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).
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
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
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?