What is the relationship between content of elemental magnesium and magnesium ʟ-threonate?

Looking at some supplements descriptions and comparing with recommendations of magnesium intake. From magnesium ʟ-threonate supplement description:



> 
> Recommended daily intake (4 capsules) contains 2000 mg of magnesium ʟ-threonate including 160 mg elemental magnesium (42% NRV\*).  
> 
> \*Nutrient reference value
> 
> 
> 


Why there is less elemental magnesium in magnesium ʟ-threonate? How to explain the relationship between both forms of magnesium for someone without a chemistry background?




[Magnesium ʟ-threonate](https://pubchem.ncbi.nlm.nih.gov/compound/Magnesium-L-Threonate) has the molecular formula $\ce{C8H14MgO10}$ with the molar mass $M = \pu{294.5 g mol^-1}$.
Magnesium molar mass is about $\pu{24.9 g mol^-1}$.


Magnesium ʟ-threonate has then percental magnesium content


$$\displaystyle\frac{\pu{24.9 g mol^-1}}{\pu{294.5 g mol^-1}} \times 100\,\% = 8.46\,\%,$$


what gives for $\pu{2000 mg}$ of magnesium ʟ-threonate just $\pu{169 mg}$ of magnesium. The big rest belongs to carbon, hydrogen and oxygen content within ʟ-threonate.


The $\pu{160 mg}$ versus $\pu{169 mg}$ discrepancy may be due possible water content of the solid product due its high supposed water affinity.


 4