How much magnesium from an Epsom salt bath is absorbed through human skin according to clinical studies?
Executive summary
Clinical studies and reviews report mixed, limited evidence that magnesium from Epsom salt (magnesium sulfate) baths can cross human skin and produce small increases in body magnesium markers after prolonged or repeated soaking, but the magnitude and clinical significance are inconsistent and not definitively quantified [1] [2]. Key positive signals come from small, uncontrolled bathing trials that measured rises in serum and urinary magnesium after multiple hot baths with high salt loads [3] [1], while other laboratory and review work finds little or no reliable systemic absorption under typical conditions [4] [2].
1. The original bathing studies that started the claim
A frequently cited pilot series from the University of Birmingham reported that 19 volunteers who soaked in hot magnesium sulfate baths on consecutive days showed modest rises in serum magnesium (from a baseline mean ~105 ppm/mL to ~114 ppm/mL after one bath, and to ~141 ppm/mL after seven days) and corresponding increases in urinary magnesium, suggesting transdermal transfer in at least some participants [1] [3]. Those investigators reported effective bath concentrations on the order of 400–600 g of Epsom salts in ~60 L of water and noted wide individual variability; the study lacked rigorous controls and used very hot, short-duration soaks, which complicates interpretation [3] [1].
2. Why sceptics demand more robust evidence
Systematic reviewers and journalists point out that the Birmingham work was small, had methodological problems (no control arm, high bath temperatures) and that more rigorous work has not confirmed large-scale, reproducible magnesium uptake from baths; National Geographic summarizes that there is “no definitive scientific evidence” showing humans absorb a significant amount of magnesium through skin in an ordinary Epsom bath [1] [2]. Controlled laboratory tests using isolated human skin have even shown that sulfates may penetrate more readily than magnesium ions, and some in vitro work found magnesium did not cross intact skin under the test conditions [4].
3. Mechanisms and plausible pathways—hair follicles and damaged skin
Laboratory permeation studies indicate that hair follicles can facilitate ion permeation and that localized delivery routes may allow some topical magnesium to enter superficial tissues [5]. This suggests a plausible mechanism for limited absorption, especially where skin barrier integrity is reduced or where follicles present a lower-resistance pathway, but it does not quantify systemic uptake in healthy people bathing under normal conditions [5].
4. How much — the practical numeric answer from available clinical data
No high-quality clinical trial provides a precise, generalizable percentage or milligram figure for magnesium absorbed per standard bath; the best available clinical signal is the modest rises in serum and urinary magnesium reported in the small Birmingham cohort after repeated, high‑concentration, hot baths (with measurable post‑bath serum increases and urinary excretion increases noted) but those findings are not universally replicated and lack rigorous controls to translate into a definitive absorption rate [3] [1]. Reviews and mainstream reporting therefore conclude that while trace absorption is possible, there is no robust evidence that a typical Epsom bath delivers a clinically meaningful magnesium dose comparable to oral supplementation [2] [1].
5. What the wellness industry and secondary summaries add — and why to be cautious
Commercial blogs and advocacy groups often amplify positive interpretations—claiming transdermal magnesium is superior or that baths are an efficient supplementation route—citing the same pilot studies or mechanistic papers and sometimes extrapolating to practical recommendations on temperature and salt amounts [6] [7]. Independent science communicators and reviews warn these summaries overreach the limited clinical evidence and emphasize variability, small sample sizes, and alternative explanations for perceived benefits such as heat, relaxation, and placebo effects [2] [1].
6. Bottom line for interpretation and next steps for science
Clinical studies to date indicate possible, small-scale magnesium transfer from very concentrated, repeated, hot Epsom baths in some people, but they do not establish a reliable absorption amount per bath or clinical benefit; higher‑quality, controlled trials measuring systemic magnesium (serum, intracellular markers) after realistic bathing regimens are needed to quantify uptake and relevance [3] [1] [2]. Until then, claims that Epsom baths meaningfully restore whole‑body magnesium compared with oral intake remain unproven rather than disproven [1] [2].