How much magnesium is typically absorbed per Epsom salt bath based on studies or estimates?
Executive summary
Small, controlled studies have measured modest rises in blood magnesium after Epsom salt (magnesium sulfate) baths, but they do not quantify a reliable, repeatable mass of magnesium absorbed per bath; mainstream science and medical outlets warn that the skin is a strong barrier and that any transdermal uptake is likely small and variable [1] [2] [3].
1. The single most-cited experiment and what it actually measured
A frequently cited experiment from researchers including Waring and Mitchell put 19 volunteers into short, hot Epsom-salt baths and reported mean plasma magnesium rising from about 104.68 ppm/mL to 114.08 ppm/mL after one 12‑minute soak and to roughly 140.98 ppm/mL after daily soaks for seven days, a change the authors interpreted as evidence that prolonged soaking can increase circulating magnesium and sulfate [4] [1].
2. Why that plasma rise does not translate cleanly into “milligrams absorbed”
The Waring study and summaries report concentration changes in blood, not a direct measurement of total body magnesium transferred from the bath (for example, by measuring total body magnesium, balance studies, or mass lost from the bath solution), so converting the recorded ppm/mL increases into an absolute milligram uptake requires assumptions that the study did not provide and therefore cannot be calculated precisely from the published figures alone [1].
3. Consensus voices: small effect, limited evidence, and caution
Major consumer-health reviews and science journalism emphasize the weak evidence base: reputable outlets and clinicians note it’s “unlikely you can absorb much magnesium through your skin” and that the health benefits of Epsom baths may largely stem from heat, relaxation and improved circulation rather than substantial mineral uptake [2] [3]. ConsumerLab’s review likewise calls the evidence preliminary while acknowledging that at least one study saw blood-level changes [5].
4. Conflicting claims, commercial hype, and the marketing agenda
Well‑sourced blogs and vendor sites often assert substantial transdermal absorption or recommend specific salt loads (500–600 g per bath, two to three times weekly) as “optimal,” but these claims extrapolate from limited experiments or rely on anatomical plausibility rather than rigorous mass‑balance data; such sites have a clear commercial incentive to amplify perceived efficacy [6] [7]. Conversely, independent reviews warn that the marketing narrative—transdermal magnesium as superior to oral supplementation—outpaces the available science [1] [8].
5. Practical takeaway: what can responsibly be said about “how much”
Based on existing, small studies, the defensible statement is that Epsom salt baths can raise circulating magnesium modestly in some people, especially with repeated soaks, but precise quantification in milligrams per bath is not available from the published data and therefore cannot be stated with confidence [1] [5]. Scientific and medical summaries characterize the likely magnitude as modest and variable and emphasize that higher‑quality trials measuring total uptake or performing mass‑balance calculations are lacking [2] [3].
6. Where the evidence needs to go next (and what readers should watch for)
A rigorous answer requires randomized, controlled trials that measure total magnesium movement (for example: bath solution concentration before/after, timed urinary excretion, and whole‑body magnesium balance) and compare realistic home‑bath conditions against placebo baths; until those data appear, both cautious clinicians and skeptical science writers recommend treating Epsom soaks as a safe, low‑cost relaxation therapy that may give a small systemic magnesium boost for some people, rather than a reliable replacement for oral supplementation when clinically indicated [1] [3].