2.1

Study participants

The Suita study is a population-based cohort study of cardiovascular disease; details of the study design have been described elsewhere . We recruited 2083 subjects to both medical check-ups and dental examinations between June 2008 and November 2013. Of these, 251 individuals were excluded due to with non-fasting blood collection (n = 109), incomplete masticatory performance data (n = 80) and incomplete periodontal data (n = 114). Finally, 1780 subjects (743 men, 1037 women; mean age, 66.5 ± 7.9 years) were included in the study. The study protocol was approved by the ethics committee of the National Cerebral and Cardiovascular Center (M19-62). Written, informed consent was obtained from all participants.

2.2

Medical examination

Well-trained nurses measured blood pressure twice in a seated position using an automated sphygmomanometer (Colin BP-I03ill; Omron, Kyoto, Japan) and an appropriately sized cuff according to a standard protocol after at least 5 min of rest before the initial blood pressure reading was obtained. Systolic blood pressure (SBP) and diastolic blood pressures (DBP) were considered as the average of 2 measurements recorded >1 min apart .

Routine blood tests were performed at baseline examination, including triglycerides, high-density lipoprotein (HDL) cholesterol, and fasting blood glucose. MetS was defined using the uniform definition proposed in the 2009 Joint Interim Statement . Each component of MetS was defined as follows: high blood pressure (SBP ≥130 mmHg and/or DBP ≥85 mmHg, and/or taking antihypertensive medication), low serum HDL cholesterol ( < 40 mg/dL in men, <50 mg/dL in women), hypertriglyceridemia (triglycerides ≥150 mg/dL, and/or taking antihyperlipidemia medication), high plasma glucose (fasting blood glucose ≥100 mg/dL, and/or undergoing diabetic therapy), and abdominal obesity as measured by waist circumference (waist circumference: ≥90 cm in men; ≥80 cm in women) in the Asia diagnostic criteria .

MetS was defined as the presence of any three or more of the five components.

2.3

Dental examination

2.3.1

Periodontal tissue examination

Periodontal status was assessed using the Community Periodontal Index (CPI) by means of records of 10 index teeth. Ten teeth were examined in total, comprising 8 designated molars (first and second molars) and 2 incisors (upper right and left central incisors), and when this test could not be performed because of the loss of one or both of the central incisors concerned, the same tooth on the opposite side was examined . No evaluation was performed when all relevant teeth were missing. Periodontal status was examined using a CPI probe (YDM, Tokyo, Japan) to evaluate each tooth with respect to six periodontal pockets according to the following criteria, and the highest-value code was recorded. CPI codes were as follows: Code 0, no sign of gingival inflammation; Code 1, evident bleeding after probing; Code 2, deposits of dental calculus (including those detected by probing up to 4 mm beneath the gingival margin); Code 3, periodontal pocket depth ≥4 mm, but <6 mm; and Code 4, periodontal pocket depth ≥6 mm. This examination was performed by five dentists who had undergone calibration in advance. Cohen’s κ value for the consistency between periodontal tissue examinations by the five dentists was 0.78. In our study, periodontitis was defined as CPI Code ≥3.

2.3.2

Masticatory performance

Masticatory performance was determined from the concentration of dissolved glucose obtained from test gummy jelly (20 × 20 × 10 mm, 5.5 g; UHA Mikakuto, Osaka, Japan), as a standardized food developed for measuring masticatory performance . Subjects were instructed to chew jelly using 30 chewing strokes on the preferred chewing side (right, left, or both sides), and to expectorate the bolus of comminuted particles as thoroughly as possible. The cotton gauze and comminuted particles were then rinsed under running water for 30 s in order to remove saliva and glucose adhering to their surfaces. The comminuted jelly alone was subsequently placed in a plastic container, and water (35 °C, 15 mL) was injected into this container, and the contents were agitated for 10 s with a magnetic stirrer (400 rpm, PC-410D Digital Stirrer; Corning, Tewksbury, MA). Immediately after this agitation, a small amount of supernatant was collected using forceps and placed in contact with the tip of a sensor fitted to a commercially available instrument for self-monitoring blood glucose (Glutest Every; Sanwa Chemical Laboratory, Nagoya, Japan), and the glucose concentration displayed after 15 s was recorded. The increase in the surface area of comminuted jelly was calculated from the glucose concentration using a regression formula (y = 15x − 250), and this was regarded as masticatory performance . Participants who wore removable prostheses kept their denture in place during measurements.

2.4

Lifestyle variables

Information on lifestyle was collected using normalized questionnaires by well-trained nurses through interviews, including drinking and smoking habits. Drinking status was divided into never drinker, former drinker, or current drinker and smoking status was divided into never smoker, former smoker, or current smoker, respectively.

2.5

Statistical analysis

We divided subjects into four groups by quartile of masticatory performance. First, we evaluated each component of MetS, smoking and drinking status, and the prevalence of periodontal disease in each group. Second, we expressed odds ratios (ORs) and 95% confidence intervals (95%CIs) for the prevalence of MetS among quartile groups by logistic regression analyses adjusted for sex, age, smoking and drinking status, and periodontal status as covariates in multivariate analysis. In addition, we divided subjects by age into four groups by quartile of masticatory performance of the all subjects and investigated the association between MetS and masticatory performance, with multivariate adjustments for sex, age, smoking and drinking status, and periodontal status. Values of P < 0.05 were considered significant for all comparisons. All analyses were performed using PASW Statistic 21 (SPSS; IBM, Tokyo, Japan).

