Sedentary behavior is an important contributor to the temporal increase in overweight and obesity (25), but only 46% of U.S. adults engage in recommended levels of physical activity associated with health benefits (6). Reasons for lack of physical activity may be explained by a constellation of individual and environmental factors such as perceived lack of time, feeling too tired, obtaining enough exercise at one's job, no motivation to exercise (4), lack of access to recreational facilities (11), and lack of sidewalks (15). Although demographic correlates of physical activity in broad adult populations have been well documented through research interventions (16,22,24), national surveys (5,9), and community-based studies (12,13), physical activity patterns specific to adults trying to lose weight or maintain weight have not been well documented.
Increasing physical activity is one of the cornerstones of a long-term, healthy weight-management program (1). Physical activity has been effective at helping people to keep from gaining weight (25) and to lose weight when combined with a decrease in caloric intake (27). Although physical activity should be an integral part of weight control practices, little is known on a population level about the amount and type of physical activity reported among people trying to control their weight. Understanding patterns of physical activity behaviors among people trying to lose weight or trying to maintain their weight may provide health care professionals with direction as to how to increase physical activity levels in the U.S. population and among people engaging in weight control behaviors. Gaining more information about the physical activity patterns in this population can also assist in eventually reducing the prevalence of overweight and obesity.
Evidence for weight loss and weight loss practices as a primary public health concern comes from several sources. There has been an increase in the availability of weight loss products and services, with Americans spending over $46 billion dollars a year in this industry (17). However, there are limited data on national estimates of weight loss and weight maintenance practices. Previous data on the prevalence of weight loss among adults have shown that 46.3% of adults were trying to lose weight in 2000 (3). Survey data collected from the National Weight Control Registry provide data on over 4000 individuals who have been successful at long-term weight loss maintenance; 89% reported both diet and physical activity for weight loss, but among this sample, only 1% reported using exercise alone for weight loss (28). Despite these multiple sources, information specific to physical activity patterns among those who have lost weight or who are trying to maintain their weight is rare. The purpose of this paper is to compare and describe leisure-time physical activity level by weight control status in a nationally representative sample of U.S. adults.
The National Health and Nutrition Examination Survey (NHANES) is a major program of the National Center for Health Statistics, Centers for Disease Control and Prevention that assesses health and nutritional status of adults and children in the United States (8). The NHANES is unique in that it combines interviews and physical examinations. Data are released on public-use data files every 2 yr. In the current study, data from two cycles (1999-2000 and 2001-2002) were combined to create NHANES 1999-2002, to increase sample size and analytic options. Data items collected in all combined years were comparable in wording and methods; therefore, the data were concatenated to form a single file (interview sample size of 21,004). To develop an adult analytic sample, we eliminated the following respondents: age < 18 (N = 9563), missing information on physical activity types (N = 17), missing body mass index (N = 1297), pregnant women (N = 613), and respondents who did not answer the questions about trying to lose weight or not trying to gain weight (N = 18). The final analytic sample was N = 9496. In 1999-2000, the overall response rate for those interviewed was 81.9% (9965 out of 12,160), and the response rate for those examined was 76.3% (9282 out of 12,160). In 2001-2002, the overall response rate for those interviewed was 83.9% (11,039 out of 13,156), and the response rate for those examined was 79.6% (10,477 out of 13,156).
We created three mutually exclusive categories of weight control (weight loss and management) status: those trying to lose weight, those trying to maintain weight, and those not trying to lose or maintain weight. In the NHANES interview, respondents were first asked their current self-reported weight and their weight 1 yr ago; if they had lost weight, they were asked whether it had been intentional (8). To assess current weight loss, respondents were then asked, "During the past 12 months, have you tried to lose weight?" Those who self-reported an intentional weight loss of 10 lb or more or who reported that they were currently trying to lose weight were categorized as trying to lose weight. Those who did not self-report a weight loss of 10 lb or more were asked the following question to assess weight maintenance status (i.e., attempt to control weight and prevent from gaining weight): "During the past 12 months, have you done anything to keep from gaining weight?" Those who indicated having tried to keep from gaining weight in the past 12 months and who were not trying to lose weight were categorized as trying to maintain their weight. If respondents said "no" to both questions, they were classified as not trying to lose or maintain weight.
