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00004872-201105000-0000700004872_2011_29_846_shi_monosodium_5article< 106_0_16_5 >Journal of Hypertension© 2011 Lippincott Williams & Wilkins, Inc.Volume 29(5)May 2011p 846–853Monosodium glutamate is related to a higher increase in blood pressure over 5 years: findings from the Jiangsu Nutrition Study of Chinese adults[Original papers: Epidemiology]Shi, Zumina,b,c; Yuan, Baojuna; Taylor, Anne Wb,c; Dai, Yuea; Pan, Xiaoquna; Gill, Tiffany Kc; Wittert, Gary AcaDepartment of Nutrition and Foodborne Disease Prevention, Jiangsu Provincial Centre for Disease Control and Prevention, Nanjing, ChinabPopulation Research and Outcome Studies Unit, Department of Health, AustraliacDiscipline of Medicine, University of Adelaide, Adelaide, South Australia, AustraliaReceived 14 October, 2010Revised 10 January, 2011Accepted 20 January, 2011Correspondence to Dr Zumin Shi, Department of Nutrition and Foodborne Disease Prevention, Jiangsu Provincial Centre for Disease Control and Prevention, 172 Jiangsu Road, Nanjing 210009, China Tel: +61 8 8313 1188; fax: +61 8 8313 1228; e-mail: zumins@vip.sina.comAbstractObjective: One large cross-sectional study across four countries suggests that glutamate intake may be inversely associated with blood pressure (BP). The aim of this analysis was to investigate a possible association between monosodium glutamate (MSG) intake and change in blood pressure over 5 years.Methods: Data from 1227 Chinese men and women who participated in the Jiangsu Nutrition Study (JIN) were analyzed. In this study, MSG intake and blood pressure were quantitatively assessed in 2002, and followed-up in 2007.Results: MSG intake was associated with a significant increase in SBP and DBP. A strong sex interaction was observed in relation to SBP change. Women with high MSG intake were more likely to have increased SBP and DBP. Total glutamate intake was also positively associated with an increase in SBP. In those chronically taking antihypertensive medications, there was a strong association between MSG intake and an increase in DBP.Conclusion: MSG intake may have independent BP-increasing effects, especially among women and those taking hypertension medications at baseline and follow-up.Abbreviations BP: blood pressure; FFQ: food frequency questionnaire; MSG: monosodium glutamate.IntroductionMonosodium glutamate (MSG) is a widely used flavor enhancer. Since the first report suggesting a possible link between MSG and ‘Chinese restaurant’ syndrome by Kwok in 1968 [1], numerous studies have been published to assess the association between MSG and health outcomes, including asthma, diabetes, obesity, and allergic rhinitis [2–4]. There is only limited data relating to the association between MSG and blood pressure (BP) and the findings are inconsistent [5–9]. The InterMap study is the only large-scale population study to have addressed this issue, although the relationship between glutamate rather than MSG, intake and BP, was examined cross-sectionally and reported as an inverse association [10].Like many other parts of the world, due to the rapid nutrition transition, the prevalence of noncommunicable chronic diseases, such as hypertension, is very high in China. Data from the 2002 Chinese National Nutrition Survey show that one in six people are hypertensive [11]. Only 19% of those with hypertension have adequate blood pressure control [11], and hypertension is one of the leading preventable risk factors for premature mortality in the Chinese population [12]. It has been established that sodium intake increases the risk of hypertension in the Chinese population [13], but the relationship between MSG and blood pressure has not previously been investigated.The objective of the study is to assess the association between MSG intake and blood pressure changes after 5 years, based on a large population-based study in China, the Jiangsu Nutrition Study (JIN).Participants and methodsSampleThe JIN cohort study of persons aged 20 years or older and the methods of sampling have been described previously [14–16]. In 2002, 2849 adults aged 20 years and above living in two cities and six rural areas had blood pressure measured and dietary information obtained. In 2007, only 1682 participants could be identified through household visit, 1492 of them participated in the follow-up interview, 190 participants refused to participate, thus height and weight measurements were obtained from 1282 (76.2%) participants and 210 participants completed the