Introduction

Evidence of a global epidemic of obesity is emerging, and the worldwide development poses a great threat to public health due to the serious, obesity-related health consequences (1). Therefore, identification of the determinants for obesity is warranted. Both BMI and obesity are under strong genetic control, as documented from twin and adoption studies (2), but environmental factors are known to operate as well and may be particularly powerful among subjects who are genetically susceptible to obesity (3). For instance, effects of dietary fat on weight change have been shown to be much stronger among those with a familial predisposition to obesity than among those without. Likewise, hereditability for weight change has been found to depend on the level of activity (4).

Work stress may be another environmental factor with a differential effect among subgroups like those with and without a predisposition to obesity. According to Karasek's Demand/Control Model (5), high job strain (workload), defined as high job demands combined with low influence in job, may be experienced as work stress by the individual and, through overeating, may eventually lead to weight gain (6). We found earlier that nurses who reported being either always busy or never busy in their job and nurses with no job influence or job control gained more weight during a 6-year period than other nurses (7). The purpose of the present study was to examine whether people genetically predisposed to obesity may be particularly prone to gain weight when exposed to high work stress.

Research Methods and Procedures

Subjects

The Danish Nurse Cohort Study was established in 1993, when all Danish female nurses above the age of 44 years who were members of the Danish Nurses' Association (n = 23,170) were mailed a questionnaire on lifestyle, health, and work conditions. In total, 19,898 (response rate 86% ) returned the questionnaire. In 1999, a similar questionnaire was sent to the same cohort. Altogether, 15,322 nurses participated in both surveys. The cohort has been further described elsewhere (8).

The present study was based on data from nurses who, in 1993, were below the retirement age (65 years) and in the labor market and had participated in both surveys: 10,263 nurses altogether.

Excluded from this population were 3441 nurses who had left the labor market since 1993 and 418 nurses who were missing information on BMI, their job situation, or covariates. The final study population comprised 6404 nurses who completed the study questionnaires and were eligible for analysis in this study.

The Danish Nurse Cohort Study

Main Exposure Variables

Information on psychological workload exposure was obtained by questionnaire in 1993. The definition of job characteristics was based on responses to the following three questions.

Busy in Job.

Do you have so much to do at work that you find it difficult to have time to manage your own work? Answers to the question on job demands were categorized into five levels: never, not often, sometimes, often, and almost always busy.

Job Speed.

What is the work pressure/work speed like at your work? Answers to the question on job demands were categorized into five levels: much too high, a little too high, suitable, a little too low, and much too low.

Job Influence.

Normally, how big is your influence on the organization of your daily work? Answers to the question on influence/control in job were categorized into four levels: major influence, a certain influence, minor influence, and no influence.

Questions on job demands and job control were repeated in 1999.

Effect-Modifying Variables

Questionnaires in 1999 included validated and previously developed pictograms of parents' body shape for information on familial obesity (9). Women were asked to identify, among nine pictograms (9, 10), their biological parents' body shape at the age of 40 (Figure 1). Nurses who were overweight and reported having at least one obese parent were considered to have a familial predisposition to obesity. Adult obesity occurring in families is considered to be largely genetically determined (2). Weight and height were self-reported, and nurses were considered overweight when BMI > 25 kg/m² (1).

Figure 1.

(A) Pictograms of body shape of biological mothers at the age of 40. What kind of body shape did your biological mother have at the age of 40? Outline one of the figures. (Reprinted with permission from Sorensen TIA, Stunkard AJ, Teasdale TW, Higgins MW. The accuracy of reports of weight: children's recall of their parents' weights 15 years earlier. Int J Obes. 1983;7:115–22.) (B) Pictograms of body shape of biological fathers at the age of 40. What kind of body shape did your biological father have at the age of 40? Outline one of the figures. (Reprinted with permission from Sorensen TIA, Stunkard AJ, Teasdale TW, Higgins MW. The accuracy of reports of weight: children's recall of their parents' weights 15 years earlier. Int J Obes. 1983;7:115–22.)

Full figure and legend (132K)

The following covariates, measured at baseline, were included as categorical variables: smoking (never, former, 1 to 14, 15 to 24, >25 cigarettes/d); physical activity in job (sedentary, standing/walking, lifting/carrying, strenuous); physical activity at leisure (competitive sport, heavy exercise at least 4 h/wk, light exercise at least 4 h/wk, sedentary); alcohol, specified in units of 12 grams (units/wk < 1, 1 units/wk < 7, 7 units/wk < 14, units/wk 14); menopause (premenopausal, postmenopausal); and parity (0, 1, 2, 3, >3 children). BMI was calculated as weight/height (kilograms per meter squared) and was included as a continuous variable in the analyses.

Outcome Variable

As an outcome measure, change in body weight was calculated as the difference in self-reported body weight between 1993 and 1999 and was measured in kilograms.

Statistics

ANOVA and ² tests were used to test for differences between predisposed nurses and other nurses; to test for interactions, a standard F test was carried out among workload variables (busyness, speed, and influence in job), familial predisposition, and weight gain. Statistical analyses were carried out using SPSS version 11.0 (SPSS Inc., Chicago, IL).

