Why I Supplement Magnesium in Most of My Patients

One supplement that I frequently reach for across nearly every patient population is magnesium. Not because it's trendy, but because the evidence is overwhelming: most Americans are not getting enough, standard testing frequently misses the deficiency, and the consequences of chronic insufficiency touch nearly every system in the body.

The Problem with Standard Magnesium Testing

Why Serum Magnesium Misses the Mark

When your conventional doctor orders a magnesium level, they generally order a serum magnesium — a measure of magnesium floating freely in the liquid portion of your blood. Most of the magnesium is stored inside cells and bone tissue so the freely available portion in the serum does not reflect the true magnesium reserves in your body.

Because the body tightly regulates serum magnesium levels, it will pull magnesium from cells and bone to maintain a 'normal' serum reading — even when true intracellular stores are critically depleted. This means you can appear 'normal' on a standard panel while being genuinely deficient in a functional sense.

Why I Use RBC (Red Blood Cell) Magnesium Instead

Red blood cell (RBC) magnesium measures magnesium inside the red blood cells, providing a more reliable picture into intracellular magnesium status. Because red blood cells live approximately 120 days, this test reflects your body's magnesium reserves over several months, not just your dietary intake from yesterday.

As serum magnesium begins to fall, the body pulls magnesium from red blood cells. So by the time RBC magnesium is low, a true deficiency is well-established. Importantly, studies have shown that magnesium supplementation improves RBC magnesium content while often having no significant effect on serum magnesium in people who are deficient — further confirming that serum levels are an unreliable indicator.

An RBC magnesium level around 6.0 mg/dL is essential for optimal health and disease prevention. While RBC magnesium levels are superior to serum testing, it is still not a perfect measurement of total body magnesium. However, this lab remains our best accessible, actionable biomarker and should always be interpreted in clinical context: symptoms, diet, medications, and chronic disease history.

References: Razzaque MS. Magnesium: Are We Consuming Enough? Nutrients. 2018; PMC6316205

DiNicolantonio JJ, O'Keefe JH. Subclinical magnesium deficiency: a principal driver of cardiovascular disease and a public health crisis. Open Heart. 2018; PMC5786912

A Nation Running on Empty: Magnesium Deficiency in the United States

Magnesium deficiency is not a rare as you might think. It is a widespread, underrecognized public health problem that touches nearly half the American population.

According to multiple analyses of National Health and Nutrition Examination Survey (NHANES) data, nearly 50% of U.S. adults consume less magnesium than the Estimated Average Requirement (EAR). The current RDA (recommended dietary allowance) is approximately 420 mg/day for adult men and 320 mg/day for adult women --- although even these figures are considered potentially outdated. Survey data consistently shows average intakes falling far short of these requirements.

Dietary magnesium intake in the U.S. has dropped dramatically over the past century — from roughly 500 mg/day in the early 1900s down to an average of just 250 mg/day today. The calcium-to-magnesium ratio in the American diet has simultaneously worsened, rising from approximately 2:1 historically to 5:1 or higher in modern diets — a shift with significant implications for cardiovascular risk and metabolic health.

The health consequences of chronic low magnesium are extensive. Low magnesium has been associated in the literature with:

•         Type 2 diabetes and insulin resistance

•         Hypertension and cardiovascular disease

•         Migraine headaches

•         Osteoporosis

•         Depression and anxiety

•         Poor sleep quality

•         Muscle cramps and chronic pain syndromes including fibromyalgia

•         Elevated C-reactive protein and systemic inflammation

Given how broadly magnesium is required — it is a cofactor in over 600 enzymatic reactions — it stands to reason that low levels would have diffuse, system-wide consequences that are easy to misattribute to other causes.

References: Rosanoff A. Perspective: US Adult Magnesium Requirements Need Updating: Impacts of Rising Body Weights and Data-Derived Variance. Adv Nutr. 2021 Mar 31;12(2):298-304. doi: 10.1093/advances/nmaa140. PMID: 33179034

You Can't Eat Your Way To Optimal Magnesium: Food Quality and Modern Farming Practices

Even patients who eat well — plenty of leafy greens, legumes, nuts, seeds, and whole grains — are often surprised to learn they may still be falling short on magnesium. The reason is not just in dietary choices, but the quality of the food that is consumed

Modern intensive agriculture has dramatically altered the mineral content of our crops. Many analyses show that both increasing yield and harvest numbers are highly significant factors explaining the down trend in mineral concentration. Essentially the modern farming practices of prioritizing yield has created a tradeoff between productivity and nutritional quality. This intensive cropping - without magnesium replenishment in the soil - further exacerbates this issue.

