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Abstract Background Loneliness is associated with poor functional ability in older people. Little is known about this association in the middle-aged. The aim is to investigate if perceived loneliness is associated with lower physical capability among middle-aged men and women and if the associations of loneliness with physical capability interact with socioeconomic position and cohabitation status. Methods 5224 participants from Copenhagen Aging and Midlife Biobank (CAMB) aged 49–62 years (mean age 54) were included. Handgrip strength (measured by a dynamometer) and maximal number of chair rises in 30 s was recorded. Multivariate linear regression analyses were adjusted for age, occupational social class, cohabitation status, morbidity and personality traits. Results No association was found between loneliness and physical capability. For example estimates for handgrip strength in ‘often’ lonely men and women compared with the ‘not lonely’ were 1.2 kg (95% CI − 0.5;2.9)/1.0 kg (−0.7;2.6). Low occupational social class was associated with poorer physical capability, and living alone was associated with poorer handgrip strength in men [−2.4 kg (95% CI − 3.2;−1.5)] and poorer chair rise test in women [−0.8 rises (95% CI − 1.6;−0.1)]. There was no support for interactions. Conclusion In contrast to earlier studies among older people, no association between loneliness and physical capability was found in this cohort of middle-aged men and women. Loneliness may not yet have resulted in detectable differences in physical capability in this age group. Further research is needed to clarify if, and at what point in the life course loneliness begins to affect physical capability. Introduction Loneliness has been described as “the social equivalent of physical pain, hunger and thirst”1 and can be described as a feeling of being socially isolated, which is often accompanied by a perceived deficiency in quantity and quality of social relations.1 Loneliness has been associated with adverse health outcomes, including mortality, depression, cardiovascular disease, metabolic syndrome.2,3 Loneliness has also been associated with functional decline in older people,4,5 but little is known about this association in middle-aged adults. In the Northern European countries the prevalence of loneliness in the 60+ years old is around 6% which is lower compared with Southern (11%) and Eastern European countries (20%).6 The prevalence of loneliness furthermore varies with age,7 gender,7 socioeconomic position,7–9 living arrangements,7 health10 and personality.1 Suggested pathways through which loneliness accelerates physical ageing and contributes to poor health11 include: higher total peripheral resistance and blood pressure (1), higher concentration of epinephrine,11 dysregulation of the inflammatory and neuroendocrine responses,12 poorer self-reported sleep quality11 A clear social gradient in physical capability among middle-aged adults has been suggested,13 and furthermore that loneliness is associated with lower socioeconomic position.8,9 According to the concept of differential vulnerability the effect of a given risk factor (here loneliness) is intensified along the social strata.14 Similarly, living alone has been associated with poorer physical capability especially among older males15 and higher levels of loneliness are experienced by those living alone.16 The aim of this study is to investigate (1) if perceived loneliness is related to lower levels of physical capability evaluated using objective measures, including handgrip strength and number of chair rises, in a middle-aged population, and (2) if the associations of loneliness on physical capability are different in respondents with a high or low socioeconomic position and for those living alone versus those not living alone. Methods Study population The Copenhagen Aging and Midlife Biobank (CAMB) is based on three longitudinal studies: the Metropolit 1953 Danish Male Birth Cohort (MP), The Copenhagen Perinatal Cohort (CPC) born 1959–61 and The Danish Longitudinal Study on Work, Unemployment and Health (DALWUH) born 1949 or 1959. In total, 5575 persons answered the questionnaire and participated in the clinical examination (response rate 30%). Compared with non-respondents, CAMB participants were more educated and were less likely to be unemployed and experienced significantly higher all-cause mortality.17 Exclusion criteria Participants with missing information on loneliness, included covariates, job information for social class classification were excluded as were participants reporting to be a student or a homemaker (0.5% of the participants). There were no specific exclusion criteria for the handgrip test, but participants with systolic >160 and/or diastolic blood pressure >100 were not asked to complete the chair rise test.13 In total, 351 (6%) were excluded due to missing values in the analyses on hand grip strength (N = 5224) and 833 (15%) in the analysis on chair rise test (N = 4742). See figure 1 in Supplementary files for a flow chart of the study population. Loneliness, occupational social class and cohabitation status Loneliness (unwanted aloneness) was measured by a single item: “Are you ever alone, when you would much rather be with