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International Journal of Obesity (2005) 29, 697–702 & 2005 Nature Publishing Group All rights reserved 0307-0565/05 $30.00 www.nature.com/ijo

PAPER Body mass index and weight change from adolescence into adulthood, waist-to-hip ratio and perceived work ability among young adults J Laitinen1*, S Na¨yha¨1,2 and V Kujala1,3 1 Oulu Regional institute of Occupational Health, Oulu, Finland; 2Department of Public Health and General Science, University of Oulu, Oulu, Finland; and 3Rehabilitation Unit, Oulu University Hospital, Oulu, Finland

OBJECTIVES: To study how body mass index (BMI, kg/m2) at 14 and 31 years (y) changes in BMI between 14 and 31 y, and waist-to-hip ratio (WHR) at 31 y are associated with poor perceived working ability at 31 y. DESIGN AND SUBJECTS: A population-based cohort, originally 11637 people, born in Northern Finland in 1966 was resurveyed at 14 and 31 y. MEASUREMENTS: Perceived work ability (measured by modified work ability index, WAI), BMI, WHR, alcohol intake, smoking, basic education and work history were recorded at 31 y and BMI also at 14 y. RESULTS: Low WAI (15% of lowest values) at 31 y showed a U-shaped association with BMI at 14 y, and also with BMI at 31 y, except in obese males. Low WAI similarly had a U-shaped association with WHR, but in males only, while in females, the probability of low WAI increased almost linearly with WHR. Low WAI was more common among smokers and people with a low level of education. The pattern for alcohol intake was more equivocal, with a nonsignificant finding in males and an almost linear decrease of low WAI with increasing alcohol intake in females. CONCLUSIONS: Work ability, health habits and anthropometric measures should be evaluated regularly in young workers, and preventive measures against diseases leading to premature retirement should be taken in time, for example by the occupational health-care service. International Journal of Obesity (2005) 29, 697–702. doi:10.1038/sj.ijo.0802936 Published online 22 March 2005 Keywords: body mass index; waist-to-hip ratio; work ability; alcohol; smoking; cohort study

Introduction Overweight and obesity are increasing in prevalence at an alarming rate worldwide, both in developed and developing countries.1 The body mass index (BMI, kg/m2) shows a J- or U-shaped association with mortality from all causes over a long follow-up time,2,3 and overweight and obesity increase the risk of disability and early retirement,4–6 mainly due to increased morbidity with respect to obesity-related conditions such as cardiovascular and musculoskeletal diseases. An increased risk of disability from mental diseases was also noted among the obese in a Swedish study of middle-aged men,5 but not in a Finnish study of 25 to 64-y-old men and women.4

*Correspondence: J Laitinen, Finnish Institute of Occupational Health, Oulu, Aapistie 1, Oulu 90220, Finland. E-mail: [email protected] Received 11 May 2004; revised 30 November 2004; accepted 6 December 2004; published online 22 March 2005

Besides economic costs and losses to individuals and societies from premature pensions and deaths, the direct costs to the health-care services caused by obesity-related illnesses have been estimated at 2–7% of total health-care costs in several developed countries. They therefore represent one of the largest items of expenditure in national health-care budgets.1 Since major diseases causing disability emerge in middle age, efforts to maintain optimal body weight should be started in childhood. We therefore used a large cohort followed up since birth and resurveyed at 14 and 31 y of age to study the association of BMI and waist-to-hip ratio (WHR) with a poor work ability measured by the work ability index (WAI), which in turn predicts disability in later life. We also focused on the significance of weight change between the two ages, and made separate analyses of overall and abdominal obesity, since the latter in particular is closely related to the development of cardiovascular diseases7 and to high levels of stress.8,9

Obesity and work ability in young adults J Laitinen et al

698

Subjects and methods Study design and population This was a longitudinal, population-based study of 11637 subjects born in Northern Finland in 1966 who were still alive at the age of 31 y.10,11 The present data were collected by means of a postal questionnaire at 14 and 31 y and a clinical examination at 31 y. The analyses are based on data for individuals who did not object to their data being used for scientific purposes and who were employed at 31 y. This study was approved by the Ethics Committees of the Finnish Institute of Occupational Health and the University of Oulu.

