WARNING: LONG POST WITH MILD SCIENCE CONTENT - BUT PROBABLY WORTH THE READ ;o)
Ottiebow, serum magnesium levels are notoriously irreflective of actual intracellular magnesium levels unless the values are close to or over the edges of the reference. The reasons for this are multiple, but the primary one is that the heart requires the magnesium level in serum to be within a fairly tight band or it will not function properly. Evolution has given this system priority over the others since heart failure self selects organisms out of the gene pool.
If you are magnesium deficient, and there is about a 90% off the cuff chance in this country that you are, then repletion of magnesium is essential and time consuming. It's depletion also took some time so please be patient, the results are worthwhile. If you absolutely need to know the true value of your intracellular magnesium status, then the Exatest is the way to go...more info at that name dot com (no connection with me). Since 99% of magnesium is intracellular, it is the gold standard for your magnesium status. You can measure progress along this route.
If you're like me and the prisoner of an HMO that doesn't offer even the most basic tests, just assume you're deficient and begin repleting to clinical effect...and keep on. Unless you have end stage kidney failure, your kidneys, via the loop of Henle, can excrete excess magnesium faster than you can get it except through a fast running high dose drip IV.
Now to your point: If you wish to increase your body's ability to hold onto magnesium, then increase your boron intake. Adequate boron will increase magnesium retention by around 33%. This is the reason country vets include it in Cal/Mag injections for livestock. They don't have time to keep driving 100 miles because Bessie is down again for not retaining her minerals. They include borogluconate in the injection; but you can get therapeutic amounts MUCH cheaper by using plain borax. That's right, I said borax. Plain off the shelf, 20 Mule Team Borax. The less than a dollar a pound stuff. It's pure sodium tetraborate decahydrate - Na2B4O7 + 10 H20.
Sodium tetraborate decahydrate (borax) is about 11% boron by weight...the rest is a trivial amount of sodium and mostly oxygen and water. It is safer than table salt per long history and much study. This is easily checked via search. The old timers had an arthritis remedy whereby they would stick a wet finger into plain borax up to the first knuckle and then lick that off. It tastes like sweet baking soda btw...probably because of all that oxygen. Anyway that supposedly kept arthritis at bay. A little checking and some measuring will produce the following: Boron intake levels are inversely correlated with arthritis rates...that is, where people get adequate boron from foods, water, or other sources, they have correspondingly less arthritis. Why is this? Because boron increases magnesium retention and the magnesium keeps calcium in an ionized state and in solution...it also prevents it from entering cells and solidifying. Magnesium dissolves solid calcifications so it also begins to reverse the arthritic disease state biochemically. Boron has a number of other positive effects that would lengthen this already long response, but I assure you, you will appreciate them...as will a significant other...and vice versa.
There are easier ways to get the boron from borax than licking one's finger...I got tired of that after about 2 days. Some accurate measuring showed me that the finger dip method was giving me an average of about .15 - .2 grams of whole borax per dip...or about 17 - 22 mgs of elemental boron...right on the high end of dietary recommendations for my size (200lbs - 91kgs). Apparently the old timers were pretty sharp...think about it...smaller people have smaller fingers, bigger ones, etc...if they dipped to the second joint it was more than needed...just to the fingernail, less. But I digress. So the trick is to get around 0.1g of borax (11mg boron) per hundred pounds. I just remeasured for this post and borax is exactly 4 grams per teaspoon (4.01 +/-). Personally I just put an eighth of a teaspoon in a gallon of magnesium bicarbonate* drinking water and call it good. It makes the water taste 'fresher' in my opinion. I average drinking about a half gallon a day, so it averages out a little on the high side.
Another critical co-factor is Vitamin B6...preferably the P-5-P version that is already immediately bioavailable, since the majority of folks on this site are almost certainly poor methylators to one extent or another. In fact, all the B vitamins should be considered essential...actually they are...imagine a bucket brigade with a mix of strong men and toddlers or absentees. The weak links will need shoring up if you're serious about putting out the fire.
Once you've helped shut the flood gates a bit with the boron and B vitamins, the repletion will begin to have much more effect. Without getting into how much you are injecting and how often, let me give you some tips on other very effective ways to replete magnesium on a more day-to-day basis. These will help you to quickly undo a Mg deficit.
