Magnesium for Constipation Relief: A Dietitian’s Guide to Dosing, Forms & Results

Why Magnesium Matters

Magnesium is a key mineral for our health; it is involved in over 300 enzymatic reactions related to muscle contraction, nerve transduction, energy production, blood sugar control, bone health, and much more (1). Unsurprisingly, deficiency has been linked to consequences such as type 2 diabetes, inflammation, obesity, hypertension, neurological conditions, migraines, and osteoporosis (2-5).  

Magnesium is an essential mineral, meaning we must get it from our diet and/or supplements, as our bodies cannot create it. Therefore, to maintain adequate health, it is important that we pay attention to the amount we consume. Some key dietary sources include  pumpkin seeds, chia seeds, almonds, leafy greens (such as spinach), cashews, peanuts, some fortified breakfast cereals, whole grains, black beans, bananas, and edamame (2, 6). Some tap and bottled water also contain magnesium, but the amount varies drastically by source and brand, ranging anywhere from 1-120 mg/dL (2). The quantity in foods also varies notably depending on the soil it was grown in. Since only 1% of magnesium is found in our bloodstream (the remainder being located in bone and inside of cells), serum levels do not reflect total body stores, making deficiency prevalence difficult to estimate (1). However, we do know that more than ⅓ of Canadian adults consume less magnesium than the recommended amount (320mg and 420mg for women and men, respectively) (7, 8). Factors that increase deficiency risk include taking proton pump inhibitors or loop diuretics, being elderly, suffering from chronic alcoholism, type 2 diabetes, and gastrointestinal diseases (such as celiac or Crohn’s Disease) (2). 

Magnesium for Constipation Relief

In addition to all these crucial roles, magnesium is also touted as a constipation remedy. It works as such because it is an osmotic laxative. This means that when present in the small intestine, it promotes the transfer of water into the intestine, softening stool and making it easier to pass. This increased stool bulk also stimulates intestinal contractions, facilitating its movement through the gastrointestinal tract (GIT) (9, 10). 

NHANES, the major health and nutritional survey in the US, noted that greater dietary magnesium intake relates to lower incidence of constipation when defined by stool frequency, but not stool consistency (11). However, getting enough just from food can be challenging. While one serving of pumpkin seeds provides 156 mg of magnesium and chia seeds 111 mg, all other top sources contain 80 mg or less, with the majority falling below 60 mg (2). This raises the question on the need for supplementation to see clinically meaningful improvements in constipation. So, let’s dive into more detail on this topic! 

Types of Magnesium Supplements for Constipation Relief

The three main forms of magnesium supplements typically considered for constipation include magnesium glycinate, magnesium citrate, and magnesium oxide. 

Magnesium glycinate is more effective in resolving deficiency and improving insomnia and stress than it is for constipation. Its laxative effects are fairly minimal as it is readily absorbed, and it is unabsorbed magnesium that is able to draw water into the GIT  (1). 

Magnesium citrate meanwhile is more useful for constipation, as less of it gets absorbed than magnesium glycinate, but still enough to be beneficial for correcting deficiency (1).  

Magnesium oxide (MgO) is very poorly absorbed, giving it strong laxative properties, but also meaning it should not be used to treat deficiency. It is also effective for heartburn (1). It has long since been used to alleviate constipation based on anecdotal evidence, but more recently, highly controlled studies have corroborated this claim. For example, one study found that supplementing MgO improved spontaneous bowel movements, transit time, abdominal symptoms, stool formation, and even quality of life compared to placebo (12). Another study found that it was as effective as the commonly prescribed laxative ingredient “senna”, but noted that both were less effective than some newer pharmaceutical laxatives like lubiprostone, linaclotide, and elobixibat. However, these options tend to be more expensive, so authors suggest MgO could be used in conjunction with them to reduce the dose needed of each (13). Of note, taking proton pump inhibitors hinder MgO’s laxative effects (9).   

Dosing

So how much magnesium should we supplement to actually see these improvements in constipation? The dose depends on which type of magnesium supplement you are taking, as they vary in two key ways: 

1. First, the three forms contain different amounts of elemental magnesium. Magnesium glycinate is only 14.1% elemental magnesium by weight, while the remainder is glycine (14). This means that a 100mg capsule contains only 14.1 mg, rather than 100 mg of magnesium. Meanwhile, magnesium citrate is roughly 14-16% and MgO 60% elemental magnesium (15, 16). The Supplements Facts Panel will indicate the amount of elemental magnesium per capsule however, so you don’t need to worry about memorizing this (but you should ensure to read the label so you know what you are taking!) (2). Typically, all three forms are sold in tablets containing 100-200mg elemental magnesium each (17). Magnesium citrate is also commonly available in liquid form (18).   
2. The second key factor in terms of dosing is the supplement’s bioavailability, meaning how much of the magnesium your body absorbs. Generally, inorganic forms (such as MgO) are less bioavailable (less absorbable) than organic forms (like magnesium glycinate and citrate) (19). In fact, only about 4% of MgO gets absorbed (20)! Once again, this is why it works as a laxative but not for resolving a deficiency. In comparison, ~25-30% of magnesium citrate and up to 80% of magnesium glycinate is absorbed. For reference, 30-40% of magnesium from food gets absorbed (2). Bioavailability also varies by supplement form, with effervescent tablets and granule formulations being better absorbed than standard tablets (19). Meanwhile, absorption decreases as dose increases, which is why it is generally recommended to split it up into 2-3 smaller doses per day rather than taking one larger dose (19). 

