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Original Research Communication |
1 From the Centre for Preventive Medicine, Unit for Dietary Studies, Glostrup University Hospital, University of Copenhagen, and the Danish Epidemiology Science Centre at the Institute of Preventive Medicine, Copenhagen, University Hospital (BLH), and the Department of Physiology, University of Odense, Denmark (LG).
2 Supported by The Danish Research Council (FREJA), the Danish Health Insurance Foundation, The Wedell-Wedellsborgs Foundation, and the Danish National Research Foundation. 3 Reprints not available. Address correspondence to BL Heitmann, Institute of Preventive Medicine, Copenhagen University Hospital, Øster Farimagsgade 5A, opg 23A 1399 København K, Denmark. E-mail: blh{at}ipm.hosp.dk.
| ABSTRACT |
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Objective: We examined the composition of weight change in relation to obesity, previous weight changes, weight-loss attempts, and physical activity.
Design: Part of the Danish MONICA (Monitoring Trends in Cardiovascular Disease) project, this was a longitudinal population study of changes in weight and body composition, with examinations in 19821983, 19871988, and 19931994. A total of 1236 men and 1200 women aged 35, 45, 55, or 65 y in 19871988 participated. Changes in fat and fat-free mass were measured by bioelectrical impedance.
Results: Before adjustment for age-related changes, fat-free mass made up 41% of weight lost and 24% of weight gained in men. In women, loss of fat-free mass (35%) was more than double that of gains (15%). After adjustment, the fractions of weight gained as fat-free mass were not significantly different from the fractions lost. These fractions were independent of age, obesity, and weight changes in the previous 5 y; successful weight-loss attempts; and physical activity. Independent of age and degree of obesity, weight changes were associated with greater changes in fat-free mass in men than in women.
Conclusions: These data do not support the theory that weight loss or weight cycling may lead to an unfavorable body composition, nor do they provide a biological explanation for why long-term weight loss is often unsuccessful. However, the metabolic and health consequences of weight change may differ in men and women.
Key Words: Body composition obesity weight change longitudinal population study Denmark fat-free mass weight cycling Danish MONICA project
| INTRODUCTION |
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72% and 65% in women and men, respectively (5). We suggested that these fractions closely resemble the composition of weight change in comparable groups of subjects. In the present study we examined the composition of weight changes in 2436 Danish adults between examinations in 19871988 and 19931994. Because many factorsage, degree of obesity, previous weight changes, successful weight-loss attempts, and physical activity during leisuremay influence the composition of the weight lost or gained, we investigated whether weight loss and gain are accompanied by different relative changes in body composition in groups of subjects characterized by differences in these factors.
| SUBJECTS AND METHODS |
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Anthropometric data
All anthropometric measurements were made in accordance with World Health Organization standards (15). Height was measured to the nearest 0.5 cm while the subjects were standing without shoes, with their heels together, and with their heads in the horizontal Frankfurter plane. Body weight was measured to the nearest 0.1 kg on a Seca scale (Copenhagen), with the subjects wearing only hospital underwear.
Measurements of electrical impedance
We used a BIA-103 body-composition analyzer (RJL Systems, Detroit) to measure electrical impedance, following the manufacturer's instructions. Measurements were taken with a tetrapolar electrode arrangement while the subjects were lying relaxed on a couch. The electrodes were placed on the dorsal surfaces of the right hand and foot at the distal metacarpals and metatarsals, respectively, and between the distal prominence of the radius and the ulna at the wrist and the medial and lateral malleoli at the ankle. In a previous study we used a subset of our population to develop an equation for estimating body fat from impedance, with measurements of total body water and potassium as a reference (16):
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Weight changes
The present study focused on changes in weight and body composition between the examinations of 19871988 and 19931994. Body weight changes of
2 kg were considered to be normal daily variations. Only a gain or loss of >3 kg was considered a significant weight change. Subjects with weight changes of >2 but <3 kg were excluded from further study (n = 229). Weight changes between the examinations in 19821983 and 19871988 were used only to describe the grouping variable, weight changes in the previous 5 y. We also examined the effects of weight change between the 19821983 and 19871988 examinations on subsequent body composition changes (gain or loss of >1 kg was chosen because weight-loss groups in particular were very small if the cutoff was >3 kg) and of whether subjects reported that they had succeeded in an attempt to lose weight.
