1) The misconception that MSG leads to extreme thirst.
Monosodium glutamate (MSG) is composed of glutamic acid and sodium, where glutamate is found naturally in many ingredients you eat on a regular basis such as tomatoes, mushroom, anchovies and cheese. Fermentation, heat application (roasting, etc.), and ageing and can enhance the umami qualities of a natural ingredient. For instance, roasted/grilled mushrooms or tomatoes have heightened umami compared to raw mushrooms or tomatoes. Do these ingredients make you thirsty?
Sodium, however, may be the culprit. Although human bodies need a small amount of sodium for good health (e.g. electrolyte balance), too much of salt disrupts the balance of fluid in our cells (think: your body tries to draw more water to combat the excessive amount of salt). Salt is the component in food that makes you thirsty.
Many foods we consume outside are heavily salted, and we often put the blame on MSG for our extreme thirst. Umami can help create a fullness of taste and increase salty and sweet perceptions; appropriate use of MSG can reduce our sodium intake. Instead of boycotting restaurants that advocate the use of MSG over salt, thank them for lowering our total sodium intake (replacing salt with MSG can lower the total sodium content by about 30% to 50%).
2) Confusing flavour with taste.
When we eat, we often speak of the flavour of the food by what we taste with our tongue (taste buds), rarely by what we smell through our noses. Taste applies to sensations arising from the taste buds, where we perceive sweetness, saltiness, bitterness, sourness, and umami, while flavour is a distinctive quality of food evaluated by multiple senses, including taste and smell.
When we are chewing our food, odour compounds are being released from the food, and chemical signals pass through the internal nares and are sent to the olfactory bulb and to brain informing it of the smell, and together with receptor cells in the mouth and on our taste buds, informing the brain of the flavour. This form of smelling is called retro-nasal olfaction (smell arising from inside the mouth), and what we are used to is ortho-nasal olfaction (external smell).