Humans absorb only about 1% to 5% of dietary manganese. Infants and children tend to absorb greater amounts of manganese than adults. In addition, manganese absorption efficiency increases with low manganese intakes and decreases with higher intakes
https://ods.od.nih.gov/factsheets/Manganese-HealthProfession...
Another interesting tidbit from the links you provide, 90% of glutamine synthesis happens in muscle cells (from glutamate and ammonia). So in theory, building more muscle mass (while getting adequate dietary glutamine and glutamate) be beneficial for regulating glutamine-to-glutamate ratios. Although it doesn't look like this has been studied specifically as of yet.
On the other hand, over-training and extreme endurance training without proper protein intake leads to depletion of circulating glutamine levels. It is suggested that depleted glutamine may be the cause of increased risks of infection and chronic fatigue during periods of over-training (even in elite athletes).