Questionnaire surveys and 24-h dietary recalls

Questionnaire surveys, dietary assessments and physical examinations were conducted among all 2111 participants at a central survey site in their residential area. The questionnaire was administered face-to-face by trained interviewers to collect socio-demographic information including age, sex, area of residence, marital status, occupation, educational status, household income and lifestyle- and health-related information on smoking, alcohol consumption, physical activity, personal and family history of major chronic diseases, and history of medication.

Dietary intake was assessed in person by using unannounced 24-h dietary recalls on 3 consecutive days (2 weekdays and 1 weekend day). Participants were asked to recall everything they had eaten in the preceding 24 h, including quantities. The foods included staple foods, snacks, fruits, vegetables, etc. Measuring guides were used to enhance recall and facilitate the estimation of an average portion size. Intakes of condiments were assessed by Weighed Diet Records. The nutrient contents of the diet were analysed, and the daily dietary nutrient intakes, including manganese, were calculated on the basis of the China Food Composition (2nd ed.) provided by the Chinese Nutrition Society⁽ 25 ⁾, which included Mn contents in 1506 foods.

Anthropometric and laboratory measurements

Anthropometric measurements including height, weight, BMI and WC were obtained for each adult participant by trained field workers, following standard protocols. WC was measured to the nearest 0·1 cm at the midpoint between the top of the iliac crest and the lower margin of the last palpable rib in the mid-axillary line using a tape measure. After 5 min of rest in the sitting position, SBP and DBP (mmHg, an accuracy of 2 mmHg) were measured using a mercury sphygmomanometer (Yuwell-Jiangsu Yuyue Medical Equipment & Supply Co., Ltd.) in the right arm with an appropriately sized cuff, in accordance with the Guidelines for Prevention and Treatment of Hypertension in China (2005); three consecutive readings each separated by 5–10 min were taken, and the mean values of the three measures were used for the analysis.

Venous blood samples were collected and measured in the local hospitals; 6 ml of blood after fasting for 10–12 h was collected into a 2-ml vacuum, lithium heparin anticoagulant tube and a 4-ml vacuum tube, respectively. Whole blood samples were centrifuged at 1500 g for 10–15 min. The serum collected in the 2-ml tube was separated for TAG (mmol/l) and HDL-cholesterol (mmol/l) measurements by an automatic biochemical analyser, and 20-μl serum was separated from the 4-ml vacuum tube for FBG measurement. After the 6-ml fasting blood samples were collected, participants took 75 g glucose dissolved in 300 ml water orally within 3 min, and another blood sample was collected into a 2-ml vacuum tube 2 h later (±3 min). An additional 20 μl of serum was separated after centrifugation at 1500 g for 10–15 min for an oral glucose tolerance test. The remaining blood samples were stored at −70°C.

The serum levels of TAG were measured using the glycerol phosphate oxidase-phenol amino phenazone (GPO-PAP) method and a Pars Azmoon kit (Pars Azmoon Inc.). Serum HDL-cholesterol levels were tested using the sedimentary method with the same kit using the Liasys autoanalyser device (Liasys). FBG was measured by the glucose oxidase method using the same auto-analyser device.

Diagnostic criteria of the metabolic syndrome

The MetS was diagnosed according to the American Heart Association and the National Heart, Lung and Blood Institute scientific statement⁽ 26 ⁾; the criterion of WC was appropriate for Asians. Participants were diagnosed as having the MetS if they met at least three of the following criteria: (1) abdominal obesity, WC≥90 cm in men and WC≥80 cm in women; (2) hypertriacylglycerolaemia, TAG≥1·7 mmol/l (150 mg/dl) or on drug treatment for elevated TAG; (3) low HDL-cholesterol, HDL-cholesterol<1·03 mmol/l (40 mg/dl) in men and HDL-cholesterol<1·3 mmol/l (50 mg/dl) in women or on drug treatment for reduced HDL-cholesterol; (4) hypertension, SBP/DBP≥130/85 mmHg or on antihypertensive drug treatment in patients with a history of hypertension; and (5) hyperglycaemia, FBG≥5·56 mmol/l (100 mg/dl) or on drug treatment for elevated blood glucose.

Major food sources of manganese

Table 2 lists different food sources of Mn and contribution percentages in the total sample and in various sampling sites. Only foods contributing to ≥2 % of Mn intake are listed. Rice (>42 %) was the major food source of Mn for both men and women, regardless of MetS status in Zhejiang, China, followed by wheat flour (approximately 3 %) and noodles (approximately 3 %).

Daily dietary intake of manganese

According to the Chinese Nutrition Society⁽ 25 ⁾, the minimum safe level of Mn intake is 2 mg/d, the maximum safe level is 11 mg/d and the recommended intake level is 4·5 mg/d in China. The average daily intake of Mn in this study population (men: 6·07 (sd 2·94) mg/d; women: 5·13 (sd 2·65) mg/d) was higher than the recommended intake level but lower than the maximum safe level. Our results indicated that Mn intake was sufficient in this Chinese adult population. Previous studies have shown that the mean intake of Mn varied substantially from 1·38 to 11·0 mg/d across different countries⁽ 15 ^– 18 ^, 27 ^– 30 ⁾. In general, the mean Mn intake level among participants in our study was similar to those of other Asian populations (e.g. 5·0 mg/d for Japanese⁽ 27 ⁾; 4·18 mg/d for Koreans⁽ 28 ⁾) but higher than European populations (e.g. 2·6 mg/d for Belgian⁽ 11 ⁾; 2·37 mg/d for Spanish⁽ 30 ⁾). The different Mn intake levels may be explained by various dietary patterns in different countries and different Mn contents in various types of foods.