Questionnaire data in 19871988
A simple ranking scale for physical activity during leisure was validated previously and found to perform well in separating sedentary and more active groups (17). The subjects were asked to rank their leisure-time level of activity as 1) mostly sitting, 2) light activity
4 h/wk, 3) active in sport
3 h/wk or heavy work during leisure, or 4) active in a competitive sport several times per week. Few of the subjects fell into the latter category; therefore, groups 3 and 4 were merged for the present analyses. The subjects were asked whether they were current or exsmokers or had never smoked. They were also asked whether in the previous 5 y they had followed a diet through which they had been successful in losing >5 kg.
Statistical methods
Analysis of variance was used to evaluate differences between groups of subjects by sex. Because adults lose fat-free mass with age (18, 19), we adjusted for age-related changes in body composition with the use of linear regression. For each combination of sex, weight-change group (loss, gain, or stable), and either age, percentage body fat, weight change in the previous 5 y, successful weight-loss attempts, or physical activity level, a model including a set of regression analyses of the change in fat-free mass relative to the change in body weight (
fat-free mass/
body weight) was tested. The estimated slopes in these models describe the part of the change in body weight that is fat-free mass. The estimated intercepts to these slopes describe the change in fat-free mass by time. Slopes and intercepts were tested for interactions between the different combinations to examine the differences in gain and loss of fat-free mass with weight changes. Differences with a P value <0.05 were considered statistically significant. The analyses were performed by using SAS statistical procedures (version 8.0; SAS Institute Inc, Cary, NC).
| RESULTS |
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Changes in body composition related to age
Mean (±SD) characteristics of subjects from the 3 examinations are shown in Table 1
by sex for the total group and by weight status (ie, stable weight, weight loss >3 kg, and weight gain >3 kg) between the 19871988 and 19931994 examinations.
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Changes in body fat and fat-free mass in subjects whose weight was stable during the 6-y period reflect the expected loss in fat-free mass and gain in body fat related to age. Before adjustment for these age-related changes in body composition, which are independent of weight change, the fraction of body weight lost as fat-free mass was greater than that gained as fat-free mass (men, 41% compared with 24%; women, 35% compared with 15%). However, after adjustment for age-related changes in body composition, the fractions of body weight lost and gained as fat-free mass were not significantly different, and these fractions made up
33% of the body weight change in men and
25% of the weight change in women. The difference between men and women was significant across all weight groups (Table 2
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| DISCUSSION |
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We found that weight changes were associated with more unfavorable relative changes in fat-free mass in men than in women, even after considering age-related changes in fat-free mass and the differences in percentage body fat between men and women. This finding agrees with that of our cross-sectional study (5). However, the finding is surprising because it was assumed that such sex differences depended on differences in body fatness and because most, but not all (21), studies failed to show such differences (2224). Experimental studies have found that physical activity decreases losses of fat-free mass (20, 25). Hence, differences in physical activity provided one speculative explanation for the differences between the sexes. However, in the present study more women than men reported being inactive, and changes in fat-free mass were not significantly different between those who were physically active and those who were inactive at baseline (data not shown).
Our results imply that in middle-aged adults, metabolic and other health consequences of weight change are dependent on sex. Few studies have had the aim or the statistical power to explore health differences between men and women related to BMI or changes in BMI. However, crude data from
1.7 million Norwegian adults suggest that compared with middle-aged women, middle-aged men have a greater mortality both at a high and at a low BMI (26). At later ages these differences seem to disappear.
The finding that the change in fat-free mass relative to the change in body weight was unrelated to percentage body fat was also surprising, because others suggested that during weight change more fat is lost or gained by obese than by lean persons (9, 21, 24, 27). Although the possibility cannot be excluded that body-composition changes in subjects who are very lean differ from those of who are very obese, the results of the present study, conducted on a large sample of adult Danes with substantial variation in body composition, provide no support for this.
