I won't discount the studies out of hand, but I do have some issues as to how they were reported...
What was the dosage given to the rats and mice, in comparison to the equivalent for an average human? Excessive amounts of anything given to lab animals will skew results and cause problems. That's why I have problems with statements like "_____ has been proven to cause ______ in laboratory animals".
"Studies have found that adding MSG to certain foods, such as soup and mashed potatoes, has been successful in increasing the food intake in institutionalized elderly populations." From my understanding (and personal experience as well) many elderly persons tend to have a reduced sense of taste to begin with. If a food tastes better to them, then their preference will be toward that food rather than one that (to them) has little or no flavor at all, possibly resulting in greater consumption.
I believe that it mentioned somewhere that Americans consume about a teaspoon of MSG per day, and scientists give rats much less in ratio to their body mass to make them obese.
As I said, I'm not discounting the studies. However, when the methodology isn't given, I begin to doubt the validity.
For example, did they give the rats 34mg (equiv. to a teaspoon in a human)? More? Less? How often?
Without that data, all it shows is a correlation but it does nothing to prove
causation.
On a similar note, I also have problems with studies that claim "_____ increases the risk of _____ by __%". Unless actual ratios are given, percentages are not only meaningless but allow the studies to capitalize on sensationalism.