interview at home but missed the measurement in the clinic. For the current analyses, we excluded those participants who had extreme values of weight change (i.e., >20 kg, n = 11) and those who had known diabetes, stroke, or cancer at baseline (n = 40). Four participants also had missing values for MSG intake at baseline. The final sample size examined for BP change consisted of 509 men and 718 women (total n = 1227). The study was conducted according to the guidelines laid down in the Declaration of Helsinki and all procedures involving human participants/patients were approved by Jiangsu Provincial Centre for Disease Control and Prevention. Written consent to participate was obtained from all the participants.Data collection and measurementsParticipants were interviewed at their homes by health workers using a standard questionnaire [14]. The health workers were intensively trained in all aspects of data collection.Outcome variablesBlood pressure was measured twice by mercury sphygmomanometer on the right upper arm of the participant, who was seated for 5 min before the measurement. The mean of these two measurements was used in the analyses. The cuff size was selected on the basis of the upper arm circumference to ensure that the cuff did not overlap [11]. Hypertension was defined as SBP above 140 mmHg and/or DBP above 90 mmHg, or using antihypertensive drugs.Dietary intakeIn 2002, dietary intake patterns during the previous year were investigated by a series of detailed questions about the usual frequency and quantity of intake of 33 food groups and beverages. The food frequency questionnaire (FFQ) has been validated and compared with weighted food records [17–19] and reported to be a useful method for the collection of individual food consumption information in face-to-face interviews, but not in self-administered surveys due to the current educational level of the majority of the Chinese population. To determine the amount of MSG and other seasonings consumed by individuals, each household was specifically asked about their usual monthly consumption of these items. Individual consumption of MSG was calculated according to the total amount of MSG consumed in the household divided by the number of individuals per household and then adjusted for the proportion of the household energy intake energy consumed by each individual. Average total glutamate intake was also calculated by adding the glutamate concentrations of all foods/seasonings consumed by an individual per day. Nutrient (e.g., sodium, potassium, fiber) and vegetable oil intakes were assessed using a 3-day weighed food diary, which recorded all foods consumed by each individual, on three consecutive days; this was done to confirm the intakes reported from the FFQ data. We did not consider underreport and overreport of energy intake to be an issue, because upon reviewing the food diaries with the participants, the health workers would clarify any intake value for a particular food that fell below or above the usual value reportedly consumed by the population within the region. Not everyone in the household was asked for dietary information at follow-up (FFQ was only used among those 15 years and above); however, the household MSG intake was determined, thus mean household MSG intake was calculated. Food consumption data were analyzed using the Chinese Food Composition Table [20]. The 2004 version of the table is an update of the previous version (2002), which is listed in the FAO website (http://www.fao.org/infoods/tables_asia_en.stm#china ). The Chinese Centre for Disease Control and Prevention (CDC) is responsible for all the analyses. It covers 757 food items.Dietary patternsDietary patterns were identified by factor analysis based on food intake measured by the FFQ, using standard principal component analysis as described elsewhere [21]. In short, four food patterns were obtained – factor 1 (‘macho’) was characterized by various kinds of animal foods and alcohol; factor 2 (the ‘traditional’ pattern) loaded heavily on rice, fresh vegetables, and inversely on wheat flour; factor 3 (‘sweet tooth’) contained cake, milk, yogurt, and drinks; and factor 4 (‘vegetable-rich’ pattern) included whole grains, fruits, root vegetables, fresh and pickled vegetables, milk, eggs, and fish. The four factors explained 28.5% of the variance in intake.Other lifestyle factorsCigarette smoking was assessed by asking the frequency of daily cigarette smoking. Eating out was assessed by asking whether individuals ate out on