Study design and participants
The Bunkyo Health Study is a prospective cohort study designed to clarify how muscle mass, muscle strength, and insulin sensitivity are associated with multiple diseases that necessitate long-term care (20). In this study, we recruited elderly subjects aged 65–84 years living in Bunkyo-ku, an urban area in Tokyo, Japan. All subjects participated in examinations over two visits to the Sportology Center from October 15, 2015, to October 1, 2018. Briefly, we evaluated cognitive function by questionnaires, physical fitness by dynamometer and physical performance tests, brain lesions by MRI, body composition and bone mineral density by dual-energy X-ray absorptiometry (DXA), arteriosclerosis by the cardio-ankle vascular index (CAVI), and abdominal fat distribution by MRI. We also carried out a 75-g oral glucose tolerance test (OGTT). This study protocol was approved by the ethics committee of Juntendo University in November 2015 (Nos. 2015078, 2016138, 2016131, 2017121, and 2019085). This study is carried out in accordance with the principles outlined in the Declaration of Helsinki. All participants gave written informed consent at the orientation meeting. Participants were told that they had the right to withdraw from the trial at any time.
This study enrolled 1612 subjects who had undergone OGTT, muscle strength testing, and brain MRI at the beginning of the Bunkyo Health Study. Missing data were relatively infrequent for OGTT (0.3%), muscle strength (0.3%), and MRI (0.4%). Among the 1612 subjects, 81 with a previous stroke event were excluded; these subjects either had a previous confirmed clinical stroke (n = 52) or newly found non-lacunar brain infarcts by MRI in the Bunkyo Health Study (n = 29). Therefore, 1531 subjects were analyzed.
Evaluation of insulin sensitivity
Insulin sensitivity was estimated by PREDIM, a recently established index for insulin sensitivity (21). PREDIM was calculated from the Oral Glucose Insulin Sensitivity (OGIS) index (22) and other parameters, including body mass index (BMI), 2-h glucose during OGTT, and fasting insulin, to achieve good correlation with insulin sensitivity (M value) measured by hyperinsulinemic euglycemic clamp.
The OGIS is calculated using the following equation:
OGIS = 1/2×(B + square (B2 + 4×p5×p6×(Glu90-GluCLAMP) ×ClOGTT))
B=(p5×(Glu90-GluCLAMP) + 1) ×ClOGTT
ClOGTT=p4×((p1×DO-V×(Glu120-Glu90)/30)/Glu90 + p3/Glu0)/(Ins90-Ins0 + p2) (22)
*Glu0, Glu90, Glu120: glucose concentration values (mmol/L), Ins0, Ins90: insulin concentration values (pmol/L), DO: glucose dose of the OGTT (mmol/m2, i.e. normalized for body surface area), V: glucose distribution volume = 104 ml/kg, GluCLAMP: clamp glucose concentration = 5 mmol/L, p1 = 6.50, p2 = 1951, p3 = 4514, p4 = 792, p5 = 11.8×10− 3, p6 = 173.
PREDIM is calculated using the following equation:
loge(PREDIM) = A + B×loge(OGIS) + C×loge(BMI) + D×loge(Glu120) + E×loge(Ins0) (21)
*Glu120: glucose concentration values (mmol/L), Ins0: insulin concentration values (pmol/L), A = 2.8846219, B = 0.5208520, C=-0.8223363, D=-0.4191242, E=-0.2427896
The equivalence test in subgroups showed that the clamp-derived M value and PREDIM value were similar for comparison of subgroups based on glucose tolerance (normal glucose tolerance, impaired fasting glucose, impaired glucose tolerance, type 2 diabetes) and BMI (lean, overweight, obesity) (21).
Muscle strength measurement
We evaluated the isokinetic muscle strength of the knee extensors using a dynamometer (BIODEX system 3 or 4: Biodex Medical Systems, Upton, NY, USA) (17). Participants were stabilized in the examination chair with shoulder and abdominal straps. The isokinetic peak torques of the knee extensors were measured at an angular velocity of 60 degrees per second. During the test, participants were encouraged to exert maximal muscle force. The isokinetic peak torques of the knee extensors were adjusted by body weight according to the following formula: isokinetic peak torques (Nm) / body weight (kg) (17).
