Like everyone, I suppose, I find myself receiving email updates from websites I once consulted and would otherwise have forgotten. One of these is www.alllooksame.com which, as far as I can gather, started life dealing with questions of Asian identity. But for some time it has also been debating the question: just how dangerous is that common combination of sodium and glutamic acid – monosodium glutamate (MSG)? On the one hand, are those whose posts - correctly - point out that the (good) published studies fail to find adverse effects of MSG consumption over and above effects produced by a placebo. In response, the contrary viewpoints tend to have a much more personal tone, based on experiences that they attribute to this alleged toxin. A recent post illustrates this:
“MSG is highly dangerous. For me, I didn't used to get reactions, but after decades of eating this stuff, now get raging migraines, heart fibrillation, insomnia, blurry vision, all from accidentally eating MSG in foods. …. As you age, you … start having to deal with some neuro damage from the stuff. I've seen family members, constant eaters of Chinese restaurant food, now have all sorts of neuro-degenerative issues …”
The origins of such beliefs make for an interesting study of how worries about modern foods combine with a fear of environmental toxins to create anxiety that requires a convenient place to be attached – in this case, MSG acts as the “hook”.
From a scientific point of view, the question of MSG toxicity/allergy is largely settled in the negative. However, considerable interest remains in the question of why glutamate acts so reliably as a flavor enhancer. It is straightforward to demonstrate, for example, that savoury foods containing added MSG are evaluated as having greater flavor intensity as well as being more liked [1]. In this regard, MSG acts much like fat and sugar and this analogy has been used to in studies in which MSG has been used to promote liking for novel flavours.
Repeatedly pairing MSG with a novel flavor in solution results in that flavor becoming more liked [2], which is the same effect produced by either fat or sugar. The proposed mechanism for these latter preference boosters is based on the use of the energy provided by fats and sugars by the body, converting the otherwise ‘meaningless’ flavor into a (liked) signal that energy is being delivered. This is variously referred to as ‘post-ingestive’ or ‘flavour-nutrient’ conditioning. Either way, the idea is that a link is learned between the flavor and a positive consequence of ingesting something that is valued by the body. This argument is well supported by demonstrations that these conditioned learning effects only occur when the energy/flavor pairings happen in a state of relative hunger – that is, when the energy is most valued.
So, is glutamate an energy source, like sugar and fat? Possibly, although there is also evidence that the glutamate that we take in our diet is used for a variety of important metabolic processes [4] that could underlie the conditioned liking. Another persistent hypothesis is that the presence of glutamate is a signal to our bodies for the presence of protein. This makes intuitive sense, since glutamic acid is a key amino acid present in proteins. But while there has yet to be convincing evidence for one hypothesis over another, any or all of these effects could support flavor preferences.
Regular readers will recall the distinction made between liking and wanting (October, 2012: Learning to want). More recent research has shown that not only does the presence of MSG promote novel flavours into preferred ones, it acts also on motivation to eat, that is, wanting. Flavours conditioned with MSG increase appetite and promote increased consumption of the flavoured foods, at least in the short term [3]. A recent publication by Una Masic and Martin Yeomans from the University of Sussex has explored further the effects that added glutamate may have on how we eat [5]. They first raise the possibility that since glutamate makes food more palatable, we might expect increased intake of glutamate enhanced foods over the longer term.
It is well known that palatable foods produce an initial appetizer effect – that is, appetite and intake increases following our first bite. Masic and Yeomans hypothesized that adding glutamate to foods also has an effect on short-term satiety and the longer-term satiation:
“MSG may lead to reduced satiation (as a consequence of its effects on palatability) but enhanced satiety, by acting as a cue for protein ingestion”.
In investigating this question, the researchers prepared three soups: a control that was low in energy, a more energy dense version and another that was higher in protein, each with or without added MSG. Initially, hunger decreased but only when there was no added MSG, and only in the context of added protein. To put this another way, adding MSG to a protein enhanced soup maintained hunger. Assessing hunger ratings for a longer period of two hours after the soup was consumed showed, as predicted, a significant suppression of hunger at least for the first hour after consumption. This time it was when MSG had been added, but again this was only evident in the protein enhanced version.
These data suggest a very important role for glutamate in regulating food intake, at least for protein-based foods. In doing this, the data also enhance the link to protein signaling, proposed as an explanation for glutamate palatability. Satiety and satiation are known to be influenced both by the nutrients present in foods and by a food’s palatability. This study steps up the complexity a notch by showing how an ingredient that affects palatability can modulate both processes when it interacts with the nutrients in the food. We know already that taste (in the broad sense of the term) is the best predictor of what we eat. This study suggests too that a taste (in the narrow, more technical sense) can also be a strong influence of how much we eat and when we are likely to feel hunger again.
As a final thought, it is worth considering how those who brand MSG a toxin could possibly account for glutamate’s complex influence on appetite, hunger and palatability. Increasing food preferences and intake is usually considered incompatible with neuro-(or any other kind of)toxicity.
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1. Fuke, S. and T. Shimizu, Sensory and preference aspects of umami. Trends Food Sci. Technol., 1993. 4: p. 246-251.
2. Prescott, J., Effects of added glutamate on liking for novel food flavors. Appetite, 2004. 42(2): p. 143-150.
3. Yeomans, M.R., et al., Acquired flavor acceptance and intake facilitated by monosodium glutamate in humans. Physiol. Behav., 2008. 93: p. 958-966.
4. Reeds, P.J., et al., Intestinal glutamate metabolism. J. Nutrit., 2000. 130: p. 978S-982S.
5. Masic, U. and M. Yeomans, Does monosodium glutamate interact with macronutrient composition to influence subsequent appetite? Physiol Behav, 2013. 116-117: p. 23-29.
