Bioavailability: why the form of an ingredient matters
You could be taking a supplement every day and absorbing very little of it.
The reason isn't the dose on the label — it's the form the ingredient comes in.
THE CONCEPT
What bioavailability actually means
Bioavailability refers to the proportion of a nutrient or ingredient that is absorbed from the digestive tract and made available for use by the body. A supplement may contain 500mg of a nutrient on the label — but if only a fraction of that reaches your bloodstream and cells, the stated dose is largely irrelevant.
The chemical form an ingredient takes has a direct impact on how well it survives the digestive process, how readily it crosses the gut wall, and how much ultimately reaches the tissues where it's needed.
In simple terms: two supplements with the same ingredient at the same dose can produce very different results — depending entirely on which form was used.
CASE STUDY ONE
Magnesium: chelated vs. oxide
Magnesium is one of the most widely studied minerals in terms of form and absorption, and the differences between forms are well documented.
Magnesium oxide — the most common form used in low-cost supplements due to its high elemental magnesium content — has been shown in multiple peer-reviewed studies to be significantly less bioavailable than organic forms. A 1990 study by Lindberg et al. published in the Journal of the American College of Nutrition found that magnesium citrate produced a substantially higher rise in urinary magnesium than oxide — a standard measure of absorbed magnesium. [2]
A 2017 randomised study published in BMC Nutrition further confirmed that organic magnesium compounds are superior to magnesium oxide in terms of bioavailability, while acknowledging that study design variations make direct numerical comparisons difficult. [3]
Chelated magnesium — where the mineral is bonded to amino acids — takes this further. The chelate bond protects the magnesium molecule as it passes through the digestive tract, reducing interactions with other nutrients and allowing absorption at the optimal point in the intestine. Research by Albion Minerals (Balchem) shows chelated magnesium bisglycinate absorbs up to 4× better than magnesium oxide. [4]
WHAT AFFECTS IT
Factors that determine absorption
Several variables determine how much of a supplement is actually absorbed. The most significant is the chemical form of the ingredient itself — whether it is bound to another molecule, and how stable that bond is through the digestive tract. Other factors include whether the supplement is taken with food, an individual's age and health status, and the presence of competing nutrients.
A systematic review published in Clinical Nutrition (2021) found that across 14 studies on magnesium supplementation alone, inorganic forms consistently showed lower bioavailability than organic forms — and that the percentage absorbed decreases further as the dose increases. [1]
Magnesium bisglycinate chelate (TRAACS®)
High
bioavailability
Used by
Indeed Nutrition
Amino acid chelate. Protected through digestion. Most clinically researched chelated form.
Magnesium
oxide
Inorganic.
High elemental content but low solubility.
May cause digestive discomfort at higher doses.
Lower bioavailability
Magnesium citrate
Organic salt.
Better solubility than oxide.
Moderate absorption, commonly studied.
Moderate
bioavailability
References
1. Veronese N et al. (2021). Bioavailability of magnesium food supplements: a systematic review. Clinical Nutrition, 40(6), 4054–4060. sciencedirect.com
2. Lindberg JS et al. (1990). Magnesium bioavailability from magnesium citrate and magnesium oxide. Journal of the American College of Nutrition, 9(1), 48–55. doi.org
3. Schuchardt JP & Hahn A (2017). Higher bioavailability of magnesium citrate as compared to magnesium oxide. BMC Nutrition, 3(1), 7. springer.com
4. Albion Minerals by Balchem (2025). Which magnesium to choose: why Albion® Minerals stands out. balchem.com