Folks with SIBO struggle with nutrient mal absorption, and magnesium is no exception. Perhaps no nutrient is more fundamental for basic physiological processes in the body. Whether its helping process Vitamin D and make it useable, or converting Glutamate to Gaba, or calming neuro excitation from over active NMDA receptors, its critical. Be it topical creams, or epsom salt baths, both appear worthy of consideration.
Yet, many struggle with getting enough magnesium from their diet, struggle with oral absorption, especially balanced against GI tolerance. I have found for most, sulfur sensitivity aside, luke warm epsom salt baths offer benefit.
In a small study of 19 people, statistically significant elevations in serum and urine magnesium levels were reported after 7 consecutive days of epsom salt baths of 12 mins in duration .
Serum Magnesium Levels
The values before the first bath were, mean 104.68 ± 20.76 ppm/ml; after the first bath the mean was 114.08 ± 25.83 ppm/ml. Continuation of bathing for 7 days in all except 2 individuals gave a rise to a mean of 140.98 ± 17.00ppm/ml. Prolonged soaking in Epsom salts therefore increases blood magnesium concentrations.
Urine Magnesium Excretion
Measurement of magnesium levels in urine showed a rise from the control level, mean 94.81 ± 44.26 ppm/ml to 198.93 ± 97.52 ppm/ml after the first bath. Those individuals where the blood magnesium levels were not increased had correspondingly large increases in urinary magnesium showing that the magnesium ions had crossed the skin barrier and had been excreted via the kidney, presumably because the blood levels were already optimal. Generally, urinary magnesium levels 24h after the first bath fell from the initial values found after day 1 (mean 118. 43 ± 51.95) suggesting some retention of magnesium in tissues after bathing as blood levels were still high.
How Much Epsom Salt in Needed In My Bath ? (about 1.5 pounds in a 15 gallon bath)
All individuals had significant rises in plasma magnesium and sulfate at a level of 1% Epsom salts . This equates to 1g MgS04/100ml water; 600g Epsom salts/60 litres, the standard size UK bath taken in this project (~15 US gallons). However, most volunteers had significantly raised Mg/S04 levels on baths with 400g MgS04 added. Above the 600g/bath level, volunteers complained that the water felt ‘soapy’. Although this project did not specifically set out to answer the question of how frequently baths should be taken, the results are consistent with saturation of the skin (and possibly the gut ) transporters .These proteins are not well understood or described but, at least for sulfate, they are believed to be high affinity but low capacity. The values obtained suggest that most people would find maximal benefit by bathing 2 or 3 times/ week, using 500-600g Epsom salts each time.
Isolation / Float Tanks
Isolation or float tanks use between 100-300 pounds of epsom salt in their tanks. We wonder why folks report a calming and peaceful experience, while others proclaim their life long anxiety disappeared. Hmm.
Ocean / Sea Water
I have had several clients tell me in our first meeting they swear by walking in the ocean, that they feel great, and just couldn't understand why:)?
Even Better - Magnesium Required For QPRT Activity 
What!? What is QPRT, its only one of the most important genes in the pathway that converts Tryptophan into NAD+. Um, oh, that seems important. What happens when QPRT gets backed up ? Quinolinic Acid builds - um, oh. Quinolinic Acid is neuro toxic, and stimulates both the NMDA and AHR receptors. Um, double uh oh. These two receptors are at the heart of CFS and LH from my view. NAD+ and NADPH are perhaps the most common needed cofactors for the most enzymatic reactions in the human body. When people are short on NAD+, they are 'tired', things just stop working. Grape Seed Extract can also help support QPRT.
In Sum: Magnesium is mostly our friend
So, there are multiple reasons we 'feel good' when we get enough magnesium - we have more energy production (NAD+), less quinolinic acid, and we block excess glutamate from NMDA receptors.
 Report on Absorption of magnesium sulfate (Epsom salts) across the skin Dr RH Waring, School of Biosciences, University of Birmingham. B15 2TT, U.K. firstname.lastname@example.org
 Quinolinic Acid: An Endogenous Neurotoxin with Multiple Targets. Rafael Lugo-HuitrónOxid Med Cell Longev. 2013; 2013: 104024. Published online 2013 Sep 5. doi: 10.1155/2013/104024 PMCID: PMC3780648. PMID: 24089628