Plant Hormones in Fish Feed: The Hidden Estrogens Reshaping Aquaculture

Global aquaculture produced roughly 185.4 million tonnes of fish in 2022, supplying high-quality protein to billions of people, and the industry’s fastest-growing challenge is what to feed all those fish. As fishmeal and other animal-based proteins become scarce and expensive, feed formulators are turning to plant ingredients such as soybeans, cottonseed, linseed, safflower, and alfalfa. But these crops carry a hidden chemical payload: phytoestrogens, polyphenolic plant compounds that can mimic the hormone estrogen in animal bodies. A new mini review published in Blue Biotechnology by Sadiya Farooq, Naila M. Bhat, Showkat Ahmad Dar, and Mohd Ashraf Malik examines whether these compounds are a friend or a foe to farmed fish, and the answer turns out to be genuinely two-sided.

Phytoestrogens are secondary metabolites that plants use for their own purposes, defending against disease and herbivores, supporting reproduction, and even coloring flowers. Chemically, they are defined by two hydroxyl groups attached to a phenolic ring, a structure close enough to 17β-estradiol that the phenolic group can bind to estrogen receptors in animals, acting as either an agonist or an antagonist. The most common forms in plants are conjugated glycosides, which are biologically inactive until gut bacteria and intestinal enzymes cleave the sugar groups, making bioavailability dependent on each species’ digestive machinery. Once absorbed, phytoestrogens are conjugated in the liver, circulate in the bloodstream, and are eventually excreted in urine.

The review organizes these compounds into two broad chemical families. Flavonoids, built on a diphenyl propane skeleton of two benzene rings closed by a heterocyclic pyran ring, include isoflavones such as genistein, daidzein, biochanin A, and formononetin; flavones such as luteolin and apigenin; and coumestans, of which coumestrol is the standout. Non-flavonoids comprise the stilbenes, including resveratrol, and the lignans, which gut metabolism converts into enterodiol and enterolactone. Coumestrol is particularly potent, showing 30 to 100 times the estrogenic activity of isoflavones because its two hydroxyl groups sit in positions matching those of estradiol, allowing it to inhibit key steroidogenic enzymes such as aromatase and 3-hydroxysteroid dehydrogenase.

Legumes are the richest sources, with many feed legumes containing phytoestrogens at 5 to 25 percent of their composition. In soybeans, genistein and daidzein together account for up to 90 percent of isoflavones, while red clover is dominated by formononetin and biochanin A. Concentrations vary with temperature, humidity, light, plant age, fertilization, and infection, and different compounds accumulate in different plant parts. Flax and sesame seeds are the most abundant lignan sources, coumestans concentrate in sprouts and fodder crops, and phytosterols such as sitosterol appear in plant oils and even in pulp mill effluents at levels with biological effects. Algae, cyanobacteria, and aquatic macrophytes may add further phytoestrogens to aquatic ecosystems, though their significance remains poorly quantified.

At the right doses, phytoestrogens can genuinely help farmed fish. Low concentrations of genistein enhanced growth in goldfish and yellow perch by modulating the growth hormone and insulin-like growth factor I axis. Their polyphenolic structure gives them antioxidant properties, scavenging free radicals and reducing oxidative stress. Moderate intake has been linked to enhanced lysozyme activity and leukocyte function, and equol, a metabolite of daidzein, promoted ovarian development and fecundity in female beluga sturgeon. Phytoestrogens can also induce vitellogenin synthesis in fish hepatocytes, hinting at potential as natural alternatives to synthetic hormones in reproductive management.

The darker side emerges when exposure is high or prolonged. As xenoestrogens, phytoestrogens bioaccumulate in fish tissues and drive reproductive abnormalities: male fish produce aberrant vitellogenin and show reduced sperm counts, while females suffer lower fecundity and egg hatchability. In rainbow trout, elevated dietary phytoestrogens impaired lipid metabolism and caused excess fat accumulation in hypertrophic adipocytes, threatening product quality. Estradiol-like signaling downregulates the GH/IGF axis in salmonids, boosting protein degradation and suppressing protein synthesis in muscle, and phytoestrogens such as genistein and daidzein slowed protein synthesis in trout muscle cells. High dietary soy isoflavones significantly reduced weight gain, feed efficiency, and whole-body lipid content in Japanese flounder. Immunity can also suffer: in beluga sturgeon, dietary phytoestrogens reduced lysozyme and IgM concentrations, echoing mammalian findings where genistein shrank the thymus in mice.