a frequent basis and was coded as yes or no. Alcohol consumption was assessed by asking the frequency and amount of alcohol consumed. Information on passive smoking was asked. Questions on daily commuting were grouped into three categories: using motorized transportation, or no work (0 min of walking or cycling); walking or bicycling 1–29 min; and walking or bicycling for 30 min or more. Daily leisure time physical activity was grouped into three categories: 0, 1–29, and at least 30 min. Time taken on sedentary activities each day (viewing television, operating computer, playing video games, and reading during leisure time) was classified into four categories: less than 1, 1–1.9, 2–2.9, and 3 h or more. Education was recoded into either ‘low’ (illiteracy, primary school), ‘medium’ (junior middle school), or ‘high’ (high middle school or higher), based on six categories of education levels in the questionnaire. Occupation was recoded into ‘manual’ or ‘nonmanual’ based on a question with 12 occupational categories. Hypertension medication use (yes/no) was asked at baseline and follow-up.Anthropometric measurementsIn both 2002 and 2007, anthropometric measurements were obtained using standard protocols and techniques [11,19]. Body weight was measured in light indoor clothing without shoes to the nearest 100 g. Height was measured without shoes to the nearest millimeter using a stadiometer. Waist circumference was measured to the nearest millimeter midway between the inferior margin of the last rib and the crest of the ilium, in the mid-axillary line in a horizontal plane. Family history of hypertension was defined as the presence of known family members with hypertension in any of three generations (siblings, parents, or grandparents).StatisticsMSG intake was recoded into quartiles. The χ2-test was used to compare difference between categorical variables and ANOVA was used to compare differences in continuous variables between groups. Fractional polynomial prediction plot (using two-way fpfitci command) was used to visually examine the relationship between MSG intake (continuous, g/day) and blood pressure change. Mixed-effects linear regression was used to determine the association between MSG intake (quartiles) and SBP and DBP change adjusted for age, education, occupation, active commuting, leisure time physical activity, smoking, passive smoking, alcohol drinking, overweight (yes/no) at baseline, change in BMI, eating out, family history of hypertension, energy, sodium, fiber, and potassium intake. These multivariate models were adjusted for household cluster using the xtmixed command. We tested for linear trend across categories of MSG intake by assigning each participant the median value for the category and modeling this value as a continuous variable. Food patterns were also put into the multivariate models to control for the residual confounding, as suggested by Imamura et al. [22]. In the provinces, a distinct definition of urban/rural dwelling is difficult due to ongoing economic development. As we have adjusted for education and job status as well as dietary patterns, the decision was made not to adjust for urban/rural. All the analyses were performed using STATA 11 (Stata Corporation, College Station, Texas, USA). A P value less than 0.05 was considered to be statistically significant (two sided).ResultsThe mean intake of MSG for the entire population was 3.8 g/day (SD 4.3). Of the 1227 participants, 72 reported no use of MSG and median intakes across the quartiles were 0.8, 2.0, 3.7, and 6.9 g/day, respectively. Table 1 shows the cross-sectional associations between MSG intake, nutrients, and specific food items or food groups. MSG intake was positively associated with fat and sodium intake (P < 0.001) but inversely associated with carbohydrate intake (P < 0.001). No significant difference in energy, potassium, or protein intake was found across MSG intake quartiles. Total glutamate was the same in quartiles 1–3 but was, on average, 37% greater in quartile 4 (P < 0.001). Rice intake was greater among individuals in the higher quartiles of MSG intake (P < 0.001), but intakes of fruit and vegetables were not different across these levels. The prevalence of smoking and alcohol consumption was also higher among individuals in the higher quartiles of MSG intake (P < 0.001). Cross-sectionally, there was no association between MSG intake and hypertension at baseline. The prevalence of hypertension at baseline across MSG intake quartiles was 30.9, 32.3, 31.6, and 