MSG is a flavor enhancer. This flavor is known as umami or savoriness and is caused by the carboxylate ion in monosodium glutamate(MSG). Anything with glutamate(one of the amino acids) or glutamate derivatives(id MSG) will have this savory taste.

Glutamate is found in any protein rich food(meat, cheese, seafood, nuts, etc)

Salt/sugar as a substitute does not have any of savory flavor and therefore will not work as a substitute.

Fish sauce is the best direct substitute. You can also use Worcestershire sauce or soy sauce if you want to add a more flavor to the dish.

Mushroom stock or ground mushrooms will do the job, and is usually what i will use. Alternately, roasted tomato paste, miso paste or roasted bone marrow

In Mediterranean cultures, the staple source of glutamine is tomatoes / tomato sauce and caramelized onions, sometimes with mushrooms. In China, it is soy sauce. In Japan, it is seaweed. All of these foods are rich in glutamine.

You might not realize this but MSG stands for monosodium glutamate. This simply means that it is consisted of one sodium (Na+ cation) and one glutamate (an amino acid anion). MSG is an ionic compounds, so once it enters your body, it essentially dissociates into its parts mentioned above.

What is sodium? It is essential for the maintenance of ionic concentrations in your body (more specifically the water contents in your cells due to osmosis). It is also important in generating the electric potential in your nerve cells. Of course in the current day and age, sodium is pretty much found in anything.

What is glutamate? Glutamate is an amino acid, meaning it is the basis of many of the proteins in your body. It is perfectly "natural"; there is really nothing "artificial" about it. All the substitutes above contains glutamic acid, which is basically a MSG molecule with H+ replacing Na+ as the cation. It is broken down in the body in the exact same way as MSG; both results in free glutamate ions.

tl;dr version 1) There is nothing "not natural" about MSG 2) Every food containing glumatic acid naturally is essentially chemically equivalent to MSG. 3) There is no scientific literature associating MSG consumption with increased health risks. All the claims of dizziness and uneasiness after MSG consumption has no statistical basis whatsoever in a double blind placebo experiment. 4) There are way worse things (with serious health consequences) that we consume on a daily basis; nevertheless, people still consume them. 5) Just stop worrying, enjoy your food.

Soy sauce is a staple of many of my meals for this exact reason.

comment number 3 is a fallacy.

However i will agree that MSG occurs naturally, but only in SMALL AMOUNTS. MSG has countless PROVEN harmful effects. MSG in packs are considered large amounts even if you put only one particle, the small amount that i'm talking about here is the naturally occuring amount already present in your food. Meaning any additional MSG is already a BIG AMOUNT!

So don't use it. MSG is just used by individuals who lack culinary skills.