Physical activity questions from the 1999-2002 NHANES queried participation in leisure-time activities, including exercise, sports, and physically active hobbies, during the past 30 d (6). Types of specific leisure-time physical activity were assessed by asking respondents six separate questions about their participation in specific vigorous- and moderate-intensity activities for at least 10 min per occasion during the past 30 d. Respondents were asked "Over the past 30 d, did you do any vigorous activities for at least 10 min that caused heavy sweating, or large increases in breathing or heart rate? Some examples are running, lap swimming, aerobics classes, or fast bicycling." Respondents were handed a card with a list of 24 vigorous activities and an "other" category. Those who reported "yes" were asked, "[Over the past 30 d] what vigorous activities did you do?" They were then asked the frequency (e.g., how many times per day, per week, or per month) and duration (e.g., how long in terms of minutes or hours) they had performed these activities. In a similar manner, respondents were also asked about their participation in moderate-intensity physical activity: "[Over the past 30 d], did you do moderate activities for at least 10 min that caused only light sweating or a slight to moderate increase in breathing or heart rate? Some examples are brisk walking, bicycling for pleasure, golf, or dancing." Individuals who reported that they had engaged in moderate-intensity activity were asked to report the frequency and duration of any of the 32 moderate activities. Activities reported for < 10 min and those reported for ≥ 12 h·d−1 were not included in the NHANES dataset.
The total time for participation in either moderate- or vigorous-intensity leisure-time physical activities in the past 30 d was summed to characterize three activity patterns. Respondents were grouped as regularly active if they reported engaging in a total of ≥ 600 min, ≥ 20 times in the last month, of a combination of moderate- or vigorous-intensity activity. These estimates approximate ≥ 150 min·wk−1 of moderate- to vigorous-intensity physical activity. Those active at a lower frequency or duration were considered irregularly active, and those who reported no moderate- or vigorous-intensity leisure-time activity in the last month were considered inactive. Activities that were similar conceptually were grouped together-for example vigorous-intensity running and jogging and moderate-intensity yard work and gardening. The median time spent in moderate leisure-time activity in the past month was calculated among those who had participated in any physical activity. To examine the physical activity further, we created two subsets of regular physical activity, a higher and a lower activity profile. The higher-activity level physical activity category consisted of respondents reporting ≥ 1800 min, ≥ 30 times, of total physical activity (moderate- or vigorous-intensity) in the last month; this is equivalent to 60 min of physical activity on most days of the week. The lower-activity level physical activity category consisted of respondents reporting ≥ 600 min·wk−1, ≥ 20 times (but less than high activity) in the last month; this is equivalent to 30 min of physical activity on most days of the week. The estimate of 30 min on most days of the week is based on the physical activity recommendation for health benefits established by the Dietary Guidelines for Americans 2005 (26), and the estimate of 60 min on most days of the week is based on the weight maintenance recommendation.
Height and weight were measured in each of the three NHANES mobile examination centers. BMI was calculated by dividing weight (kg) by squared height (m2). Four BMI categories were created: normal weight (BMI < 25), overweight (BMI = 25-29.9), obese class I (BMI = 30-34.9), and obese class II (BMI ≥ 35). Three categories of BMI were used for analyses (normal weight, overweight, obese) because of small sample sizes in the obese class II category. Compared with those whose BMI values were not missing, those whose BMI values were missing were more likely to be female (55.4%, P = 0.02), to be older than 65 yr (32.8%, P < 0.01), to have less than a high school education (29.6%, P < 0.01), to be non-Hispanic white (71.3%, P < 0.01), to not be doing anything about their weight (56.0%, P < 0.01), and to be inactive (54.2%, P < 0.01).
Data were age standardized to the 2000 U.S. Census (18-29 yr, 30-44 yr, 45-64 yr, ≥ 65 yr). Sample weights were used to adjust for unequal selection probabilities, nonresponse, and adjustments to independent population controls (http://www.cdc.gov/nchs/data/nhanes/nhanes_03_04/nhanes_analytic_guidelines_dec_2005.pdf). Standard errors were estimated using Taylor series linearization. Prevalences were calculated to describe the sample characteristics (gender, age, race/ethnicity, education, BMI, physical activity level) and the prevalence of specific types of physical activities among each weight control status (those trying to lose weight, those trying to maintain weight, and those not trying lose or maintain weight). Chi-square was calculated to determine prevalence differences between physical activity levels (e.g., inactive, irregularly active, and regularly active), and differences were considered significant at P < 0.05. Polytomous regression was used to calculate odds ratios (OR) and 95% confidence intervals (95% CI) of the association between physical activity level (inactive was the referent group; thus, regularly active was compared with inactive, and irregularly active was compared with inactive) and weight control status (not trying to lose or maintain weight was the referent group). Gender, age, race/ethnicity, education, and BMI were included as covariates in the model. To describe the distribution of respondents who were categorized as regularly active at recommended levels, we created a low- and a high-activity group. All analyses were conducted using SUDAAN (windows version 9.0; Research Triangle Institute, Research Triangle Park, NC) software package to account for the complex sampling design.
