You can certainly over consume magnesium (Mag) if you are taking in supplements. What your colleague may be referring to, is that by ingesting Mag from food alone, it would be near impossible to consume too much. Magnesium is a commonly under-consumed nutrient in western nations including the US. In fact almost 70% of the US population fall short of the Mag RDA and shortfalls are associated with early aging/disease and therefore a complete multivitamin and mineral formula (CMVM) should help shore up needs. The tolerable upper limit (UL)* of Mag is actually very low but that’s because it has the unusual distinction of only applying to supplement/drug intake and not foods. Therefore, the suggestion is that supplement intake should not exceed 350mg/d but most experts think it could be slightly higher but not too much because chronic ingestion of too much can cause adverse reactions. Orally, high doses of magnesium can cause gastrointestinal irritation, nausea, vomiting, and diarrhea. Hitting close or at the RDA (320-420 mg/d) of Mag thru supplements alone is a good idea and the extra from the diet will be complementary only.
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Magnesium – cofactor in over 300 enzymes that utilize nucleotide triphosphates, such as ATP to synthesize body components, produce energy, help transport nutrients including calcium absorption, nerve conduction, protein synthesis, maintenance of metabolic rate. RDA: 320-420 mg; UL: 350mg (UL is established only for intake from pharmacological agents/supplements -i.e. supplements should not exceed this number).
*UL: Tolerable Upper Intake Level - the highest level of nutrient intake that is likely to pose no risk of adverse health effects for almost all individuals in the general population