Weight loss may be voluntary or involuntary, and the composition of the weight lost may depend on this. We addressed this question by examining whether a different fraction of weight lost was fat-free mass in subjects who reported successful weight-loss attempts than in those who did not. Although we found no significant difference once age-related changes in body composition were accounted for, the data showed that less fat-free mass was lost by subjects with a history of successful weight-loss attempts than by subjects without such a history. The subjects did not provide information on whether the successful weight-loss attempts were the consequences of diet, physical activity, drugs, or a combination of these; hence, we cannot explore this further.
We anticipated that the fraction of fat-free mass lost during a loss of body weight would be larger than the fraction gained with weight gain. The present study provided no support for this when age-related changes in body composition that are independent of weight change were accounted for. Hence, the results of the present study, which measured body-composition changes over time in a large and representative sample of adult Danes with a high response rate does not seem to provide a biological explanation for why long-term weight loss is often unsuccessful. Nor do the results offer support for the theory that weight cycling results in a low percentage of fat-free mass, which may be associated with increased mortality (28).
Some limitations to the present study should be noted. First, it may be argued that impedance measurements are not sufficiently precise to detect small changes in body composition. However, several studies have documented the reliability and validity of this method of measuring body composition (29). In addition, the measured difference in fractions of fat-free mass gained and lost were small. Therefore, it is not likely that we overlooked small significant differences. Furthermore, several studies have shown that impedance performs well in measuring body composition at the group level in epidemiologic surveys but not as well when measuring individuals in clinical settings (29). Furthermore, the particular equation used for computing fat and fat-free mass in the present study was developed specifically for the present population sample (16). When we reexamined the data, we found the fractions of weight change made up by fat-free mass to be independent of the size of the weight change (data not shown). Hence, measurement error did not seem to be a problem in the present study.
Second, we cannot exclude the possibility that the measurement of leisure-time physical activity may have been subject to misclassification and was too crude to differentiate between sedentary and more active individuals. However, the questionnaire was found to have high validity when compared with fitness and other measures of activity (17), has been used for >30 y in Scandinavia, and was shown to be a strong predictor of risk factors for all causes of death and total mortality (30, 31). Although we cannot exclude the possibility that the crude measure of physical activity was partly responsible for the lack of difference in results between the active and inactive subjects, the present data do not provide evidence that differences in physical activity are responsible for the sex difference.
A third limitation of the present study may be the lack of information about the time course of the weight changes. Rapid weight changes may involve a relatively larger change in fat-free mass than do gradual weight changes (25); these 2 types of weight changes were not analyzed separately in the present study. Hence, any possible metabolic differences between adapted and transitional changes cannot be ascertained. Indeed, whether men experience more rapid weight changes than do women is unknown, but the possibility cannot be excluded that the sex differences may relate to this. On the other hand, during the 6 y between the examinations in 19871988 and 19931994, the women generally reported more weight-loss attempts that resulted in a weight loss of >5 kg than did the men, suggesting that the women may have experienced more periods of rapid weight loss than did the men.
Finally, the reliability of self-reported information on intentionality of weight loss may be limited, as pointed out by French et al (32). Furthermore, it is likely that in a population sample such as that in the present study, subjects tend to cycle in weight above and below an average level with an unknown periodicity, which may not be equivalent to the 6-y weight changes reported here. Hence, the measurement time points may be considered "snapshots," catching some on their way up in weight and others on their way down. Indeed, surveys done at different times are likely to catch different people in weight-gaining and -losing groups, as indicated by the fact that twice as many subjects who reported weight gains in the 5 y before the study fell into the weight-loss group than into the weight-gain group at subsequent examinations. In other words, regression toward the mean may be part of the observed result. However, the fact that weight changes in the previous 5 y did not influence the composition of the subsequent weight changes does not support the importance of regression toward the mean.
The present study, in which there was a high response rate from a large and representative sample of the Danish population, showed no significant within-sex difference in the fraction of weight lost or gained as fat-free mass. The hypothesis that weight cycling may result in adverse health effects by causing a relative loss of fat-free mass cannot be supported by these data. On the other hand, even after taking into account percentage body fat and age-related changes in fat-free mass, weight changes were found to be associated with a more unfavorable relative change in fat-free mass in men than in women, suggesting that the metabolic and health consequences of weight change may be dependent on sex to some degree.
| ACKNOWLEDGMENTS |
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| REFERENCES |
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