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Anecdotal evidence blames the consumption of MSG for several symptoms such as headache, nausea, weakness, heart palpitations etc. Most migraine literature cites MSG as a potential trigger (example) but usually without any scientific basis. A quick google search gives plenty more reasons that it might be bad for your health.

Are there any reliable studies that support MSG being associated with any kind of health problem, major or minor?

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It should also be noted that MSG or glutamic acid occurs naturally in high concentration in some food (e.g. tomatoes or parmesan). –  Konrad Rudolph Jun 6 '11 at 9:23
    
@Konrad Good point. Perhaps the issue is when it is isolated, or added in higher concentrations than occur naturally. –  Andrew Jun 6 '11 at 11:51
    
Until I find references as a comment: nothing major, but MSG causes an irritated and sore tongue when I eat them, and one of my collegues essentially has the same problem. That is why I try to avoid them. –  johanvdw Sep 26 '11 at 10:53
    
There also have been claims on the web that MSG is a neurotoxin (excitotoxin), but I couldn't find a solid peer-reviewed scientific study on it though. –  Memming Nov 12 '12 at 21:15
    
The difference between medicine and poison is the dosage, and this dosage varies for everyone. –  Michael Lai Jun 6 at 4:46

3 Answers 3

DISCLAIMER: I've just ripped this, almost verbatim, from my answer to the Is Soy bad for you? question.


Some people are exquisitely sensitive to glutamates (e.g. MSG).

From wikipedia:

Australia and New Zealand

Food Standards Australia New Zealand (FSANZ) cites "overwhelming evidence from a large number of scientific studies" to explicitly deny any link between MSG and "serious adverse reactions" or "long-lasting effects", declaring MSG "safe for the general population". It does, however, describe that in less than 1% of the population, sensitive individuals may experience "transient" side effects such as "headache, numbness/tingling, flushing, muscle tightness, and generalised weakness" to a large amount of MSG taken in a single meal.

[Emphasis mine.]

United States

Monosodium glutamate is one of several forms of glutamic acid found in foods, in large part because glutamic acid, being an amino acid, is pervasive in nature. Glutamic acid and its salts can also be present in a wide variety of other additives...

The toxicity of MSG is an area of some debate. The noted scientist John Olney (of Olney's lesions fame) was an outspoken critic of the addition of MSG to foods.

There is some evidence that MSG may be linked to obesity.

Association of Monosodium Glutamate Intake With Overweight in Chinese Adults: The INTERMAP Study

This study examines the association between MSG intake and overweight in humans. We conducted a cross-sectional study involving 752 healthy Chinese (48.7% women), aged 40–59 years, randomly sampled from three rural villages in north and south China. [...] With adjustment for potential confounders including physical activity and total energy intake, MSG intake was positively related to BMI. Prevalence of overweight was significantly higher in MSG users than nonusers. [...] This research provides data that MSG intake may be associated with increased risk of overweight independent of physical activity and total energy intake in humans.

On the other side of the coin, a study funded by a Japanese MSG manufacturer (Ajinomoto) found:

MSG intake suppresses weight gain, fat deposition, and plasma leptin levels in male Sprague–Dawley rats

Monosodium l-glutamate (MSG), an umami taste substance, may be a key molecule coupled to a food intake signaling pathway, possibly mediated through a specific l-glutamate (GLU) sensing mechanism in the gastrointestinal tract. Here we investigated the effect of the spontaneous ingestion of a 1% MSG solution and water on food intake and body weight in male Sprague–Dawley rats fed diets of varying caloric density, fat and carbohydrate contents. Fat mass and lean mass in the abdomen, blood pressure, and several blood metabolic markers were also measured. Rats given free access to MSG and water showed a high preference (93–97%) for the MSG solution, regardless of the diet they consumed. Rats ingesting MSG had a significantly smaller weight gain, reduced abdominal fat mass, and lower plasma leptin levels, compared to rats ingesting water alone. Naso-anal length, lean mass, food and energy intakes, blood pressure, blood glucose, and plasma levels of insulin, triglyceride, total cholesterol, albumin, and GLU were not influenced by the ingestion of the MSG solution. These same effects were observed in a study of adult rats. Together, these results suggest that MSG ingestion reduces weight gain, body fat mass, and plasma leptin levels. Moreover, these changes are likely to be mediated by increased energy expenditure, not reduced energy intake or delayed development. Conceivably, these effects of MSG might be mediated via gut GLU receptors functionally linked to afferent branches of the vagus nerve in the gut, or the afferent sensory nerves in the oral cavity.