2.1

Study participants

The Suita study is a population-based cohort study of cardiovascular disease; details of the study design have been described elsewhere . We recruited 2083 subjects to both medical check-ups and dental examinations between June 2008 and November 2013. Of these, 251 individuals were excluded due to with non-fasting blood collection (n = 109), incomplete masticatory performance data (n = 80) and incomplete periodontal data (n = 114). Finally, 1780 subjects (743 men, 1037 women; mean age, 66.5 ± 7.9 years) were included in the study. The study protocol was approved by the ethics committee of the National Cerebral and Cardiovascular Center (M19-62). Written, informed consent was obtained from all participants.

2.2

Medical examination

Well-trained nurses measured blood pressure twice in a seated position using an automated sphygmomanometer (Colin BP-I03ill; Omron, Kyoto, Japan) and an appropriately sized cuff according to a standard protocol after at least 5 min of rest before the initial blood pressure reading was obtained. Systolic blood pressure (SBP) and diastolic blood pressures (DBP) were considered as the average of 2 measurements recorded >1 min apart .

Routine blood tests were performed at baseline examination, including triglycerides, high-density lipoprotein (HDL) cholesterol, and fasting blood glucose. MetS was defined using the uniform definition proposed in the 2009 Joint Interim Statement . Each component of MetS was defined as follows: high blood pressure (SBP ≥130 mmHg and/or DBP ≥85 mmHg, and/or taking antihypertensive medication), low serum HDL cholesterol ( < 40 mg/dL in men, <50 mg/dL in women), hypertriglyceridemia (triglycerides ≥150 mg/dL, and/or taking antihyperlipidemia medication), high plasma glucose (fasting blood glucose ≥100 mg/dL, and/or undergoing diabetic therapy), and abdominal obesity as measured by waist circumference (waist circumference: ≥90 cm in men; ≥80 cm in women) in the Asia diagnostic criteria .

MetS was defined as the presence of any three or more of the five components.

2.3

Dental examination

2.3.1

Periodontal tissue examination

Periodontal status was assessed using the Community Periodontal Index (CPI) by means of records of 10 index teeth. Ten teeth were examined in total, comprising 8 designated molars (first and second molars) and 2 incisors (upper right and left central incisors), and when this test could not be performed because of the loss of one or both of the central incisors concerned, the same tooth on the opposite side was examined . No evaluation was performed when all relevant teeth were missing. Periodontal status was examined using a CPI probe (YDM, Tokyo, Japan) to evaluate each tooth with respect to six periodontal pockets according to the following criteria, and the highest-value code was recorded. CPI codes were as follows: Code 0, no sign of gingival inflammation; Code 1, evident bleeding after probing; Code 2, deposits of dental calculus (including those detected by probing up to 4 mm beneath the gingival margin); Code 3, periodontal pocket depth ≥4 mm, but <6 mm; and Code 4, periodontal pocket depth ≥6 mm. This examination was performed by five dentists who had undergone calibration in advance. Cohen’s κ value for the consistency between periodontal tissue examinations by the five dentists was 0.78. In our study, periodontitis was defined as CPI Code ≥3.

2.3.2

Masticatory performance

Masticatory performance was determined from the concentration of dissolved glucose obtained from test gummy jelly (20 × 20 × 10 mm, 5.5 g; UHA Mikakuto, Osaka, Japan), as a standardized food developed for measuring masticatory performance . Subjects were instructed to chew jelly using 30 chewing strokes on the preferred chewing side (right, left, or both sides), and to expectorate the bolus of comminuted particles as thoroughly as possible. The cotton gauze and comminuted particles were then rinsed under running water for 30 s in order to remove saliva and glucose adhering to their surfaces. The comminuted jelly alone was subsequently placed in a plastic container, and water (35 °C, 15 mL) was injected into this container, and the contents were agitated for 10 s with a magnetic stirrer (400 rpm, PC-410D Digital Stirrer; Corning, Tewksbury, MA). Immediately after this agitation, a small amount of supernatant was collected using forceps and placed in contact with the tip of a sensor fitted to a commercially available instrument for self-monitoring blood glucose (Glutest Every; Sanwa Chemical Laboratory, Nagoya, Japan), and the glucose concentration displayed after 15 s was recorded. The increase in the surface area of comminuted jelly was calculated from the glucose concentration using a regression formula (y = 15x − 250), and this was regarded as masticatory performance . Participants who wore removable prostheses kept their denture in place during measurements.

2.4

Lifestyle variables

Information on lifestyle was collected using normalized questionnaires by well-trained nurses through interviews, including drinking and smoking habits. Drinking status was divided into never drinker, former drinker, or current drinker and smoking status was divided into never smoker, former smoker, or current smoker, respectively.

2.5

Statistical analysis

We divided subjects into four groups by quartile of masticatory performance. First, we evaluated each component of MetS, smoking and drinking status, and the prevalence of periodontal disease in each group. Second, we expressed odds ratios (ORs) and 95% confidence intervals (95%CIs) for the prevalence of MetS among quartile groups by logistic regression analyses adjusted for sex, age, smoking and drinking status, and periodontal status as covariates in multivariate analysis. In addition, we divided subjects by age into four groups by quartile of masticatory performance of the all subjects and investigated the association between MetS and masticatory performance, with multivariate adjustments for sex, age, smoking and drinking status, and periodontal status. Values of P < 0.05 were considered significant for all comparisons. All analyses were performed using PASW Statistic 21 (SPSS; IBM, Tokyo, Japan).