26.1%, respectively. However, the mean DBP tended to decrease across quartiles of MSG intake (P = 0.005). When we used all the available data including those lost to follow-up, the baseline association between MSG and hypertension was not significant (data not shown). Seventy-six participants (6.2%) reported taking hypertension medication (at both baseline and follow-up).Table 1 Baseline sample characteristic according to monosodium glutamate intake quartiles among Chinese adultsaThe mean 5-year BP change among all participants was 4.5 mmHg (SD 19.2) for SBP and 3.0 mmHg (SD 11.2) for DBP. The prevalence of hypertension at follow-up across each of the MSG intake quartiles was 38.8, 42.4, 44.0, and 41.2%, respectively (P = 0.610). Figure 1 shows the association between MSG intake (as continuous variable) and BP changes without any adjustment for covariates. There was a clear trend of dose–response relationship between MSG intake and BP changes.Fig. 1 No caption available.Table 2 shows the associations between MSG intake and BP changes using regression analyses. There were positive associations between MSG intake and changes in both SBP and DBP. In the full model including dietary pattern adjustment (model 3), the β values and 95% confidence interval (CI) for BP changes across quartiles of MSG intake were 0, 0.09 (−3.06 to 3.24), 3.05 (−0.32 to 6.41), and 3.15 (−0.29 to 6.60) (P for trend = 0.040) for SBP and 0, −0.35 (−2.17 to 1.47), 2.94 (1.01–4.88), and 1.67 (−0.30 to 3.64) (P for trend = 0.037) for DBP. After excluding 46 participants who reported change in diet at baseline in order to control chronic diseases, the β values and 95% CI for BP changes across quartiles of MSG intake were 0, 0.12 (−3.11 to 3.35), 4.05 (0.61–7.49), and 3.28 (−0.23 to 6.79) (P for trend = 0.040) for SBP and 0, −0.44 (−2.31 to 1.43), 2.96 (0.97–4.96), and 1.38 (−0.65 to 3.41) (P for trend = 0.037) for DBP (data not shown). Total glutamate intake was positively associated with SBP change but not DBP change (data not shown).Table 2 Linear regression β coefficients (95% confidence interval) for quartiles (Q1–Q4) of the monosodium glutamate intake predicting 5-year change in blood pressure in 1227 adults participating in the Jiangsu Nutrition StudyTable 3 shows that there were significant interactions for MSG intake with sex, medication status, and smoking in relation to SBP change. The associations between MSG intake and SBP change were stronger among women, nonsmokers, and those regularly taking hypertension medications (medication at both baseline and follow-up). A significant association was also found between MSG intake and DBP change among those regularly taking hypertension medications (Table 4). Comparing extreme quartiles of MSG intake among those on medication, the change in DBP was 19.41 (95% CI 10.49–28.33) mmHg. In the multivariate analysis, adjusting for age and sex, the odds ratio for regular hypertension medication intake across quartiles of MSG intake was 1, 1.31 (0.62–2.76), 1.47 (0.70–3.10), and 2.19 (1.05–4.57) (P for trend = 0.030).Table 3 Stratified regression coefficients (95% confidence interval) for SBP change according to the monosodium glutamate intake quartiles (β coefficients and 95% confidence intervals) among Chinese adults (n = 1227)aTable 4 Stratified regression coefficients (95% confidence interval) for DBP change according to the monosodium glutamate intake quartiles (β coefficients and 95% confidence intervals) among Chinese adults (n = 1227)aWhen we used the mean household MSG intake at follow-up as the exposure variable, the associations between MSG intake and BP change were consistent with those of baseline MSG intake. After adjusting for the covariates in model 3 of Table 2, the β (95% CI) for BP changes across quartiles of mean household MSG intake was 0, 2.38 (−0.71 to 5.47), 4.86 (1.52–8.20), and 4.44 (1.19–7.69) (P for trend 0.002) for SBP; 0, 1.84 (0.09–3.59), 2.77 (0.89–4.66), and 3.75 (1.91–5.58) (P for trend <0.001) for DBP (data not shown).DiscussionIn this longitudinal study, we found a positive association between MSG intake and changes in both SBP and DBP over 5 years. A strong sex interaction was observed in relation to SBP change. Women with high MSG intake were more likely to have increased SBP and DBP. The association between MSG intake and increased DBP was stronger among those taking hypertension medication at both baseline and follow-up. A similar association between total glutamate intake and blood pressure change was also found. To