Assume that you're thirsty after eating food that didn't taste salty while you were eating it. I know that MSG contains sodium.
“Chinese food makes me so thirsty—it’s the MSG.” — Well, chances are, if you’re thirsty after eating Chinese food (or any food), you’ve probably consumed a good dose of sodium in the food or you’ve not had enough liquids recently. And, while MSG does contain sodium, it has only 1/3 the sodium of table salt. Chinese food contains a lot of high-sodium ingredients, including soy sauce, broths and other sauces. In fact, there are lots of foods (snack crackers and flavored chips, condiments, soups and sauces, commercially-prepared entrees) we eat all the time that typically contain much more salt than MSG. It’s time to stop blaming Chinese food (and MSG) for your thirst. Many Americans are chronically under-hydrated, and a salty meal of any ethnicity can send us all racing for water afterward.
But Hungry Onion says:
But since the food didn’t feel overly salty at the time, the only reason I can think of for the thirst is MSG.
and Wired.com impute the thirst to the MSG:
The exact flavor MSG confers is difficult to describe, and many just say it increases the "taste intensity" of food. One thing is certain: It makes people thirsty, encouraging them to eat and drink more. Americans consume about 28,000 tons of MSG per year, according to one estimate reported in the June 1995 Journal of Environmental Health.