Results

Table 1A shows characteristics of nurses according to their own overweight and parental obesity. Those who were overweight and had at least one obese parent were considered to be predisposed for obesity. Nurses with obese parents gained, on average, more weight between 1993 and 1999 than nurses without obese parents (p < 0.001). There were minor differences in workload in 1993 among the four groups of nurses, but predisposed nurses reported that they felt more busy (p = 0.007) and had less influence in their job than other nurses (p = 0.01).

Table 1A. - Characteristics of Danish nurses according to own overweight and to parental obesity; the results are given as means standard deviation or percentage.

Full table

Among overweight nurses who reported that they were often/almost always busy in their job, those with two obese parents gained 5.2 kg in weight, whereas those with one obese parent gained only 3.2 kg (p = 0.007). Nurses who felt they had major influence in their job and had two obese parents gained more weight (4.8 vs. 2.4 kg, p = 0.03) than nurses with one obese parent (data not shown).

Table 1B shows mean or percentages of different characteristics by levels of busyness in job. Nurses with both high and low busyness in job generally experienced greater weight gain than nurses who were sometimes busy (both p < 0.04, data not shown).

Table 1B. - Characteristics of Danish nurses between 44 and 65 years of age according to levels of busyness in job in 1993; the results are given as means standard deviation or percentage.

Full table

For smoking or alcohol intake, no significant differences were found between the levels of busyness. For activity in job, nurses who were often/nearly often busy were more physically active than were other nurses.

In adjusted analysis, there was a tendency (p = 0.08) that predisposed nurses who were either often/almost always busy or never/not often busy gained more weight than the nurses who were sometimes busy (Table 2A) . Nurses with high busyness gained 1.2 kg more weight than nurses who were sometimes busy, and nurses with low busyness gained 1.6 kg more weight than nurses who were sometimes busy. We have made analyses adjusted for BMI at baseline, age, marital status, smoking, weekly hours of work, physical activity in job and in leisure, alcohol intake, shift work, job category, menopause, and parity.

Table 2A. - Adjusted mean body weight gain in kilograms according to nurses' own body weights, parental obesity, and level of busyness in job in 1993; standard error; N = number of nurses.

Full table

Furthermore, the difference in the busyness in job weight gain association among the predisposed nurses and the other groups was significant. For adjusted analyses, low influence in job was generally associated with higher weight gain than higher levels of influence in job. This trend was most pronounced for nurses who were predisposed to obesity. Table 2B adjusted analyses showed that although low influence in job was only borderline significant associated with higher weight gain (p = 0.07), the trend was more pronounced for the predisposed nurses than for other nurses. Further adjustment for the same variables from 1999 also gave virtually similar results (data not shown).

Table 2B. - Adjusted mean body weight gain in kilograms according to nurses' own body weights, parental obesity, and level of influence in job in 1993; standard error; N = number of nurses.

Full table

Discussion

In the present study, psychological workload was found to influence subsequent weight gain in general and, in particular, among those with familial obesity, suggesting a synergy between predisposition to obesity and the psychological workload environment.

In particular, compared with overweight nurses without a familial history of obesity, those with a familial history who also reported a high or low degree of busyness and low influence in their job gained the most weight. We could not, however, identify a similar synergy for job speed, changes in weight gain, and familial obesity.

It could be argued that familial resemblance in obesity in the present study was not an indicator of a predisposition to obesity that was genetic but rather the consequence of learned behavior and shared environment. Several studies on twins and adoptees, however, have shown that the familial resemblance in adult obesity is almost entirely due to shared genes rather than shared environment (11), so that the familial resemblance in BMI in these adult women in the present study is most likely an indicator of genetic rather than environmental predisposition to obesity. Hence, we propose that the results of the present study suggest that a genetic predisposition to obesity may modify the association between psychological workload and weight gain in such a way that high or low degree of busyness combined with low influence in job leads to development of obesity.

Although the present study does not identify the genes involved, it is possible that the larger weight gain among predisposed nurses, as compared with others, is a result of either metabolic alterations or a potential appetite stimulation that works differentially in predisposed vs. other nurses. Alternatively, the obesity-promoting genes may be up-regulated by psychological workload stimulus.

Nurses of normal weight with obese parents comprise an interesting group because they may be obesity-susceptible but remain of normal weight due to voluntary weight management. Hence, differences in lifestyle between these nurses and overweight nurses with obese parents may provide information on how nurses who are genetically predisposed avoid being overweight. In the present study, smoking differed between the two groups, suggesting that smoking may be one way for these nurses to limit weight gain, a method that unfortunately cannot be recommended for other reasons. It should be noted, however, that although initial body weight for predisposed nurses and normal-weight nurses with obese parents was different, their 6-year weight change was similar (Table 1A).

A biological explanation linking weight gain to the experience of workload may be based on a workload-induced increase in cortisone levels, which may subsequently lead to abdominal and general obesity (13). However, job strain may also cause overeating, so-called stress-induced eating, and, thereby, lead to weight gain (6). In this context, Greeno (6) found that women seem to be more prone to stress-induced eating than men, and both nurses and school teachers have been found previously to react to work stress by changing food intake.

