- PURPOSE OF REVIEW: To review the prevalence, causes and functional significance of vitamin B12 deficiency in vulnerable subpopulations including older adults and the developing embryo. RECENT FINDINGS: It is becoming increasingly recognized that the susceptibility to vitamin B12 deficiency may change throughout the life cycle, with the developing embryo and older adults exhibiting elevated risk. Recent data implicate low vitamin B12 status as a risk factor for birth defects resulting from improper neural tube development. The potential for vitamin supplementation and/or food fortification to ameliorate the risk of deficiency in these subpopulations is discussed. SUMMARY: The prevalence and impact of vitamin B12 deficiency varies throughout the life cycle, with older adults and potentially the developing embryo having the greatest risk and susceptibility. Additional research is needed to develop effective public health interventions that address the unique causes of this nutritional deficiency, which differ among at-risk subpopulations.