Magnesium supplements, the different forms and when they help

What magnesium does and how much you need

Magnesium is a mineral the body uses in more than 300 biochemical reactions, including nerve and muscle function, a steady heartbeat and the production of energy and protein [1]. In the UK the reference intake for adults is 300 mg a day for men and 270 mg a day for women [2].

Green leafy vegetables, nuts, seeds, beans, whole grains and milk are good sources [1]. A true lack of magnesium is rare. It is more likely with bowel disease or surgery that reduces absorption, with type 2 diabetes, with heavy drinking and in older age [1].

How the forms differ

In a supplement, magnesium is always bound to another substance, which gives the form its name, such as oxide, citrate, glycinate, chloride or hydroxide. The forms differ in how well they dissolve and how much magnesium the gut takes up. When researchers tested 15 products on sale, dissolution and absorption in the laboratory varied very widely, and in 30 volunteers the product that performed poorly in the laboratory also raised blood magnesium less [3].

Some forms work mainly in the bowel. Magnesium hydroxide is used as a saline laxative for occasional constipation, because it holds water in the stool [4]. Loose stools are the side effect people notice most, so the form and the dose affect how comfortable a supplement is to take.

A food supplement label must state the amount of each mineral the product contains, with the recommended daily dose and a warning not to exceed it [5]. That stated amount is the figure to compare between products.

What the evidence shows

For night cramps in older adults, a Cochrane review of 11 trials concluded that magnesium is unlikely to give a clinically meaningful reduction in cramps, while the evidence in pregnancy is conflicting [6].

For sleep, a review of 3 small trials in 151 older adults with insomnia found that people fell asleep about 17 minutes sooner with magnesium, though the trials were at moderate to high risk of bias and the evidence was of low to very low quality [7].

For blood pressure, a meta-analysis of 34 trials with 2,028 people found that a median dose of 368 mg a day lowered blood pressure by 2.00 mm Hg systolic and 1.78 mm Hg diastolic on average [8]. That is a small change, and it is no substitute for treatment your doctor has advised.

These findings come from research, and a product sold as a food supplement in Great Britain may carry only the health claims on the authorised register, which for magnesium include that it contributes to normal muscle function and to the reduction of tiredness and fatigue [9].

Safety, and who should check with a doctor first

Too much magnesium from food is uncommon, because healthy kidneys remove the excess. It most often comes from supplements or magnesium laxatives, and the risk is greatest when kidney function is significantly reduced [1]. UK labelling guidance asks for a warning on products giving more than 400 mg of magnesium a day that they may cause mild stomach upset in sensitive people [10]. In the Cochrane trials, the side effects of oral magnesium were mostly digestive, such as diarrhoea [6].

Ask a doctor or pharmacist before you start if any of these apply to you.

• You have kidney disease, or have been told your kidney function is reduced [1].
• You take other regular medicines, because the advice for magnesium hydroxide is to take other medicines at least 2 hours before or after it [4].
• You have taken an acid-reducing medicine called a proton pump inhibitor for a long time, which can lower magnesium on its own [11].
• You are pregnant, planning a pregnancy or breastfeeding [4].

When low magnesium needs a doctor

Low magnesium is usually a sign of something else, such as a medicine or a condition affecting absorption. The MHRA has reported severe low magnesium in people taking proton pump inhibitors, mostly after more than a year of treatment, and the symptoms can begin gradually and be overlooked [11]. In most reported cases, levels recovered after magnesium replacement and stopping the medicine, which is a decision for your doctor [11].

If you take one of these medicines and notice muscle twitches, tremors, vomiting, tiredness or loss of appetite, see a doctor rather than treating yourself [11]. If you have a seizure, collapse, chest pain or a racing or irregular heartbeat, call 999 in the UK or the local emergency number.

How The Wellness can help

A GP appointment gives you time to go through what you take, why you take it and whether it still makes sense alongside your medicines. If tiredness is the reason you reached for magnesium, the Core Wellness blood test looks at iron stores, thyroid function and vitamin D, and a doctor explains the results with you.

If a supplement does suit you, your doctor can help you choose a form and a dose, and The Wellness shop stocks supplements you can buy with that advice in hand. Every doctor is GMC registered, our clinics are on Baker Street in Marylebone and Eccleston Street in Belgravia, and a doctor can also come to your home or hotel in London.