Because physical activity level in multivariate analyses did not influence the associations between workload and weight gain, it is still possible that the found associations are due to physical activity levels associated with high workload. One potential limitation of the present study is that the nurses reported their perception of workload; hence, our information of workload is subjective, rather than objectively measured.

However, perceived workload may, indeed, be the measure of importance in relation to subsequent obesity development. Secondly, we found earlier (7) a good agreement between the variable busyness in job and the objective measurement of the number of hours of work, suggesting that objective and subjective measures of workload may not differ greatly.

Another limitation relates to the fact that all information on height and weight was based on self-reports. Especially obese women are known to under-report their body weight more than normal-weight women (12). We assume that the associations would have been stronger than those observed considering reporting bias because the group of predisposed nurses, who were all obese, may under-report more than the other groups of nurses. Therefore, the observed differences between nurses with and nurses without a familial predisposition may potentially have been stronger. However, the consequence of such bias is that our results are, in fact, conservative.

Finally, it may be considered a limitation that only 8.6% of the nurses (n = 548) were categorized as predisposed to obesity according to the definition. It may be argued that this way of defining familial obesity is fairly crude and may be subject to substantial misclassifications. However, the fact that we were able to demonstrate significant interactions (p = 0.046 for busyness and p = 0.020 for influence) using this crude measure suggests that these interactions may truly be present because such misclassification bias may have attenuated our results.

In conclusion, perceived high workload was generally associated with an increase in body weight among Danish nurses, in general, and for nurses predisposed to obesity, in particular. High or low busyness in job, combined with low influence in job but not with job speed, was found to be especially predictive of weight gain among predisposed nurses. Hence, high psychological workload may have an obesity-promoting effect that is particularly prevalent among women with a familial predisposition to obesity.

References

1. World Health Organization (1997) Obesity: preventing and managing the global epidemic: report of a World Health Organization consultation on obesity World Health Organization Geneva, Switzerland.
2. Bouchard, C. (1994) Genetics of obesity: overview and research directions, Bouchard, C eds.. The Genetics of Obesity 223–233. CRC Press, Inc Boca Raton, FL.
3. Heitmann, B. L., Kaprio, J., Harris, J. R., Riisanen, A., Korkeila, M., Koskenvuo, M. (1997) Are genetic determinants of weight gain modified by leisure-time physical activity? A prospective study of Finnish twins. Am J Clin Nutr. 66: 672–678. | PubMed | ChemPort |
4. Heitmann, B. L., Lissner, L., Sørensen, T. I. A., Bengtsson, C. (1995) Dietary fat intake and weight gain in women genetically predisposed for obesity. Am J Clin Nutr. 61: 1213–1217. | PubMed | ISI | ChemPort |
5. Karasek, R., Theorell, T. (1990) Healthy Work: Stress, Productivity and the Reconstruction of Working Life, Basic Books New York. 1–381.Healthy Work: Stress, Productivity and the Reconstruction of Working Life , Basic Books New York.
6. Greeno, C., Wing, RR. (1994) Stress-Induced Eating. Psychol Bull 115: 444–464. | Article | PubMed | ISI | ChemPort |
7. Overgaard, D., Gamborg, M., Gyntelberg, F., Heitmann, BL. (2004) Psychological workload is associated with weight gain between 1993 and 1999: analyses based on the Danish Nurse Cohort Study. Int J Obes. 28: 1072–1081.
8. Hundrup Andersen, Y. (2000) Use of hormone replacement therapy among Danish nurses in 1993. Acta Obstet Gynecol Scand. 79: 194–201. | Article | PubMed | ISI | ChemPort |
9. Sorensen, T. I. A., Stunkard, A. J., Teasdale, T. W., Higgins, MW. (1983) The accuracy of reports of weight: children's recall of their parents' weights 15 years earlier. Int J Obes. 7: 115–122. | PubMed |
10. Magnusson, C., Baron, J., Persson, I., et al (1998) Body size in different periods of life and breast cancer risk in post-menopausal women. Int J Cancer 76: 29–34. | Article | PubMed | ChemPort |
11. Sorensen, T. I. A., Holst, C., Stunkard, A. J., Skovgaard, LT. (1992) Correlations of body mass index of adult adoptees and their biological and adoptive relatives. Int J Obes. 16: 227–236.
12. Perry, G. S., Byers, T. E., Mokdad, A. H., Serdula, M. K., Williamson, D. (1995) The validity of self-reports of past body weights by U.S. adults. Epidemiology 6: 61–66. | Article | PubMed | ChemPort |
13. Björntorp, P. (2000) Endocrine abnormalities in obesity. Diabetes Rev. 5: 52–58.

Acknowledgments

The Wedell-Wedellborg Foundation supported the statistical analysis in this study. In addition, financial support was obtained from Cabinetmaker Sophus Jacobsen Foundation, The Nurses' Research Foundation, and The Lundbeck Foundation.