Manganese is a neurotoxic chemical which, owing to the processes involved in manufacture, is found in much higher concentrations in infant formulae, particularly soy-based formulae, than in breast milk. According to the Environmental Working Group, soy-based formulas contain about 80 times as much manganese as mothers' milk, while animal-based formulas contain about 30 times more. Elevated manganese levels have been linked to behavioural problems, such as Attention Deficit Hyperactivity Disorder (ADHD) later in life. Babies are not able to absorb and excrete excess manganese during the first year of life, say the EWG.

Manganese occurs naturally in breastmilk, at a concentration of about 1 μg/100 kcal, and some manganese is necessary for healthy skin, bone, and cartilage formation, as well as glucose tolerance.

The European Directive on the composition of infant formula specifies that it should be at a minimum level of 1 μg/100 kcal and at a maximum Level of 50 μg/100 kcal for infant formulae based on milk protein and its hydrolysates, and of 100 μg/100 kcal for infant formulae based on soy protein. However the taking into account that there is no major difference in manganese bioavailability between breast milk and formulae, surely these very high manganese intakes should be avoided, because manganese excretion is immature in infants and hence very high intakes may cause accumulation in tissues including brain with potential adverse consequences, such as neurodevelopmental abnormalities observed in newborn animals. Feeding of a soy protein-based infant formula with 300 μg manganese/L (which is about 48 μg manganese/100kcal ie 2μg / 100kcal lower than the Directive's maximum level) to infant rhesus monkeys, tended to alter neurotransmitter levels in cerebrospinal fluid and led to signifcantly less play behaviour, more affiliative clinging in social dyadic interactions (ie between mum and baby), shorter wake cycles, and shorter periods of daytime inactivity, than observed in controls fed a soy protein based infant formula with 50 μg manganese/L, or a cows milk protein based infant formula. Given these indications of marked effects of very high manganese intakes on early neurodevelopment, it is suggested that a maximum value should be set which should be no higher thann can be achieved with reasonable efforts and standard manufacturing technology.