What is MSG?  

Monosodium glutamate (MSG) is a flavor enhancer that is commonly used to boost the savory taste associated with meat flavor or Umami as the Japanese call it.   MSG is probably the most important taste-bearing compound humans use to promote food consumption.  MSG can be naturally present in cooked foods or added to foods.

MSG is a sodium salt of amino acid called glutamic acid.   Glutamic acid is an integral part of all sorts of naturally occurring proteins.   When proteins are decomposed chemically or enzymatically, they release glutamic acid.  Glutamic acid per se does not taste savory or meaty or brothy.   But when sodium ions are present, glutamic acid more or less is converted into MSG, which renders the savory or meaty taste or umami.  

That is why in order to make the food you are cooking taste good, you do two things.  You cook it so proteins can release glutamic acid, and you add some salt so the released glutamic acid canform MSG with sodium coming from table salt.  Without MSG, naturally occuring or added, foods do not taste umami!   MSG is present in all cooked natural whole foods.  You cannot avoid it if you like to eat cooked svory foods with protein.

Some foods taste better than others.  That is because they contain different levels of proteins, more specifically different amounts of glutamic acid.   Proteins are naturally present in foods derived from plants or animals.  Glutamic acid rich foods include cheese, meat, fish, soybeans, mushrooms, ripe tomatoes, broccoli and peas.   These foods when processed adequately can provide high levels of glutamic acid, which is why they often taste very good.  Peas often do not taste as good because MSG is hard to form if it is not properly processed.

(Note that foods must be processed to release glutamic acid.   For instance, soybeans with high levels of glutamic acid rich protein would taste nothing if they are not processed.  Soy sauce tastes savory because fermented soybeans contain free forms of proteins, peptides, and glutamic acid taste good when salt is present.)

Is MSG harmful?

First, it should be understood that some natural things in one food or another can be toxic.  In reality, no one can fully avoid all toxic compounds from foods.   The good news is, food consumers do not have to avoid all sorts of toxins.  Firstly, not all toxic compounds pose any real risk because  they are present in very low doses.  Secondly, humans have built-in defense systems to handle low levels of many naturally occuring toxic compounds in foods.   In certain cases, exposure to certain levels of toxin can even help prime our defense system and make us stronger physiologically.

MSG can elicit excitotoxicity or neurotoxicity at certain concentrations.  It may also have a potential effect on the liver, kidney and reproductive system.  Some call them MSG side effects.   When some people eat too much of MSG, they may experience "Chinese restaurant syndrome", MSG reaction or MSG syndrome.   At high levels, MSG may have an effect on neurons.  Added MSG may cause the problem if the usage level is not controlled.   Foods with naturally occurring MSG which at a level is merely enough to render a savory taste should not be a concern.   If there is some risk associated with such foods because of presence of naturally occurring MSG, the risk should be tolerated because no one likes savory foods without any savory taste.

Here are some comments.  

1) The first thing to know is that MSG as a pure flavoring compound is produced as a flavor enhancer by a fermentation process.  It is a natural product.   There is no chemical difference between added MSG and naturally occuring MSG.

2) You can use MSG which can be purchased from all Asian grocery stores.   But make sure to use it in an amount that just delivers the savory taste.  Do not use overuse it.  For instance, vegetables often do not taste as good (savory) as meat and cheese.  So when you cook vegetables, you may add a bit of MSG.

3) When you cook meat such as beef, chicken, pork or fish or seafood, you do not have to use any MSG.  A bit of sauce which may contains MSG may be enough to boost the taste.   Spices and seasonings are more important for these foods and they can minimize use of savory sauces or pure MSG or salt.  To increase the savory note, meat can be aged for some time.  The aging process releases glutamic acid, which along with sodium ions provides the umami or savory taste.

4) Do not get fooled by natural sauces or seasonings or flavorings without added MSG.  Those products contain high levels of MSG even though it is naturally present in the products.    But still, these products can also be used.  Just to remember that they are not superior to other sauces with MSG added.

5) If you are sensitive to MSG, do not eat highly savory foods.  If a food tastes very savory, it contains a high level of MSG.  Chinese foods served at Chinese restaurants often contain added MSG.  Nowadays, Chinese restaurants often do not use pure MSG, but a product called Chicken Essence, which is a mix of chemicals including added MSG and other savory ingredients such as IMP and GMP.

In one word, you can use pure MSG, but do not overuse it.  Avoid processed foods with high levels of MSG if you are sensitive to MSG. MSG is safe to use as long as you d onot overuse it.    Added MSG is commonly consumed in China and other Asian countries and few adverse effects associated with MSG use have ever been reported.

However, if you do not use a balanced diet which is full of protective nutrients such as vitamin C and E and other phytochemicals, you may use some supplements.  Studies have indicated that some supplements such as curcumin or curry and vitamin C among others may help reduce the potential otxicity of MSG.  (David Liu)