Here's a question most people have never thought to ask: what does the number on your magnesium supplement actually mean?
If your label says "500mg Magnesium Oxide," you might assume you're getting 500mg of magnesium. You're not. You're getting roughly 12mg of magnesium that your body can actually use. The rest passes through.
This isn't a minor discrepancy. It's the reason most people try magnesium, feel nothing, and conclude that supplements are a waste of money — when the real issue was the form they were taking.
The Label Problem
Every magnesium supplement is magnesium bound to something else — a carrier molecule. The number on the label is the weight of the entire compound, not just the magnesium.
Magnesium oxide is the most common form on Australian shelves, largely because it looks impressive. You can pack a lot of elemental magnesium into an oxide molecule — roughly 60% by weight. But what the label doesn't tell you is that magnesium oxide has approximately 4% bioavailability. That means your body absorbs about 4 cents in every dollar.
Magnesium glycinate, by contrast, contains about 14% magnesium by weight — which sounds lower. But it absorbs at around 80%. That 500mg of magnesium glycinate delivers close to 70mg of magnesium your body can actually use. The 500mg of oxide delivers about 12mg.
The form isn't a minor detail. It's the whole story.
What to Look for on the Label
The only number that matters is the elemental magnesium figure — the actual amount of magnesium delivered per dose, after accounting for the carrier.
A well-labelled supplement will state it clearly: "Magnesium (as magnesium glycinate) — 200mg." That 200mg is your actual dose. If a label only shows the compound weight and doesn't specify elemental magnesium separately, that's a signal to look more carefully.
The upper limit for supplemental magnesium from the TGA and NHMRC is 350mg elemental per day for adults. Most quality glycinate products are dosed between 200–400mg elemental, right in that range. Most oxide products, despite their impressive-looking labels, deliver a fraction of that.
Why Glycinate Is the Form Worth Choosing
Magnesium glycinate is magnesium bound to glycine — an amino acid. This matters beyond just the absorption rate.
Glycine is an inhibitory neurotransmitter in its own right. It activates GABA receptors independently — the same receptors your brain uses to wind down and transition into sleep. When you take magnesium glycinate, you're getting two calming mechanisms working together: the magnesium, and the glycine it's paired with.
The research reflects this. A randomised controlled trial published in Nature and Science of Sleep found that 250mg elemental magnesium as bisglycinate over four weeks produced a statistically significant reduction in insomnia severity compared to placebo. A 2025 study also demonstrated reduced anxiety-like behaviour with magnesium glycinate specifically, suggesting a genuine stress-modulating role.
This is what separates glycinate from other forms. Citrate absorbs reasonably well but can cause GI upset at higher doses. Malate is better suited to energy and muscle performance taken in the morning. Oxide, as established, barely absorbs at all. Glycinate is the form purpose-built for the outcomes most people are actually seeking — better sleep, reduced stress, and consistent magnesium repletion across the board.
When and How to Take It
The timing question is simple: evening. The glycine component supports the transition into sleep, and magnesium's muscle-relaxing and nervous system-calming effects are most useful in the hours before bed.
Take it 30–60 minutes before you want to sleep. With or without food — glycinate is well tolerated either way.
One practical note: magnesium and calcium compete for absorption. If you're supplementing both, take calcium in the morning and magnesium in the evening. They'll absorb better, and the timing makes logical sense for each one's function.
What to Expect
Magnesium isn't melatonin. It doesn't knock you out. What most people notice over two to four weeks of consistent use is that the quality of sleep changes — it's easier to fall asleep, easier to stay asleep, and the sleep feels more restorative. Stress response tends to soften. Muscle tension eases.
Deficiency is more common than most people realise. Australian diets frequently fall short of the recommended daily intake of 310–420mg, particularly in people who eat a lot of processed food, drink regularly, or exercise heavily — all of which deplete magnesium faster than it's replaced.
The fix isn't complicated. But it does start with choosing the right form.