others?” Response categories were “Yes, often”,” Yes, sometimes”,” Yes, but seldom” and” No”, where “No” was used as the reference. For the analyses investigating the potential differential vulnerability, loneliness was dichotomized into either lonely (‘often’ or ‘sometimes lonely’) or not lonely (‘seldom’ or ‘not lonely’). Occupational social class was classified based on occupation in accordance with the Danish Occupational Social Class Measurement into occupational social class I–VI.18 Occupational social class I–V represent employed individuals ranging from professional occupation in social class I, to unskilled occupation in social class V. Social class VI represents people on transfer income, including unwaged persons and those receiving social security benefits or sickness benefits. Occupational social class was also dichotomized into high (social class I, II and III) and low (IV, V and VI). Loneliness and occupational social class/cohabitation status were combined into two joint variables with the following four levels; “lonely/low social class or living alone”, “lonely/high social class or not living alone”, “not lonely/low social class or living alone” and “not lonely/high social class or not living alone” (reference group). Physical capability Handgrip strength was measured with a dynamometer (model G100, Biometrics Ltd, Newport, UK) with the best maximal handgrip strength in kg recorded out of three to five attempts.13 During a 30-s chair rise test, the maximal number of chair rises was recorded (chair with a seat height of 45 cm and the seat wired to a computer providing information on mechanical contact).13 Covariates Potential confounders were identified based on prior knowledge and the method of directed acyclic graphs (see Supplementary file).19 Covariates included age, gender, occupational social class, cohabitation status, number of chronic physical and mental disorders and personality as they have been suggested as potential causes of both loneliness1,7–10 and physical capability/physical function.13,20 Loneliness is associated with high neuroticism and low conscientiousness and agreeableness,1 and both conscientiousness and neuroticism are associated with physical functioning and physical activity.21 Depressive symptoms can act both as a mediator and a potential confounder and was therefore included in a separate model.16 Body mass index (BMI), alcohol intake, smoking habits, physical activity were all identified as potential mediators of the relation between loneliness and physical functioning and they were added in a separate model. Gender and age were assessed from social security numbers given to all Danish citizens. The CAMB participants were asked to declare the following medical conditions: (1) allergies, (2) diabetes, (3) hypertension, (4) myocardial infarction, (5) stroke, (6) lung disease (incl. asthma), chronic bronchitis, emphysema, (7) autoimmune disease and arthritis (8) cancer incl. leukaemia and (9) other mental disorder than depression. These conditions were compiled to a morbidity index counting the number of medical conditions reported (0–9). Personality was assessed with the short Danish version of the NEO Five Factor Inventory.21,22 Depressive symptoms were assessed using the Major Depression Inventory with a score ranging from 0 to 50.23 For detailed description of measurement of BMI, alcohol intake, smoking habits, physical activity please see Supplementary file. Statistical methods The associations between loneliness and handgrip strength/number of chair rises were analysed with multiple linear regression, PROC GLM, using SAS 9.3. Data were stratified by gender because the chair rise test was non-normally distributed in the combined data. Plots of the residuals for the association between hand grip strength and chair rise, respectively, as well as loneliness were judged symmetric. Potential interaction (differential vulnerability) was assessed by analysing the association between joint variables of loneliness and cohabitation status/occupational social class, respectively, in regards to physical capability, as well as by the inclusion of product terms between loneliness and occupational social class and cohabitation status, respectively, in the regression analyses. Initially, crude multivariate regression models were run followed by a model adjusted for the identified confounders (Model 1). In Model 2, depressive symptoms at baseline were added, and in Model 3 health behaviour variables were added. In the joint effect analyses, the main effects of each of the joint effect variables (loneliness, occupational social class and cohabitation status) were not included. Weights were not used in this study as this method builds on the understanding that those who do in fact participate (given their measured variables) are representative of those who do not. This is difficult to validate and hence does not solve the problem of selection bias. Results Population demographics Sixteen per cent of the population reported ‘often’ or ‘sometimes’ feeling lonely, 3% often felt lonely. Approximately one-third of the population was men (n = 3596 men and n = 1628 women), with no gender differences in loneliness. Age ranged from 48 to 62 years, and mean age did not differ between the four categories of