Interindividual variations in manganese retention can be large (Davidsson et al., 1989b). Ten days after giving ⁵⁴Mn in an infant formula to 14 healthy men and women, manganese retention ranged from 0.6 to 9.2 percent. Mean retention in these subjects was 2.9 ± 1.8 percent (standard deviation [SD]). Intraindividual variation was not as large, and retention values of 2.3 ± 1.1, 3.3 ± 3.1, and 2.4 ± 1.4 percent (SD) were observed for three repeated doses in six subjects (Davidsson et al., 1989b).

In one study, seven healthy men, aged 19 to 22 years, were fed a purified low-protein diet containing 0.01 mg/day of manganese for days 1 to 10, followed by a protein-adequate diet containing 0.11 mg/day of manganese until day 39. Using a factorial method, the authors estimated that the minimum requirement for manganese was 0.74 mg/day and estimated on the basis of the percentage of manganese retention that 2.11 mg/day would be required (Friedman et al., 1987). Subsequently, five young men were fed a diet of ordinary foods (1.21 mg/day of manganese) supplemented with manganese sulfate or placebo at the evening meal to create five different levels of manganese intake (Freeland-Graves et al., 1988). Total manganese intakes were 2.89 mg/day for days 1 to 21, 2.06 mg/day for days 22 to 42, 1.21 mg/day for days 43 to 80, 3.79 mg/day for days 81 to 91 (repletion), and 2.65 mg/day for days 92 to 105. The mean manganese balances for the corresponding days were 0.083, -0.018, -0.088, +0.657, and +0.0136 mg/day, respectively.

An 8-week balance study conducted by Hunt and coworkers (1998) showed that women, aged 20 to 42 years, were in slightly positive mean balance when consuming 2.5 mg/day of manganese.

Some adolescent girls were observed to be in negative or slightly positive balance when consuming 3 mg/day of manganese (Greger et al., 1978a, 1978b).

Balance studies are problematic for investigation of manganese requirement because of the rapid excretion of manganese into bile and because manganese balances during short- and moderate-term studies do not appear to be proportional to manganese intakes (Greger, 1998, 1999). For these reasons, a number of studies have achieved balance over a wide range of manganese intakes (Table 10-1). Therefore, balance data were not used for estimating an average requirement for manganese.