Soy protein isolate: a small, measured cholesterol effect

Across 39 randomized trials, soy protein isolate lowered LDL cholesterol by 7.3 mg/dL (95% CI 5.4 to 9.3; metric: 0.19 mmol/L, 0.14 to 0.24), and across 9 trials diastolic pressure by 1.99 mm Hg, while other soy products in the same 2008 meta-analysis did not. Doses and durations varied, and no trial in it measured heart attacks or strokes. A 2025 trial in 55 Singapore adults aged 60 to 80 found the same direction, with 20 g a day for 16 weeks lowering total cholesterol by 11 mg/dL (0.28 mmol/L) within the soy group of 18 to 19 people. Regulators split on it. US labeling rules still carry a model claim that 25 g of soy protein a day, in a diet low in saturated fat and cholesterol, may reduce heart disease risk, and the FDA proposed revoking it in 2017. The EU did not authorize the claim.

The numbers, per 3.5 oz (100 g)

What your body absorbs

• After a protein-matched drink, soy protein isolate raised blood essential and branched-chain amino acids less than whey or krill protein over 3 hours in 10 young men. — 10 healthy young men, four test days (n = 10) Evidence B source
  AUC for EAA and BCAA ranked whey > krill > soy; whey higher than soy at 20 to 60 min
• In 15 postmenopausal women, a month on soy protein isolate cut urinary calcium to about 80 to 85 mg a day from 121 mg on milk protein, with no change in calcium absorption or retention. — 15 postmenopausal women, three 1-month controlled diets (n = 15) Evidence B source
  Urinary calcium 85 and 80 mg/d on soy diets vs 121 mg/d control (P < 0.01); fractional absorption and retention unchanged

What it does to your health

• Across 39 randomized trials soy protein isolate lowered LDL cholesterol by 7.3 mg/dL (0.19 mmol/L), and across 9 trials diastolic pressure by 1.99 mm Hg, while other soy products did not. — adults in randomized trials of flavonoid-rich foods, 133 trials in total (n = 39 trials (LDL), 9 trials (DBP)) Evidence A source
  LDL -7.3 mg/dL (95% CI -9.3 to -5.4; metric: -0.19 mmol/L, -0.24 to -0.14); diastolic BP -1.99 mm Hg (-2.86 to -1.12)
• In Singapore adults aged 60 to 80 on a healthy diet, adding 20 g of soy protein isolate a day for 16 weeks lowered total cholesterol by 11 mg/dL (0.28 mmol/L) within the soy group. — 55 older adults in Singapore, 16 weeks, healthy dietary pattern (n = 55) Evidence B source
  Total cholesterol -0.28 +/- 4.6 mg/dL (0.12 mmol/L) in the soy group; control group triglycerides +29 mg/dL (0.33 mmol/L)
• A daily 20 g soy protein isolate drink for 2 years after prostate removal did not lower prostate cancer recurrence in 177 high-risk men (HR 0.96). — 177 men at high risk of recurrence after radical prostatectomy, 7 US centers, up to 2 years (n = 177) Evidence B source
  Biochemical recurrence 27.2% soy vs 29.5% casein placebo, HR 0.96 (95% CI 0.53 to 1.72)
• With 12 weeks of resistance training, 26 g of soy protein isolate built lean mass (about 3.3 lb (1.5 kg)) and strength as well as 19 g of whey matched for leucine. — 48 untrained men and women aged 18 to 35 who completed the trial, training 3 times a week (n = 48) Evidence B source
  Lean body mass +3.4 lb (95% CI 2.1 to 4.7; metric: 1.54 kg, 0.94 to 2.15) across groups, leg extension torque +40.27 Nm; no difference soy vs whey (p > 0.05)
• In 44 postmenopausal women, a soy protein isolate drink with added isoflavones for 24 weeks reduced facial wrinkle severity by 7.1% compared with baseline, more than casein. — 44 postmenopausal women, Fitzpatrick skin types I to III, 6 months (n = 44) Evidence B source
  Wrinkle severity -5.9% at 16 weeks and -7.1% at 24 weeks (p < 0.0001 vs casein at 24 weeks); cheek hydration +39% and +68%