loneliness among women but differed slightly for men (in the ‘often lonely’ mean age was 1 year lower). The association between loneliness and each of the covariates showed a dose-dependent pattern except for alcohol intake. More men and women who often felt lonely, were in the lowest occupational social class group VI compared with the other groups. Fifteen per cent of both men and women were living alone. Among men in the often lonely group 61% were living alone, for often lonely women 34% was living alone. Respondents scoring high on loneliness were in general more likely to suffer from three or more chronic diseases, to be depressed, obese or being smokers, to be physical active less than 3–4 h/week (stronger for men) and to be either non-drinkers or heavy drinkers (men only). The often lonely scored higher on neuroticism and lower on conscientiousness (tables 1 and 2). Table 1 Men: distribution of demographic, personality, health and health behaviour variables by loneliness (N = 3596) Loneliness Yes, often Yes, sometimes Yes, seldom No P-value N 110 (3%) 460 (13%) 1095 (31%) 1931 (54%) Age, mean (years) 54 (SD 3.5) 55 (SD 3.2) 55 (SD 3.3) 55 (SD 3.2) 0.0083 Cohabitation status <0.0001 Alone 67 (61%) 174 (38%) 193 (18%) 91 (5%) Not alone 43 (39%) 286 (62%) 902 (82%) 1840 (95%) Socioeconomic position <0.0001 Social class I 12 (11%) 71 (15%) 204 (19%) 385 (20%) Social class II 15 (14%) 89 (19%) 255 (23%) 549 (28%) Social class III 23 (21%) 98 (21%) 244 (22%) 466 (24%) Social class IV 15 (14%) 72 (16%) 178 (16%) 269 (14%) Social class V 6 (6%) 51 (11%) 90 (8%) 166 (9%) Social class VI 39 (36%) 79 (17%) 124 (11%) 96 (5%) Number of chronic illnessesa <0.0001 0 29 (26%) 252 (31%) 411 (38%) 763 (40%) 1–2 59 (54%) 252 (55%) 557 (51%) 998 (52%) ≥3 22 (20%) 67 (15%) 127 (11%) 170 (9%) Depressionb <0.0001 Yes 27 (25%) 34 (7%) 36 (3%) 27 (1%) No 83 (75%) 426 (93%) 1059 (97%) 1904 (99%) Personality Neuroticism Median 25 Median 19 Median 17 Median 15 <0.0001 Q1c 17 Q1 15 Q1 13 Q1 11 Q3c 30 Q3 25 Q3 22 Q3 18 Agreeableness Median 32 Median 32 Median 32 Median 33 <0.0001 Q1 28 Q1 29 Q1 29 Q1 30 Q3 35 Q3 35 Q3 36 Q3 37 Conscientiousness Median 31 Median 32 Median 33 Median 35 <0.0001 Q1 25 Q1 28 Q1 30 Q1 31 Q3 35 Q3 35 Q3 37 Q3 38 BMI 0.0022 Underweight (<18.5) 2 (2%) 2 (1%) 6 (1%) 5 (1%) Normal (18.5–25) 41 (37%) 183 (40%) 391 (36%) 727 (38%) Overweight (25–30) 38 (35%) 204 (44%) 513 (47%) 929 (48%) Obese (>30) 29 (26%) 71 (15%) 185 (17%) 270 (14%) Smoking status <0.0001 Smoker 42 (38%) 138 (30%) 256 (23%) 400 (21%) Non-smoker 68 (62%) 322 (70%) 839 (77%) 1531 (79%) Alcohol <0.0001 Not drinking 19 (17%) 54 (12%) 90 (8%) 141 (7%) 0 units/week 50 (45%) 223 (48%) 562 (51%) 1109 (57%) Low risk drinking 12 (11%) 75 (16%) 211 (19%) 313 (16%) 14 units/weekd 29 (26%) 108 (24%) 232 (21%) 368 (19%) Elevated risk drinking 15–21 units/weekd High risk drinking >21 units/weekd Physical activitye 0.0312 ≤3–4 h/week 48 (44%) 177 (38%) 366 (33%) 645 (33%) ≥5 h/week 62 (56%) 283 (62%) 729 (67%) 1286 (67%) Loneliness Yes, often Yes, sometimes Yes, seldom No P-value N 110 (3%) 460 (13%) 1095 (31%) 1931 (54%) Age, mean (years) 54 (SD 3.5) 55 (SD 3.2) 55 (SD 3.3) 55 (SD 3.2) 0.0083 Cohabitation status <0.0001 Alone 67 (61%) 174 (38%) 193 (18%) 91 (5%) Not alone 43 (39%) 286 (62%) 902 (82%) 1840 (95%) Socioeconomic position <0.0001 Social class I 12 (11%) 71 (15%) 204 (19%) 385 (20%) Social class II 15 (14%) 89 (19%) 255 (23%) 549 (28%) Social class III 23 (21%) 98 (21%) 244 (22%) 466 (24%) Social class IV 15 (14%) 72 (16%) 178 (16%) 269 (14%) Social class V 6 (6%) 51 (11%) 90 (8%) 166 (9%) Social class VI 39 (36%) 79 (17%) 124 (11%) 96 (5%) Number of chronic illnessesa <0.0001 0 29 (26%) 252 (31%) 411 (38%) 763 (40%) 1–2 59 (54%) 252 (55%) 557 (51%) 998 (52%) ≥3 22 (20%) 67 (15%) 127 (11%) 170 (9%) Depressionb <0.0001 Yes 27 (25%) 34 (7%) 36 (3%) 27 (1%) No 83 (75%) 426 (93%) 1059 (97%) 1904 (99%) Personality Neuroticism Median 25 Median 19 Median 17 Median 15 <0.0001 Q1c 17 Q1 15 Q1 13 Q1 11 Q3c 30 Q3 25 Q3 22 Q3 18 Agreeableness Median 32 Median 32 Median 32 Median 33 <0.0001 Q1 28 Q1 29 Q1 29 Q1 30 Q3 35 Q3 35 Q3 36 Q3 37 Conscientiousness Median 31 Median 32 Median 33 Median 35 <0.0001 Q1 25 Q1 28 Q1 30 Q1 31 Q3 35 Q3 35 Q3 37 Q3 38 BMI 0.0022 Underweight (<18.5) 2 (2%) 2 (1%) 6 (1%) 5 (1%) Normal (18.5–25) 41 (37%) 183 (40%) 391 (36%) 727 (38%) Overweight (25–30) 38 (35%) 204 (44%) 513 (47%) 929 (48%) Obese (>30) 29 (26%) 71 (15%) 185 (17%) 270 (14%) Smoking status <0.0001 Smoker 42 (38%) 138 (30%) 256 (23%) 400 (21%) Non-smoker 68 (62%) 322 (70%) 839 (77%) 1531 (79%) Alcohol <0.0001 Not drinking 19 (17%) 54 (12%) 90 (8%) 141 (7%) 0 units/week 50 (45%) 223 (48%) 562 (51%) 1109 (57%) Low risk drinking 12 (11%) 75 (16%) 211 (19%) 313 (16%) 14 units/weekd 29 (26%) 108 (24%) 232 (21%) 368 (19%) Elevated risk drinking 15–21 units/weekd High risk drinking >21 units/weekd Physical activitye 0.0312 ≤3–4 h/week 48 (44%) 177 (38%) 366 (33%) 645 (33%) ≥5 h/week 62 (56%) 283 (62%) 729 (67%) 1286 (67%) a Number of chronic illnesses includes: (1) Allergies, (2) Diabetes, (3) Hypertension, (4) Myocardial infarction, (5) Stroke, (6) Lung disease (incl. asthma, chronic bronchitis, emphysema) , (7) Autoimmune disease and arthritis, (8) Cancer incl. leukaemia and, (9) present other