Having spent many evenings and weekends working in a retail pharmacy with a thriving vitamin and supplement aisle, I’m no longer surprised at the purchasing habits of consumers. The setting gave me not only the opportunity to learn about the latest supplement fads, but also to speak with consumers that place great faith in these products. A recent paper suggests 25% of CAM users account for 75% of all expenditures on CAM. And that’s consistent with what I saw in the pharmacy. Some consumers spend hundreds per month on supplements, so the belief that these products are beneficial must be very strong. So I like to ask about reasons for use. Many attribute their current health status to supplements, and usually tell me that their supplements are for preventing disease, optimizing health, or “boosting” the immune system. And while I’m encouraged to see these consumers take an active interest in maximizing their own health, I’m often puzzled by the decision-making that’s occurring. In some cases, beliefs about health, medicine, or the effectiveness of their supplements are misguided, or flat-out wrong. They’re getting expensive urine and positive feelings, but there’s little evidence to suggest the supplementing is having any objective benefits. More surprisingly, some consumers take products for which there is no medical indication at all – it’s intended as “primary prevention” of a medical condition they believe they can avoid through supplement use. Afraid of macular degeneration? Take lutein. Or worried about Alzheimer’s? Grab some ginkgo biloba. No matter your anxiety, there’s a supplement marketed for your. One supplement with enduring popularity is magnesium.  It cures cancer. And depression. Throw away your inhalers, because it cures asthma too. Read enough online, and you’ll conclude that magnesium is quite possibly the prevention and the cure to all disease.

Magnesium really is everywhere in the body – it’s the 4th most abundant mineral. It’s involved in hundreds of different biochemical reactions and enzyme systems, supporting processes like protein synthesis, cell growth, and energy production. It has roles in nerve function, muscle control, and blood pressure. Even blood sugar regulation depends on magnesium.  The body has several mechanisms to stabilize levels. We’re carrying about 25 grams in total, much of which of which is locked in the skeleton. Some magnesium sits on the surface of bone and helps (slowly) regulate levels in the rest of the body: Only a tiny fraction of the total body’s magnesium (1%) is actually in the blood.

We obtain magnesium in our diet, with an “average intake” of 360mg per day. Most foods contain magnesium, though some have more than others. After ingestion, magnesium enters the blood stream through a combination of diffusion and also active absorption at sites throughout the bowel. Some of this effect appears to be influenced by vitamin D. Kidneys manage most of the excretion of magnesium and actively control the amount that is eliminated to maintain steady levels. Interestingly, almost all magnesium filtered out initially, and then about 95% of it reabsorbed. Drugs which inhibit the regular function of the kidney, like diuretics (“water pills”) can affect magnesium levels. Some diuretics promote excretion of magnesium, while others promote retention – it depends on their mechanism of action. There are a long list of other drugs that can influence magnesium levels, most by affecting the kidney’s filtration process. Proton pump inhibitors (e.g., Nexium) are among the offenders, and can cause hypomagnesemia in some. Given its multiple actions, low magnesium levels are associated with conditions like hypertension, cardiovascular disease, and diabetes. Other medical conditions can affect magnesium levels, either through action on absorption or elimination. Alcoholism, kidney or liver disease, and various gastrointestinal malabsorbtion conditions (e.g., Crohn’s disease) can all reduce magnesium levels.

Does magnesium have medicinal effects when consumed? In medical and pharmacy practice, it has several demonstrated uses – the most well known are probably as an antacid or laxative. Good old milk of magnesia (MOM) works because magnesium is partially absorbed – enough stays in the bowel, drawing water into the colon and softening and dislodging anything below.  Injectable magnesium, sold as a prescription drug,  allows large doses to be given quickly, and is ued for the treatment of preeclampsia (pregnancy-induced hypertension) and eclampsia (the resulting seizures) – it’s well tolerated and better than other drugs.  Magnesium also seems to be effective for treating some forms of arrhythmia, as well as asthma exacerbations. Some types of migrane and cluster headaches also seem to respond to magnesium supplements.