Safety, limits and interactions

• The FDA label for levothyroxine says soybean flour may bind the drug and cut its absorption from the gut. Soy protein isolate is made from soybean flour but the label does not name it. — People taking levothyroxine sodium tablets (n = -) Evidence E source
  Direction only: soybean flour may bind levothyroxine and reduce its absorption; no size, no amount of soy, no timing in this section
• Taking 19.2 g of soy protein isolate a day for 18 months lowered total testosterone and SHBG in men after prostatectomy but did not change free testosterone. — men aged 47 to 74 after radical prostatectomy, samples from a 2-year RCT (n = subset of 159) Evidence B source
  Total testosterone and SHBG reduced vs casein; free testosterone, estradiol and IGF-1 unchanged
• In people on hemodialysis, isoflavones from one soy protein isolate drink stayed in the blood about 10 times longer than in healthy people. — 23 hemodialysis patients and 10 healthy subjects; clearance in 3 patients and 2 controls (n = 33) Evidence C source
  Half-life about 10-fold longer; morning genistein 1271 vs 425 and daidzein 1304 vs 292 (P < 0.05); dialysis did not remove them

What people believe that the data does not support

• US labeling rules still carry a model health claim that 25 g of soy protein a day, in a diet low in saturated fat and cholesterol, may reduce heart disease risk. — US food labels (n = -) Evidence E source
  Authorized model claim wording; FDA proposed revoking it in 2017
• The EU did not authorize the claim that isolated soy protein lowers blood cholesterol and so may reduce heart disease risk. — EU food labels (n = -) Evidence E source
  Status non-authorized

Who this works differently for

• In 15 adults on hemodialysis, CRP was 2.1 mg/L lower during 4 weeks of a 20 g whey supplement than during 4 weeks of soy protein isolate. — 15 adults on chronic hemodialysis, 20 g protein after each session, 4 weeks per arm (n = 15) Evidence B source
  Hs-CRP 2.1 mg/L lower in whey period vs soy period (P = .04); sequence effect on albumin
• In 750 stunted Ugandan children aged 1 to 5, swapping soy protein isolate for milk protein in a 12-week nutrient supplement added no extra height (0.01 in / 0.03 cm). — 750 stunted children aged 12 to 59 months in Uganda, 100 g/day LNS for 12 weeks (n = 750) Evidence B source
  Milk protein vs soy protein isolate: +0.01 in (0.03 cm) height (95% CI -0.10 to 0.16); LNS itself +0.22 in (0.56 cm)

The bottom line

For muscle, one trial found soy isolate kept pace with whey at a larger dose. Over 12 weeks of resistance training in 48 untrained adults, 26 g of soy protein isolate built lean mass and strength as well as 19 g of whey, both matched for 2 g of leucine, with lean mass up about 3.3 lb (1.5 kg) across groups. The soy dose had to be larger to get there. After a single protein-matched drink in 10 young men, soy raised blood essential amino acids less than whey or krill protein over 3 hours. In a paste fed to 750 stunted Ugandan children aged 1 to 5 for 12 weeks, swapping soy isolate for milk protein made no difference to height (0.01 in (95% CI -0.04 to 0.06; metric: 0.03 cm, -0.10 to 0.16)). The cancer hope did not hold up. A daily 20 g soy isolate drink for 2 years after prostate removal did not lower recurrence in 177 high-risk men (hazard ratio 0.96, 95% CI 0.53 to 1.72), and the trial stopped early for futility. In 15 postmenopausal women, a month on soy isolate cut urinary calcium to 80 to 85 mg a day from 121 mg on milk protein, with no change in how much calcium they absorbed. A skin trial in 44 postmenopausal women reported 7.1 percent less wrinkle severity after 24 weeks, using an industry-supplied drink with added isoflavones and an image-based score. Men and people on dialysis should know these findings. In men after prostate removal, 19.2 g a day for 18 months lowered total testosterone and SHBG against casein, and free testosterone did not change. In people on hemodialysis, isoflavones from one soy isolate drink stayed in the blood about 10 times longer than in healthy people, from a clearance subgroup of 3 patients and 2 controls, and what those high levels mean clinically is unknown. In 15 adults on hemodialysis, CRP was 2.1 mg/L lower during 4 weeks of whey than during 4 weeks of soy isolate. If you are on dialysis, the choice of protein supplement is one for your renal team. If you take any regular medicine, are pregnant or live with a chronic illness, a daily protein supplement is a question for your doctor first. The doses above are what studies used, not a recommended intake, and this page holds no card on an upper limit.