mental disorder. b Depression according to Major Depression Inventory-scale (41;42) ‘Yes’ includes moderate and severe depression and ‘no’ includes no and mild depression. c Q1: lower quartile, Q3: upper quartile. d Low-risk recommendations 1–14 units/week for men. Elevated risk drinking 15–21 units/week for men. High-risk drinking >21 for men. e Physical activity includes sports, physical training, house- and garden work, walks and bicycle rides incl. between home and work. Table 2 Women: distribution of demographic, personality, health and health behaviour variables by loneliness (N = 1628) Loneliness Yes, often Yes, sometimes Yes, seldom No P-value N 44 (3%) 232 (14%) 509 (31%) 843 (52%) Age, mean (years) 53 (SD 4.6) 52 (SD 4.1) 53 (SD 4.2) 53 (SD 4.4) 0.77 Cohabitation status <0.0001 Alone 15 (34%) 86 (37%) 96 (19%) 63 (7%) Not alone 29 (66%) 146 (63%) 413 (81%) 780 (93%) Socio-economic position <0.0001 Social class I 1 (2%) 13 (6%) 53 (10%) 95 (11%) Social class II 8 (18%) 64 (28%) 152 (30%) 240 (28%) Social class III 4 (9%) 49 (21%) 110 (22%) 234 (28%) Social class IV 6 (14%) 45 (19%) 109 (21%) 170 (20%) Social class V 4 (9%) 18 (8%) 41 (8%) 68 (8%) Social class VI 21 (48%) 43 (19%) 44 (9%) 36 (4%) Number of chronic illnessesa <0.0001 0 7 (16%) 85 (37%) 192 (38%) 332 (39%) 1–2 14 (57%) 107 (46%) 270 (53%) 462 (55%) ≥3 12 (27%) 40 (17%) 47 (9%) 49 (6%) Depressionb <0.0001 Yes 18 (41%) 30 (13%) 18 (4%) 17 (2%) No 26 (59%) 202 (87%) 491 (96%) 826 (98%) Personality Neuroticism Median 28 Median 23 Median 19 Median 17 <0.0001 Q1c 22 Q1 18 Q1 15 Q1 13 Q3c 31.5 Q3 27.5 Q3 24 Q3 21 Agreeableness Median 35 Median 35 Median 35 Median 35 0.08 Q1 30 Q1 31 Q1 32 Q1 32 Q3 39 Q3 38 Q3 38 Q3 39 Conscientiousness Median 30 Median 32 Median 34 Median 35 <0.0001 Q1 26 Q1 28 Q1 30 Q1 32 Q3 35 Q3 36 Q3 37 Q3 38 BMI 0.20 Underweight (<18.5) 1 (2%) 6 (3%) 8 (2%) 23 (3%) Normal (18.5–25) 18 (41%) 119 (51%) 292 (57%) 440 (52%) Overweight (25–30) 15 (34%) 71 (31%) 145 (28%) 278 (33%) Obese (>30) 10 (23%) 36 (15%) 64 (13%) 102 (12%) Smoking status 0.01 Smoker 17 (39%) 60 (26%) 116 (23%) 168 (20%) Non-smoker 27 (61%) 172 (74%) 393 (77%) 675 (80%) Alcohol <0.0001 Not drinking 21 (48%) 47 (20%) 81 (16%) 137 (16%) 0 units/week 11 (25%) 104 (45%) 231 (45%) 374 (44%) Low-risk drinking 7 (16%) 53 (23%) 142 (28%) 219 (26%) 7 units/weekd 5 (11%) 28 (12%) 55 (11%) 113 (13%) Elevated risk drinking 8–14 units/weekd High-risk drinking >14 units/weekd Physical activitye 0.1116 ≤3–4 h/week 13 (30%) 69 (30%) 113 (22%) 225 (27%) ≥5 h/week 31 (70%) 163 (70%) 396 (78%) 618 (73%) Loneliness Yes, often Yes, sometimes Yes, seldom No P-value N 44 (3%) 232 (14%) 509 (31%) 843 (52%) Age, mean (years) 53 (SD 4.6) 52 (SD 4.1) 53 (SD 4.2) 53 (SD 4.4) 0.77 Cohabitation status <0.0001 Alone 15 (34%) 86 (37%) 96 (19%) 63 (7%) Not alone 29 (66%) 146 (63%) 413 (81%) 780 (93%) Socio-economic position <0.0001 Social class I 1 (2%) 13 (6%) 53 (10%) 95 (11%) Social class II 8 (18%) 64 (28%) 152 (30%) 240 (28%) Social class III 4 (9%) 49 (21%) 110 (22%) 234 (28%) Social class IV 6 (14%) 45 (19%) 109 (21%) 170 (20%) Social class V 4 (9%) 18 (8%) 41 (8%) 68 (8%) Social class VI 21 (48%) 43 (19%) 44 (9%) 36 (4%) Number of chronic illnessesa <0.0001 0 7 (16%) 85 (37%) 192 (38%) 332 (39%) 1–2 14 (57%) 107 (46%) 270 (53%) 462 (55%) ≥3 12 (27%) 40 (17%) 47 (9%) 49 (6%) Depressionb <0.0001 Yes 18 (41%) 30 (13%) 18 (4%) 17 (2%) No 26 (59%) 202 (87%) 491 (96%) 826 (98%) Personality Neuroticism Median 28 Median 23 Median 19 Median 17 <0.0001 Q1c 22 Q1 18 Q1 15 Q1 13 Q3c 31.5 Q3 27.5 Q3 24 Q3 21 Agreeableness Median 35 Median 35 Median 35 Median 35 0.08 Q1 30 Q1 31 Q1 32 Q1 32 Q3 39 Q3 38 Q3 38 Q3 39 Conscientiousness Median 30 Median 32 Median 34 Median 35 <0.0001 Q1 26 Q1 28 Q1 30 Q1 32 Q3 35 Q3 36 Q3 37 Q3 38 BMI 0.20 Underweight (<18.5) 1 (2%) 6 (3%) 8 (2%) 23 (3%) Normal (18.5–25) 18 (41%) 119 (51%) 292 (57%) 440 (52%) Overweight (25–30) 15 (34%) 71 (31%) 145 (28%) 278 (33%) Obese (>30) 10 (23%) 36 (15%) 64 (13%) 102 (12%) Smoking status 0.01 Smoker 17 (39%) 60 (26%) 116 (23%) 168 (20%) Non-smoker 27 (61%) 172 (74%) 393 (77%) 675 (80%) Alcohol <0.0001 Not drinking 21 (48%) 47 (20%) 81 (16%) 137 (16%) 0 units/week 11 (25%) 104 (45%) 231 (45%) 374 (44%) Low-risk drinking 7 (16%) 53 (23%) 142 (28%) 219 (26%) 7 units/weekd 5 (11%) 28 (12%) 55 (11%) 113 (13%) Elevated risk drinking 8–14 units/weekd High-risk drinking >14 units/weekd Physical activitye 0.1116 ≤3–4 h/week 13 (30%) 69 (30%) 113 (22%) 225 (27%) ≥5 h/week 31 (70%) 163 (70%) 396 (78%) 618 (73%) a Number of chronic illnesses includes: (1) Allergies, (2) Diabetes, (3) Hypertension, (4) Myocardial infarction, (5) Stroke, (6) Lung disease (incl. asthma, chronic bronchitis, emphysema), (7) Autoimmune disease and arthritis, (8) Cancer incl. leukaemia and (9) present other mental disorder. b Depression according to Major Depression Inventory-scale (41;42); ‘yes’ includes moderate and severe depression and ‘no’ includes no and mild depression. c Q1: lower quartile, Q3: upper quartile. d Low-risk recommendations 1–7 units/week or women. Elevated risk drinking 8–14 units/week for women. High-risk drinking >14 for women. e Physical activity includes sports, physical training, house- and garden work, walks and bicycle rides incl. between home and work. Loneliness