*Oral: The absolute best way I have found to replete orally is via magnesium bicarbonate water. You have to make it by mixing a measuring capful of plain Milk of Magnesia (45ml) into a liter of cold carbonated water (seltzer water), then diluting the reaction into 3 gals of plain water, but it is very easy to do and extremely effective. There are plenty of how to recipes online (search 'magnesium bicarbonate water') so I will spare the step-by-step and share some costs and results. Using plain WM Equate MoM, and generic seltzer it costs me about $0.28/gal to make and is worth a million dollars.
Magnesium bicarbonate water got me off of narcotics..powerful narcotics..quickly. I was actually just trying to find a way to get magnesium into the mitochondria and read where this is the mitos preferred molecule, the incredible pain relief was just a Godsend after 3 decades of torment. It also saved my sister's career and she is now a Mg bicarb evangelist...I get cell calls from people thanking me for turning them on to this...the recipe is spreading through my old job of 3,200 cops...so please forgive our enthusiasm, but it's hard to watch lives being changed and not get excited. It helped give us back a large chunk of our lives. Mg bicarb water has about 125mg of Mg per liter and is completely ionized and dissolved in the water. This will give about a 50% absorption rate or about 12x the absorption from plain Mg oxide tablets. I can say the pain relief came as much from the bicarb as the Mg. Increasing the amount of bicarb with sodium and/or potassium bicarb enhances that effect, just not as much without the Mg.
Transdermally: I use two types and two methods. Mg Sulfate first, this is plain old Epsom Salts. The cheapest places to get it are WM, Costco, and Big Lots...if they're charging much more than $.50/lb it's a rip. The first method is the familiar soak. In the tub use about a pound/15 gallons of water as warm as tolerable...so an average tub might be 2 lbs worth. A buck. The Brits studying this recommend 3x/week to replete with soak times of at least 20 minutes. I notice the 'magnesium effect' (utter calmness) hitting me around the 20 - 30 minute mark. The other soak method is using a large porcelain on steel roasting pan with enough water to cover the upper ankles and about a pound of ES dissolved in it. I put it on an induction plate on the lowest setting to keep the water warm with a folded towel on the bottom of the inside of the pan to keep from burning the soles of my feet. If you're diabetic and can't feel your feet, do not use the coil...you could cook your feet. Not good.
The other method is to make 'mag oil' out of the ES, by dissolving it into just enough distilled water to hydrate all the crystals into solution...about 50/50 ES to water. This makes a great rub or spritzer when put into a spray bottle. Put the spray bottle in the shower and spray your arms and legs and rub that in for a great effect. If you have a migraine or can't sleep, just walk into the shower and spray your feet or shoulders and rub it in. It'll hit you in about a half hour or less and the migraine will go away (Mg relaxes the spasms in the arteries) and you will feel better and/or be able to drift asleep. Works amazing. Cramps..same thing + potassium.
Another benefit of ES is that it also contains a high ratio of sulfate, which CFS'ers are likely helped by if their transulfuration systems are compromised or they need additional free sulfur to make glutaTHIOne (thio in the name indicates sulfur) or meTHIOnine, etc...
The other type of transdermal mag I use is magnesium chloride. It is more expensive than ES (search 'food grade' 'refined nigari'), so I reserve it for mag oil (3:7 ratio with water) and used to use it in drinking water before I found the Mg bicarb. It is highly bioavailable and makes a better mag oil, IMO, than ES since it doesn't leave a residue when it dries. It does sting sensitive areas (use your imagination) and some sensitive people, because of the chloride and the fact that the stability constant of the molecule is zero. Which means it almost immediately disassociates into elemental Mg and elemental Cl ions upon entering the body. Both of these are critical essential elements to human biochemistry.
Together with your injections, these tips should hopefully help you get your magnesium levels into a helpful and healthy range. Once the repletion begins taking hold you will see the serum levels increase, but until then do not be alarmed if drifts around in a range unless it is alarmingly low. In that case ask for (rather insist upon) IV magnesium.
To your health.
Randy