Nature's Bounty Magnesium 500 mg Review: The Oxide Problem and the 350 mg Limit

Magnesium is one of the few supplements where a lifter can make a reasonable case that many people are not getting enough, and one of the few where the cheapest product on the shelf is often the wrong one. A $7 bottle with "500 mg" on the front is a good example. The number is large, the price is small, and the form is the part that decides whether you get what you paid for.

This review looks at one specific product, Nature's Bounty Magnesium 500 mg tablets. We read the Supplement Facts panel on the listing to settle what the 500 mg actually is, compare it with the official intake numbers, look at how well magnesium oxide is absorbed, and then at the evidence for blood pressure, performance, and the rest. We have already written a broad primer on recovery and sleep, so for the physiology and food sources see our magnesium complete guide. This article is about the product.

What You Are Buying

The details below come from the Amazon listing for Nature's Bounty Magnesium 500 mg Tablets, 200 count (ASIN B00H5PJ0HW). The listing's text fields do not include the Supplement Facts, so they are read from the listing's label image, along with the manufacturer's product comparison image. Check the bottle you receive against them.

One inconsistency is worth flagging. The text of the listing's directions says to take "one (1) softgel daily," while the product is a tablet and the label image says tablet. That looks like a copy error, so go by the label image, but it is a reminder to read the bottle instead of the web copy.

500 mg: What the Number Means

The question with any mineral supplement is whether the milligram figure is the weight of the element or the weight of the compound. The NIH Office of Dietary Supplements (ODS) states that the Supplement Facts panel on a dietary supplement label declares the amount of elemental magnesium in the product, not the weight of the entire magnesium-containing compound. That would mean 500 mg is elemental magnesium, and the arithmetic agrees. The ODS gives the magnesium Daily Value as 420 mg, and 500 divided by 420 is 119 percent, which matches the percentage on the label. Because magnesium oxide is about 60 percent magnesium by weight, a tablet with 500 mg of elemental magnesium has to contain a bit more than 800 mg of magnesium oxide. The listing does not state the tablet's size or weight.

In short, the listing states exactly what it appears to: one tablet, 500 mg of elemental magnesium, in the form of magnesium oxide. The manufacturer's own comparison image, as we read it, shows its other magnesium products as glycinate at 240 mg and 360 mg, citrate at 270 mg, and a 400 mg oxide softgel, with this 500 mg oxide tablet as the highest-potency option.

500 mg Against the RDA and the Upper Limit

ODS, in its current fact sheet, lists the Recommended Dietary Allowance for magnesium at 400 mg a day for men aged 19 to 30 and 420 mg for men 31 and older, and 310 mg and 320 mg for women in those age ranges. It sets the Tolerable Upper Intake Level for supplemental magnesium at 350 mg a day for adults, and explains that the upper limit counts magnesium from dietary supplements and medications only. It does not include the magnesium naturally present in food and beverages, which is the reason for the apparent oddity that the upper limit can be lower than the RDA.

It helps to be precise about what these numbers do and do not say. The RDA is the total you aim for from all sources, and a lifter eating a typical diet already gets a good share of it from food. The upper limit is on the supplement portion, and it exists because of what happens in the gut, not because of toxicity in healthy people. ODS says that too much magnesium from food poses no health risk to healthy people because the kidneys eliminate the excess, but that high doses from supplements or medications often cause diarrhea that can come with nausea and abdominal cramping. A tablet that supplies 500 mg of supplemental magnesium is above the line where that becomes more likely.

Magnesium Oxide: The Absorption Problem

Magnesium oxide is the cheapest, most compact, and most concentrated form, which is exactly why a 500 mg tablet is feasible. It is also dissolved poorly. ODS says that forms of magnesium that dissolve well in liquid are more completely absorbed than less soluble forms, and that small studies have found the aspartate, citrate, lactate, and chloride forms to be absorbed more completely and to be more bioavailable than the oxide and sulfate forms.

Two older studies put numbers on it. Firoz and Graber (2001) measured bioavailability as the increment in urinary magnesium excretion in normal volunteers given about 21 mEq a day of four commercial preparations. They found relatively poor bioavailability of magnesium oxide, with a fractional absorption of 4 percent, and significantly higher and equivalent bioavailability for magnesium chloride, lactate, and aspartate. Walker and colleagues (2003) randomized 46 healthy people to 300 mg a day of elemental magnesium as an amino acid chelate, citrate, or oxide, or placebo for 60 days. The organic forms (citrate and chelate) showed greater absorption at 60 days than the oxide as measured by 24-hour urinary magnesium. Citrate produced the highest serum magnesium after both acute and chronic dosing, and the authors reported that magnesium oxide resulted in no differences compared with placebo on the measures they looked at.

