I posed the question to Pim Dekker, a product expert from Solgar. This was his response:

"In general, there is little research on the bioavailability of various mineral compounds in humans. This is due to the fact that there is little scientific interest in such studies, they are exceedingly expensive, and individual variations are significant. Only for various calcium and magnesium compounds is there some comparative material available.

Many inorganic compounds, such as oxides, phosphates, sulfates, and carbonates, are not absorbed at all. For example, the biological availability of calcium carbonate or magnesium oxide is 0.0. However, in the stomach, a portion of such compounds will ionise (break down into a mineral ion and an acid residue). That free ion can potentially be absorbed. This is the case for young people (who still have relatively high stomach acid) for about 20%, so the biological availability can be estimated at around 20%. In older individuals (with low stomach acid), following stomach reduction, or when using acid suppressants/binders, this is much lower and absorption will be no more than 5%. Citrate compounds can easily ionise in a non-acidic environment.

The remarkable thing about amino acid compounds or picolinates is that these compounds can be absorbed intact through the intestine into the bloodstream. As a result, the biological availability is much greater.
Manganese glycinate is not listed in Annex II, and is therefore not permitted, but it does not actually exist (at least, if it is meant that one manganese atom is bound to one glycine molecule). A product that labels itself as such is incorrect, unless it is meant that there is a (large) complex of glycine molecules and manganese present. This would lead to very large molecules, which are too big to be effectively absorbed through the intestinal wall.

The company Albion, from whom we source our amino acid complexes, guarantees that there is a bond of one atom to two (bis) glycine molecules. This results in molecules with relatively low weight, which are small enough to be well absorbed.

Unfortunately, I cannot provide exact figures, given the above. I hope this has sufficiently informed you."

According to the Vitamin Information Bureau, the maximum safe dose is 11 mg per day (for long-term use; taking a higher dose occasionally or for a short period is not a problem). An excess of manganese can eventually cause damage to the nervous system. So, 20 mg is quite above that. Chelated Manganese

In itself, manganese is permissible during pregnancy, but this product contains a rather high dosage. Since no research has been conducted on its effects during pregnancy, it is advised against using it.

If it is important for your own health to continue using manganese, please consult your doctor or specialist first.

A common dosage of manganese during pregnancy is approximately half a milligram per day, such as in the Prenatal Nutrients. The Chelated Manganese has 16 times that amount (8 mg).