In a 2019 study in rats comparing several magnesium supplements, the researchers found that magnesium malate provided the most bioavailable magnesium. Five different magnesium compounds (magnesium sulfate, magnesium oxide, magnesium acetyl taurate, magnesium citrate and magnesium malate) were administered as a single dose 400 mg/70 kg to Sprague Dawley rats. Bioavailability was evaluated by examining time-dependent absorption, tissue penetration, and the effects on the behavior of the animals. Magnesium oxide and magnesium citrate had the lowest bioavailability when compared to the control group.
Uysal, N., Kizildag, S., Yuce, Z., Guvendi, G., Kandis, S., Koc, B., Karakilic, A., Camsari, U. M., & Ates, M. (2019). Timeline (Bioavailability) of Magnesium Compounds in Hours: Which Magnesium Compound Works Best?. Biological trace element research, 187(1), 128–136. https://doi.org/10.1007/s12011-018-1351-9