Mineral nutrients, vitamins, and trace elements are essential for the growth and development of a multicellular organism. Today, an adequate supply of nutrients is often unattainable solely through a well-balanced diet, so a targeted, individually designed dietary supplement regime is necessary. Nutrient deficiency, which is impossible to detect through plasma levels alone, is reliably detected through the intracellular measurement of the nutrient levels in the blood. Two case studies presented here indicate the need for supplementation as improvement in nutritional behavior could not replenish already exhausted nutrient reservoirs. Only supplementation was able to significantly boost nutrient levels and confer beneficial effects on general welfare, physical performance, and resistance to infections. Therefore, it appears that nutritional supplements are advisable for everyone, but more research is needed, especially on an intracellular level, to corroborate these findings.