In 1999-2002, 45.6% of NHANES participants reported trying to lose weight; 40.2% were men and 59.8% were women (Table 1). Among people trying to lose weight, 71.7% were white, 55.3% had more than a high school education, and 17.6% had BMI ≥ 35. In contrast, 9.5% reported trying to maintain weight, among whom 48.3% were women, 82.5% white, 67.9% had more than a high school education, and 10.2% had BMI ≥ 35.
Table 2 shows the percentages of physical activity levels across selected demographic and weight control characteristics; 37.4% of respondents were inactive, 34.3% were irregularly active, and 28.3% were regularly active. We also found that men (29.7%) had a higher prevalence of regular activity than women (26.9%; P < 0.001). Among those ages 18-29 yr, the prevalence of being regularly active (34.1%) was significantly higher than for those older than 65 yr (24.0%; P < 0.001). Regular activity was also significantly higher among those with more than a high school education (35.4%) than among those with less than a high school education (16.0%; P < 0.001). Approximately 32.6% of those trying to lose weight and 37.9% of those trying to maintain weight were regularly active, compared with 21.8% of those not trying to lose or to maintain weight (P < 0.001).
Compared with those not trying to control their weight, those trying to lose weight were almost three times more likely to be regularly active (OR = 2.7, 95% CI = 2.3-3.3) than inactive and almost two times more likely to be irregularly active (OR = 1.8, 95% CI = 1.5-2.1) than inactive (Table 3). Respondents who were trying to maintain weight, compared with those not trying to control weight, were over three times more likely to be regularly active (OR = 3.3, 95% CI = 2.5-4.3) than inactive and over two times more likely to be irregularly active (OR = 2.3, 95% CI = 1.6-3.4) than inactive.
Walking was the most common activity reported across all weight control categories, although the prevalence of walking was greater among those trying to maintain weight (45.3%) compared with those trying to lose (38.3%) or those not trying to lose or maintain weight (24.0%) (Table 4). Common physical activities reported by those trying to control their weight were yard work, biking, running, and weight lifting.
Using data from the 1999-2002 NHANES, the median minutes of physical activity in the past month within each weight loss group, stratified by activity level, did not differ significantly (Table 5). Among those who were regularly active, the median minutes of those trying to lose weight were lower than for those not trying to lose or maintain weight (1692 vs 1794). Similarly, among those who were irregularly active, the median minutes of those trying to lose weight were lower than for those not trying to lose or maintain weight (385 vs 401).
As shown in Figure 1, approximately 18% of individuals who reported trying to lose weight and 23% of those trying to maintain weight met the minimum Dietary Guidelines (26) recommendation (i.e., 30 min) designed for health benefits. Fewer than 15% of those trying to maintain weight reported engaging in physical activity at the recommended level for weight maintenance (i.e., 60 min). When examining the differences in meeting the minimum guidelines, those trying to lose weight or maintain weight were significantly different from those who were not trying to lose or maintain weight (P < 0.001). Statistical differences in meeting the higher activity profile were also noted between those trying to lose or maintain weight compared with those not trying to lose or maintain weight (P < 0.001).
Approximately 32.6% of the U.S. adult population in 1999-2002 who were trying to lose weight engaged in regular physical activity. Our findings regarding those who were trying to lose weight are similar to the weight loss practices reported in the 1989 Behavioral Risk Factor Surveillance System, which found that 36.4% of men and 38.8% of women who were trying to lose weight met regular activity patterns (at least three times per week for at least 20 min per session, at less than 60% cardiorespiratory capacity) (9). Although physical activity participation rates are low, people trying to lose weight or maintain weight were more likely to engage in regular physical activity than people who reported not doing anything to lose or maintain weight. It is possible that people trying to lose weight or to maintain their current weight are more health conscious and seek information and advice from health care providers to assist them in weight loss or maintenance (20). It is also interesting that a substantial proportion of individuals who were overweight or obese were not trying to lose or maintain weight. Further research is needed to determine whether body weight acceptance among chronic dieters increases weight control attempts (2).
