Technical Abstract: It is clear that many people do not consume recommended intakes of calcium (Ca) and magnesium (Mg), yet biochemical and/or functional changes indicative of deficiencies in these nutrients have been rare. This prompted two series of studies: one addressing an apparent Ca-deficiency rickets in children in southeastern Bangladesh; another addressing Mg balance in healthy adults in Grand Forks, ND. The Ca study was a double-blind, clinical trial with 1-5 yr. children randomized to four supplements providing graded levels (50, 250 or 500 mg) of Ca. It was conducted in six villages in a rickets-endemic area in southeastern Bangladesh. During a 7 mo. pre-trial period in 12% of the screening cohort of 1749 children developed rachitic leg signs, 158 asymptomatic children remained asymptomatic after 13 mos. of intervention, suggesting that even the lowest supplement of Ca (50 mg/d) was useful in supporting normal bone development in this high-risk population. The Mg studies were controlled feeding ones conducted with post-menopausal women living in a metabolic unit. Each followed a depletion-repletion regime using Western-type diets that relatively low amounts (100-146 mg/day) of Mg with Mg repletion effected by using supplemental Mg to levels of 320-350 mg Mg/day. Magnesium depletion resulted in functional (cardiac arrhythmias) and biochemical changes (increased Ca balance and urinary phosphorous excretion, decreased urinary potassium excretion). These studies represent cases demonstrating beneficial responses to Ca or Mg supplementation to diets low in those essential minerals.