Definition of outcome and explanatory variables The outcome was work ability, measured in terms of the modified WAI, developed by the Finnish Institute of Occupational Health in conjunction with a follow-up study of ageing municipal workers and now established as a commonly used method for assessing perceived work ability in health examinations and workplace surveys.12 It has been said to be a reasonably good predictor of work disability among workers of 50 y of age, since approximately one-third of the subjects with a low WAI were granted a work disability pension during a follow-up period of 4 and 11 y13,14 and also mortality.14 The present study used a modified WAI, which was calculated by summing the items 1–6. Subjects at or below the sex-specific 15th percentile of the index were classified as having poor work ability. A poor level of each item was defined on the basis of our earlier study.15 The original version also includes a question on the number of current diseases diagnosed by a physician. Since the latter diseases include obesity, this item was omitted. The items used here were (1) current work ability compared with the subject’s lifetime best (0–10 points; poor level r7 points), (2) work ability in relation to the demands of the job, separately for physically and mentally demanding work (both 1–5 points; poor level o4 points), (3) estimated work impairment due to diseases (1–6 points; poor level r4 points), (4) sick leave during the past year (12 months) (1–5 points; poor level o4 points), (5) personal prognosis of one’s work ability in terms of health 2 y from now (1, 4 or 7 points; poor level r4 points) and (6) mental resources (three questions: have you been able to enjoy your regular daily activities recently, have you been active and alert recently, have you felt yourself to be full of hope for the future recently), the scores being summed and the sum classified on a scale of 1–4 points (poor level r2 points). Measures of body weight included BMI at 14 and 31 y, weight change from 14 to 31 y and WHR at 31 y. Body weight and height, self-reported at 14 y and measured at the clinical examination at 31 y, were converted to BMI. In 30% of the subjects, body height or weight had not been recorded, and these figures were replaced by corresponding information obtained in the postal questionnaire at 31 y. To allow for International Journal of Obesity

nonlinear relationships, BMI was classified in 5 kg/m2 intervals (20, 20–24,y,35 þ ) at 31 y and 3 kg/m2 intervals (15.4, 15.5–18.4, y, 27.5 þ ) at 14 y, with similar classifications for both sexes. Weight change between the ages of 14 and 31 y was classified as (1) always normal, (2) gained weight, (3) lost weight and (4) always overweight or obese. The latter coding was based on a comparison of the subjects’ weight classes (normal, overweight and obese) at both ages, these being defined as less than 21.5, 21.5–24.0 and 24.0 kg/ m2 or more at 14 y, and less than 25.0, 25.0–29.9 and 30.0 kg/ m2 or more at 31 y, respectively. WHR was measured at 31 y as the ratio between the circumference of the waist (at a level midway between the margin of the lowest rib and the iliac crest) and the hip (at the widest trochanters). WHR was classified using the same interval throughout (0.05) but different cutoff points for males and females (0.84, 0.85– 0.89, y, 1.00 þ for males; 0.74, 0.75–0.79, y, 0.90 þ for females). Individuals smoking on at least one day a week at the age of 31 y were classified as smokers, and those smoking less often or only occasionally or not smoked at all as nonsmokers. Basic education was assessed as low (primary school or less) vs high (matriculation examination or more). The postal questionnaire administered at 31 y included questions on the frequency of consumption of beer, wine and spirits during the last year, and the usual amount of each per drinking occasion. The amount of alcohol consumed per day was calculated using the following alcohol contents (vol%): beer 4.8, light wine 5.0, table wine 14.5 and spirits 37.0. The subjects were classified into quartiles of alcohol consumption. This method was validated against 7-day food records.16

Statistical methods Univariate associations between poor work ability and the explanatory factors were examined by means of simple tabulations and the w2 test. We also evaluated the association between explanatory variables and poor level of each of the six items of a modified WAI by crosstabulation and the w2 test. Logistic regression was then used to assess the probability of low WAI in relation to BMI, WHR and weight change, using smoking, alcohol intake and education as potential confounding factors. As BMI, WHR and weight change were correlated, they were analyzed separately. The significance of each term in the model was tested using the likelihood ratio test.