From this national survey, we found that 37.9% of those trying to maintain weight reported regular physical activity. Regardless of weight control status, physical activity is important for the improvement and maintenance of health. Although the present study did not investigate the effects of accompanying weight control strategies (e.g., diet practices), this research adds insight into physical activity patterns by weight control subgroups. Because weight control is a complex behavior, for long-term weight loss and maintenance, the American College of Sports Medicine (1) recommends that overweight and obese individuals progressively increase the amount of time they spend being physical active.
To fully understand weight control status, we looked at a model that included a four-category measure of weight loss: those trying to lose weight with BMI < 25.0, those trying to lose weight with BMI ≥ 25.0, those trying to maintain weight, and those not trying to lose or maintain weight. Using this four-category model, the odds of being regularly active among those trying to lose weight with BMI < 25.0 were 2.5 (95% CI = 1.9-3.3) and were 2.9 (95% CI = 2.3-3.7) among those trying to lose weight with BMI ≥ 25.0, compared with those not trying to lose or maintain weight (data not shown). Because there was no significant difference between the two BMI categories, we decided to keep those trying to lose weight as one group. We also examined demographic characteristics of those with BMI < 25.0: 39.8% were male, 36.9% were ages 18-29, 48.1% were white (data not shown). Because more than one quarter of those with normal BMI were trying to lose weight, these findings suggest that it may be important to examine cultural perceptions of inappropriate weight loss. Those trying to lose weight with BMI < 25.0 may be trying to lose weight for cosmetic reasons (21), because their BMI is within normal weight guidelines.
These findings suggest that health-promotion efforts should encourage increased levels of physical activity among all adults. Physical activity prevalence tended to be higher among men than women and to be higher among 18- to 29-yr-olds than among other age groups. The prevalence of regular physical activity was lowest among those age 65 yr and older. These findings are consistent with those from other national surveys (10,23) that have provided demographic correlates of people who engage in weight control practices.
A significant association between weight control status and being regularly active (vs inactive) and irregularly active (vs inactive) was found. Compared with those not trying to lose or maintain weight, those trying to maintain their weight had slightly higher odds of regular physical activity (OR = 3.3) than those trying to lose weight (OR = 2.7). We compared models with and without BMI to examine the impact of BMI on physical activity, and the model that included BMI was a better fit, so we decided to leave BMI in the model. Although we were not able to determine why activity pattern differences exist, those trying to maintain their weight may be incorporating physical activity into their daily life in a different manner from those trying to lose weight. Perhaps maintainers have had previous experience with weight loss and recognize the importance of physical activity in keeping weight off. It is also possible that those trying to lose weight are engaging in caloric-restriction practices only, without integrating physical activity (18).
We found that walking was the most prevalent physical activity; approximately one in three adults who were trying to lose or maintain weight walked in their leisure time. The prevalence of walking among those trying to lose or maintain weight was similar to the overall national estimates reported from the 1998 Behavioral Risk Factor Surveillance System (38.6%) (19). Because walking was the most prevalent physical activity, this finding provides direction for health-promotion campaigns to promote the most popular activity-walking-to increase the number of adults who engage regularly, preferably daily, in moderate physical activity for ≥ 30 min·d−1. In this study, we examined physical activity patterns among people trying to lose or maintain weight during their leisure time; future studies could examine opportunities for increased physical activity in nonleisure contexts (i.e., occupation, transportation, and household).
The median minutes of activity reported by each weight control group within a physical activity level were comparable. Increased doses of physical activity can assist in improving overall health. Future research could focus on overall volume by types of activity and could include questions on walking, active transportation, and household activities.
