Human breast milk is a complex biological fluid containing essential nutrients, immunological factors, and a variety of bioactive hormones, including estrogen, progesterone, and prolactin¹². While these hormones are physiologically active in infants, their potential effects on adult consumers remain largely unexplored. This review critically examines the endocrinological implications of human breast milk consumption in adults, emphasizing female hormones. Evidence indicates that the digestive system of adults effectively degrades peptide and protein hormones, significantly limiting systemic absorption⁹. Lipophilic hormones such as estrogen and progesterone may partially survive digestion, but the concentrations are far below thresholds required to induce hormonal changes³. Clinical and experimental studies reveal no significant impact on testosterone, estrogen, prolactin, or reproductive function in adult men or women⁶¹³. Observed effects of breast milk intake in adults are primarily nutritional or immunological, such as improved gut health and mild circadian regulation through melatonin⁷¹². This review provides a mechanistic understanding of why female hormones in breast milk do not affect adult endocrine physiology and highlights potential areas for future research in bioactive milk components for adult health.

Human breast milk is the optimal source of nutrition for infants, supplying carbohydrates, proteins, fats, vitamins, and bioactive molecules that support growth and immune development¹². Among its components, hormones such as estrogen, progesterone, prolactin, cortisol, and growth factors play critical roles in neonatal development and metabolic programming¹²⁹. While extensively studied in the context of infant health, the potential endocrine effects of these hormones in adult consumers have generated interest in translational and clinical research. The endocrine system in adults is tightly regulated. Hormones circulate in physiologically defined ranges, and systemic exposure to exogenous hormones is usually required to elicit measurable effects¹³. Therefore, investigating whether bioactive hormones in breast milk can influence adult endocrine physiology is necessary for understanding potential health implications [712].

2.1 Hormonal Composition of Breast Milk

Human breast milk contains hormones from maternal circulation and local mammary synthesis [5]. Key female hormones include:

Estrogen (estradiol, estrone) – involved in reproductive development [4]

Progesterone – supports mammary gland function [2]

Prolactin – primarily a peptide hormone influencing lactation [6]

Cortisol and melatonin – modulate metabolism and circadian rhythm [7]

Concentrations of estrogen and progesterone in breast milk are extremely low (picogram levels), several orders of magnitude below pharmacologically active doses for adults.[93]

2.2 Digestion and Absorption of Hormones in Adults

Peptide hormones such as prolactin and growth factors are largely denatured by gastric acid and proteolytic enzymes, limiting systemic absorption⁹. Lipid-soluble steroid hormones may survive partially, but their levels are insufficient to produce endocrine effects². Adult gastrointestinal permeability and enzymatic activity contrast with neonatal physiology, which allows some hormone bioavailability.[5]

2.3 Clinical Observations

No clinical studies report feminizing effects, alterations in testosterone, prolactin elevation, or reproductive dysfunction in adult men consuming breast milk[13]. Nutritional and immunological benefits, such as gut microbiota modulation and anti-inflammatory activity, have been observed, but these are independent of female hormone activity[717].

This review synthesizes data from:

Peer-reviewed journals on breast milk hormone composition [12]

Endocrinology textbooks detailing adult hormone metabolism [59]

Clinical case reports and experimental studies assessing adult consumption of breast milk [613], Databases searched include PubMed, Scopus, and Web of Science using keywords: "breast milk," "adult consumption," "female hormones," "estrogen," "progesterone," "prolactin","endocrineeffects."

Table 1: Hormonal Levels in Human Breast Milk and Potential Effects in Adults

These data demonstrate that hormone levels in breast milk are orders of magnitude below adult endocrine thresholds. No systemic hormonal effects are expected.

Figure 1: Fate of Female Hormones from Breast Milk in Adults

Source: Created By Haider. Et. Al 2025

Adult consumption of breast milk does not meaningfully alter endocrine function. Female hormones are either destroyed in the digestive tract or absorbed at sub-physiological levels⁹. Even occasional consumption is unlikely to affect testosterone, estrogen, or prolactin levels³⁶¹³. Observed effects of breast milk intake are primarily nutritional or immune-related, such as modulation of gut microbiota, anti-inflammatory properties, or minor circadian rhythm effects from melatonin[712].

This highlights the specificity of breast milk’s hormonal activity for neonatal physiology. Potential research avenues include isolating bioactive milk peptides for adult metabolic or immunological applications, rather than endocrine modulation²⁹.

Female hormones in human breast milk are biologically active in infants but do not impact adult endocrine physiology, even if consumed occasionally. Adults derive nutritional, immunological, and minor circadian benefits, not hormonal effects. Understanding hormone degradation and bioavailability in adults clarifies the limited role of breast milk in adult endocrine modulation[137].

The authors express their sincere gratitude to Dr. Naweed Imam Syed, Professor, Department of Cell Biology, University of Calgary, for valuable guidance and scientific insights throughout this project.

All authors contributed equally to the study design, data evaluation, writing, and final approval of the manuscript.

The authors declare no conflict of interest.

The authors received no financial support for the research, authorship, or publication of this article