The ability to adapt to a large daily intake of unleavened bread made from wheaten wholemeals of high extraction rate was examined in two young Americans who had not previously consumed fiber, phytate-, and phosphate-rich bread of this type. Adaptation was evaluated by comparing balances of zinc, calcium, magnesium, and phosphorus for a 12 day period after ten days of bread consumption with those during a similar period after 50 days and also by observing changes in blood composition during this period of bread consumption. Significantly negative balances of zinc, magnesium and phosphorus were observed in the initial period. However, all had become positive or nearly so after 50 days. By contrast, calcium balances which were also negative during the initial period remained negative at the end of the study. Plasma calcium and zinc concentrations had declined to low normal or subnormal values at this time. The failure to overcome the disturbing effects of consumption of wholemeal bread upon calcium metabolism occurred despite daily exposure to sunshine for many hours under conditions favorable for Vitamin D biosynthesis.