Sources

1. spi-r6-06 · RCT crossover
   Area under the curve (AUC) analyses revealed that whey resulted in the highest postprandial serum concentrations of essential AAs (EAAs) and branched chain AAs (BCAAs), followed by krill and soy, respectively.
   Nutrients, 2021 · checked 2026-09-29
2. spi-r6-07 · RCT crossover
   Urinary calcium excretion was significantly (P < 0.01) less with consumption of either of the soy diets (soy-plus diet: 85 +/- 34 mg/d; soy-minus diet: 80 +/- 34 mg/d) than with consumption of the control diet (121 +/- 63 mg/d), but fractional calcium absorption was unaffected by treatment.
   Am J Clin Nutr, 2005 · checked 2026-09-29
3. spi-r6-01 · meta-analysis
   Soy protein isolate (but not other soy products or components) significantly reduced diastolic blood pressure (-1.99 mm Hg; -2.86, -1.12; 9 studies) and LDL cholesterol (-0.19 mmol/L; -0.24, -0.14; 39 studies).
   Am J Clin Nutr, 2008 · checked 2026-09-29
4. spi-r6-02 · RCT
   In contrast, both HPD groups showed maintenance of the lipid-lipoprotein profile and CVD risk predictors, with further significant reduction in the total cholesterol level in the HPD-SP group (-0.28 ± 0.12 mmol L-1).
   Food Funct, 2025 · checked 2026-09-29
5. spi-r6-03 · RCT
   The resulting hazard ratio for active treatment was 0.96 (95% CI, 0.53-1.72; log-rank P = .89). Adherence was greater than 90% and there were no apparent adverse events related to supplementation.
   JAMA, 2013 · checked 2026-09-29
6. spi-r6-05 · RCT
   All participants engaged in supervised resistance training 3×/week and consumed 19 grams of whey protein isolate or 26 grams of soy protein isolate, both containing 2 g (grams) of leucine. ... No differences between groups were observed (p > 0.05).
   Int J Environ Res Public Health, 2020 · checked 2026-09-29
7. spi-r6-11 · RCT
   The average wrinkle severity was decreased in the SPII intervention group at week 16 and week 24 by 5.9% and 7.1%, respectively, compared to the baseline.
   Nutrients, 2023 · checked 2026-09-29
8. sf-soylev-r5-3 · agency advice
   Soybean flour, cottonseed meal, walnuts, and dietary fiber may bind and decrease the absorption of levothyroxine sodium from the gastrointestinal tract.
   Levothyroxine sodium tablets, FDA-approved label, section 7.9, effective 2026-03-24 · checked 2026-10-08
9. spi-r6-04 · RCT secondary analysis
   Compared with a casein-based placebo, 18 mo, of consumption of 19.2 g/day of whole soy protein isolate containing 24 mg genistein-reduced circulating testosterone and SHBG, but not free testosterone, and did not affect serum concentrations of estradiol, VEGF, IGF-1, IGFBP-3, IGF-1/IGFBP-3 ratio, soluble Fas, Fas-ligand, and sFas/Fas-ligand ratio.
   Nutr Cancer, 2022 · checked 2026-09-29
10. spi-r6-08 · controlled clinical study
    The half-life of both compounds was estimated to be 10-fold longer in the ESRD patients than in the healthy subjects.
    J Am Soc Nephrol, 1999 · checked 2026-09-29
11. spi-r6-12 · regulation
    (1) 25 grams of soy protein a day, as part of a diet low in saturated fat and cholesterol, may reduce the risk of heart disease. A serving of [name of food] supplies ____ grams of soy protein.
    21 CFR 101.82, Health claims: Soy protein and risk of coronary heart disease (CHD) · checked 2026-09-29
12. spi-r6-13 · regulatory decision
    POL-HC-6237 Isolated soy protein Protein-rich soybean component has been shown to lower/reduce blood cholesterol; blood cholesterol lowering may reduce the risk of (coronary) heart disease. -/- Non-authorised
    EU Register on nutrition and health claims, claim POL-HC-6237, snapshot 2026-09-21 · checked 2026-09-29
13. spi-r6-09 · RCT crossover
    After accounting for period and sequence, high-sensitivity C-reactive protein levels were 2.1 mg/L lower during the whey period compared with the soy period (P = .04), independent of total supplemental protein consumed.
    J Ren Nutr, 2026 · checked 2026-09-29
14. spi-r6-10 · RCT
    The main effects of MP were 0.03 (95% CI [-0.10, 0.16]; p = 0.662) cm in height and 0.2 (95% CI [-0.3, 0.7]; p = 0.389) mm in knee-heel length.
    PLoS Med, 2023 · checked 2026-09-29