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I recently came across a video (that is anti-MSG) that made the claim that scientist use MSG to fatten lab rats/mice. I can't seem to find any source for this information. I can find examples of studies where baby mice were given large amounts of MSG, but I can't seem to find a "how-to fatten your lab rats? give them MSG" example.

Link to the video: http://vimeo.com/39562192#t=925s

Transcript:

It actually excites a part of your brain that's in charge of the fat programs. The chemical of MSG excites the brain. And due to that excitation, your body activates the fat programs and gets fatter, and everybody knows this!

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Yes, there are studies where monosodium glutamate (MSG) is used to induce obesity in rodents, but using this fact as an anti-MSG argument is very questionable.

Example of papers using rodent models of MSG-induced obesity

The induction of obesity in rodents by means of monosodium glutamate. - Bunyan et al., Br J Nutr. 1976

or

The monosodium glutamate (MSG) obese rat as a model for the study of exercise in obesity. - Gobatto et al., Res Commun Mol Pathol Pharmacol., 2002

Of course this is rodent data and as much as animal research is absolutely indispensable to understand human physiology and pathology, it is necessary to critically interpret its results. Note that:

Newborn mice were injected subcutaneously with 3 mg MSG/g body-weight at 1, 2, 3, 6, 7, and 8 d of age

and, in the second study

Obesity was induced by MSG administration (4 mg/g, each other day, from birth to 14 days old)

Which is definitely not the same as an adult human eating MSG.

So, how much MSG is consumed by an average person daily?

A 1979 US survey reported 550 mg/die
GRAS Committee on GRAS List Survey. Estimating distributions of daily intake of monosodium glutamate (MSG). Washington, DC: Food and Nutrition Board, Division of Biological Sciences, Assembly of Life Sciences, 1979
(sorry, could not really find a link for this, but it is cited in many other publications...)

A 1991 UK survey reported 580 mg/die
A survey of the monosodium glutamate content of foods and an estimation of the dietary intake of monosodium glutamate. - Rhodes et al., Food Addit Contam., 1991

Another source reports 5 to 12 g/die in European countries

Consensus meeting: monosodium glutamate - an update. - Eur J Clin Nutr., 2007

Taking the larger 12 g/die figure, for a 70kg person that comes to approximately 0.17 mg/g body weight, that is about 17 times less than what was used in the article. The 550 mg/die figure would give 0.007 mg/g body weight, which is 380 times lower.

That said, I could find one study linking consumption of MSG with obesity in the Chinese population

Consumption of monosodium glutamate in relation to incidence of overweight in Chinese adults: China Health and Nutrition Survey (CHNS) - He et al., Am J Clin Nutr., 2011

although the results have been criticized:

A lack of epidemiologic evidence to link consumption of monosodium L-glutamate and obesity in China. - Bursey et al. - Am J Clin Nutr., 2011

So, in summary altough there are studies showing that injection of MSG in neonatal rodents cause adult obesity:

  1. this is definitely not the most common model of obesity used in a laboratory setting

  2. the fact that MSG had to be injected rather than given orally to get the effect may indicate that the obesity is rather a consequence of the stress due to the injection. Exposure to high level of glucocorticoids (the "stress hormones") during the perinatal period (pregnancy and lactation) has been linked to adult cardiovascular diseases and obesity in both rodents and human.

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    I did the math and concluded I would have to inject about 3/4 of a lb of MSG under my skin each day to to get the equivelant of the mice in the first study (240lb adult weight). That seemed pretty extreme. The problem is the video specifically stated that scientists use MSG to increase the weight of mice if they need to do a study with overweight mice. This seemed bogus to me and all I can seem to find are studies where you just want to find out what happens when given large quantities. Whoever makes these types of videos, I find them quite baffling. :( – Hackmodford Oct 22 '13 at 19:06
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    @Hackmodford: two important things to note: 1) those injections were done in the first 2 weeks of age (i.e. sucklings). This is a very delicate period and many different stressors applied at the time can cause obesity. I am not sure if the same protocol works on adult mice. 2) Although this obesity protocol does exist it is not a commonly used one. If you need an obesity model you either get genetically obese animals (e.g. ob/ob mice) or you feed them an high-fat diet. Much easier, and more relevant to human health. – nico Oct 22 '13 at 19:38
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    If eaten, glutamate stimulates the umami taste receptor (the one making protein tasty: it is an important amino acid in proteins). Glutamate - obesity correlation studies should IMHO discuss the taste reception: is the control food as tasty as the glutamate-rich food? The first article had control mice being fed (instead of injected, and 20 mg/kg body weight) with MSG - and they did not get obese. The EurJClinNutr paper says that we (Europeans) get about 0.4 g of free glutamate for every 10 g of glutamate in protein - so the main glutamate source is normal protein. – cbeleites unhappy with SX Oct 22 '13 at 21:03
  • I miss another control experiment in the "obese mouse suckling" paper: what about injecting other amino acids/hydrolysed protein? Maybe it isn't specific to glutamate - I could readily imagine that the immune system doesn't like amino acid injections? And how do the sucklings react to the same Na⁺ concentration injected as NaCl? – cbeleites unhappy with SX Oct 22 '13 at 21:04
  • @cbeleites: 100% agree with you. It is very well known that perinatal stress correlates with adult obesity in rodents and humans and daily injections can definitely be a stress for pups. In any case I updated the answer with some of the comments. – nico Oct 22 '13 at 22:26

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