No previous studies have looked at transdermal absorbency of Mg 2+ in human subjects. In this pilot study, transdermal delivery of 56 mg Mg/day (a low dose compared with commercial transdermal Mg 2+ products available) showed a larger percentage rise in both serum and urinary markers from pre to post intervention compared with subjects using the placebo cream, but statistical significance was achieved only for serum Mg 2+ in a subgroup of non-athletes. Future studies should look at higher dosage of magnesium cream for longer durations.

In this single blind, parallel designed pilot study, n = 25 participants (aged 34.3+/-14.8y, height 171.5+/-11cm, weight 75.9 +/-14 Kg) were randomly assigned to either a 56mg/day magnesium cream or placebo cream group for two weeks. Magnesium serum and 24hour urinary excretion were measured at baseline and at 14 days intervention. Food diaries were recorded for 8 days during this period. Mg test and placebo groups’ serum and urinary Mg did not differ at baseline. After the Mg 2+ cream intervention there was a clinically relevant increase in serum magnesium (0.82 to 0.89 mmol/l,p = 0.29) that was not seen in the placebo group (0.77 to 0.79 mmol/L), but was only statistically significant (p = 0.02)) in a subgroup of non-athletes. Magnesium urinary excretion increased from baseline slightly in the Mg 2+ group but with no statistical significance (p = 0.48). The Mg 2+ group showed an 8.54% increase in serum Mg 2+ and a 9.1% increase in urinary Mg 2+ while these figures for the placebo group were smaller, i.e. +2.6% for serum Mg 2+ and -32% for urinary Mg 2+. In the placebo group, both serum and urine concentrations showed no statistically significant change after the application of the placebo cream.

Competing interests: In the future, Dr Andrea Rosanoff may receive royalties on a magnesium cream for which she has signed an agreement. The cream in which she will have an interest is different than the one used in this pilot study as the magnesium concentration is more than double that used in the pilot study and it is a different formulation. The name of the cream is Natural Calm Cream. The U.S. Patent title is: Cream Transdermal Magnesium Supplement. Publication No. US-2016-0317576-A1. Publication Date: 11/03/2016. This does not alter our adherence to all the PLOS ONE policies on sharing data and materials.

Copyright: © 2017 Kass et al. This is an open access article distributed under the terms of the Creative Commons Attribution License , which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Introduction

Mineral elements, such as magnesium (Mg2+), are required by the human body in modest amounts for the maintenance of health and the development of optimal functioning[1]). Mg2+ is an important mineral element and it is the fourth most abundant cation in living organisms, with Mg2+ being a cofactor to over 325 enzymatic reactions within the body [2]. Around 99% of total body Mg2+ is located in the bone, muscles and non-muscular soft tissue [3]. Mg2+ supplementation has been shown to significantly improve blood pressure [4–6] as well as modifying vascular tone by regulating endothelium and smooth muscle cell function [4].

To maintain an adequate Mg2+ status humans must consume Mg2+ at regular intervals [7]. The daily recommendation for Mg2+ is controversial, as the literature is conflicting and varies between countries, although values of ≥ 300 mg/day are usually reported for healthy adults with adjustment for age, sex and nutritional status [7].

Oral magnesium supplementation has been shown to affect various parameters, such as blood pressure [5,8], immune function [9], cardiovascular system [10] and metabolic syndrome [11]. A recent meta-analysis by Zhang et al (2016) in the Journal of Nutrition [12] found that in a healthy population group there was a dose dependent increase in both serum Mg concentration and urinary Mg excretion with supplemental oral magnesium intake ranging from 250–500mg/day. Kass, Weekes and Carpenter (2012) identified that a supplement of >370 mg/day of Mg+ shows greater efficacy than a lower dose in improving blood pressure and that magnesium supplementation gives a dose dependent response with regards to blood pressure [5].

An alternative method of perhaps attaining recommended magnesium intakes might be through topical application. Current formulations include magnesium oils and trans-dermal creams, from which the magnesium may be absorbed across the skin and into the systemic circulation. However, in contrast to gastrointestinal epithelium, a primary function of the skin is to act as a barrier, which restricts the absorption of exogenous chemicals into the body. The barrier function of the skin is thought to lie predominately in the outermost layer of the epidermis, the stratum corneum. The stratum corneum is thought to largely present a hydrophobic barrier to the absorption of transdermal creams. The dermis, immediately below the epidermis, contains the blood vessels that are able to transport substances that have permeated the skin into the systemic circulation.

Although less studied than organic molecules, metal ions are known to be able to cross the skin, with the literature having focussed on metals that are known to cause irritant/toxic effects [13]. The lower resistance to permeation of the skin appendages, skin structures that serve a particular function including sensation, lubrication and heat loss, and the ionised nature of metals means that their permeation across these skin appendages is considered to be the most likely route of absorption [13,14]. However, the low surface area available for this in human skin means that metal ion absorption across skin is expected to be relatively low. Therefore, it has been questioned that a transdermal route of administration might provide sufficient Mg2+ absorption to help meet systemic Mg2+ requirements. This investigation aims to determine if transdermal absorption of Mg2+ from a topical cream occurs “in vivo” in humans.

To date, no study has investigated the absorbency of transdermal magnesium cream in human subjects. Commercially available Mg2+ topical applications range from 75mg to 400mg depending on the dosage recommended by manufacturers. This ranges from 5–30 sprays of magnesium oil or 2–4 teaspoons of magnesium cream, which can be applied in one application or throughout the day. Disappointingly, many commercial topical creams and oils do not state the concentration of magnesium in the product.

This study was designed as a first time, pilot study to ascertain whether such a topical Mg2+ preparation might affect urinary or serum Mg2+. Since less than 1% of magnesium is contained in the blood, assessment by serum status may be problematic [3]. It is often considered that 24-hr urine excretion of Mg2+ may be a better indicator of intestinal absorption than serum concentration; however, urinary Mg2+ excretion is also highly variable and it is questionable whether it can be used reliably to assess an individual’s Mg2+ status [15]. However, serum Mg2+ can reflect a longer term dietary Mg2+ status over weeks or months whilst urinary Mg2+ can be a better marker of one’s recent dietary intake [16]. These studies using serum and urinary Mg2+ markers in dietary Mg2+ research could not be assumed to be helpful in the design of this study. Djurhuus et al,[17], however, reported that although it is unlikely that a single determination of serum Mg2+ can be used in assessing whole-body Mg2+ status in an individual, serial determinations of serum Mg2+ might prove useful as an indicator of changes in whole body Mg2+ status. These authors also found that 24-hr urinary Mg2+ excretion is unlikely to be a reliable measure of whole body Mg2+ status and is not a good marker to measure changes in whole body Mg2+ status. Nonetheless, we decided to use 24-hr urinary Mg2+ as well as serum Mg2+ in this pilot study.

Therefore, the purpose of this pilot study was to investigate whether a 56 mg/day dose of magnesium in a cream, applied transdermally to humans, would affect either serum magnesium levels or urinary excretion over a two-week period and to measure effect, if any, and variance to inform a properly powered future study, if warranted.