Is soy bad for men? Soy, testosterone and oestrogen

The objection

“Soy is full of phytoestrogens, plant chemicals that act like oestrogen. Doctors have written up men who grew breasts after drinking soy milk, so eating a lot of it has to mess with a man’s hormones.”

The answer

Across 41 clinical studies pooled in 2021, soy made no significant difference to men’s hormones, though one later trial found a fall in total testosterone with no change in free testosterone. Reed and colleagues (2021) pooled studies in which men ate soy foods, soy protein or isoflavone extracts from soy or red clover, and found no significant effect on total or free testosterone, oestradiol, oestrone or sex hormone binding globulin (SHBG), whichever statistical model they used [1]. Total testosterone was measured in 1,753 men, and oestrone, the thinnest result, in 239 [1]. Grouping the studies by isoflavone dose or by how long they ran showed no effect either [1]. The analysis updated a 2010 meta-analysis by partly the same team (Hamilton-Reeves et al.), which had found no significant effect on testosterone, free testosterone or SHBG [2].

The 2021 analysis was funded by the United Soybean Board [1], through which US soybean farmers pay for research and promotion meant to strengthen demand for soy [3]. That is a fair reason to check its conclusion against work by other groups, including the case reports and the sperm study that critics cite. A randomised trial by a different team, published after the analysis’s April 2020 search cut-off [1], did find a change: in middle-aged and older men who had had surgery for prostate cancer, 18 months of 19.2 g a day of soy protein isolate supplying 24 mg of genistein lowered total testosterone and SHBG compared with a casein (milk protein) placebo, but did not change free testosterone or oestradiol (Bosland et al., 2022) [17]. It was one trial in a narrow group of men, and its abstract does not say how large the fall was.

The worry does start from real chemistry. Soy is rich in isoflavones, and in laboratory tests genistein and several other phytoestrogens bound the body’s oestrogen receptors, the beta form more strongly than the alpha form, and could switch both on (Kuiper et al., 1998) [4]. Cells in a dish can’t show what a normal diet does to a man, which is why the studies in men carry more weight. The American Cancer Society (2025) says human studies have found soy’s oestrogen-like effects to be either absent or linked to lower breast cancer risk, possibly because isoflavones can block the body’s own, more potent oestrogens [5].

Soy fear attaches most of all to breast cancer, and there the evidence leans the other way. A pooled analysis of 9,514 US and Chinese breast cancer survivors (Nechuta et al., 2012) found that eating 10 mg or more of isoflavones a day after diagnosis, compared with less than 4 mg, was linked to a 25% lower risk of recurrence, and to fewer deaths by a margin that was not statistically significant [6]. The Global Cancer Update Programme (2023), which took over the World Cancer Research Fund and American Institute for Cancer Research’s Continuous Update Project, took its recurrence result from that same pooled analysis [7]. Adding two more studies to the three pooled cohorts, it found that each extra 2 mg of isoflavones a day was linked to a 4% lower risk of death from any cause, a result that was not statistically significant (relative risk 0.96, 95% CI 0.92 to 1.02), and its independent expert panel graded the evidence as limited-suggestive [7]. Both rest on observational studies, so they show a link without proving that soy caused it, and the pooled analysis reports relative risks only. The American Cancer Society calls soy foods healthy and safe, and does not recommend soy supplements for cancer prevention [5]. If you have had breast cancer, ask your oncology team before taking any isoflavone supplement.

Can soy make men grow breasts?

Large amounts of soy may have done so in a few men: at least five published case reports blame heavy soy intake for breast growth or low testosterone in men and one boy, and in four of them the problem eased or cleared once the soy was cut out. A 60-year-old man seen at a US Army hospital in Texas for breast enlargement had oestrone and oestradiol levels four times the top of the normal range, and turned out to be drinking 3 quarts of soy milk a day, nearly three litres; once he stopped, his breast tenderness went and his oestradiol slowly returned to normal (Martinez and Lewi, 2008) [8]. His doctors called it a very unusual case. A 19-year-old man with type 1 diabetes lost his libido and developed erectile dysfunction after eating large quantities of soy products on a vegan-style diet; his free and total testosterone were low, and they and his sexual function returned to normal within a year of his giving up that diet, so soy was not the only thing that changed (Siepmann et al., 2011) [18]. A 54-year-old Japanese man who had drunk about 1.2 litres a day for three years, estimated from the label at about 310 mg of isoflavones, developed low testosterone, erectile dysfunction and breast growth, and his hormone levels improved after he gave it up (Imai et al., 2022) [9]. An 8-year-old boy seen by paediatric endocrinologists at UCLA developed breast growth on one side that cleared completely once foods with a lot of soy were cut from his diet, the first such case before puberty that his doctors knew of (Sea et al., 2021) [19]. A 33-year-old man in Indonesia who had drunk half a litre to a litre of homemade soy milk a day during a bodybuilding regime still had enlarged breasts two years after stopping, and had them treated surgically (Widjajahakim et al., 2025) [20]. A case report records what happened to one person and cannot say how often it happens, which is why the studies above carry more weight.