and physical capability In men (crude models), loneliness was associated with lower handgrip strength [‘Often lonely’ −1.3 kg (95% CI −2.9;0.3) ‘Sometimes lonely’ −2.0 (−2.8;−1.1) reference group ‘Not lonely’] and for both genders with fewer chair raises [‘Often lonely’ women −3.3 rises/30 s (95% CI −5.5; −1.5, ‘Often lonely’ men −0.8(−2.0;0.4) ‘Sometimes lonely’ men −1.1 (−1.7;−0.5)] (table 3). After adjustment for confounders (Model 1), the observed associations attenuated. In women, there were no clear associations between loneliness and handgrip strength in any of the models (table 3). Adjusting for depressive symptoms in Model 2 and BMI, smoking status, alcohol consumption and physical activity in Model 3, did not change these conclusions (see Supplementary file). Table 3 Multivariate linear regression analysis of loneliness and handgrip strength/chair rise test, crude and adjusted models Handgrip strength Chair rise test Loneliness N Mean (kg) Estimate (kg) 95% CI Loneliness N Mean (rises/30 s) Estimate (rises/30 s) 95% CI Men Crude model Often lonely 110 48.9 −1.3 (−2.9;0.3) Often lonely 89 21.1 −0.8 (−2.0;0.4) Sometimes lonely 460 48.2 −2.0 (−2.8;−1.1) Sometimes lonely 403 20.7 −1.1 (−1.7;−0.5) Seldom lonely 1095 49.2 −0.9 Seldom lonely 986 21.5 −0.4 (−0.8;0.1) Not lonely 1931 50.2 Ref. (−1.5;−0.3) Not lonely 1720 21.9 Ref. P-value <0.0001 0.004 Adjusted model Often lonely 110 47.9 1.2 (−0.5;2.9) Often lonely 89 20.0 0.2 (−1.0;1.4) Sometimes lonely 460 46.2 −0.4 (−1.3;0.5) Sometimes lonely 403 19.4 −0.3 (−0.9;0.3) Seldom lonely 1095 46.4 −0.3 (−0.9;0.4) Seldom lonely 986 19.7 −0.1 (−0.5;0.4) Not lonely 1931 46.7 Ref. Not lonely 1720 19.8 Ref. P-value 0.23 0.77 Women Crude model Often lonely 44 30.8 −0.8 (−2.5;0.8) Often lonely 43 18.3 −3.3 (−5.0; −1.5) Sometimes lonely 232 31.2 −0.4 (−1.2;0.4) Sometimes lonely 222 20.1 −1.5 (−2.3; −0.6) Seldom lonely 509 31.1 −0.6 (−1.2;0.0) Seldom lonely 489 21.3 −0.3 (−0.9;0.4) Not lonely 843 31.6 Ref. Not lonely 790 21.6 Ref. P-value 0.25 <.0001 Model 1 Often lonely 44 31.3 1.0 (−0.7;2.6) Often lonely 43 20.9 −0.3 (−2.0;1.4) Sometimes lonely 232 30.6 0.3 (−0.6;1.1) Sometimes lonely 222 20.9 −0.3 (−1.1;0.6) Seldom lonely 509 30.0 −0.4 (−1.0;0.2) Seldom lonely 489 21.2 0.1 (−0.5;0.7) Not lonely 843 30.4 Ref. Not lonely 790 21.2 Ref. P-value 0.21 0.79 Handgrip strength Chair rise test Loneliness N Mean (kg) Estimate (kg) 95% CI Loneliness N Mean (rises/30 s) Estimate (rises/30 s) 95% CI Men Crude model Often lonely 110 48.9 −1.3 (−2.9;0.3) Often lonely 89 21.1 −0.8 (−2.0;0.4) Sometimes lonely 460 48.2 −2.0 (−2.8;−1.1) Sometimes lonely 403 20.7 −1.1 (−1.7;−0.5) Seldom lonely 1095 49.2 −0.9 Seldom lonely 986 21.5 −0.4 (−0.8;0.1) Not lonely 1931 50.2 Ref. (−1.5;−0.3) Not lonely 1720 21.9 Ref. P-value <0.0001 0.004 Adjusted model Often lonely 110 47.9 1.2 (−0.5;2.9) Often lonely 89 20.0 0.2 (−1.0;1.4) Sometimes lonely 460 46.2 −0.4 (−1.3;0.5) Sometimes lonely 403 19.4 −0.3 (−0.9;0.3) Seldom lonely 1095 46.4 −0.3 (−0.9;0.4) Seldom lonely 986 19.7 −0.1 (−0.5;0.4) Not lonely 1931 46.7 Ref. Not lonely 1720 19.8 Ref. P-value 0.23 0.77 Women Crude model Often lonely 44 30.8 −0.8 (−2.5;0.8) Often lonely 43 18.3 −3.3 (−5.0; −1.5) Sometimes lonely 232 31.2 −0.4 (−1.2;0.4) Sometimes lonely 222 20.1 −1.5 (−2.3; −0.6) Seldom lonely 509 31.1 −0.6 (−1.2;0.0) Seldom lonely 489 21.3 −0.3 (−0.9;0.4) Not lonely 843 31.6 Ref. Not lonely 790 21.6 Ref. P-value 0.25 <.0001 Model 1 Often lonely 44 31.3 1.0 (−0.7;2.6) Often lonely 43 20.9 −0.3 (−2.0;1.4) Sometimes lonely 232 30.6 0.3 (−0.6;1.1) Sometimes lonely 222 20.9 −0.3 (−1.1;0.6) Seldom lonely 509 30.0 −0.4 (−1.0;0.2) Seldom lonely 489 21.2 0.1 (−0.5;0.7) Not lonely 843 30.4 Ref. Not lonely 790 21.2 Ref. P-value 0.21 0.79 Crude model: outcome is solely dependent on loneliness. Model 1: Adjusted for age, occupational social class, cohabitation status, number of chronic illnesses and personality (neuroticism, agreeableness, conscientiousness). Joint effect of loneliness and occupational social class/cohabitation Occupational social class was associated with physical capability in both men and women. On average, men and women in the high occupational social class had 1 kg greater handgrip strength and could raise themselves two times more from the chair than those in the low occupational social class. Men who lived alone had, on average, 2 kg weaker grip strength, than men who did not live alone. Women who lived alone could on average raise themselves one fewer times from the chair, than women who did not live alone. Men who were jointly exposed to low occupational social class/living alone and loneliness generally showed lower hand-grip strength [Lonely/low social class −1.8 kg (95% CI −3.0;−0.6), Lonely/living alone −1.5 (−2.7;−0.4)] and fewer chair raises [Lonely/low social class −2.4 rises/30 s (95% CI −3.2;−1.6)] compared with the reference group of non-lonely persons with high occupational social class or who was not living alone. Of notice, lonely men living alone did not experience a poorer chair-rise test compared to the reference group [0.1 rises/30 s (95% CI −0.8;0.8)]. For women the joint variables were not associated with hand grip strength, but lonely women in low social class had fewer chair rises than the reference group (−2.3 rises/30 s (95% CI −3.3;−1.2)]. However, in conclusion the associations were no more than expected from the combination of the two exposures (P-values of the interaction term range 0.13–0.67) (table 4). Sensitivity analyses with those reporting loneliness ’often’, ’sometimes’ and ’seldom’ categorized together were run, but this did not change the overall conclusion. Table 4 Multivariate linear regression analysis Hand grip strength N = 5224 Men Women N Mean (kg) Est (kg) 95% CI Test for interaction* N Mean (kg) Est (kg) 95% CI Test for interaction* Loneliness 1. lonely (often/sometimes lonely) 570 46.2 −0.1 (−0.9;0.7) 276 30.3 0.4 (−0.3;1.1) 2. not lonely (seldom/not lonely) 3026 46.3 Ref. 1352 29.9 Ref. Occupational social class 1. low social class (IV, V, VI) 1185 45.6 −1.3 (−1.9;−0.7) 605 29.6 −1.1 (−1.6;0.6) 2. high social class (I, II, III) 2411 46.9 Ref. 1023 30.7 Ref. Cohabitation status 1. Living alone 525 45.0 -2.4 (-3.2;-1.5) 260 30.0 -0.2 (-0.9;0.5) 2. Not living alone 3071 47.4 Ref. 1368 30.2 Ref. Loneliness/social class 1. Lonely/low social class 262 45.1 −1.8 (−3.0;−0.6) 0.44 137 30.4 −0.1 (−1.1;0.9) 2. Lonely/high social class 308 47.2 0.3 (−0.7;1.4) 139 30.2 −0.4 (−1.3;0.6) 3. Not lonely/low social class 923 45.8 −1.1 (−1.8;−0.4) 468 29.1 −1.4 (−2.0;−0.8) 4. Not lonely/high social class 2103 46.9 Ref. 884 30.5 Ref. 0.13 Loneliness/co-habitation status 1. Lonely/living alone 241 46.3 −1.5 (−2.7;−0.4) 0.14 101 30.9 0.6 (−0.5;1.7) 2. Lonely/not living alone 329 47.3 -0.6 (-1.5;0.4) 175 30.7 0.4 (-0.4;1.3) 3. Not lonely/living alone 284 45.1 -2.7 159 30.1 -0.2 (-1.1;0.7) 4. Not lonely/not living alone 47.8 Ref. (-3.8;-1.7) 1193 30.3 Ref. 0.64 Chair rise test N = 4742 N Mean rises pr. 30 s Est 95% CI Test for interaction* N Mean rises pr. 30 s Est 95% CI Test for interaction* Loneliness 1. Lonely (often/sometimes lonely) 492 19.2 −0.3 (−0.8;0.3) 265 20.2 −0.6 (−1.3;0.2) 2. Not lonely (seldom/not lonely) 2706 19.5 Ref. 1279 20.8 Ref. Occupational social class 1. Low social class (IV, V, VI) 1033 18.4 −2.0 (−2.4;−1.5) 573 19.5 −2.0 (−2.6;−1.4) 2. High social class (I, II, III) 2165 20.3 Ref. 971 21.5 Ref. Chair rise test N = 4742 Men Women N Mean rises pr. 30 s Est 95% CI Test for interaction* N Mean rises pr. 30 s Est 95% CI Test for interaction* Cohabitation status 1. Living alone 462 19.2 −0.2 (−0.8;0.3) 249 20.1 −0.8 (−1.6;−0.1) 2. Not living alone 2736 19.5 Ref. 1295 20.9 Ref. Loneliness/social class 1. Lonely/low social class 220 18.0 −2.4 (−3.2;−1.6) 0.35 129 19.5 −2.3 (−3.3;−1.2) 2. Lonely/high social class 272 20.4 0.0 (−0.8;0.7) 136 20.9 −0.9 (−1.9;0.1) 3. Not lonely/low social class 813 18.6 −1.9 (−2.3;−1.4) 444 19.6 −2.1 (−2.8;−1.5) 4. Not lonely/high social class 1893 20.4 Ref. 835 21.8 Ref. 0.26 Loneliness/co-habitation status 1. Lonely/living alone 204 19.8 0.1 (−0.8;0.8) 0.31 98 20.5 −1.1 (−2.2;0.1) 2. Lonely/not living alone 288 19.4 −0.4 (−1.1;0.3) 167 21.2 −0.4 (−1.2;0.5) 3. Not lonely/living alone 258 19.6 −0.2 (−0.9;0.5) 151 20.9 −0.7 (−1.6;0.2) 4. Not lonely/not living alone 2448 19.8 Ref. 1128 21.6 Ref. 0.67 Hand grip strength N = 5224 Men Women N Mean (kg) Est (kg) 95% CI Test for interaction* N Mean (kg) Est (kg) 95% CI Test for interaction* Loneliness 1. lonely (often/sometimes lonely) 570 46.2 −0.1 (−0.9;0.7) 276 30.3 0.4 (−0.3;1.1) 2. not lonely (seldom/not lonely) 3026 46.3 Ref. 1352 29.9 Ref. Occupational social class 1. low social class (IV, V, VI) 1185 45.6 −1.3 (−1.9;−0.7) 605 29.6 −1.1 (−1.6;0.6) 2. high social class (I, II, III) 2411 46.9 Ref. 1023 30.7 Ref. Cohabitation status 1. Living alone 525 45.0 -2.4 (-3.2;-1.5) 260 30.0 -0.2 (-0.9;0.5) 2. Not living alone 3071 47.4 Ref. 1368 30.2 Ref. Loneliness/social class 1. Lonely/low social class 262 45.1 −1.8 (−3.0;−0.6) 0.44 137 30.4 −0.1 (−1.1;0.9) 2. Lonely/high social class 308 47.2 0.3 (−0.7;1.4) 139 30.2 −0.4 (−1.3;0.6) 3. Not lonely/low social class 923 45.8 −1.1 (−1.8;−0.4) 468 29.1 −1.4 (−2.0;−0.8) 4. Not lonely/high social class 2103 46.9 Ref. 884 30.5 Ref. 0.13 Loneliness/co-habitation status 1. Lonely/living alone 241 46.3 −1.5 (−2.7;−0.4) 0.14 101 30.9 0.6 (−0.5;1.7) 2. Lonely/not living alone 329 47.3 -0.6 (-1.5;0.4) 175 30.7 0.4 (-0.4;1.3) 3. Not lonely/living alone 284 45.1 -2.7 159 30.1 -0.2 (-1.1;0.7) 4. Not lonely/not living alone 47.8 Ref. (-3.8;-1.7) 1193 30.3 Ref. 0.64 Chair rise test N = 4742 N Mean rises pr. 30 s Est 95% CI Test for interaction* N Mean rises pr. 30 s Est 95% CI Test for interaction* Loneliness 1. Lonely (often/sometimes lonely) 492 19.2 −0.3 (−0.8;0.3) 265 20.2 −0.6 (−1.3;0.2) 2. Not lonely (seldom/not lonely) 2706 19.5 Ref. 1279 20.8 Ref. Occupational social class 1. Low social class (IV, V, VI) 1033 18.4 −2.0 (−2.4;−1.5) 573 19.5 −2.0 (−2.6;−1.4) 2. High social class (I, II, III) 2165 20.3 Ref. 971 21.5 Ref. Chair rise test N = 4742 Men Women N Mean rises pr. 30 s Est 95% CI Test for interaction* N Mean rises pr. 30 s Est 95% CI Test for interaction* Cohabitation status 1. Living alone 462 19.2 −0.2 (−0.8;0.3) 249 20.1 −0.8 (−1.6;−0.1) 2. Not living alone 2736 19.5 Ref. 1295 20.9 Ref. Loneliness/social class 1. Lonely/low social class 220 18.0 −2.4 (−3.2;−1.6) 0.35 129 19.5 −2.3 (−3.3;−1.2) 2. Lonely/high social class 272 20.4 0.0 (−0.8;0.7) 136 20.9 −0.9 (−1.9;0.1) 3. Not lonely/low social class 813 18.6 −1.9 (−2.3;−1.4) 