Results The WAI exhibited a left-skewed distribution, the mean, median and mode lying close to the upper tail (Table 1). All measures of central tendency showed little difference between males and females, but the cutoff point for the

Obesity and work ability in young adults J Laitinen et al

699 15th percentile was slightly higher in the males (29) than in the females (25).

Univariate associations The associations of work ability with measures of body weight were more or less U-shaped, both lean and fat individuals showing a high prevalence of poor work ability (Table 2). The one major exception was WHR in the females, with which poor work ability had an almost linear association. The prevalence of poor work ability was lowest among

Table 1

Descriptive statistics related to the modified WAI

Males Females

Mean

Median

Mode

15th percentile

Range

No. of cases

36 34

38 37

39 40

29 25

7–42 6–42

2674 2948

those who had lost weight since 14 y, and was almost twice as high in the females who had either gained weight after 14 y or had always been overweight. The associations between measures of body weight and poor level of each item of a modified WAI (data not shown) were quite similar to the results of body measures and a modified WAI (a sumscore of six items). Poor work ability was more prevalent in smokers and in those with low education (Table 3). The pattern for alcohol intake was U-shaped in the males, but an almost linear decrease in poor work ability with increasing alcohol intake was seen in the females.

Adjusted analyses The associations of poor work ability with the four measures of body weight are shown in the form of adjusted odds ratios and their 95% confidence intervals in Figure 1. Adjustments were made for smoking, alcohol intake and basic education.

Table 2 Distributions of subjects according to measures of body weight (BMI at 14 and 31 y, WHR at 31 y and weight change from 14 to 31 y) and percentages of subjects with a low modified WAI (15% of lowest values) Males BMI (kg/m2), 14 y 15.49 15.5018.49 18.5021.49 21.5024.49 24.50+ All w2 (df ¼ 4) PB BMI (kg/m2), 31 y 19.99 20.0024.99 25.0029.99 30.0034.99 35.00+ All w2 (df ¼ 4) PB

N

Percent with low WAI

65 885 1113 263 87 2413

23.1 14.5 14.7 12.2 20.7 14.8 7.46 0.114

BMI (kg/m2), 14 y 15.49 15.5018.49 18.5021.49 21.5024.49 24.50+ All 2 w (df ¼ 4) PB

106 1275 1061 184 45 2671

24.5 14.2 14.8 17.4 17.8 15.1 9.23 0.056

BMI (kg/m2), 31 y 19.99 20.0024.99 25.0029.99 30.0034.99 35.00+ All w2 (df ¼ 4) PB

357 836 829 446 183 2651

16.8 13.5 13.4 15.5 24.6 15.0 17.31 0.002

All w2 (df ¼ 4) PB

1163 884 53 295 2395

15.0 14.8 13.2 14.6 14.8 0.14 0.986

Weight change, 14–31 y Always normal Gained weight Lost weight Always overweight All 2 w (df ¼ 4) PB

WHR, 31 y 0.84 0.850.89 0.900.94 0.950.99 1.00+ All w2 (df ¼ 4) PB Weight change, 14–31 y Always normal Gained weight Lost weight Always overweight All 2 w (df ¼ 4) PB

