In terms of studies, MSG is used to reliably cause obesity and glaucoma in mice. The claims that it doesn’t affect humans are based on per kilogram of body weight calculations, which works if MSG can simply dissipate evenly throughout your body, but inaccurate if it doesn’t or if it doesn’t in some people.

The natural argument is really pretty silly. Alcohol is natural. Salt is natural. Even mint flavor is natural, but deadly in large amounts.

Also, you’d think that if the Japanese have been adding it to everything for thousands of years then anyone that can’t handle it probably didn’t live very long or had a very poor quality of life and the gene wasn’t passed on. Then it’s just self-fulfilling that no Japanese has a problem with eating MSG.

MSG in Taiwan also was not commonly added to foods in the amounts that they were until around World War II. When you’re poor, MSG is pretty expensive. Elsewhere in the world, it is much less common.

In the US, your biggest sources of MSG are going to be Chinese food, chips (Doritos, Lays, etc), KFC, some McDonald’s, some Subway, sausage ham and processed meats, and a general hodge-podge of processed foods. But your average restaurant doesn’t use it. So it’s pretty conceivable that you can be eating it in the US once in awhile and having problems, but not knowing what is casuing it. In contrast, in Taiwan, basically all restaurants use it, especially in soups and fried foods and dumplings–I don’t know if they add it to cooking oil, but oyster sauce, used as a flavoring and cooking oil, is very high in MSG. All those canned foods are also high in it. So there’s a huge jump in the amounts you’d be ingesting moving from a country that doesn’t use MSG (mostly) to one that adds it to every dish.

By the way, consistent with the natural foods you mentioned, a lot of tomato sauce or Parmesan cheese affect me the same way that

My conclusion is that there are probably people avoiding it that don’t need to be, but there is definitely variation, and it’s a serious problem for a certain segment of the population, and I don’t think it’s good to just simply declare it generally safe for everyone.

I asked a doctor I’m acquainted with to explain it to me, and he said that it’s because an enzyme that most people have is lacking in my blood. In most people, that enzyme breaks down the MSG very quickly, but in me, it just circulates for hours.

I sometimes even get MSG in sweet snacks in Taiwan. It often shows up mixed in with peanut power, such as you get with mwa2 jee1 or gua1 bao4. It is often added in containers that just say salt (in chinese) on the label, and to flavoring/spice mixtures. Which means that cooks are often adding MSG, even when they assure you that they aren’t. Deep fried foods in Taiwan almost always have MSG, even those doughnut-like sweet sesame seed balls, as I found out the hard way. Maybe they used that same oil the day before to fry chicken. A Taiwanese friend also told me he thought that many brands of cooking oil used by restaurants here are sold with MSG already added.

Since it was Ajinomoto that spread MSG around the world, it’s no surprise that its name became synonymous with MSG in places other than Japan.

One final thing, due to the nature of Wikipedia, I don’t think it’s the best place to look for information on subjects for which a lot of misinformation has been popularized. That’s why I chose to link to the European Food Information Council instead.