In other medical conditions, magnesium’s role is less clear. Magnesium levels may be related to osteoporosis, but the role of supplements is still being established. Diets high in magnesium seem to reduce the risk of stroke, but supplementation doesn’t seem to offer the same benefit. The same can be said for managing high blood pressure – effects with supplements are inconsistent and unimpressive.  For treatments of conditions like anxiety, ADHD, muscle cramps, and restless leg, more data is required before we can determine if supplements are beneficial.

In cases where drugs or diseases have reduced blood magnesium levels, supplementation is appropriate. Magnesium supplements effectively treat and prevent low magnesium. There’s debate over which type of oral supplement is “best”, fueled by a lack of studies that have actually examined how much magnesium is actually absorbed. Magnesium gluconate and magnesium chloride seem to be preferred over magnesium oxide and magnesium sulfate, which are associated with more diarrhea. Some pharmacies manufacture  magnesium glycinate, which is claimed to be superior. There is little credible information to guide selection, unless you are a rat. Given the influence of food and other factors, the specific version used may depend more on individual preferences and side effects. Magnesium is often combined with calcium in tablets, sometime in specific ratios (e.g., (3:1). There’s no scientific basis for these proportions.

Supplements or Diet?

Having established magnesium’s bona fides as a drug, with medicinal uses, what does this suggest about supplementation in the absence of a confirmed deficiency? Given the number of systems and processes that are controlled by magnesium, supplementation could plausibly have biological effects. However, for it to do so, it would need to act against all of our regulatory systems that work to keep our levels fairly constant. Magnesium supplements are generally well tolerated and toxicity is rare. But here’s where we run into questions about correlation and causation. Without prospective data from randomized controlled trials, we cannot infer that supplements have the same effects as dietary changes. Benefits in studies seem to be related consuming foods rich in magnesium, not necessarily magnesium supplements.

Magnesium is abundant on earth and so it’s also abundant in food.  In general, the absorption of magnesium from the diet is estimated at about 50%. Despite the prevalence in food there is information to suggest that dietary magnesium consumption can still be insufficient. Plants, and particularly grains, have more magnesium than meat or dairy. A simple rule of thumb is that magnesium usually accompanies fiber. The water supply can also contribute to magnesium intake, particularly in areas with “hard” water. There’s epidemiological evidence that connects water hardness with cardiovascular disease. While the data are interesting, few studies have measured consumption and correlated it with magnesium levels. Given that most of our magnesium consumption is from food, the current consensus does not support minimum hardness levels being established in the water supply.

Risk and Benefit

No harms or adverse effects have been observed when magnesium is consumed as a food ingredient. Supplements, however can cause adverse effects – diarrhea being the most common when daily doses exceed 350mg. High levels, hypermagnesemia, is uncommon but has been reported, particularly with regular consumption of magnesium supplements in people with decreased kidney function. In the absence of risk factors, particularly undiagnosed kidney disease, magnesium supplementation is generally considered safe.


Magnesium supplementation has an established role for some diseases and medical conditions, and looks promising for others. Whether supplementation is required in the absence of a relevant disease or documented deficiency is unclear. While it is tempting to extrapolate the relationship between low magnesium and various diseases into an argument for supplements, the issue of confounding in dietary studies cannot be ignored.  As we have seen with other vitamin studies, the long-term benefits of supplementation should not be assumed until they have demonstrated.  Given the complexity of magnesium’s role in the body, the best way to answer questions about the benefit of supplements is with randomized trials. But there’s another approach that has an attractive risk/benefit perspective: Boosting magnesium intake through dietary changes. Helpfully, foods rich in magnesium are already recommended in diets associated with  positive health outcomes: Green vegetables, beans, whole grains, nuts and seeds are all good sources of magnesium. Supplementation may not turn out to be harmful, but it may be unnecessary. And it may distract people from making positive changes to their diet. Without a confirmed deficiency, or specific medical need, magnesium supplementation seems unnecessary.


Posted by Scott Gavura