The present study reports a significant association between the risk of the metabolic syndrome and MSG daily consumption in a rural area of Thailand where people cook their own meals with the typical eating habit of sharing food among family members. Importantly, the association is independent of major confounding factors such as age, sex, smoking, family history, physical activity, or calorie intake. This unique context allowed the measurement of MSG consumption for 10 days to be more accurate than the use of questionnaires.
The metabolic syndrome is currently considered as the most prominent risk factor for cardiovascular diseases worldwide. Data from the Third National Health and Nutrition Examination Survey (NHANES III) on 8,814 men and women suggest that 20 to 30% of adults in developed countries meet the syndrome criteria. In addition to an elevated calorie intake, specific dietary factors have been linked with the incidence of the syndrome with conflicting results. MSG is a common flavor enhancer and naturally occurring amino acid utilized in increasing amounts in food industry and preparation. Despite its optimal safety profile, a large cross-sectional study revealed that MSG consumption is related to a higher increase in blood pressure. Moreover, MSG has been linked with obesity, type II diabetes, and the metabolic syndrome as its intake in healthy Chinese adults correlates with the resulting increase in body mass index regardless of energy intake[3, 4].
We found a significant trend for increasing insulin levels and prevalence of insulin resistance across tertiles of MSG intake. However, the change did not alter the glucose homeostasis significantly as fasting blood sugar did not differ among different levels of MSG intake. This is in agreement with the observation that the administration of MSG to fasting human subjects increases insulin levels without altering glucose concentration and glucose tolerance. While glutamate is known to stimulate insulin secretion via glutamate receptor, the optimal glucose regulation in healthy volunteer may overcome the intermittent rising of insulin level[19, 20].
Further evidence of a link between MSG and the metabolic disorders is obtained from animal models. High doses of MSG injected in rodents during the neonatal period lead to the development of glucose intolerance, insulin resistance, and obesity along with adipose tissue hypertrophy, hyperinsulinemia, hyperglycemia, hyperleptinemia, and decreased insulin stimulated glucose transport in adipocytes and muscle[22–24]. These changes in glucose metabolism and insulin resistance were not observed when MSG was administered to adult animals, thus suggesting that the endocrine regulation in these animals is very sensitive to MSG during the early postnatal period. The metabolic disturbances found in this present study might be the sum of exposure during early life via the maternal diet including in their later life.
MSG is a common additive used in Thai cuisine and we observed an average MSG intake of 4.0 g/day which was over two times higher than an older survey of Thai adults. The MSG intake in this study was similar to that recent report in Chinese population (3.8 g/day) by the Jiangsu Nutrition Study. However, our recorded MSG intake was 12 and 2 times higher compared to that of the two Chinese population nutritional surveys, i.e. the INTERMAP study and the China Health and Nutrition Survey (CHNS), respectively. While we can only speculate on the impact of different eating habits, our measurement of MSG consumption based on common cooking habits in a limited area with uniform dietary and physical activity regimens. Further, to be included in the present study families had to prepare their meals twice a day thus limiting the impact of commercially available food. Lastly, inhabitants of Thai rural areas rarely eat restaurant food compared to urban areas, thus minimizing this potential confounding factor.
Additional observations strengthen our hypothesis. First, we should note that daily energy intake and physical activity levels are not significantly different among tertiles of MSG consumption, possibly based on the fact that 66% of participants are agricultural workers with a vigorously active lifestyle. Second, the prevalence rates of hypertriglyceridemia and low HDL cholesterol levels are not associated with MSG intake, while the overall prevalence of hypertriglyceridemia is similar to data from Thai adults in rural areas. Our data demonstrate the significant trends of insulin resistance, overweight, and metabolic syndrome towards levels of MSG intake. However, after adjusting for the confounding factors such as sex, age, calories intake, physical activity, smoking status, and history of diabetes, the MSG consumption does not significantly increase risk of insulin resistance. Since there were no differences in glucose level and energy intake in different tertiles of MSG consumption, the metabolic syndrome associated with MSG intake is probably not due to the energy intake-induced obesity leading to insulin resistance. The finding of MSG consumption significantly correlated with BMI in our study agreed with the two studies in Chinese population, INTERMAP and CHNS[3, 4], through a mechanism possibly mediated by white adipose tissue deposition, as suggested in animal models[7, 23, 29]. MSG may enhance shifting the dietary glucose towards lipid synthesis, increasing the rate of lipogenesis and activating gene expression of enzymes involved in lipid biosynthesis and storage in adipose tissue.
Our results demonstrate that MSG intake is associated with the risk of having the metabolic syndrome. A longitudinal study with a larger sample size is needed for confirmation the contribution effect of MSG on the development of metabolic syndrome, a rising global metabolic emergency.