Reproductive disruption is the most dramatic effect. Genistein inhibits ovarian aromatase expression while raising serum testosterone and estradiol, yet reduced estrogen receptor expression means vitellogenin production falls, leaving female common carp with smaller, less vitellogenic ovaries. In channel catfish, chronic dietary genistein at 4 and 8 milligrams per gram produced more phenotypic males and intersex individuals, likely because the compound both mimics and blocks estrogen by inhibiting aromatase. Sperm quality is vulnerable too: genistein-enriched diets reduced sperm motility and concentration in rainbow trout in a dose-dependent manner, and in vitro genistein exposure cut sperm motility, ATP content, and fertilization rates in channel catfish and walleye. Yet the picture is not uniform. In fighting fish, waterborne genistein and β-sitosterol had no effect on sperm quality, and soy protein concentrate showed no estrogenic effect on trout juveniles.

These contradictions reflect a deeper truth the review emphasizes: responses are intensely species-specific. Carnivorous sturgeon metabolize phytoestrogens slowly, so the compounds accumulate in tissues and perturb sex differentiation, while omnivorous common carp show more moderate responses, likely thanks to metabolic adaptability. Aromatase inhibition by phytoestrogens varies across species, tissues, and developmental stages, and even within a species, individuals differ by sex, age, and genetic background in the expression of metabolic enzymes such as cytochrome P450s. The authors point to striking parallels beyond fish: Southern White Rhinoceroses show marked estrogen receptor activation on phytoestrogen exposure that their one-horned relatives do not, a difference implicated in captive breeding failures.

Fortunately, mitigation is feasible. Extensive pre-cooking before crushing removes 50 to 80 percent of the initial isoflavone content of soy products, soaking and boiling leach daidzein and genistein into water, and heating triggers Maillard-type reactions and auto-degradation that lower genistein levels. Growing soybeans at higher temperatures reduces isoflavone accumulation. Enzymatic treatments using laccases and peroxidases can degrade isoflavones and lignans into less bioavailable forms, lactic acid bacteria fermentation modifies genistein and daidzein during processing, and selective breeding is producing low-phytoestrogen soy, clover, and legume cultivars without sacrificing yield. Alternative plant proteins such as pea, lupine, and beans carry lower phytoestrogen loads.

The review closes with a regulatory warning. While the fungal mycoestrogen zearalenone has guidance values in livestock feed, ranging from 0.1 to 0.5 milligrams per kilogram under European Commission Recommendation 2006/576/EC, no such thresholds exist for naturally occurring phytoestrogens in aquafeeds, despite teleost fish being especially sensitive to hormonal perturbation during early development and gonadal differentiation. The authors call for species-specific toxicological studies, dose-response relationships, and maximum residue limits tailored to aquaculture, alongside long-term exposure research and ecological assessments of farm effluents, which can carry phytoestrogens into wild fish populations. As plant-based feeds become the backbone of sustainable aquaculture, managing these invisible hormones may prove as important as the protein itself.

Subject of Research: Effects of phytoestrogens in plant-based aquafeeds on fish growth, reproduction, and sex differentiation

Article Title: Phytoestrogens in aquaculture: friend or foe to fish growth and reproductive health?

Article References: Phytoestrogens in aquaculture: friend or foe to fish growth and reproductive health?. (n.d.). https://doi.org/10.1186/s44315-025-00037-7

Image Credits: AI Generated

DOI: 10.1186/s44315-025-00037-7

Keywords: phytoestrogens, aquaculture, aquafeed, genistein, isoflavones, endocrine disruption, fish reproduction, sex differentiation, vitellogenin, aromatase, soybean meal, fish health