Why does magnesium work for some people but not others?
Whether magnesium works for you depends strongly on which form you take and how high your dose is. Preferably choose an organic form such as citrate rather than oxide, and give it at least two months.
Which form you take makes a big difference. Magnesium oxide, the cheapest and most widely sold variety, was not absorbed any better than a placebo in a 60-day randomised study involving 46 healthy adults. Organic forms such as citrate, gluconate and amino acid chelate are demonstrably better absorbed: serum levels rose by around 6% with the most soluble form, while oxide barely moved.
How high your dose is also plays a role. The percentage of magnesium that the gut actually absorbs decreases as you take more. Anyone who assumes that a higher dose delivers more may be mistaken: the gut absorbs relatively less, and the surplus magnesium ends up in the intestines, causing diarrhoea.
Your physical condition partly determines how much benefit you get. Healthy people without a deficiency seem to be able to maintain their normal magnesium level with almost any form. In older adults or people with a medical condition or already reduced magnesium levels, that is less certain. In people who had part of their intestine removed, magnesium chelate proved better absorbed and better tolerated than oxide, although the group was too small to draw firm conclusions.
A newer variant, sucrosomial magnesium, showed higher concentrations than oxide and bisglycinate in a small crossover study. The researchers themselves emphasise that larger studies are needed before it becomes clear whether this actually delivers a meaningful benefit in practice.
If you want to try magnesium, the bioavailability studies suggest the following: choose an organic form such as citrate and allow at least 60 days for a stable effect on your magnesium level. A common dose in the studies is 300 mg of elemental magnesium per day. Check the label for the content of 'elemental magnesium', not the total weight of the salt. Oxide consistently scores worst, and there is a real chance you will get little out of it.
This answer is based on a systematic review of 14 studies, several randomised trials in humans, an animal experiment in rats and a number of smaller crossover studies in healthy volunteers. The human studies are largely small (most include tens of participants), which means the findings are plausible but not yet conclusive. The systematic review rated the evidence on bioavailability in older adults and people with illness as uncertain. For healthy people with a normal magnesium status the results are consistent: oxide works poorly, organic forms work better.
Can you take too much magnesium?
Yes, too much magnesium can be dangerous, although serious toxicity through ordinary supplements is rare in people with healthy kidneys. With kidney problems or the use of intravenous magnesium, medical supervision is genuinely necessary.
Which form of magnesium is best to take?
Magnesium citrate has the strongest evidence for absorption into the body. Magnesium oxide, the cheapest option, demonstrably performs the worst and is best avoided.
What exactly does magnesium glycinate do in your body?
Magnesium glycinate is well absorbed, but for most healthy people it is not demonstrably better than citrate. For those with poor intestinal function or a sensitive stomach, it is a worthwhile choice.
What is the difference between magnesium bisglycinate and magnesium glycinate?
Magnesium bisglycinate and magnesium glycinate are two names for the same supplement. Whether it is absorbed better than magnesium citrate is not yet clear: the studies in healthy people contradict each other, and citrate currently has the strongest evidence base.
Does magnesium really help you sleep better and keep your mind sharp?
Magnesium may be worth trying for sleep problems, but the evidence is limited and the studies are partly contradictory. At normal doses it is safe; in any case, make sure you get enough through your diet.
What does magnesium do for your bone strength?
Getting enough magnesium appears to be good for your bones, but whether supplements truly prevent fractures has not yet been convincingly demonstrated. First make sure your intake through diet is in order; consider supplementation only if you know you are deficient, and discuss this with your doctor if in doubt.