Abstract Aim The World Health Organisation (WHO) magnesium (Mg) estimated average requirement (EAR) is not adjusted for rise in human body weight (BW) and neglects body Mg stores depletion. Cereal grain food processing results in Mg loss and reduces dietary Mg intake which mainly originates from cereals. Here we reassess human dietary Mg deficiency risk considering actual human BWs and modern levels of cereal grain food processing. Methods Human Mg requirement was adjusted for rising BW plus low and high estimates to prevent body Mg store depletion. Magnesium supply was recalculated for cereal grain (maize, millet, rice, oats, sorghum, and wheat) food processing of none, 25%, 50%, 75% and 100%. Resulting Mg deficiency risks in 1992 and 2011 were calculated at national, regional, continental and global scales using the EAR cut-point method. Results Globally, human Mg requirement increased by 4–118% under the three Mg requirement scenarios compared to the WHO EARs set in 1998. However, dietary Mg supply declined with increased cereal grain food processing. At 100% cereal grain processing, dietary Mg supply was reduced by 56% in 1992 and 51% in 2011. Global human Mg deficiency risk reached 62% in 2011 with 100% cereal grain processing and largest EAR set to prevent depletion of body Mg stores and corrected for BW rises. Conclusion Global dietary Mg Supply adequately meets human Mg requirement given the global obesity epidemic. But, Mg intakes preventing body Mg store depletion plus high Mg losses due to cereal grain food processing start to show noteworthy risks of potential Mg deficit in populations consuming diets with >50% cereal grain food processing. These findings have ramifications for the global spread of the major chronic, non-communicable diseases associated with nutritional Mg deficiencies such as cardiovascular diseases and type 2 diabetes.