14 June 2020
This letter refers to the article “Oral Monosodium Glutamate Administration Causes Early Onset of Alzheimer’s Disease-like Pathophysiology in APP/PS1 Mice” by Fuchsberger et al. , reporting that oral administration of MSG accelerated the onset of pathophysiological and behavioral symptoms in APP/PS1 mice, a genetically modified animal model of Alzheimer’s disease (AD). The authors speculate that circulating glutamate originating from dietary monosodium glutamate (MSG) entered the brains of APP/PS1 model mice by crossing the blood-brain barrier (BBB), and exerted an excitotoxic effect that led to AD-related hallmarks such as accumulation of amyloid-β (Aβ), and tau phosphorylation.
It is, however, well established that glutamate ingested with food is metabolized, mainly serving as an energy source for enterocytes [2, 3] and normal dietary glutamate ingestion has just a small effect on blood glutamate concentration in humans [3-5]. Further studies have elucidated that the profile of circadian variation in the glutamate concentration in blood is not significantly different on diets with or without added MSG , and that the basal glutamate concentration in humans is not affected by long-term MSG consumption . Rodent studies show that ingestion of a diet containing MSG hardly influences blood glutamate levels and suggest that the macronutrients in diets, particularly carbohydrate, is most effective in attenuating any rise in blood glutamate levels induced by oral administration of MSG . These research results imply that experimental animals including mice and rats, which have free access to a normal diet combined with an MSG solution, will not show any increase in the blood glutamate concentration. In fact, some studies conducted under standard feeding conditions demonstrate that rats provided with water containing MSG did not show any rise in the basal blood concentrations of amino acids. Even if MSG is consumed in large amounts under these conditions, blood levels of amino acids including glutamate were maintained at almost constant levels [9, 10]. As described by the authors, blood concentrations of glutamate ranging from 50 µM to 100 µM are markedly higher than those in cerebral extracellular fluid (CEF) . If the authors’ speculation that blood glutamate enters the brains of APP/PS1 mice by crossing BBB is scientifically accurate, APP/PS1 mice must have a high glutamate concentration in the CEF and exhibit pathophysiological changes at 16 weeks of age independent of the experimental condition. It is therefore quite surprising that only the APP/PS1 mice provided with a 1% MSG solution as drinking water appear to show pathophysiological changes with associated changes in cognitive function.
Many types of murine model have been created using genetic modification and used in studies of AD. These model mice have specific characteristics that depend on the modified genes and have been developed to improve applicability to studies of AD. There have been many reviews [12-15] of the pros and cons of the various models, including APP/PS1 transgenic mice, allowing selection of suitable and valid models in studying AD. In these reviews, the disadvantages of the APP/PS1 transgenic mouse, such as relatively unstable Aβ expression and absence of neurofibrillary tangles are noted. These findings imply that onset of AD-like pathophysiology varies depending on each individual. In the current study, the values of biomarkers related to AD were obtained from just three or four samples. I consider this sample size to be insufficient to evaluate the effects of MSG. To overcome these disadvantages, an adequate sample size is required with each verification. Regarding cognitive function assessment, time in open arms should be measured before an intervention to evaluate effect of the intervention.
Recently, it has been suggested that environmental enrichment can reduce accumulation of Aβ deposits and suppress the progression of memory impairment in AD model animals [16, 17]. Effects on AD of interventions associated with the environment including diet and aroma have also been investigated and amelioration of cognitive function has been observed in many clinical trials. A flavor enhancer, MSG is well known to make food more palatable and enhance the enjoyment of eating. Kouzuki et al. reported that an intervention involving ingestion of meals seasoned with MSG for 12 weeks improved some scores related to cognitive function in patients with dementia, including AD . As the authors mention in the report, the average intake of glutamate as a flavoring agent is estimated to be 0.3-0.5 g/day in European countries and 1.2-1.7 g/day in Asian countries, suggesting that people in Asian countries consume a far greater amount of MSG daily . If the authors’ speculation is scientifically accurate, early onset and prevalence of AD and dementia would be prominent in Asian countries. However, this geographical disproportion in onset and prevalence of AD and dementia has not been observed so far [20, 21]. Therefore, in order to suggest a potential role of dietary glutamate in aggravating AD pathophysiology, further studies seem to be needed and the authors should at least discuss the discrepancies between their study results and clinical and epidemiological facts.
Yoshida Shintaro, PhD, DVM
International Glutamate Technical Committee
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