Take those results with proper caution. Both are small, both are older, and both rely on urinary and blood markers as proxies for absorption and tissue status, which is imperfect for a mineral the body regulates tightly. ODS itself describes the evidence as small studies. Still, they point the same way, and they come from different groups using different methods. If you apply the 4 percent figure from Firoz and Graber naively to 500 mg, you get about 20 mg of absorbed magnesium, which is simple arithmetic and not a prediction for any individual, since absorption varies with dose, food, and the person. The honest conclusion is that you are probably absorbing a small fraction of the 500 mg label, and that fraction is a lot less than what you would assume from the big number on the front.

The Digestive Side

The same property that limits absorption drives the main side effect. ODS notes that the forms of magnesium most commonly reported to cause diarrhea include magnesium carbonate, chloride, gluconate, and oxide. Unabsorbed magnesium draws water into the bowel, which is why magnesium is the primary ingredient in some laxatives, and why many people notice loose stools at doses that supplement labels treat as normal. The label's advice to take the tablet with a meal is sensible for that reason. It does not remove the effect for everyone.

Practically, this means two things for a lifter. If you take one tablet and your stools loosen or you get cramping, that is a dose and form signal, not a sign you are detoxing. And if you take the tablet at night for sleep, a laxative effect is a poor trade. No specific alternative is reviewed here, but the research points toward better-absorbed forms as the way to reduce the problem, and the manufacturer itself sells some.

What the Research Shows for the Claims

Blood pressure

Magnesium has its best-documented effect outside the gym. Zhang and colleagues (2016) pooled 34 randomized, double-blind, placebo-controlled trials with 2,028 participants. At a median dose of 368 mg a day for a median of three months, magnesium supplementation reduced systolic blood pressure by 2.00 mm Hg (95% CI 0.43 to 3.58) and diastolic by 1.78 mm Hg (95% CI 0.73 to 2.82), with a 0.05 mmol/L rise in serum magnesium. A dose of 300 mg a day or a duration of one month appeared sufficient to raise serum magnesium and lower blood pressure, though the authors noted that residual heterogeneity may exist. That is a small effect that matters more for people with elevated pressure than for healthy young lifters. The abstract does not break results out by magnesium form, so it does not tell us whether oxide performs as well as better-absorbed forms.

Athletic performance and recovery

Heffernan and colleagues (2019) reviewed 130 experiments on minerals and trace elements in exercise and athletic performance, including 22 on magnesium. They concluded that there is little evidence supporting mineral supplementation for improving physiological markers of performance, with the possible exceptions of iron in particular situations and magnesium, the two nutrients with experiments of sufficient quality to be rated strong. They did not claim that magnesium improves performance; they said that its evidence base is of better quality than the others, and that more high-quality research is needed. That is a measured finding. It is not an endorsement of any particular form or dose, and it does not say that lifters with adequate magnesium intake will benefit.

On sleep, cramps, and soreness, no verified trial supports a specific effect of magnesium oxide at 500 mg, so none is cited. Our magnesium guide covers the broader literature, including where it is mixed.

What It Costs

Price is the clear strength. At $7.47 for 200 tablets, one tablet costs about 3.7 cents.

Cost per label milligram is exceptionally low, but cost per absorbed milligram is a different calculation. If only a small fraction of the 500 mg is absorbed, which the oxide studies suggest, the effective price of usable magnesium is higher than the label implies, though it cannot be quantified for any individual. If you compare, convert everything to elemental magnesium per serving first, check the form, and note the serving size, since some products require two or three capsules to deliver the amount shown on the front.

Safety and Interactions

For healthy adults, the main risk from this product is digestive, and the listing advises that anyone who is pregnant, nursing, taking medication, or has a medical condition should consult a healthcare professional first. ODS states that very high doses of magnesium can cause toxicity, with hypotension, vomiting, difficulty breathing, and irregular heartbeat, and that a few cases of fatal hypermagnesemia have been reported. Healthy kidneys clear excess magnesium, so anyone with reduced kidney function should ask a clinician before using a high-dose product.

ODS also lists medications that can interact with magnesium supplements or affect magnesium status: oral bisphosphonates, tetracycline and quinolone antibiotics, and, in terms of status, diuretics and proton pump inhibitors. Because these are interactions, timing may matter, and a pharmacist can tell you how far apart to take them. The same fact sheet notes that very high zinc doses of 142 mg a day from supplements can interfere with magnesium absorption, which is far above typical use.

Quality and Third-Party Testing

The listing's feature bullets and description make no claim of third-party testing for this product, and none appears in the label images. The bullets cover dose, the coating for easier swallowing, and the absence of artificial flavors and sweeteners, and the safety text includes a tamper-seal warning. A single-ingredient mineral is simple to verify by lab testing, so if lot-level assurance matters, for example for tested sport, ask the company for a certificate of analysis or look for a certification mark on the bottle and confirm it with the certifier.

