Why is magnesium oxide rated D?

Magnesium oxide has only approximately 4% bioavailability — meaning your body absorbs just 4% of the elemental magnesium on the label. It is primarily used as a laxative rather than a therapeutic magnesium supplement. Despite being the most common form on pharmacy shelves, it is the least effective for correcting magnesium deficiency.

What is the best form of iron?

Iron bisglycinate (ferrous bisglycinate) is the best tolerated and well absorbed form — it causes significantly less GI distress than ferrous sulfate while maintaining good absorption. Ferrous sulfate is the most common form and adequately absorbed but causes nausea and constipation in many people.

What fillers should I avoid in supplements?

Key fillers to avoid include titanium dioxide (banned as a food additive in the EU), artificial colors like Red 40 and Blue 1, carrageenan (linked to GI inflammation in some research), and hydrogenated oils. Magnesium stearate and silicon dioxide at normal amounts are generally considered safe.

Supplement Quality Checker — Grade A to D

Q: What is the best form of magnesium?

Magnesium bisglycinate (also called magnesium glycinate) is the best form for most people — approximately 80% absorbed, gentle on the stomach, and does not cause the laxative effect of magnesium oxide. Magnesium malate is also excellent at approximately 70% absorbed and is particularly good for energy and muscle function.

Q: Does the supplement brand matter?

Brand reputation matters less than form. A generic magnesium bisglycinate from a lesser known brand is significantly better than a premium brand magnesium oxide. Focus on the form of the active ingredient first, then consider brand reputation and third party testing certificates like USP or NSF.

Not all supplements are equal. The form of the active ingredient matters more than the brand, the price, or the marketing claims. Check quality grade A-D before you buy — free, instant, based on published sources including NIH ODS, FDA drug labels, and peer-reviewed literature.

Grade A-D quality

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What the grades mean

EXCELLENT

High bioavailability, clean label, NIH verified

GOOD

Solid absorption, minor concerns only

AVERAGE

Moderate bioavailability or some fillers

POOR

Low absorption or concerning fillers detected

Magnesium forms compared

Magnesium is one of the most commonly purchased supplements — and one of the most commonly purchased in the wrong form. Here is how the major forms compare:

Form

Grade

Absorbed

Verdict

Magnesium Bisglycinate

~80%

Best

Magnesium Malate

~70%

Excellent

Magnesium Glycinate

~80%

Best

Magnesium Citrate

~25%

Good

Magnesium Chloride

~20%

Good

Magnesium Carbonate

~12%

Average

Magnesium Oxide

~4%

Avoid

Zinc forms compared

Form

Grade

Absorbed

Verdict

Zinc Bisglycinate

~60%

Best

Zinc Picolinate

~60%

Excellent

Zinc Gluconate

~40%

Good

Zinc Sulfate

~20%

Average

Zinc Oxide

~10%

Avoid

Iron forms compared

Form

Grade

Tolerability

Verdict

Iron Bisglycinate

Excellent

Best

Ferrous Bisglycinate

Excellent

Best

Ferrous Gluconate

Good

Ferrous Sulfate

Poor

Average

Ferric Iron

Poor

Avoid

Vitamin D forms compared

Form

Grade

Type

Verdict

Vitamin D3 (Cholecalciferol)

Animal-derived

Best

Vitamin D3 (Lichen-based)

Vegan

Best vegan

Vitamin D2 (Ergocalciferol)

Plant-derived

Less effective

Why form matters more than brand

The supplement industry is largely unregulated in the United States. Unlike prescription drugs, supplements do not need to prove efficacy before being sold. This means the form of the active ingredient — which determines how much your body actually absorbs — varies enormously between products at similar price points.

Magnesium oxide costs approximately the same per serving as magnesium bisglycinate — but delivers 20 times less elemental magnesium to your bloodstream. A well-known brand name on a bottle of magnesium oxide is still magnesium oxide. The form is the most important factor by a significant margin.

Elthio grades supplements based primarily on the bioavailability of the active ingredient form, cross-referenced against published absorption studies and NIH Dietary Supplement Label Database records.

Fillers to avoid

CONCERNING ADDITIVES ELTHIO DETECTS

Titanium Dioxide — High Concern

Banned as a food additive in the EU since 2022. Still present in thousands of US supplements as a whitening agent. No nutritional purpose.

Artificial Colors — High Concern

Red 40, Blue 1, Yellow 5, Yellow 6. No nutritional value. Associated with hyperactivity in children. Common in gummy vitamins and chewable supplements.

Carrageenan — Moderate Concern

Derived from seaweed. Used as a thickener and emulsifier. Some research links it to gastrointestinal inflammation. Common in softgel capsules.

Hydrogenated Oils — Moderate Concern

Trans fat source used in some supplement formulations. No nutritional purpose.

Magnesium Stearate — Low Concern

Flow agent used in tablet and capsule manufacturing. Generally considered safe at normal supplement amounts. Some people prefer to avoid it.

Frequently asked questions

What is the best form of magnesium?

Magnesium bisglycinate or glycinate — approximately 80% absorbed, gentle on the stomach, no laxative effect. Magnesium malate is also excellent at approximately 70% absorbed and is particularly good for energy and muscle function.

Why is magnesium oxide so common if it is rated D?

Magnesium oxide is the cheapest form to manufacture and contains the highest percentage of elemental magnesium by weight — which looks good on a label. Most consumers do not know that absorption percentage matters more than the mg on the label. It sells well despite being poorly absorbed.

What is the best form of zinc?

Zinc bisglycinate and zinc picolinate are the best absorbed at approximately 60% bioavailability. Zinc gluconate is adequate. Zinc oxide at approximately 10% absorbed is best avoided.

Does the supplement brand matter?

Form matters more than brand. A generic magnesium bisglycinate is significantly better than a premium brand magnesium oxide. That said reputable brands with third party testing certificates like USP, NSF, or Informed Sport offer additional assurance of label accuracy.

What fillers should I avoid?

The most concerning fillers are titanium dioxide (banned in the EU), artificial colors like Red 40 and Blue 1, carrageenan, and hydrogenated oils. Magnesium stearate and silicon dioxide at normal amounts are generally considered safe.

Is Elthio free to use?

Yes — completely free. No account required. Type any supplement name and get an instant quality grade with filler detection and NIH verification.

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ABOUT THIS TOOL

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Quality grades are based on published bioavailability research, NIH DSLD label data, and peer-reviewed absorption studies. Filler detection references EU food safety regulations and published safety literature. How we grade

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Data sources

NIH Dietary Supplement Label Database (DSLD) — Label verification for 207,000+ products
NIH Office of Dietary Supplements (ODS) — Bioavailability and safety research
PubMed — National Library of Medicine — Peer-reviewed absorption studies
European Food Safety Authority (EFSA) — Filler and additive safety evaluations

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Based on published sources including NIH ODS, FDA drug labels, and peer-reviewed literature and peer-reviewed bioavailability research
Educational only — not medical advice.
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