Summary

For some people, low doses of MSG may cause health problems1 such as obesity, headache, flushing, sweating, facial pressure or tightness, numbness, tingling or burning in face, neck and other areas, rapid, fluttering heartbeats (heart palpitations), chest pain, nausea, weakness.2


1I have retinitis pigmentosa. One of the prominent aspects of the disease is severe photopsia. MSG is the best trigger factor I have found for photopsia. I realize this is an n=1 study group size but I thought I'd put it out there.

2Mayo Clinic: My favorite Chinese restaurant has a sign that says "No MSG." What is MSG? Is it bad for you?

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Excellent answer, However, you forget to discuss a very relevant fact that glutamate activates a dedicated umami (savoriness) taste receptors on the tongue. Addition of MSG, while not nutritionally important in those that are not sensitive, stimulates these receptors and can make a food more desirable causing overeating of foods that would otherwise be less palatable. –  crasic Jun 6 '11 at 4:13
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Yep, for this question, that answer is appropriate and gets my vote :) –  Lagerbaer Jun 6 '11 at 5:16
    
@crasic - This is true. It's also worth pointing out that glutamate is an excitatory neurotransmitter that acts in the brain. Other neurotransmitters: adrenaline, serotonin, GABA, dopamine, acetylcholine, etc. When you eat MSG, you're eating a neurotransmitter. –  user2466 Jun 6 '11 at 6:16
    
Nice answer. Are you aware of any studies that support MSG causing adverse effects, other than obesity? –  Andrew Jun 6 '11 at 11:03
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@boehj: however, when you eat MSG it doesn't go to the brain, so the fact that it is a neurostransmitter is of no consequence... –  nico Oct 2 '11 at 11:57

No. Carefully blinded trials don't reproduce the effects claimed

It would be extremely surprising if MSG was dangerous to people as glutamic acid the amino acid exists in all our proteins and we will all therefore contain some glutamate (the salt of the free acid).

But it is possible that some people could be particularly sensitive to large amounts of it if added to food. The problem is that studies done with good scientific controls (double blinded) have never consistently shown that even those people who claim such sensitivity suffer the adverse effects when actually given MSG (and don't suffer them when even something other than MSG). As one review put it:

Work over the past 17 years has consistently failed to reveal any objective sign accompanying the transient sensations that some individuals experience after the experimental ingestion of monosodium glutamate and it is questionable whether the term 'Chinese Restaurant Syndrome' has any validity. When some common food materials are used in the same experimental setting, similar symptoms can be produced in a limited number of people. Double-blind testing of individuals who identify themselves as suffering the 'syndrome' has failed to confirm the role of monosodium glutamate as the provocative agent.

I think that is pretty solid evidence that it isn't bad for you.

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Your money quote is more an indictment of self-diagnosis. –  horatio Feb 22 '12 at 22:26
    
-1 … this answer doesn’t address the general claim that MSG is bad for you, just that specific reported averse reactions couldn’t be replicated. However, there is a more general body of research potentially showing toxicity of MSG. The mere fact that glutamic acid also acts as an amino acid says nothing about its toxicity. The dose makes the poison. As a related example, consider phenylalanine intolerance. In the context of MSG, its excitotoxicity is particularly relevant. –  Konrad Rudolph Jun 11 '13 at 11:59
    
@KonradRudolph Hypothetical. Moreover, the reason why people are worried about MSG is the specific effects it is supposed to have. If those effects can't be replicated in controlled experiments, then we have good reason to doubt them. The onus of proof is on you to show that does in specific foods are large compared to the natural occurrence of MSG from protein breakdown. Besides is there a significant population group that doesn't metabolise MSG? –  matt_black Jun 11 '13 at 23:35
    
@matt_black My context is this question which cites several solid-looking peer-reviewed research showing a clear MSG toxicity. However, I haven’t had time to look at the articles in detail or whether they have been followed up or disproved, and I don’t know how relevant these studies are for the dosage commonly used in food. –  Konrad Rudolph Jun 11 '13 at 23:57

I did look this up sometime ago, in the context of pregnancy. Almost all the health concerns are skeptical based on random and possible connections I have not found any documented scientific experiments/studies with proven results; either way. On the other hand, asian cuisine has used MSG for years.

The only thing to be careful of, is if you are allergic to MSG. There have been recorded incidents of serious reactions to MSG..

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