Who Should Buy It and Who Should Skip It

It may make sense if your diet is low in magnesium-rich foods and you want a very cheap supplement, you tolerate oxide well, and you understand you are probably absorbing a modest portion of what is on the label. At about 4 cents a tablet, it is a low-stakes purchase.

Skip it, or look for a different form, if you have a sensitive stomach, you want it for sleep and cannot afford a laxative effect at bedtime, you take any of the interacting medications, or you have kidney problems. Anyone aiming for the maximum benefit per milligram should look at better-absorbed forms, and should keep total supplemental magnesium under the 350 mg upper limit unless a clinician says otherwise. Our electrolytes guide covers how magnesium fits with sodium and potassium during training, and our sleep guide covers what actually moves recovery.

What Buyers Report

Buyer reviews on supplement retailer pages for the 500 mg tablets come down to tablet size, stomach comfort and price.

• Swallowing and taste: Many buyers say the coated tablets are easy to swallow and free of chalky texture or aftertaste, while a few still call the tablet big.
• Stomach comfort: This is the biggest complaint. Several buyers say it upset their stomach, caused acidity, or was hard on them, and one notes the tablets are not coated.
• Price and packaging: Buyers often call it good value, with a few suggesting a low price signals a lower-quality form, or that a pricier magnesium suits them better. One wished the bottle were not plastic.
• Other: Some report taking more than one tablet with little change, and others say they are happy with it.

Takeaway: buyers like the easy-to-swallow coating and the low price, and the main caution they raise is stomach upset. Buyer reports are anecdotal and are not the same as the research above; they are not evidence about what a supplement does and not advice.

How HonestLifter Is Reviewing This Product

This is a research review built on the manufacturer's listing and published studies; HonestLifter's own notes will be added later as dated updates at 30 days, 60 days, 90 days, 6 months, and 12 months of use. The program is described in our longitudinal review program post.

Checkpoint Update: 30-Day, 60-Day, 90-Day and 6-Month Notes (October 2026)

HonestLifter's owner reports the following from regular use. These are personal notes, not measurements.

• Use: Daily, in the morning, swallowed as a tablet or capsule with other morning supplements.
• Verdict: Would buy again.

Several supplements are taken together, so none of these notes isolates the effect of this product. No effects, good or bad, are being claimed here. The 12-month update will be added when it is written.

The Bottom Line

Nature's Bounty Magnesium 500 mg is exactly what the label image says: 500 mg of elemental magnesium, as magnesium oxide, in one coated tablet, for about 4 cents. The problems are about fit, not honesty. The dose is 150 mg above the 350 mg supplemental upper limit, and the form is a poorly absorbed one, with 4 percent fractional absorption in one study and no difference from placebo on the measures in another. The poorly absorbed fraction is also what is most likely to loosen your stools.

Magnesium itself is worth caring about, and meta-analysis shows a small blood-pressure benefit at doses around 368 mg a day. If you want to use this product, take it with a meal as the label says, watch for GI effects, and keep total supplemental magnesium in view. If you want the most for your money, compare other forms on elemental magnesium per serving.

References

1. National Institutes of Health, Office of Dietary Supplements. Magnesium: Fact Sheet for Health Professionals (page updated 2026). ods.od.nih.gov/factsheets/Magnesium-HealthProfessional/.
2. Firoz, M., & Graber, M. (2001). Bioavailability of US commercial magnesium preparations. Magnesium Research, 14(4), 257-262. PMID 11794633.
3. Walker, A.F., Marakis, G., Christie, S., & Byng, M. (2003). Mg citrate found more bioavailable than other Mg preparations in a randomised, double-blind study. Magnesium Research, 16(3), 183-191. PMID 14596323.
4. Zhang, X., Li, Y., Del Gobbo, L.C., Rosanoff, A., Wang, J., Zhang, W., & Song, Y. (2016). Effects of magnesium supplementation on blood pressure: a meta-analysis of randomized double-blind placebo-controlled trials. Hypertension, 68(2), 324-333. doi:10.1161/HYPERTENSIONAHA.116.07664. PMID 27402922.
5. Heffernan, S.M., Horner, K., De Vito, G., & Conway, G.E. (2019). The role of mineral and trace element supplementation in exercise and athletic performance: a systematic review. Nutrients, 11(3), 696. doi:10.3390/nu11030696. PMID 30909645.

Product details were taken from the manufacturer's Amazon listing, including its images, and may change. HonestLifter carries no affiliate links. This article is educational and is not medical advice.