Magnesium is involved in over 300 enzymatic reactions in the body - energy production, muscle contraction, nerve signalling, DNA synthesis, blood pressure regulation, and sleep. Most adults aren't getting enough of it from food. The average Western diet provides about 70% of the recommended daily intake.
If you've decided to supplement, the form matters. Here's a clear breakdown.
Why Form Matters
Magnesium supplements are sold as compounds - magnesium bound to another molecule. That molecule affects how well the magnesium is absorbed (bioavailability), how fast it's released, and whether it has any side effects.
Magnesium oxide - the form in most cheap generic "magnesium" supplements - has roughly 4% bioavailability. That means only 4 in every 100mg reaches the bloodstream. The rest passes through the gut and commonly causes diarrhoea.
Magnesium glycinate and magnesium citrate are both significantly better absorbed. But they have different profiles.
Magnesium Glycinate
Magnesium bound to glycine - a calming amino acid that is itself important for sleep and nervous system regulation.
Bioavailability: High - generally considered the best absorbed form. The glycine transporter in the gut absorbs it efficiently, and absorption isn't as dependent on stomach acid as some other forms.
Laxative effect: Very low. Glycinate is well-tolerated in the gut and rarely causes loose stools at standard doses.
Best for:
- Sleep quality and insomnia
- Anxiety and stress management
- Muscle cramps and restless legs
- People with sensitive digestion
- Daily high-dose supplementation without GI side effects
Why glycinate helps sleep: Magnesium activates GABA receptors (the same receptors targeted by sleep medications, through a different mechanism) and reduces cortisol. The glycine component adds to this - glycine itself has evidence for improving sleep quality in clinical trials, lowering core body temperature at night and improving sleep onset.
A 2012 study in Magnesium Research found that magnesium supplementation (500mg/day) significantly improved sleep quality, sleep efficiency, and early morning wake symptoms in elderly adults compared to placebo. Most sleep-focused studies use glycinate specifically because of its tolerability at the doses required.
Magnesium Citrate
Magnesium bound to citric acid.
Bioavailability: High - second only to glycinate in most bioavailability comparisons. Well-absorbed in the presence of food.
Laxative effect: Moderate. Citrate draws water into the gut (osmotic laxative effect), which makes it useful for constipation but can cause loose stools at higher doses. A 2005 study in European Journal of Clinical Nutrition found citrate had higher bioavailability than oxide and caused more GI side effects at equivalent doses.
Best for:
- Constipation relief
- General magnesium deficiency
- People who want an affordable, widely available form
- Pre-colonoscopy bowel prep (at high doses prescribed by a doctor)
Why citrate is popular: It's cheaper than glycinate, widely available, and works well for most people at standard doses (150-250mg elemental magnesium). The mild laxative effect is often a benefit for the significant proportion of people who have both magnesium deficiency and constipation.
Head-to-Head Comparison
| Feature | Magnesium Glycinate | Magnesium Citrate |
|---|---|---|
| Bioavailability | Very high | High |
| GI tolerance | Excellent | Moderate (can cause loose stools) |
| Laxative effect | Minimal | Moderate |
| Best use | Sleep, anxiety, muscle | Constipation, general |
| Cost | Higher | Lower |
| Availability | Widely available | Very widely available |
| Elemental Mg per capsule | Check label (typically 100-200mg) | Check label (typically 100-150mg) |
Other Forms Worth Knowing About
Magnesium malate: Magnesium + malic acid. Good bioavailability, gentle on the gut. Particularly favoured in fibromyalgia research because malic acid plays a role in energy production. A reasonable alternative if glycinate causes any issues.
Magnesium taurate: Magnesium + taurine. Emerging evidence for cardiovascular benefits - both magnesium and taurine support heart rhythm and blood pressure. Less studied than glycinate or citrate but a legitimate option for cardiovascular-specific concerns.
Magnesium threonate: The only form clinically shown to cross the blood-brain barrier efficiently. A 2010 MIT study found magnesium-L-threonate significantly improved cognitive function and memory in animal models. Human data is still limited, but it's the most interesting form for cognitive purposes. Also the most expensive.
Magnesium oxide: Cheap and poorly absorbed. Fine as a very occasional laxative at high doses. Not a good choice for regular supplementation.
Signs You Might Be Deficient
Signs low in magnesium covers this in detail, but the main indicators: muscle cramps (especially nocturnal leg cramps), poor sleep quality, anxiety and irritability, fatigue, and headaches. Hypertension and insulin resistance are also associated with chronic magnesium deficiency.
Standard blood serum magnesium tests are relatively poor at detecting deficiency - only 1% of body magnesium is in blood. Red blood cell (RBC) magnesium is a better measure if you want to test specifically for deficiency.
Dosage Guidance
The Recommended Dietary Allowance (RDA) for magnesium is 310-420mg/day for adults (varies by sex and age).
Most magnesium supplements provide 100-400mg of elemental magnesium. Important: check the label for elemental magnesium, not the total compound weight. 1000mg of magnesium glycinate doesn't mean 1000mg of magnesium - the glycine adds to the total weight.
For sleep benefits, research doses cluster around 200-400mg elemental magnesium daily before bed. For general supplementation, 150-250mg is a common starting dose.
Start at the lower end with citrate to assess GI tolerance. With glycinate, most people tolerate 200-400mg daily without issues.
Which Should You Buy?
Choose glycinate if: Your goal is sleep improvement, anxiety management, muscle relaxation, or if you've had GI issues with other magnesium forms.
Choose citrate if: You want a general supplement at a lower price point, you benefit from the mild laxative effect, or glycinate isn't available.
Avoid oxide in either case if you want an effective supplement rather than an occasional constipation remedy.
For sleep specifically, taking magnesium glycinate 1-2 hours before bed at 200-400mg elemental magnesium is the best-evidenced approach in the current literature.