Examination of physical activity by weight control status suggests significantly different patterns. Among those trying to maintain weight, a higher proportion reported being regularly active at the lower activity level (i.e., 30 min) than at the higher activity level (i.e., 60 min). Because fewer than 15% of those trying to maintain their weight met the Dietary Guidelines for Americans 2005 physical activity recommendation for weight maintenance (26) health care professionals are encouraged to emphasize physical activity guidelines specific to maximize health and to maintain a healthy weight. Future research needs to determine whether people do not meet recommendations because of a lack of knowledge of the recommendations or because of a lack of understanding of how to accumulate recommended amounts of activity. The Task Force on Community Preventive Services of the U.S. Centers for Disease Control and Prevention (7) has suggested that environmental, economic, and social support barriers influence exercise behavior. To overcome barriers to physical activity, those trying to control their weight are encouraged to seek opportunities for physical activity near where they live, such as public open spaces and parks. Increasing convenient opportunities to engage in lifestyle physical activity (i.e., access to parks and trails) may increase enjoyment and promote lifelong participation in physical activity. Prior research findings suggest that the presence of trails and access to places for physical activity may increase the proportion of people meeting the recommended amounts of leisure-time activity (14).
Our findings have some limitations to consider because the data are cross-sectional. First, weight history and physical activity status are collected via self-report. Second, the questions used on the NHANES have not been validated to date. Although it is difficult to validate weight control status, similar physical activity questions were used in the Behavioral Risk Factor Surveillance system and have been shown to be reliable and valid (21,29). From the findings of this paper, we cannot infer the extent to which weight loss history influences physical activity patterns or weight control status. Third, the questions on weight control and physical activity might prompt a socially desirable answer and overestimate the prevalence of weight loss or weight maintenance attempts or physical activities. Fourth, the focus of this paper was on leisure-time physical activity, and only sports, aerobic, and anaerobic activities during leisure time were included in our analysis. It is unclear whether the distribution of physical activity patterns would be different if alternative domains such as transportation and household activities were included. Fifth, there may be bias because of the time frame specified in the questionnaire. The questions on weight control status referred to behavior during the past 12 months, and the questions on physical activity referred to behavior during the past 30 d. Therefore, we do not know how long respondents had engaged in their current physical activity pattern during the last 12 months. Sixth, there were unfortunately not enough people in the sample to examine race/ethnicity differences among those trying to lose weight, trying to maintain weight, or not trying to lose or maintain weight to make any truly meaningful interpretation of minority weight control behavior.
Efforts are needed to increase the awareness of physical activity recommendations among people who are trying to control their weight, because approximately 60% are not regularly active. To understand the relationship between physical activity behavior and weight control status, future studies need to examine racial/ethnic differences in conjunction with other social class indicators. To increase physical activity level in the population, further research on the complex relationship between the political, social, and environmental factors and physical activity is needed to guide interventions. Findings from this paper are useful for tracking physical activity patterns and creating messages to encourage various types of physical activities among all those trying to control their weight. For many people starting an activity program, walking is a form of moderate-intensity physical activity that is relatively easy to engage in regularly.
On the basis of these findings, roughly only 40% of adults trying to lose or maintain weight engaged in regular physical activity. Regardless of weight control attempt status, the most common type of physical activity was walking. Given the acceptability of walking among people trying to lose or maintain weight, all population subgroups should be encouraged to walk. People trying to lose weight or to keep from gaining weight should minimally try to meet the physical activity recommendation for overall health of at least 30 min of moderate-intensity physical activity on most days of the week.
The findings and conclusions of this paper are those of the authors and do not necessarily represent the CDC.
1. American College of Sports Medicine. Appropriate intervention strategies for weight loss and prevention of weight regain for adults. Med. Sci. Sports. Exerc.
2. Bacon, L., J. S. Stern, M. D. Van Loan, and N. L. Keim. Size acceptance and intuitive eating improve health for obese, female chronic dieters. J. Am. Diet Assoc.
3. Bish, C. L., H. M. Blanck, M. K. Serdula, M. Marcus, H. W. Kohl, and L. K. Khan. Diet and physical activity behaviors among Americans trying to lose weight: 2000 Behavioral Risk Factor Surveillance System. Obes. Res.
4. Brownson, R. C., E. A. Baker, R. A. Housemann, L. K. Brennan, and S. J. Bacak. Environmental and policy determinants of physical activity in the United States. Am. J. Public Health
5. Casperson, C. J., G. M. Christenson, and R. A. Pollard. Status of the 1990 physical fitness and exercise objectives-evidence from NHIS 1985. Public Health Rep.
6. Centers for Disease Control and Prevention (CDC). Adult participation in recommended levels of physical activity-United States, 2001 and 2003. MMWR Morb. Mortal. Wkly. Rep.
7. Centers for Disease Control and Prevention (CDC). Increasing physical activity: a report on recommendations of the Task Force on Community Preventive Services. MMWR Recomm. Rep.