Synthetic fertilizers have compounded the problem even further. Standard NPK (nitrogen, phosphorus, potassium) fertilization practices do not include magnesium. Potassium and ammonium ions compete with magnesium for uptake at the root of the plant (a phenomenon called cation competition). These leads to crops that are grown in progressively magnesium poorer soil.

Best Dietary Sources of Magnesium

While whole food sources remain important and should always be encouraged, here are the richest dietary sources of magnesium to prioritize:

•         Pumpkin seeds, almonds, cashews, hazelnuts, and Brazil nuts

•         Dark leafy greens: Swiss chard and spinach

•         Legumes: black beans and edamame

•         Whole grains: amaranth and brown rice

•         Avocado

The challenge is that to hit even 320–420 mg/day from food alone requires a remarkably nutrient-dense diet that most Americans don't come close to achieving — and even then, soil depletion and high yield farming practices means the actual magnesium content of those foods may be less than expected.

References: Davis DR et al. Changes in USDA food composition data for 43 garden crops, 1950–1999. JACN. 2004

Fan MS, Zhao FJ, Fairweather-Tait SJ, Poulton PR, Dunham SJ, McGrath SP. Evidence of decreasing mineral density in wheat grain over the last 160 years. J Trace Elem Med Biol. 2008;22(4):315-24. doi: 10.1016/j.jtemb.2008.07.002. Epub 2008 Sep 17. PMID: 19013359.

Choosing the Right Magnesium: A Guide to Formulations

Not all magnesium supplements are created equal. The form of magnesium matters enormously for bioavailability, tolerability, and clinical application. Here's how I navigate formulation choices with my patients:

Magnesium Glycinate (Bisglycinate)

This is my ‘go-to’ recommendation for most patients. Magnesium glycinate is chelated to the amino acid glycine, giving it excellent absorption and a very gentle GI profile — no laxative effect. Glycine itself has calming, sleep-supportive properties, making this an ideal choice for patients dealing with anxiety, poor sleep, muscle tension, and stress. It is well-tolerated even at higher doses.

Best for: Sleep, anxiety, stress, general magnesium repletion, patients with sensitive GI tracts.

Magnesium L-Threonate

This form is chelated to L-threonate, a metabolite of vitamin C. Its distinguishing feature is its ability to cross the blood-brain barrier more effectively than other forms. Animal studies have demonstrated an ability of magnesium threonate to enhance learning abilities, working memory, and short- and long-term memory in rats. Human data is limited, however there may be potential benefits for memory, cognitive function, depression, and neuroplasticity.

Best for: Cognitive support, mood disorders, neurological concerns, memory, aging patients.

Magnesium Malate

Bound to malic acid, a compound involved in cellular energy production (the Krebs cycle), magnesium malate is well-absorbed and can be considered “mildly energizing” rather than sedating. It can significantly increase the magnesium levels in skeletal muscle therefore suggesting a potential advantage for muscle function and physical performance.  

Best for: Fatigue, fibromyalgia, muscle pain, athletic recovery, morning/daytime supplementation.

Magnesium Citrate

One of the most widely available and affordable forms, magnesium citrate is reasonably well-absorbed but has a notable laxative effect at moderate-to-higher doses. This can be a feature for patients dealing with constipation, but a drawback for those with normal or loose bowel function.

Best for: Constipation, general repletion in patients without GI sensitivity, cost-conscious supplementation.

Magnesium Oxide

This is the form most commonly found in cheap, mass-market multivitamins and basic supplements. Despite containing a high percentage of elemental magnesium by weight, it is poorly absorbed by the GI tract and primarily acts as an osmotic laxative. I generally do not recommend it for the purpose of correcting magnesium deficiency. Its main utility is as a short-term treatment for heartburn, indigestion, or constipation.

Best for: Acute GI complaints (heartburn, constipation). Not recommended for magnesium repletion.

My Clinical Approach

In practice, my approach to magnesium looks like this:

•   I order RBC magnesium on virtually every new patient with a goal optimal range of around 6.0 mg/dL.

•   I assess for magnesium-depleting factors: diuretic use, proton pump inhibitors, high alcohol intake, high-dose calcium supplementation, chronic diarrhea, poorly controlled diabetes, and high stress.

•   I counsel patients on dietary sources and the limitations of food-based magnesium in the context of depleted soils and agricultural practices.

•   I supplement nearly all patients — my default is magnesium glycinate at 200–400 mg of elemental magnesium daily, adjusted based on RBC levels, symptoms, and tolerability.

•   I retest RBC magnesium after 3–4 months of supplementation to assess response and adjust dosing.

The therapeutic window for magnesium is wide and the risk of supplementation at appropriate doses is low for most patients (always use caution in those with kidney disease). The upside — addressing a deficiency that may be quietly contributing to fatigue, anxiety, poor sleep, muscle dysfunction, cardiovascular risk, and metabolic dysregulation.