Messina M, Li SS, 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. The Journal of Nutrition. 2019. https://doi.org/10.1093/jn/nxz020
- Zhan S, Ho SC. Meta-analysis of the effects of soy protein containing isoflavones on the lipid profile. The American Journal of Clinical Nutrition. 2005. https://doi.org/10.1093/ajcn.81.2.397
- Taku K, Melby MK, Kronenberg F, et al. Extracted or synthesized soybean isoflavones reduce menopausal hot flash frequency and severity: systematic review and meta-analysis of randomized controlled trials. Menopause. 2012. https://doi.org/10.1097/gme.0b013e3182410159
- Franco OH, Chowdhury R, Troup J, et al. Use of Plant-Based Therapies and Menopausal Symptoms: A Systematic Review and Meta-analysis. JAMA. 2016. https://doi.org/10.1001/jama.2016.8012
- Lethaby A, Marjoribanks J, Kronenberg F, et al. Phytoestrogens for menopausal vasomotor symptoms. Cochrane Database of Systematic Reviews. 2013. https://doi.org/10.1002/14651858.cd001395.pub4
- Chen M, Rao Y, Zheng Y, et al. Association between Soy Isoflavone Intake and Breast Cancer Risk for Pre- and Post-Menopausal Women: A Meta-Analysis of Epidemiological Studies. PLoS ONE. 2014. https://doi.org/10.1371/journal.pone.0089288
- Wei Y, Lv J, Guo Y, et al. Soy intake and breast cancer risk: a prospective study of 300,000 Chinese women and a dose-response meta-analysis. European Journal of Epidemiology. 2020. https://doi.org/10.1007/s10654-019-00585-4
- Nechuta SJ, Caan BJ, Chen WY, et al. Soy food intake after diagnosis of breast cancer and survival: an in-depth analysis of combined evidence from cohort studies of US and Chinese women. The American Journal of Clinical Nutrition. 2012. https://doi.org/10.3945/ajcn.112.035972
- Qiu S, Jiang C. Soy and isoflavones consumption and breast cancer survival and recurrence: a systematic review and meta-analysis. European Journal of Nutrition. 2019. https://doi.org/10.1007/s00394-018-1853-4
- Applegate CC, Rowles JL, Ranard KM, et al. Soy Consumption and the Risk of Prostate Cancer: An Updated Systematic Review and Meta-Analysis. Nutrients. 2018. https://doi.org/10.3390/nu10010040
- Reed KE, Camargo J, Hamilton-Reeves J, et al. Neither soy nor isoflavone intake affects male reproductive hormones: An expanded and updated meta-analysis of clinical studies. Reproductive Toxicology. 2021. https://doi.org/10.1016/j.reprotox.2020.12.019
- Hamilton-Reeves JM, Vazquez G, Duval SJ, et al. Clinical studies show no effects of soy protein or isoflavones on reproductive hormones in men: results of a meta-analysis. Fertility and Sterility. 2010. https://doi.org/10.1016/j.fertnstert.2009.04.038
- Otun J, Sahebkar A, Östlundh L, et al. Systematic Review and Meta-analysis on the Effect of Soy on Thyroid Function. Scientific Reports. 2019. https://doi.org/10.1038/s41598-019-40647-x
- AbbVie Inc. SYNTHROID (levothyroxine sodium) tablets, prescribing information. DailyMed, US National Library of Medicine. 2024. https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=1e11ad30-1041-4520-10b0-8f9d30d30fcc
- Bell DS, Ovalle F. Use of soy protein supplement and resultant need for increased dose of levothyroxine. Endocrine Practice. 2001. https://doi.org/10.4158/ep.7.3.193
- Persiani S, Sala F, Manzotti C, et al. Evaluation of Levothyroxine Bioavailability after Oral Administration of a Fixed Combination of Soy Isoflavones in Post-menopausal Female Volunteers. Drug Research. 2015. https://doi.org/10.1055/s-0035-1555784
- Gatta E, Maltese V, Cavadini M, et al. Interference or Noninterference Between Soy and Levothyroxine: That Is the Question. A Narrative Review of Literature. Endocrine Practice. 2023. https://doi.org/10.1016/j.eprac.2023.08.009
- Gupta RS, Warren CM, Smith BM, et al. Prevalence and Severity of Food Allergies Among US Adults. JAMA Network Open. 2019. https://doi.org/10.1001/jamanetworkopen.2018.5630
- Martinez J, Lewi JE. An unusual case of gynecomastia associated with soy product consumption. Endocrine Practice. 2008. https://doi.org/10.4158/ep.14.4.415
- Sea JL, Abramyan M, Chiu HK. Prepubescent unilateral gynecomastia secondary to excessive soy consumption. Journal of Pediatric Endocrinology and Metabolism. 2021. https://doi.org/10.1515/jpem-2020-0397