9. Centers for Disease Control and Prevention (CDC). Prevalence of recommended levels of physical activity among women-Behavioral Risk Factor Surveillance System, United States, 1992. MMWR Morb. Mortal. Wkly. Rep.
44:105-7, 113, 1995.
10. DiPietro, L., D. F. Williamson, C. J. Caspersen, and E. Eaker. The descriptive epidemiology of selected physical activities and body weight among adults trying to lose weight: the Behavioral Risk Factor Surveillance System survey, 1989. Int. J. Obesity
11. Eyler, A. A., D. Matson-Koffman, D. R. Young, et al. Quantitative study of correlates of physical activity in women from diverse racial/ethnic groups: The Women's Cardiovascular Health Network Project-summary and conclusions. Am. J. Prev. Med.
25(3 Suppl. 1):93-103, 2003.
12. Folsom, A. R., T. C. Cook, J. M. Sprafka, G. L. Burke, S. W. Norsted, and D. R. J. Jacobs. Differences in leisure-time physical activity levels between blacks and whites in population-based samples: the Minnesota Heart Survey. J. Behav. Med.
13. Ford, E. S., R. K. Merritt, G. W. Heath, et al. Physical activity behaviors in lower and higher socioeconomic status populations. Am. J. Epidemiol.
14. Huston, S., K. R. Evenson, P. Bors, and Z. Gizlice. Neighborhood environment, access to places for activity, and leisure-time physical activity in a diverse North Carolina population. Am. J. Health Promot.
15. Jaffee, L., J. M. Lutter, J. Rex, C. Hawkes, and P. Bucaccio. Incentives and barriers to physical activity for working women. Am. J. Health Promot.
16. King, A. C., S. N. Blair, D. E. Bild, et al. Determinants of physical activity and interventions in adults. Med. Sci. Sports Exerc.
24(6 Suppl.):S221-S236, 1992.
17. Marketdata. U.S. weight loss market worth $46.3 billion in 2004-forecast to reach $61 billion by 2008. Available at: www.marketdataenterprises.com
. Accessed March 16, 2007.
18. McGuire, M. T., R. R. Wing, M. L. Klem, H. M. Seagle, and J. O. Hill. Long-term maintenance of weight loss: do people who lose weight through various weight loss methods use different behaviors to maintain their weight? Int. J. Obes.
19. Rafferty, A. P., M. J. Reeves, H. B. McGee, and J. M. Pivarnik. Physical activity patterns among walkers and compliance with public health recommendations. Med. Sci. Sports Exerc.
34: 1225-1261, 2002.
20. Reas, D. L., R. M. Masheb, and C. M. Grilo. Appearance vs. health reasons for seeking treatment among obese patients with binge eating disorder. Obes. Res.
21. Reis, J. P., K. D. Dubose, B. E. Ainsworth, C. A. Macera, and M. M. Yore. Reliability and validity of the occupational physical activity questionnaire. Med. Sci. Sports Exerc.
22. Sallis, J. F., and M. F. Hovell. Determinants of exercise behavior. Exerc. Sports Sci. Rev.
23. Serdula, M. K., A. H. Mokdad, D. F. Williamson, D. A. Galuska, J. M. Mendlein, and G. W. Heath. Prevalence of attempting weight loss and strategies for controlling weight. JAMA
24. Stephens, T., D. R. Jacobs, Jr., and C. C. White. A descriptive epidemiology of leisure-time physical activity. Public Health Rep.
25. U.S. Department of Health and Human Services. Clinical Guidelines on the Identification, Evaluation, and Treatment of Overweight and Obesity in Adults: The Evidence Report
. Washington, DC: U.S. Department of Health and Human Services, 1998. Publication 98-4083.
26. U.S. Department of Health and Human Services and U.S. Department of Agriculture. Dietary Guidelines for Americans, 2005
. 6th ed. Washington, DC: U.S. Government Printing Office, 2005. Publication 0-16-072398-1.
27. Williamson, D., M. Serdula, R. Anda, A. Levy, and T. Byers. Weight loss attempts in adults: goals, duration and rate of weight loss. Am. J. Public Health
28. Wing, R. R., and S. Phelan. Long-term weight loss maintenance. Am. J. Clin. Nutr.
29. Yore, M. M., H. R. Bowles, B. E. Ainsworth, C. A. Macera, and H. W. Kohl III. Single versus multiple item questions on occupational physical activity. J. Phys. Act. Health