Some people experience loose stool with 1 tsp of (4.6 grams) of magnesium sulfate 7 hydrate supplementation, which is about 460 mg of magnesium, which is slightly more than the estimated daily requirement of 300-350 mg. This is just 19 mmoles of magnesium sulfate 7 hydrate or just less than 2% of a normal body pool of 25 grams of magnesium, which is just over 1 mole of magnesium.
This means either (1.) they are replete with the mineral (deficiencies are best detected in bone, and also possibly in saliva or skin, but definitely not in plasma, where magnesium is one of at least four cations – sodium, potassium, calcium being the others- essential for maintaining a safe and steady heart beat; this means that magnesium in plasma will be in the reference range up until the deficiency is so great that the body can no longer maintain plasma magnesium at a safe level or 2. they have a deficiency in magnesium AND one or more deficiencies in substances involved in the uptake of the mineral. Like all other nutrients, magnesium has its own ADME (absorption, distribution, metabolism, and excretion) network; the deficiency referred to here is a deficiency in substances required for A, absorption.
A person who takes 2 grams of magnesium sulfate and has no diarrhea is probably at least somewhat deficient in the mineral but is not deficient in substances required for uptake of the mineral. The longer he can take 2 grams a day of magnesium sulfate by itself with water and still not get a loose stool would be a direct measure of how deficient he was. If he takes 2 grams daily with nothing else but water and eventually he gets diarrhea, he is probably now surfeit in magnesium and is beginning to pass excess magnesium into the stool.
Does this make sense?