Why the form on the label decides the dose in your blood

What is bioavailability?

Pick up two magnesium supplements. Both say Magnesium 400 mg in the same confident type. One is magnesium oxide; the other is magnesium citrate. At a glance they look the same. To your gut, they are not.

Bioavailability is the fraction of a dose that reaches your circulation in a form the body can use, after it dissolves, crosses the gut wall and passes the liver's first round of processing. It never appears on the label. The label counts what is in the capsule; absorption counts what makes it through. Those are rarely the same number.

Is magnesium citrate absorbed better than magnesium oxide?

Yes. Magnesium has to be bound to something to become a stable, swallowable compound, and that partner changes how much you absorb. Magnesium oxide is cheap and dense, so it packs a lot of magnesium into each pill, but it dissolves poorly. In one comparison of US preparations, only about 4 percent of the magnesium in oxide was absorbed, well below magnesium chloride, lactate and aspartate [1].

In a 60-day randomized, double-blind trial, magnesium citrate and an amino-acid chelate were absorbed better than oxide, citrate produced the highest blood levels, and oxide did no better than placebo [2].

There is a second trap: elemental versus compound weight. 1,000 mg of magnesium glycinate is not 1,000 mg of magnesium, because the glycine accounts for much of that weight. Look for the elemental figure. A label that prints only the compound weight lets you over-count.

Folate: why the word on the label is not the whole story

Folate is the textbook case. The synthetic form, folic acid, has no coenzyme activity of its own. The body must first reduce it, starting with the enzyme dihydrofolate reductase, on the way to L-5-methyltetrahydrofolate (5-MTHF), the form that circulates and does the work [3].

Common variants in the MTHFR gene make part of that conversion less efficient for some people, and when folic acid intake outpaces the conversion, unmetabolized folic acid can appear in the blood [4]. Supplementing 5-MTHF skips those steps. At equal doses, both forms raise blood folate well [3]; the difference is the conversion folic acid needs.

Vitamin B12, vitamin D and the active-form pattern

The pattern repeats across the cabinet. Vitamin B12 is sold most cheaply as cyanocobalamin, a synthetic form the body converts into the active forms methylcobalamin and adenosylcobalamin [5].

Vitamin D comes as D2 (ergocalciferol) or D3 (cholecalciferol). A systematic review and meta-analysis found that D3 raised blood 25-hydroxyvitamin D more than D2, with the clearest difference when it was given in large, infrequent doses [6].

What helps a hard-to-absorb compound?

Some compounds absorb poorly whatever form they come in, and the formulation becomes the lever. Curcumin, the active fraction of turmeric, is the classic example. In a human study, 2 g of curcumin on its own produced blood levels that were undetectable or very low; adding 20 mg of piperine, a black-pepper extract, raised its bioavailability by about 2,000 percent [7].

Other delivery systems, such as phytosomes and liposomes, aim at the same problem: helping more of a hard-to-absorb compound reach the blood.

How do you read a label for bioavailability?

The front of the label gives you a number. The form on the back decides how much of it you absorb.

• Read the form, not just the nutrient: magnesium citrate, not just magnesium; methylfolate, not just folate.
• For minerals, find the elemental amount: milligrams of magnesium, not of the whole compound.
• On hard-to-absorb compounds such as curcumin, look for a delivery system.
• Match the dose to the research. The best form is wasted at a fraction of the studied dose.
• Look for outside testing. Bioavailability does not matter if the contents do not match the label.