Sarcopenia is associated with insomnia in Japanese older adults: a cross-sectional study of data from the Nagasaki Islands study

The NaIS is a prospective cohort study that has been performed since 2014 in Goto City in the Nagasaki prefecture, located on the western edge of Japan. The participants were recruited upon medical check-ups, and members of the general population aged 40 years or greater living in Goto City were targeted for enrollment. The recruitment process has been described elsewhere [15]. The present cross-sectional study was conducted in 2019 using data collected from May 2017 to June 2018, when the evaluation of sarcopenia was possible owing to the introduction of body composition analysis. Among the 2246 participants enrolled in the NaIS, the number of older adults aged 65 years or greater was 1637. A total of 45 participants were excluded from analysis in the current study. These included 40 participants whom body composition analysis was not performed, 4 whom grasp power was not performed, and 1 whom body composition analysis and grasp power were not performed. Consequently, a total of 1592 participants (575 men, 36.1%) were analyzed in the current study (Fig. 1). Approval for this study was granted by the Ethics Committee of Nagasaki University Graduate School of Biomedical Sciences (approval no. 14051404–8). Written informed consent was obtained from all participants.

Trained interviewers obtained information on the participants’ demographic and clinical characteristics, including sex, smoking status, drinking status, comorbidities, and care-related problems. Habitual drinkers were defined as people with a frequency of drinking ≥2 days per week and an alcohol intake ≥20 g of ethanol per day. Care-related problems were obtained in the following areas: staying at home all day, no outings, no hobbies, weakening of neighbor relations, weakening of human relations, falls, no long-distance walking, visual disturbance, stumbling, fear of falling, recent hospitalization, appetite loss, chewing difficulty, weight reduction, and muscle and fat wasting.

The symptoms of insomnia, including difficulty initiating sleep (DIS) and difficulty maintaining sleep (DMS), were assessed. Participants who had any of the symptoms were defined as having difficulty initiating and/or maintaining sleep (DIMS). Sleep duration was calculated from light-out time and get-up time.

Psychological distress was assessed using the Japanese version of the 6-item Kessler Psychological distress scale (K6). The score of each item ranges from 0 to 4, with global scores ranging from 0 to 24. The cut-off point for anxiety/emotional disorders screening was estimated at 4/5 with a sensitivity of 76–100% and specificity of 69–80% [16, 17].

A tendency toward malnutrition was defined as a body mass index (BMI) ≤ 20 kg/m², for which a higher risk of mortality has been statistically observed in Japanese older adults [18]. Blood pressure at rest was measured using the cuff oscillometric method with a PASERA AVE-1500 (Shisei Datum, Tokyo, Japan). Individuals were considered as having hypertension if they had a systolic blood pressure of ≥140 mmHg and/or diastolic blood pressure of ≥90 mmHg, if they took blood pressure-lowering medications. Individuals were considered as having diabetes if their hemoglobin was A1c ≥ 6.5% or if they took blood glucose lowering medications. Individuals were considered as having dyslipidemia if their HDL-cholesterol was < 40 mg/dl or of if they took cholesterol- and/or triglyceride-lowering medications. The severity of chronic kidney disease was categorized into 5 categories based on the estimated glomerular filtration rate: G1–5 (G1 ≥ 90, G2 60–89, G3 30–59, G4 15–29, G5 < 15 mL/min/1.73m²) [19].

Measurement of handgrip strength was performed twice in each hand and mean strength was calculated. Skeletal muscle mass (SMM) was measured by using bioelectrical impedance analysis with an InBody 770 (InBody Japan, Tokyo, Japan). Skeletal muscle mass index (SMI) was calculated as follows: SMI = SMM/height² (kg/m²) [20]. Sarcopenia was defined using the cut-off points of handgrip strength and SMI, but not gait speed, based on the criteria of the Asian Working Group for Sarcopenia (AWGS) as having both lower handgrip strength and lower SMI: handgrip strength (men < 26 kg; women < 18 kg) and SMI (men ≤7 kg/m²; women ≤5.7 kg/m²) [21].

R version 3.5.2 (https://​www.​r-project.​org/​) and EZR version 1.40 (http://​www.​jichi.​ac.​jp/​saitama-sct/​SaitamaHP.​files/​statmed.​html) were used for statistical analysis [22]. Continuous variables were presented as medians and interquartile ranges (IQR) where non-normally distributed. Comparisons of continuous variables were performed using Mann–Whitney U test. Categorical variables were presented as counts and percentages. Frequency analyses for categorical data were performed using Fisher’s exact test. The two-sided alpha level was set at 0.05.

Odds ratios (ORs) for sarcopenia were calculated via logistic regression analysis with sarcopenia as a dependent variable. The following were used as independent variables: age, sex, BMI, IHD, DIMS, sleep duration, and care related problems, including appetite loss, no hobbies, recent hospitalization, weakening of neighbor relations, visual disturbance, and without long-distance walking. Age was categorized based on the following quartiles: ≤ 68 years (reference group; ref), 69–73 years, 74–79 years, and ≥ 80 years. BMI was categorized into three groups: ≤ 20 kg/m², 20–25 kg/m² (ref), and > 25 kg/m². Smoking was categorized into three groups: never (ref), current smoker, and ex-smoker. Sleep duration was categorized into 5 groups: < 6 h, 6–7 h, 7–8 h (ref), 8–9 h, and ≥ 9 h. The global score of K6 was categorized into two groups: < 5 (ref) and ≥ 5 (psychological distress group).

Sarcopenia and insomnia are prevalent with advancing age. Therefore, subgroup analysis was performed based on the following tertiles of age: 65–70 years, 71–78 years, and 79–98 years. In the second set logistic regression analyses for sarcopenia stratified by tertiles of age, the independent variables were sex, BMI, and DIMS.

The present study possesses some limitations of note. Gait speed was not assessed in the present study. The evaluation of physical performance is an important factor to diagnose sarcopenia. Low gait speed and/or low handgrip strength were included in AWGS 2014 criteria. Therefore, participants whose gait speed and muscle mass were low, but handgrip strength was within normal range, were classified as the “Non-sarcopenia” group. In AWGS 2019 criteria, either calf circumference (CC) or the SARC-F or SARC-CalF questionnaires is recommended for case-finding, and either 5-time chair stand test or 6-m walk or short physical performance battery is recommended for assessment of physical performance [26]. In the present study, these items were not assessed. Further studies are required based on the AWGS 2019 criteria.

In the present data collection using questionnaires by interviewers, actual total sleep duration could not be confirmed; thus, an objective evaluation using polysomnography is needed to measure total sleep time accurately. Additionally, the severity of insomnia symptoms was not considered. Sleep/wake disorders related to insomnia, including chronic insomnia disorder, sleep disordered breathing, circadian rhythm sleep/wake disorders, restless legs syndrome/Willis-Ekbom disease, and rapid eye movement sleep behavior disorder were not examined systematically.

Finally, although the relationship between sarcopenia and DIMS was observed, it remains unknown whether insomnia induces sarcopenia in the present cross-sectional study. To confirm the effects of insomnia on developing sarcopenia, it is necessary to conduct large prospective cohort studies and longitudinal studies targeting the patients with insomnia.