our knowledge, this is the first longitudinal population study to investigate a possible association between MSG intake and blood pressure change.The findings relating to the association between MSG and BP are inconsistent with some small short-term randomized control studies that showed no association [5–8], but consistent with one recent double-blinded, placebo-controlled, cross-over study among 14 healthy men that showed a positive association between MSG intake and BP [9]. These discrepancies could be due to the differences in study design, sample size, and characteristics of participants. Methodological problems in early studies of MSG have been criticized, for example, inadequate sample size, not double blind, and recruiting healthy volunteers (which may exclude those sensitive to MSG) [23,24]. The study undertaken by Yang et al. [5] was double-blind, placebo-controlled, and randomized, but those receiving beta-blocker therapy or with uncontrolled hypertension were excluded.The cross-sectional InterMap study showed an inverse association between glutamate intake and both SBP and DBP, [10] an association not supported by the link between the ‘Super Size Me’ diet and blood pressure [25]. The inverse association between glutamate intake and hypertension in the Chinese population in the InterMap study also seems to contradict the south/north difference in the prevalence of hypertension in China; hypertension is higher in the north than in the south [11]. The staple food is wheat flour in the north and rice in the south, and wheat flour has approximately eight times the glutamate concentration as rice (3600 vs. 450 mg/100 g dry weight of food).It is interesting that the association between MSG and BP is independent of obesity. We have previously reported the absence of an association between MSG intake, obesity, and 5-year weight gain [14]. Selective leptin resistance may exist in the obese population, which could explain the seemingly inconsistent findings in the relationships between MSG and obesity and BP change [26].A significant interaction was found between sex and MSG intake in relation to SBP increase. Women with a high intake of MSG were more likely to have increased SBP. Higher levels of leptin in women [27] may render them more sensitive to the effects of MSG. Estrogen may also play a direct role in the sex difference. It has been found, for example, that there is a significant sex-specific effect in the response of masseter muscle afferent fibers to activation of peripheral NMDA receptors that is strongly correlated with plasma estrogen levels [28]. One of the mechanisms by which estrogen promotes vasodilation is by influencing L-type calcium channel currents [29]. 17 beta-Estradiol inhibits the voltage-dependent L-type Ca2+ currents in aortic smooth muscle cells [30].The positive association between MSG intake and DBP increase among those taking hypertension medication at baseline and follow-up was substantial, although the CI is wide because of the small number of participants in this group. We do not have detailed information on the types of hypertension medication used in each individual, but the use of calcium channel blockers is common in China. Glutamate opens the L-type calcium channel and causes Ca2+ influx [31]. MSG may, therefore, antagonize the effect of dihydropyridine calcium channel antagonists. Further research on the interaction of MSG and specific types of hypertension medication is needed.The null association between MSG intake and hypertension at baseline could be due to the change in food habits in order to control chronic diseases among some participants. Chinese websites contain abundant information claiming MSG intake is not good for hypertension. A search for ‘MSG and hypertension’ in Chinese using Google generates more than 500 000 results (e.g. the title of this website says ‘there is hidden sodium in MSG and soy sauce, overconsumption can cause hypertension’ http://health.people.com.cn/GB/10166719.html ). Nevertheless, after excluding those reporting diet change at baseline, the association between MSG intake and BP change remained significant. This may also suggest that MSG intake is not one of the main contributors of hypertension in the population. Despite this, the association between MSG intake and the increase in BP has public health significance especially for those who are taking hypertension medication.The strength of this study is its large sample size, long time frame of follow-up (5 years) and the ability to adjust