How much magnesium is absorbed through the skin during an Epsom salt bath according to clinical studies?
Executive summary
Clinical studies offer mixed and limited evidence that some magnesium from Epsom salt (magnesium sulfate) can cross human skin: a small University of Birmingham pilot reported rises in blood and urine magnesium after repeated hot baths with high salt loads, but larger reviews and major health outlets conclude there is no definitive proof of meaningful systemic absorption sufficient to treat deficiency [1] [2] [3] [4].
1. What the clinical studies actually did—and what they measured
The most-cited human experiment involved 19 volunteers who took short, hot full‑body soaks in magnesium sulfate solutions for seven consecutive days and showed increases in urinary magnesium (mean urine magnesium rose from 94.81 ± 44.26 ppm/mL to 198.93 ± 97.52 ppm/mL after the first bath) and occasional rises in plasma magnesium, leading the authors to conclude that magnesium ions crossed the skin barrier in at least some participants [1] [2]. Other controlled laboratory work has shown that sulfates can penetrate human skin relatively readily in vitro, while results for the magnesium cation itself are inconsistent—some ex vivo skin experiments reported little magnesium permeation under physiological conditions [5] [6].
2. How much magnesium was absorbed—numbers, context, and why they don’t translate easily
The Birmingham pilot gives the clearest numeric signals: urine magnesium roughly doubled in aggregate after bathing and some participants had measurable increases in blood magnesium, but the study did not provide a rigorous, reproducible per‑bath “milligrams absorbed” figure nor a pharmacokinetic model to convert urine changes into a precise absorbed dose [1] [2]. Subsequent summaries and reviews emphasize that the observed biochemical shifts were small, variable between subjects, and measured after repeated, unusually hot baths with very high salt loads (400–600 g in a ~60 L bath)—conditions that differ from most home use and that complicate extrapolation to standard practice [1] [5].
3. Methodological red flags that limit confidence in numeric absorption estimates
Independent reviewers and clinicians flag major limitations: the Birmingham work and similar small studies often lacked robust controls, were not peer‑replicated at scale, used high water temperatures that may change skin permeability, and didn’t fully rule out oral ingestion or gastrointestinal absorption from accidental swallowing or skin‑to‑mouth transfer [2] [7]. Reviews in the medical literature note sparse and heterogeneous data; mainstream outlets and hospitals conclude that there are no definitive clinical trials demonstrating sufficient transdermal magnesium uptake to correct deficiency [2] [3] [4]. Laboratory findings that hair follicles and sweat glands can facilitate ion entry point to a plausible pathway but do not quantify clinically meaningful uptake [6].
4. What experts and reviews say about clinical significance
Systematic critiques and health organizations take a cautious stance: while some studies show measurable changes in urine or occasional serum increases, the evidence is inconsistent and insufficient to endorse Epsom baths as a reliable magnesium replacement therapy for deficiency—therapeutic claims remain stronger in marketing than in peer‑reviewed science [2] [3] [4]. Enthusiastic blogs and advocacy pages cite the small positive studies to support topical magnesium use, but major scientific reviews emphasize that increased relaxation or reduced soreness after a soak may be due largely to heat, buoyancy and placebo effects rather than systemic magnesium repletion [8] [9] [10].
5. Bottom line — what can be stated with confidence and what remains unknown
Empirical clinical work demonstrates that some magnesium and sulfate can appear in body fluids after repeated, high‑concentration Epsom salt soaks, but rigorous clinical studies have not quantified a consistent, clinically meaningful absorption rate per bath and important methodological gaps remain [1] [2] [5]. Therefore the best current, evidence‑based statement is narrow: occasional Epsom salt baths may produce minor, variable increases in urinary or serum magnesium under specific experimental conditions, but there is no definitive clinical proof that standard Epsom baths reliably deliver enough magnesium through the skin to correct deficiency—further controlled, dose‑response human trials are needed before precise milligram‑per‑bath figures can be asserted [2] [3] [4].