Females

WHR, 31 y 0.74 0.750.79 0.800.84 0.850.89 0.90+

N

Percent with low WAI

67 938 1243 354 97 2699

29.9 12.3 12.6 12.4 23.7 13.3 26.81 0.000

376 1625 653 186 97 2937

14.6 11.6 14.9 17.7 29.9 13.7 30.95 0.000

506 959 680 305 319 2769

8.9 12.6 13.5 18.0 18.8 13.5 22.94 0.000

1707 561 112 299 2679

11.7 16.2 8.9 17.4 13.2 14.14 0.003

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Obesity and work ability in young adults J Laitinen et al

700 A BMI of less than 15.5 kg/m2 at 14 y was predictive of a twoto three-fold risk of poor work ability later on in both the Table 3 Distributions of subjects by explanatory factors and percentages of subjects with a low WAI (15% of lowest values) Males

N Current smoker Noa Yesb All

Alcohol intake I (lowest quartile) II III IV (highest quartile) All

Basic education Lowc Highd All

Females

Per cent with low WAI

1699 928 2627 w2 (df ¼ 1) ¼ 13.94;

668 669 631

13.2 2201 18.6 717 15.1 2918 PB0.000 w2 (df ¼ 1) ¼ 12.40;

12.5 17.7 13.8 PB0.000

17.4 13.2 13.3

18.2 15.5 10.5

611 16.2 2579 15.0 w2 (df ¼ 4) ¼ 6.82; PB0.078

1877 763 2640 2 w (df ¼ 1) ¼ 34.27;

Per cent with low WAI

N

676 761 740

Discussion

680 10.7 2857 13.7 w2 (df ¼ 4) ¼ 24.92; PB0.000

17.6 8.7 15.0 PB0.000 w2

1482 1447 2929 (df ¼ 1) ¼ 11.80;

16.0 11.6 13.8 PB0.001

a Smokes less often than 1 day a week, or occasionally. bSmokes on at least 1 day a week. cPrimary school or less. dMatriculation examination or more.

OR

BMI 14 yr

In this population of young adults, both thin and obese individuals reported poor work ability more often than those of normal weight, independently of whether BMI had been measured concurrently with work ability, or 17 y previously. The association varied depending on whether BMI or WHR was used, and was closer among the women than the men. The limitations of the study include the fact that we were unable to differentiate between the effects of BMI and WHR, due to the intercorrelation between the two and we only measured self-reported work ability. The results could nevertheless explain some of the factors underlying work

BMI 31 yr

WHR 31 yr

WEIGHT CHANGE 14 to 31 yrs

5

5

5

5

4

4

4

4

3

3

3

3

2

2

2

2

1

1

1

1

-15.4 15.5- 18.5- 21.5- 24.5-

OR

males and females, and an increased risk was also observed in the girls having a BMI of 24.5 kg/m2 or more. A BMI of less than 20 kg/m2 at the age of 31 y had some effect on poor work ability, but only in the males with any certainty, while the females showed a more than two-fold increase in the frequency of low WAI from the normal range (20–24 kg/m2) to the very obese (higher than 35 kg/m2). Males having a WHR of at least 1.00 or less than 0.85 showed an increased risk of poor work ability, while an almost linear increase in low WAI with increasing WHR was seen in the females. Weight change after 14 y had no effect in the males, but the risk was significantly increased in the females who had gained weight, as also in those women who had always been overweight.

-19

20-

25-

30-

Always Gained Lost Always normal overw.

-0.84 0.85- 0.90- 0.95- 1.00-

35-

5

5

5

5

4

4

4

4

3

3

3

3

2

2

2

2

1

1

1

1

-15.4 15.5- 18.5- 21.5- 24.5-

-19

20-

25-

30-

35-

-0.74 0.75- 0.80- 0.85- 0.90-

Always Gained Lost Always normal overw.

Figure 1 The associations between four measures of body weight (BMI at 14 and 31 y, WHR and weight change from 14 to 31 y) and poor work ability at 31 y (odds ratios (OR) and their 95% confidence intervals from logistic regression adjusting for smoking, alcohol intake and basic education). Upper panel: males; lower panel: females.