for a variety of confounding factors including dietary patterns.The main limitation of the study is that the measurement of MSG consumption was determined by the total amount of MSG consumed in the household divided by the number of individuals per household and then adjusted for the proportion of the household energy intake energy. This method may underestimate or overestimate MSG intake, although in a large epidemiological study, this would be the most practical way of measuring MSG consumption. Other limitations include the inability to account for a change in MSG consumption during the 5-year follow-up period, which may also affect the BP change. However, using mean household MSG intake at follow-up as exposure variable, we found a similar pattern in the association between MSG and BP change. Nevertheless, the large variation in intake of MSG and the high mean intake provides the power to adequately examine the association between MSG and BP gain. Although we did not have measurements of urinary sodium excretion, the three-fold increase in prevalence of hypertension in China over the past 30 years has not been accompanied by a parallel increase in salt consumption [32].There was a relatively high attrition rate in the study. Loss to follow-up occurred mainly because of job migration in rural areas and construction in urban areas. In China, the number of ‘migrant rural workers' has risen to 229.78 million [33], which makes population cohort studies very difficult. In fact, among the 31 provinces that participated in the Chinese National Nutrition and Health Survey 2002, Jiangsu is the only province that performed a follow-up survey. It may affect the ability to generalize the findings, as the baseline association between MSG intake and BP differed between those followed-up and the general population. More research, especially in different regions and populations, are needed to confirm our findings.In conclusion, these findings from a Chinese sample from Jiangsu province indicate that MSG intake is positively associated with both SBP and DBP change over 5 years, even after adjustment for a number of covariates, including dietary patterns. There is significant sex and medication interaction related to SBP change. Among women and those taking hypertension medications over the 5 years, the positive association between MSG intake and increased DBP is highly significant. It suggests that MSG intake may need to be a consideration when making a choice of antihypertensive medication. When initiating treatment for hypertension or when reviewing therapy in the setting of persistently or recurrently elevated blood pressure, it would be prudent to review MSG intake. Whether there is an effect that is greater for any particular antihypertensive medication requires further study.AcknowledgementsThe authors thank the participating regional Centers for Disease Control and Prevention in Jiangsu province, including the Nanjing, Xuzhou, Jiangyin, Taicang, Suining, Jurong, Sihong, and Haimen centers for their support for data collection.The study is supported by Jiangsu Provincial Natural Science Foundation (BK2008464, Z.S., Principal Investigator) and the Jiangsu Provincial Health Bureau, China.There are no conflicts of interest.References1 Kwok RHM. Chinese-restaurant syndrome. N Engl J Med 1968; 278:796. 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[Context Link] blood pressure change; dietary glutamate; epidemiology; longitudinal study; nutritionovid.com:/bib/ovftdb/00004872-201105000-0000700006024_1968_278_796_kwok_restaurant_|00004872-201105000-00007#xpointer(id(R1-7))|11065213||ovftdb|SL00006024196827879611065213P66[CrossRef]10.1056%2FNEJM196804042781419ovid.com:/bib/ovftdb/00004872-201105000-0000700006024_1968_278_796_kwok_restaurant_|00004872-201105000-00007#xpointer(id(R1-7))|11065405||ovftdb|SL00006024196827879611065405P66[Medline Link]ovid.com:/bib/ovftdb/00004872-201105000-0000700002739_2009_39_640_williams_monosodium_|00004872-201105000-00007#xpointer(id(R3-7))|11065213||ovftdb|00002739-200905000-00005SL0000273920093964011065213P68[CrossRef]10.1111%2Fj.1365-2222.2009.03221.xovid.com:/bib/ovftdb/00004872-201105000-0000700002739_2009_39_640_williams_monosodium_|00004872-201105000-00007#xpointer(id(R3-7))|11065404||ovftdb|00002739-200905000-00005SL0000273920093964011065404P68[Full 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adultsShi, Zumin; Yuan, Baojun; Taylor, Anne W; Dai, Yue; Pan, Xiaoqun; Gill, Tiffany K; Wittert, Gary AOriginal papers: Epidemiology529