A float tank contains a lot of epsom salt. Like, a lot a lot. Eight hundred to a thousand pounds each, to make the water so dense you literally can’t help but float.
People sometimes wonder, though, why epsom salt? It’s hardly the easiest thing we could lay our hands on in bulk. Most importantly, it’s harmless to soak in for long periods, and it doesn’t cause the itchy, pruny feeling you get from soaking in sea salt. But there are other benefits.
Epsom salt is called that because it was first produced from natural springs at Epsom, England, around 1618, and from 1695 chemists and pharmacies were selling purified “bitter salts” all over England. For three hundred years since it’s been used to cure just about anything, from muscle aches to skin health, foot odor, wrinkles, psoriasis, eczema, mosquito bites, bruises, inflammation, hangovers, migraines, constipation, and the common cold.
Do any of these really work? Let’s look at the science.
Chemically speaking, Epsom salt is magnesium sulfate, MgSO4. Magnesium and sulfates are both ingredients in essential biological processes. Both are available in many foods — especially green leafy vegetables — but both can have issues with absorption in the digestive tract.
According to the National Institutes of Health, magnesium is a component or cofactor in more than 300 critical body processes:
- protein synthesis
- muscle and nerve function
- blood glucose control
- blood pressure regulation
- energy production
- oxidative phosphorylation
- the structural development of bone
- synthesis of DNA, RNA, and the antioxidant glutathione
- transport of calcium and potassium ions across cell membranes, and thus: nerve impulse conduction, muscle contraction, and normal heart rhythm.
Further, studies consistently show that Americans do not get their recommended intake of magnesium. Indeed, the availability of magnesium in the food supply may have decreased substantially over the past century, though I can’t find a proper source for that claim.
Magnesium deficiency leads to fatigue, weakness, and eventually loss of appetite, nausea and vomiting. It may also be a contributing factor to certain heart arrhythmias, blocked arteries, high blood pressure, diabetes, weak bones, migraines, insomnia, pre-eclampsia, and muscle cramps.
People particularly at risk of magnesium deficiency include:
- People with gastrointestinal diseases such as Crohn’s, celiac disease, and regional enteritis can have difficulty absorbing enough magnesium from food.
- People with insulin resistance or type-2 diabetes can have excessive magnesium loss in urination.
- Those taking diuretics or antibiotics. Additionally, some medications for chronic diseases interfere with magnesium absorption.
- Older adults may experience decreased absorption and increased excretion of magnesium.
- People with alcohol dependence can have a whole spectrum of problems causing magnesium deficiency (among other things!)
A review from the American Academy of Family Physicians recommends that magnesium is an effective treatment for:
- eclampsia and preeclampsia,
- severe asthma,
- dyspepsia, and
and is possibly effective for:
- lowering risk of metabolic syndrome,
- improving glucose and insulin metabolism,
- preventing osteoporosis,
- improving symptoms of leg cramps in pregnant women, and
- alleviating dysmenorrhea.
Sulfate is equally essential to life — it’s critical to the functioning of the central nervous system, to the mucus membranes lining the digestive tract, to producing digestive enzymes, and in lubricating the sliding surfaces of our joints.
Under normal circumstances, the body can manufacture about 80% of its own sulfate needs. The remainder needs to come from external sources, generally foods such as meat, fish, and eggs. Most people have no trouble.
However, direct absorption of sulfate is slow at the best of times, and can be easily interfered with by bowel diseases. Also, people are not all equally efficient at manufacturing their own sulfate needs. Reduced efficiency, leading to sulfate deficiency, is a risk factor for rheumatoid arthritis and primary biliary cirrhosis.
What do you absorb from a bath?
Can you supply your body with additional magnesium and sulfate from an epsom salt bath? Perhaps surprisingly, there is some controversy.
The skin, of course, is generally extremely effective as a waterproof barrier, and that generally includes blocking water-soluble minerals like epsom salt. However, in the only direct (though not peer-reviewed) study I’m aware of, Rosemary Waring at the University of Birmingham tested the magnesium and sulfate levels of 19 subjects before and after 12-minute epsom salt baths. She found increases of varying degrees in almost all subjects, as much as 40% for magnesium.
In another lab test using excised skin, her group found that sulfate does penetrate across the barrier. “This is quite rapid so probably involves a sulfate transporter protein.”
This result is not entirely conclusive, though, as the experiment has not been replicated and some contradictory evidence also exists. The Israeli army has developed a skin cream to be used as a protectant against chemical warfare agents that contains a large amount of magnesium sulfate. They wanted to be sure the cream would not lead to excessive or even toxic concentrations of Mg in soldiers’ blood. After testing, they found “no significant differences in magnesium levels between the placebo and the study groups in any of the applications”, and concluded they could rule out permeation of magnesium through the skin. That’s the opposite of Waring’s conclusions.
There may of course be important differences between application in a skin cream and soaking in a bath, such that both results could be right. But for the moment, there are open questions.
 Guerrera, Volpe, and Mao, “Therapeutic Uses of Magnesium”, American Family Physician (2009) 80(2):157-62.
 Rosemary Waring, “Report on Absorption of magnesium sulfate (Epsom salts) across the skin”, unpublished report (2006).
 A. Eisenkraft et al, “Phase I study of a topical skin protectant against chemical warfare agents”, Mil Med (2009) 174(1):47-52.