444 19.6 −2.1 (−2.8;−1.5) 4. Not lonely/high social class 1893 20.4 Ref. 835 21.8 Ref. 0.26 Loneliness/co-habitation status 1. Lonely/living alone 204 19.8 0.1 (−0.8;0.8) 0.31 98 20.5 −1.1 (−2.2;0.1) 2. Lonely/not living alone 288 19.4 −0.4 (−1.1;0.3) 167 21.2 −0.4 (−1.2;0.5) 3. Not lonely/living alone 258 19.6 −0.2 (−0.9;0.5) 151 20.9 −0.7 (−1.6;0.2) 4. Not lonely/not living alone 2448 19.8 Ref. 1128 21.6 Ref. 0.67 Main and joint effects of loneliness, occupational social class and cohabitation status on physical capability Models have been adjusted for age, number of chronic illnesses and personality (neuroticism, agreeableness, conscientiousness). The loneliness/occupational social class model was also adjusted for cohabitation status, while the loneliness/cohabitation status model was adjusted for occupational social class. * Test for interaction by the inclusion of a product term: (1) loneliness + occupational social class + loneliness×occupational social class + covariates, (2) loneliness + cohabitation status + loneliness×cohabitation status + covariates, P values for the product term. Discussion In a large cross-sectional study of middle-aged adults we do not find evidence of poorer physical capability in lonely men and women. Men and women in low occupational social class or who were living alone had weaker hand grip strength and were limited in their chair raise ability, but there were no support for the differential vulnerability hypothesis. This study is among the first to investigate the association between loneliness and objectively measured physical capability among the middle-aged (50–60 years). In older populations the findings are mixed. A previous cross-sectional study among older people showed only a negligible association between loneliness and self-reported functional ability measured as independence from help in daily activities, which paradoxically showed that higher score on independence was associated with higher degree of loneliness.24 In contrast, a longitudinal study of middle-aged and older men and women (mean age 67 years), loneliness was found to increase the risk of self-reported functional limitations after taking into account the reciprocal association between functional limitations and loneliness as well as socio-demographic, health and health behaviour.5 Compared with this study, the cohort were older, and the findings were based on both self-reported measures of loneliness and physical limitation leaving a risk of dependent measurement error due to personality differences in reporting. Hence, studying objectively measured physical capability and adjusting for personality factors may produce results less prone to bias. In a longitudinal study, among older people (mean age 79 years), a strong association between loneliness and objectively measured motor decline was reported after adjustment for sociodemographic, baseline disability, mental and physical health and health behaviours.4 Despite its lack of adjustment for personality factors, this study lends support for a causal relation between loneliness and objectively measured physical function in older people. The younger age of the Copenhagen Aging and Midlife Biobank participants potentially in part explain our null-finding as the suggested physiological pathways11,12 between loneliness and health may not yet have resulted in detectable differences in physical capability. Furthermore, the prevalence of loneliness in middle age is rather low in the Northern European countries which we also find in this study (3% reports often to feel lonely), leaving only a smaller group exposed. A reason for this may be that most people by midlife have developed stable social networks, are well established on the labour market and most often have not suffered from social losses such as the death of a spouse or friend.8 The prevalence of loneliness is known to increase with age due to social losses, which may explain the negative association with physical capability in older people. Our findings suggest that even among the potential vulnerable groups in low occupational social class and those who live alone the effects of loneliness are not intensified. A wide variety of instruments have been used to measure loneliness, spanning from single item questions to comprehensive scales of many items e.g. the UCLA loneliness scale25 or de Jong Giervald scale.26 However, regardless of measurement differences there seems to be overall support for an association between loneliness and adverse health outcomes.1 The single item used in this study sheds light on the quantitative aspect of loneliness, where contact with others is sparse contrasting other measures evaluating feelings of loneliness.25 A Danish study among the elderly found that 43% who report unwanted aloneness are often lonely, while 59% of the often lonely also feel unwanted aloneness,7 indicating some difference in the unwanted aloneness-measure and the measure of loneliness. A meta-analysis found that older persons were more likely to associate loneliness with lack of activity, while middle-aged were more likely to recognize loneliness as absence of people.11 Therefore, the item used in this study may well encapsulate loneliness among the middle-aged. This one-item assessment has been used in prior