Taking these two points together, you can estimate the dose of magnesium you are taking by multiplying the amount of elemental magnesium in your supplement by its bioavailability. For example, if you take two 400mg capsules of MgO, each contains ~240 mg of magnesium (60% of 400). By absorbing 4% of this, you receive ~ 19.2 mg of magnesium (0.04 x 2 x 240).

 So, taking all of this into account, what dose should be used? It is important not to surpass the upper limit of 350mg per day (this limit only refers to the amount from supplements and does not include dietary sources) as this can pose health risks (more on this below!) (8). While daily supplements of <350mg are usually considered safe, the amount to take to see improvements in constipation symptoms will vary by person (6). We recommend starting at the lowest dose possible, and gradually increasing it if needed. This should be done in conjunction with monitoring your serum magnesium levels to ensure you are not approaching toxicity levels. 

Risks

You may be wondering why it’s so important to monitor your magnesium intake. Although magnesium plays many essential roles, there can be too much of a good thing, and there are actually risks associated with excess consumption. 

First, is the risk of GI upset. Given that magnesium can help alleviate constipation, it makes sense that too much can cause the opposite problem. Diarrhea, nausea, and abdominal pain are all fairly commonly reported side effects, especially from MgO (2). Magnesium glycinate is much less likely to cause GI symptoms as the majority gets absorbed (13). Other common consequences include weakness, dizziness, lightheadedness, flushing, hypotension (low blood pressure), and vomiting. In more serious cases, when blood magnesium levels rise even further, people experience respiratory paralysis, loss of deep tendon reflexes, heart node blocks, heart rhythm abnormalities, and eventually cardiac arrest (9) (21). 

Another, but more unusual side effect, is milk alkali syndrome. This refers to the combination of hypercalcemia (high blood calcium), metabolic alkalosis (high blood pH), and decreased kidney function. It is caused by excessive levels of both calcium and an alkali such as MgO (magnesium citrate and glycinate are less alkaline and thus do not pose a significant risk). This is most often seen in the elderly, as it is common for this population to simultaneously be taking magnesium for constipation and vitamin D for osteoporosis. Vitamin D increases calcium absorption, thus increasing hypercalcemia risk. Notably, vitamin D also increases the risk of hypermagnesemia in general, as it also promotes magnesium absorption (9).    

These toxicity effects reiterate the importance of monitoring your serum levels if you chose to take magnesium supplements. These symptoms begin to appear when levels surpass 2.2mEq/L (1.1mmol/L), the level used to diagnose “hypermagnesemia” (high blood magnesium) (21). Hypermagnesemia is a greater risk in people with compromised kidney function, for whom it is particularly essential to discuss with your doctor before initiating supplementation (9).    

The bottom line is that for safety purposes, it is best to combine your knowledge of appropriate dosing with monitoring your serum magnesium levels, as each capsule and person will be unique.

Other Considerations

Interactions with Other Medications

Magnesium can alter the absorption and excretion of other drugs. Specifically, it decreases the efficacy of: 

• Tetracycline
• New quinolones
• Bisphosphonates
• Iron supplements
• Fexofenadine
• Some antibiotics
• Loop diuretics
• Proton pump inhibitors
• Others 

In some cases, taking them at a different time in the day from each other will limit magnesium’s interference effect (2) (9). However, be sure to consult your doctor regarding the protocol for their co-supplementation if you do take any of these (or other) pharmaceutical drugs. 

Method of Administration

It is generally recommended to take magnesium supplements with food. This not only minimizes unwanted side effects such as abdominal cramping, but can also enhance its absorption (22).   

Fibre

Now what about fibre? You might have heard that fibre is one of the most effective constipation remedies, with strong evidence supporting the use of both dietary and supplemental sources. And this is completely true! In particular, kiwifruits, prunes, rye bread, and psyllium husk may be particularly effective (23). It is important to remember that taking a magnesium supplement does not replace the need for fibre. Fibre not only helps with regularity, but is protective against a range of chronic diseases. Implementing a higher fibre diet may also lower the dose of magnesium supplementation you need to see improvements in constipation. 

Make sure to remember however, that there is a sweet spot for the amount of fibre. Both too much and too little can worsen constipation. Increase your daily intake slowly and in conjunction with increasing your water intake too to see the best results. Luckily, most high magnesium foods are also high fibre foods (such as leafy greens, whole grains, and nuts), so by focusing on increasing your magnesium intake, you likely will naturally increase your fibre intake as well (2)! 

However, it is important to note that fibre can affect magnesium absorption. In particular, high amounts of some fibres such as hemicellulose, cellulose, and lignan can reduce magnesium absorption (25). Similarly, phytates and oxalates, which are types of plant compounds known as “anti-nutrients” found in some fibre-rich foods, can also reduce magnesium absorption. Rich sources of phytates include some whole grains, legumes, nuts, and seeds, while oxalates are mostly found in spinach, chart, beets, and rhubarb (24) (25). This is not to scare you off eating high fibre foods however! In fact, some other types of fibres, namely resistance starches, inulin, and oligosaccharides, can actually enhance magnesium absorption (25)! Instead, just take note if you are going to have a large amount of fibres that may reduce absorption, and consider spreading out that meal from when you take your magnesium supplement by at least 2 hours (22). Overall, your best bet for managing constipation is likely to use both magnesium supplements and fibre as tools! 

Takeaway Message

All in all, magnesium supplementation is a useful strategy for addressing constipation. Like with any supplement, it is important to consider your unique health situation to determine if it is appropriate to use. Nonetheless, at doses below 350mg, it is an extremely effective tool that is safe for most individuals, and has the potential to significantly improve quality of life! If you have more questions about using magnesium supplementation to manage your digestive symptoms, contact us or book a consultation today.

References

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