MSG is a salt, so when you put it in water, it dissolves into sodium and glutamate. Glutamate is an amino acid found naturally in a variety of tasty foods. So,

MSG is a salt, so when you put it in water, it dissolves into sodium and glutamate. Glutamate is an amino acid found naturally in a variety of tasty foods. So, you already consume it and you're fine. To be more sure of yourself, you can run an controlled, blinded experiment where you have group A consume food with monosodium glutamate added and group B consume food without the additive. So long as you don't tell which food is which, then group A won't have a different response from group B. But these things don't actually answer your question. They answer the question "How do we know it is safe?" To explain "why MSG is safe" seems like it would be impossible for the same reason that proving a negative is impossible: you would have to go through all of the possible metabolic processes and show that and it does not screw with them.