research and the prevalence of often feeling lonely identified in this study (∼3%) is comparable to the prevalence in the middle-aged (4–5%) identified in National population-based surveys27 as well as that of a recent study in Denmark using the Three Item Loneliness UCLA Scale(4%).7 The characteristics of those who reported often to feel lonely in the CAMB participants were consistent with prior research,1,7,9,10,16,28 which may support the comparability of the single item measure of loneliness to the more comprehensive scores. Selection bias may have affected the results as CAMB non-respondents had lower education and were more likely to be unemployed, as well as they experienced higher all-cause mortality.17 These factors are associated with loneliness,4,5,8,9,28–30 and further it has been suggested that persons feeling lonely may be less inclined to participate in surveys.8 It is also possible that people, who are less physically capable, are less likely to participate in studies such as CAMB because of mobility limitations. Per design, we condition on participation which is a common cause of both loneliness and physical capability leaving a risk of selection bias.31 Considering the above-mentioned selection mechanisms, it is likely that lonely individuals that did in fact participate in CAMB will also be less likely to be physical incapable, and hence our findings are likely to underestimate the association between loneliness and physical capability. Strengths of this study include the large sample with comprehensive information on the participants, and that physical capability was measured by objective tests, which increases reliability and permits cross-cohort comparisons.20 A limitation of this study is the cross-sectional design which increases the risk of any associations being due to reverse causality. Due to demographic changes the number of 65+ years olds will increase dramatically in the years to come of which a considerable amount will be living alone, and thereby also in increased risk of loneliness and social isolation which potentially may harm the disadvantaged the most. Niedzwiedz et al. found loneliness to be most prevalent among the least wealthy groups across 14 European countries. This indicates the necessity for a continued focus on the potential health consequences of loneliness, social isolation in general among older people but especially among the least advantaged. In contrast to earlier studies among older people, we did not find an association between loneliness and physical capability in a large sample of middle-aged men and women, and there was no support for an interaction between loneliness and occupational social class and cohabitation status, respectively. This may be explained by the harmful effects of loneliness not yet to have had influence on physical capability in this age group. Selection bias may however have caused a true association to be underestimated. Further longitudinal studies are needed to clarify if and at what point in the life course loneliness begins to affect physical capability. Acknowledgements The authors thank the staff at Department of Public Health and National Research Centre for the Working Environment, who undertook the data collection. Further thanks to Kirsten Avlund, Helle Bruunsgaard, Nils-Erik Fiehn, Åse Marie Hansen, Poul Holm-Pedersen, Rikke Lund, Erik Lykke Mortensen and Merete Osler, who initiated and established the Copenhagen Aging and Midlife Biobank from 2009 to 2011. The authors acknowledge the crucial role of the initiators and steering committees of the Metropolit Cohort, The Copenhagen Perinatal Cohort and The Danish Longitudinal Study on Work, Unemployment and Health. The authors also want to thank professor Per Kragh Andersen, Section of Biostatistics for valuable advice regarding the statistical analyses. Disclaimers None. Supplementary data Supplementary data are available at EURPUB online. Funding This work and The Copenhagen Aging and Midlife Biobank were supported by grants from the VELUX FOUNDATION (VELUX26145, VELUX31539). Conflicts of interest: None declared. Key points In older people, loneliness has been associated with higher risk of decline in physical functioning. Little is known about this association in middle-aged individuals and if specific vulnerable groups can be identified. No association between loneliness and physical capability was found in this cohort of middle-aged men and women. This may be due to the harmful effects of loneliness not yet having resulted in detectable differences in physical capability in this age group. Lower occupational social class and living alone was associated with poorer physical capability, but there was no interaction between these factors and loneliness on physical capability. Further longitudinal studies are needed to clarify if and at what point in the life course loneliness begins to affect physical capability. References 1 Hawkley LC, Cacioppo JT. Loneliness. In: Reis HT, Sprecher S, editors. Encyclopedia of Human Relationships . Thousand Oaks: Sage, 2009: 985– 90. 2 Christiansen J, Larsen FB, Lasgaard M. 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The European Journal of Public Health – Oxford University Press
Published: Feb 1, 2018
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