Magnesium is essential for health. But can supplements really help with digestion, muscle pain, anxiety and sleep? | Antiviral

Dietitian Danielle Shine says her Instagram inbox was filled with questions after actor Scarlett Johansson was quoted in the Daily Mail and other publications recently saying she takes magnesium to improve her digestion and sleep.“I know that everybody is taking magnesium now. It’s so helpful,” Johansson reportedly told beauty publication Byrdie.Shine says celebrity endorsement of supplements is persuasive and always leads to an uptick in questions.“The general sentiment was, ‘If she [Johansson] takes magnesium and says it works for her, then maybe I should be taking it too,’” Shine says.But celebrity and influencer endorsement aside, what does the evidence say?Magnesium is importantMagnesium is an essential mineral that the body needs for blood pressure regulation, bone health and many other important functions. Because the body doesn’t produce it, diet is critical, with nuts, seeds, legumes, leafy greens and brown rice among good sources.But magnesium deficiency is not common in Australia, and is more likely in people with certain conditions such as diabetes or bowel disease. Despite this, it is promoted widely as a good supplement anybody can take for a range of conditions.Limited evidence for sleepShine says some studies have reported improvements in certain aspects of sleep when taking magnesium supplements, “but the findings are inconsistent”.“Studies are generally small, short-term and use different forms and doses of magnesium, which makes it difficult to draw firm conclusions about whether supplementation reliably improves sleep,” she says.Sports dietitian Dr Evangeline Mantzioris says another difficulty is studies measure sleep quality differently, with some examining sleep duration, time taken to fall asleep, or depth of sleep. But there are not enough consistent, strong findings to show strong cause and effect, she says.If someone was struggling with sleep, “We would want to look at sleep hygiene first, such as lights and devices off, and no caffeine,” she says, along with stress and other factors.What about digestion and gut health?Shine says some forms of supplemental magnesium draw water into the intestine, and this has a laxative effect. The British Dietetic Association names magnesium oxide as an evidence-based treatment option for adults with constipation.Mantzioris says people who exercise regularly need to be careful of this, because taken wrongly, magnesium can cause nausea, abdominal cramping and diarrhoea.This is because intense exercise jolts the intestines while diverting blood away from them, making the gut extra sensitive to magnesium and giving a laxative double-whammy effect.“We always say in sports that you don’t want to take magnesium when you know you’re going to be out training the next day,” she says.Can magnesium help exercise recovery?Magnesium is important for metabolism and muscle contraction, Shine says, “but its role in these processes doesn’t mean that supplementation will improve recovery”, including from muscle pain and damage.“The Australian Institute of Sport classifies magnesium as a Group C supplement, a category where the scientific evidence doesn’t support a benefit among athletes or is insufficient to draw firm conclusions,” she says.There’s also no good evidence magnesium supplementation prevents muscle cramps, Mantzioris says. While soaking in a warm magnesium salt bath may bring relief, it is more likely the warm water rather than the magnesium doing the heavy lifting.Will it calm my nerves?Shine says marketing of magnesium products exploits it being “essential” to calming the nervous system, with suggestions supplementation is also essential to preventing and treating stress and anxiety.“Magnesium is involved in processes including nerve transmission and regulation of neuronal activity, helping nerves communicate throughout the body,” she says.“However, there’s no strong evidence that magnesium supplementation provides broad benefits for nervous-system function.”Mantzioris says supplement companies play on people’s real-life stressors and “they capture our fear that we might be deficient in something, and try to sell that this might explain why we’re stressed or tired”.“But the reason many of us are stressed and tired is because we work too much, which magnesium can’t fix.”Those with a genuine deficiency may benefitPeople at risk of deficiency or worried can see a doctor who can order a blood test and recommend the best way to increase intake. There is also no strong evidence to show that routine oral magnesium supplementation during pregnancy is beneficial.Taking magnesium in the absence of a known deficiency has some risks, Shine says.“Very high doses can cause toxicity, including low blood pressure, breathing problems, abnormal heart rhythms and, in severe cases, cardiac arrest,” she says. Melissa Davey is Guardian Australia’s medical editor Antiviral is a fortnightly column that interrogates the evidence behind the health headlines and factchecks popular wellness claims