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Obesity and work ability in young adults J Laitinen et al

701 disability, and they also provide guidelines for health-care practices. A number of studies report a U-shaped association between BMI and mortality, mainly from cardiovascular diseases,2 and also from musculoskeletal diseases.4 A J-shaped association between BMI and disability from mental disorders has been observed,5 but this finding was less unequivocal in a national Finnish study.4 A significant proportion of mental disorders remain undiagnosed, however, and such cases, together with social and psychological disadvantages related to overweight,4 could explain the high prevalence of low WAI among the most obese in the present cohort. One specific factor may be mental depression, which is associated with both overweight,17,18 weight gain19 and abdominal obesity.20 Thin people often have higher mortality than those who are of normal weight, but opinions are divided as to whether or not this is due to smoking or occult disease such as cancer.2 There is no increase in disability pensions among people having a BMI of less than 22.5 kg/m2 in Finland,4 but such an increase was found specifically for disability arising from musculoskeletal conditions among people with a BMI of less than 20 kg/m221 F a cutoff point also used in the present study. Little information is available on the occurrence of mental depression among the thin, but malnutrition associated with an anorectic state increases depression, anxiety and obsessiveness.22 Our findings give some indication that WHR is a more consistent marker of work disability than BMI, although we could not separate their effects with any certainty. Even men who were classified as severely obese in terms of BMI (435 kg/m2) did not report any more disability than the other men, whereas the association was unequivocal at the high end of the WHR scale. The failure of high BMI to indicate poor work ability in men F despite a marked association in women F could be differing perceptions of body image among men and women, overall obesity being more acceptable among men but socially and psychologically distressing for women. The existing longitudinal studies on obesity and its social and economic consequences suggest that adverse outcomes seem to emerge more easily for women than for men.23,24,25 In contrast, a high WHR, which indicates a large amount of visceral fat, was associated with poor work ability in both sexes. Abdominal obesity is an indicator of a process leading to the metabolic syndrome and cardiovascular diseases.7,26,27 The latter conditions as such are rarely the cause of disability among young adults, but a possibility exists that individuals reporting impaired work ability might be suffering from work stress and increased secretion of cortisol, factors which are in turn related to abdominal obesity.8,9 We also noted a somewhat closer association of obesity with poor work ability in the women than in the men, and the association between WHR and poor WAI in particular was markedly linear among the female subjects. The sex difference could be explained in several ways. A higher proportion of the

women in this cohort had high-strain work (high demands and low job control) than of the men;28 women have reported higher levels of work overload and stress than men, and they are often more stressed by their greater unpaid workload and by a greater responsibility for duties related to the home and family.29,30 The situation of women on the labour market in this country was insecure at the time of when this investigation was carried out, and for the first time ever, unemployment was higher among them than among men. Also, a higher proportion of the women in this cohort had short-term or part-time jobs, even though their level of education was in general higher than that of the men.31 Thus, smoking, heavy alcohol consumption, obesity and poor work ability may be consequences and indicators of underlying common factors related to workers’ mental resources, for example, passive coping skills, which are related to obesity and unhealthy habits, poor psychological functioning and even burnout.28,32–34 In conclusion, the waist and hip circumferences, body weight and health habits of young workers, in addition to their work ability, should be evaluated regularly in occupational health examinations, and preventive measures against diseases leading to premature retirement should be taken in time. This study suggests that not only unhealthy habits but also high WHR, indicating abdominal obesity, both a low and a high BMI and also weight gain during young adulthood can be used to identify young workers who run a risk of later poor work ability. Young workers form a challenging group for occupational health-care services, and there is an evident need to develop methods and forms of intervention that are suitable for use with young workers who are still healthy but are faced with many simultaneous challenges at work and in their private lives.

Acknowledgements This study was supported financially by the Academy of Finland.

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