Evaluation of silent lacunar infarcts
The whole brain of each subject was scanned with a 0.3-T clinical MR scanner (AIRIS Vento, Hitachi, Tokyo, Japan). Sequences included axial three-dimensional (3D) time-of-flight magnetic resonance angiography (repetition time (TR), 35 ms; echo time (TE), 7.1 ms; and slice thickness, 1.2 mm), T2*-weighted gradient echo (T2*-WI) imaging (TR, 1000 ms; TE, 45 ms; flip angle, 20°; and slice thickness, 5 mm) and fluid-attenuated inversion recovery (FLAIR) imaging (TR, 11,000 ms; TE, 100 ms; inversion time (TI), 2000 ms; and slice thickness, 5 mm). The evaluation of lacunar infarcts was conducted by an experienced neuroradiologist based on axial T2*-WI and FLAIR images. The radiologist was blinded to all clinical data. Lacunar infarcts were defined as infarcts of 3–15 mm in diameter located in the deep cerebral white matter, basal ganglia, or pons, and that were presumed to result from the occlusion of a single, small, perforating artery supplying the subcortical area of the brain (23, 24).
Other measurements
BMI was calculated as weight in kilograms divided by the square of height in meters (kg/m2). Physical activity was evaluated using the International Physical Activity Questionnaire (IPAQ), which assesses different types of physical activity, such as walking and moderate- and high-intensity activities (25, 26). Hypertension was defined as systolic blood pressure (SBP) ≥ 140 mmHg and/or diastolic blood pressure (DBP) ≥ 90 mmHg or current treatment with antihypertensive medications. Diabetes was defined as a combination of fasting plasma glucose ≥ 126 mg/dL and/or a 2-hour glucose level ≥ 200 mg/dL after the 75-g OGTT, and hemoglobin A1c ≥ 6.5%, or currently treatment with medication for diabetes mellitus. Dyslipidemia was defined as low-density lipoprotein cholesterol (LDL-C) ≥ 140 mg/dL and/or high-density lipoprotein cholesterol (HDL-C) < 40 mg/dL and/or triglycerides (TG) ≥ 150 mg/dL, or current treatment with lipid-lowering agents. Cardiovascular disease was defined by a medical history of ischemic heart disease or heart failure.
Statistical analysis
Male and female participants were separately divided into three groups (High, Medium, and Low) based on the standard deviation (SD) of sex-specific values of PREDIM and divided three groups (High, Medium, and Low) based on the tertiles of sex and age (65–69, 70–74, 75–79, and 80–84 years)-specific value of muscle strength (Table 1). Then, participants of both sexes were categorized into nine groups according to the combination of these values. Data are presented as mean ± SD or number (%). Characteristics of the three insulin sensitivity groups were analyzed for trend by the Jonckheere–Terpstra test (continuous variables) and Cochran–Armitage trend test (categorical variables). All statistical tests were two-sided with a 5% significant level. Logistic regression analysis was used to estimate odds ratios (ORs) and 95% confidence intervals (CIs) for the associations between the prevalence of silent lacunar infarcts and insulin sensitivity, muscle strength, and their combination, with adjustment for age (continuous variable), sex (male or female), and other potential risk factors. In this study, three models were applied for regression analysis: model 1 was adjusted for age and sex; model 2 was adjusted for muscle strength (continuous variable) or insulin sensitivity (continuous variable), physical activity level (continuous variable), and cigarette smoking (never, past and current smoker), and incorporated model 1; and model 3 was adjusted for hypertension (yes or no), diabetes (yes or no), dyslipidemia (yes or no), and cardiovascular disease (yes or no), and incorporated model 2.
Table 1
Cut-off values for insulin sensitivity and muscle strength
|
Male
|
Female
|
High
|
Medium
|
Low
|
High
|
Medium
|
Low
|
A. Insulin sensitivity
|
|
|
|
|
|
|
65–84 years
|
≥ 8.29
|
8.29–3.90
|
≤ 3.90
|
≥ 9.34
|
9.34–4.47
|
≤ 4.47
|
B. Muscle strength (Nm/kg)
|
|
|
|
|
|
|
65–69 years
|
≥ 1.83
|
1.83–1.53
|
≤ 1.53
|
≥ 1.50
|
1.50–1.23
|
≤ 1.23
|
70–74 years
|
≥ 1.70
|
1.70–1.40
|
≤ 1.40
|
≥ 1.39
|
1.39–1.13
|
≤ 1.13
|
75–79 years
|
≥ 1.50
|
1.50–1.25
|
≤ 1.25
|
≥ 1.27
|
1.27–1.00
|
≤ 1.00
|
80–84 years
|
≥ 1.36
|
1.36–1.10
|
≤ 1.10
|
≥ 1.17
|
1.19–0.90
|
≤ 0.90
|
A. Categorized for three groups based on the standard deviation (SD) of sex (male/female)-specific value of PREDIM. |
B. Categorized for three groups based on the tertile of age (65–69, 70–74, 75–79, 80–84) and sex (male/female)-specific value of knee extensor muscle strength. |