Major food sources of manganese in Chinese populations

Rice (>42 %) was the main food source of Mn in this study. Similar results were also observed in a study conducted among Korean children, which found that cereals (57·3 %) contributed the most to Mn intake⁽ 31 ⁾. This is because Mn can be accumulated in rice through incorporation into proteins with superoxide dismutase activity⁽ 32 ⁾, and Chinese dietary patterns, particularly those in southern China, typically have rice as the staple supplemented by wheat. Zhejiang Province is well known as a centre for rice production, and this may explain why an adequate intake level of Mn was found in this study and also why higher mean intake levels were found in our study population compared with European populations.

Associations between manganese intake and metabolic syndrome and its components

Our study found that a higher Mn intake was associated with a decreased risk of the MetS for men but an increased MetS risk for women. To our knowledge, only a few studies have examined the associations between Mn intake and the risk of the MetS and its components⁽ 19 ^, 22 ^, 23 ⁾. No association was observed in two cross-sectional studies conducted among Korean adults, using data from the Korea National Health and Nutrition Examination Survey⁽ 22 ^, 23 ⁾. The inverse association observed among men in our study was consistent with the results from another cross-sectional study conducted by Li et al.⁽ 19 ⁾ among 550 Chinese adults. In that study, participants in the highest quartile of Mn intake had a 53 % (95 % CI 21, 71 %) lower risk of the MetS than those in the lowest quartile⁽ 19 ⁾. However, sex difference was not further explored in the study by Li et al.⁽ 19 ⁾ because of the limited number of participants⁽ 19 ⁾. Differences in CVD or metabolic disease risk by sex have been suggested due to differences in oestrogen and androgen levels, HDL dysfunction and gene–environment interactions⁽ 33 ^, 34 ⁾. With a larger sample size, we were able to conduct a stratified analysis and found that the association between Mn intake and MetS risk was modified by sex. More research is needed to confirm the potential effect modification by sex observed in our study.

For MetS components, we found a significant inverse association between Mn intake and the risk of abdominal obesity and hypertriacylglycerolaemia for men, but a positive association with low HDL-cholesterol, and no association with hypertension and hyperglycaemia for both men and women. Our results differ from those obtained by Choi & Kim⁽ 22 ⁾, who found no association for obesity, TAG, glucose and HDL-cholesterol, but observed that participants with higher Mn intake had lower BP. However, the findings from our study are consistent with those of previous small intervention trials, in which Mn supplementation has been found to be important in reducing obesity and regulating blood lipid metabolism. A small clinical trial with fourteen adults reported that Mn gluconate supplementation of diets, combined with either calcium phosphate or calcium carbonate, increased faecal excretion of fat, thus decreasing body fat⁽ 35 ⁾. Another clinical trial found that a Mn-adequate diet of conventional foods could decrease plasma levels of cholesterol in seven men aged 19–22 years⁽ 36 ⁾. Evidence from animal studies also provides biological support for the potential role of Mn in MetS prevention. Mn supplement or intake could reduce abdominal fat accumulation by decreasing fatty acid synthase and malate dehydrogenase activities in the liver⁽ 37 ⁾ and glycerol in adipose tissue⁽ 38 ⁾, as well as decrease total cholesterol, LDL-cholesterol and HDL-cholesterol because Mn may be an essential component of the lipoprotein structure⁽ 13 ^, 39 ⁾. However, to our knowledge, no study has examined the different effects of Mn intake on the MetS and its components by sex. Given the biological evidence and the inconsistent results of the cross-sectional studies, more studies, particularly large cohort studies with long-time follow-up and full adjustment of confounders, are needed to further confirm the association between Mn intake and the risk of the MetS and its components, particularly for males and females.

The authors acknowledge the local governments that supported the survey. The authors also thank the survey participants and the interviewers from local Centres of Disease Control and Prevention who implemented the survey and conducted data entry.

The Chinese National Nutrition and Health Survey was supported by the China Ministry of Health Special Fund and the China Ministry of Science and Technology Key Research Fund. The sponsor had no role in the design, analysis or writing of this article.

G. D. was responsible for conception and design, acquisition of funding and overseeing the study, interpretation of data and manuscript editing. B. Z. and X. S. were in charge of the design, data collection, analysis and interpretation of data and manuscript writing. D. S., F. Z. and M. H. W. contributed to data analysis and interpretation. L. H., E. H., Y. Z., D. Z., D. H. and X. Z. contributed to fieldwork data collection and cleaning and manuscript drafting. E. Y. contributed to interpretation of the data and manuscript editing. R. Z. was in charge of study coordination and quality control, interpretation of the data and manuscript editing.

The authors declare no conflicts of interest.