Breast growth in men and boys is common and has other causes, which the NHS (2024) lists as including puberty, the hormone changes of men over 50, being overweight, certain medicines, anabolic steroids, an overactive thyroid, kidney disease and cirrhosis; it advises seeing a GP if you are worried about the size of your breasts, or have a lump, pain that is not going away, discharge from a nipple or changes to the skin of the breast or nipple [21].

Japan’s Food Safety Commission (2006) reviewed the studies in men and found that most of the hormone changes reported were not clinically significant, with no obvious harm at up to 74 mg of isoflavones a day in hormone studies, most of them short [10]. It also recorded breast changes in men with prostate cancer given isoflavone capsules, judged it possible that men are about as sensitive to isoflavones as women, and set the safety limits in the table below, adding that going over the everyday limit would not cause health effects of immediate concern [10]. The limits do not cover pregnancy, infants or small children: the Commission said it could not set a safe intake for them because of a lack of data, and that extra isoflavones in health foods could not be recommended for them [10].

Who or what	Isoflavones a day
Japanese males aged 15 and over, median (2002 survey) [10]	18 mg
Japanese males, 95th percentile (2002 survey) [10]	76 mg
Japan’s safe upper limit for everyday eating, not set for pregnancy, infants or small children (2006) [10]	70 to 75 mg
Japan’s limit for extra isoflavones in “foods for specified health use”, not set for pregnancy, infants or small children (2006) [10]	30 mg
Japanese man with low testosterone (case report, 2022) [9]	about 310 mg
Capsules given to men with prostate cancer, with breast changes [10]	398 mg, then 796 mg

The survey figures and the limits are aglycone equivalents, while the case figure came from the soy milk’s label and the capsule doses are as the trial gave them, so the rows are not all on exactly the same basis.

Japan’s limit for extra isoflavones taken in health foods on top of an ordinary diet is 30 mg a day [10], a useful yardstick for anyone reading the label of an isoflavone supplement. For a sense of scale, the Commission’s figures for Japanese foods put tofu at an average of 20.3 mg of isoflavones per 100 g across four samples, and soy milk at 24.8 mg per 100 g, though its three soy milk samples ranged from 6.9 to 53.8 mg [10].

If erection problems keep happening, see a GP. The NHS says frequent erectile dysfunction may be caused by high blood pressure, high cholesterol, diabetes, depression or anxiety, or a hormone problem, and can be a sign of a health condition that can be treated [11].

Does soy affect sperm or fertility?

The evidence on sperm is thin and mixed: a 2008 study linked eating more soy to lower sperm concentration, while a 2015 study by the same Harvard group found men’s soy intake was not linked to IVF outcomes. The first looked at 99 men at a fertility clinic and found that those eating the most soy, a median of 0.54 servings a day, had 41 million fewer sperm per millilitre than men who ate none (95% CI 8 to 74 million fewer), with no difference in how the sperm moved, their shape or semen volume; the authors said its cross-sectional design limits what it can say about cause (Chavarro et al., 2008) [12]. The second followed 184 men whose partners were having IVF, and their soy intake was not linked to fertilisation, implantation, pregnancy or live birth (Mínguez-Alarcón et al., 2015) [13]. A 2017 systematic review of observational studies listed soy foods, along with processed meat and full-fat dairy, among the foods linked to poorer semen quality in some studies, and said such links need confirming in cohort studies and trials (Salas-Huetos et al.) [14].

Can I eat soy if I take thyroid medication?

Yes, though if you take levothyroxine for an underactive thyroid, keep a steady gap between the tablet and food, soy included. A 2021 systematic review of 63 studies on levothyroxine and food (Wiesner et al.) found limited evidence that soy products reduce how much of the drug is absorbed, and said keeping a proper interval between the tablet and food is an effective way to avoid such interactions [22]. A 2023 review in Endocrine Practice (Gatta et al.) found the evidence came mainly from case reports, noted that the only randomised crossover study found no difference in absorption when levothyroxine and soy isoflavones were taken together, and judged that any effect seems to have little clinical impact [23]. Speak to your GP before changing or stopping any medicine.

Is soy bad for men’s health in other ways?

Broad reviews judge soy more likely to help than harm overall, though one linked high miso soup intake to a higher risk of stomach cancer in men. A 2020 umbrella review of 114 meta-analyses and systematic reviews covering 43 health outcomes, from a team at West China Hospital, judged soy and isoflavones more beneficial than harmful, and the only harmful association it found was in men who had 1 to 5 cups of miso soup a day, whose relative risk of stomach cancer was 1.17 (95% CI 1.02 to 1.36) [15]. A 2026 umbrella review led from the Chinese Center for Disease Control and Prevention reached the same overall verdict for soy foods, but found the evidence on fermented soy products inconclusive, with some studies pointing to possible harm [16].