to give you what you need.

Mixtures are basically almost ALL oxide (sometimes they list chelate on them, BUT when you call them, they say...we don't know how much of each there really is). Magnesium oxide remains because it is historically known, and is VERY CHEAP.

Now Oxide was the first available. So it ended up in doctor's therapeutic handbooks (this is what they consult before they talk to YOU).

It turns out with studies done on people who took various forms of magnesium that oxide is very poorly absorbed. (blood levels were drawn and compared to pre-dose levels in humans).

Chelates are amino acids bound to the magnesium and they do not impair absorption. Examples are citrate, lactate, malate, glycinate.

If you click on the links I give on the Vitamin Mag thread, you will see explanations for it. I really DOUBT that your doctor knows the difference.
You can get SlowMag..which was made for doctors' use in patients, and feel pretty confident that it will work for you.
Two of them provide 1/2 of the RDA for magnesium.(this recommendation is typical and it assumes you eat some magnesium daily.)

You can get magnesium from foods. 3oz of unsalted almonds have 270mg in them. Also Edamame beans, navy beans, are also good sources.

This is confusing. For example 250mg of ferrous sulfate contain
about 27mg of elemental iron. The new carbonyl version of iron supplements contain 50mg of elemental iron.

The FDA requires elemental on only iron products' labels so far.
Basically with supplements unless you are shown the difference you don't know it exists. That is why I made the magnesium thread. And most doctors do not know either--a very sad truth.

If you took 400mg of magnesium oxide (poorest form) you would get into your body about 8mg of elemental magnesium.
This is like spitting in the ocean. (an egg has the same value, the lowest in the tables made for magnesium sources).
To get 320mg a day you would have to eat ---40 eggs!

Compare to 270mg for 3oz of unsalted almonds! Big difference.
Low magnesium is common in about 70% of the US population. And when it gets low from
taking certain drugs (like diuretics, caffeine), or chemo, or from poor diet...you can have your heart stop! If you have a QT tendency this is more critical than for regular folks.
It is a SERIOUS deficiency.