overweight and obesity are as public health problems , due to both their rapid growth in recent decades and to their related health disorders , such as cardiovascular diseases , diabetes , some cancers , and other diseases [ 18 ] . studies have also showed relationships between obesity and chronic pain   and alzheimer 's disease .   in 2002 , nearly half a billion of the world population considered to be overweight or obese . obesity is assuming epidemic proportions in both developed and developing countries [ 1215 ] . in 2003 - 04 , 17.1% of us children and adolescents were overweight and 32.2% of adults were obese . almost one third of adult canadians are at increased risk of disability , disease , and premature death due to obesity . obesity is relatively common in europe , especially in southern and eastern countries , and studies from repeated surveys suggest that the prevalence of obesity has been increasing last years . up to 2000 , the prevalence of obesity in western countries was suggested to vary between 15 percent and 20 percent . in recent years , the prevalence of obesity in usa , canada , australia , united kingdom , iran , and egypt was 31.8 , 24.3 , 25.1 , 24.9 , 21.6 , and 34.6 , respectively . overall , more than one out of ten of the world adult population was obese . nearly 1.5 billion adults , 20 and older , are now considered to be overweight or obese . of these , nearly 300 million women and more than 200 million men were obese . in 2010 , almost 43 million children ( 35 million in developing countries and 8 million in developed countries ) were estimated to be overweight or obese . most studies have investigated the relationship between sociodemographic factors and obesity . it has found a significant association between weight gain and aging [ 2326 ] . sobal and stunkard   found a strong inverse relationship between socioeconomic status and obesity in women in affluent societies , with a higher proportion of obese women in lower socioeconomic groups . in low - income countries , obesity is more common among middle - age women , people of higher socioeconomic status , and people living in urban communities [ 28 , 29 ] . although the association of overweight with smoking , alcohol consumption , dietary habits , and physical activity has been analyzed in many studies , the findings are not consistent . we aimed to assess the associations between some factors with weight gain across age groups among women by using cross - sectional data from the national health survey in iran ( nhsi ) . the national health survey in iran ( nhsi ) is a survey designed to gain comprehensive knowledge and information about health care problems and difficulties throughout the country , 1999 - 2000 . the survey was conducted under the supervision and with the financial support of the iranian ministry of health and medical education . the population sample of the survey consisted of one thousandth of the total iranian population ; non - iranian were excluded . comprises of 8 households . the choice of 8 households for the cluster size was based on one - day performance capacity of the data collection group : four persons ( 2 physicians , 1 interviewer , and 1 lab technician ) . the statistical framework was based on the household lists available with every health department in the provinces , usually updated annually . data from the national health survey were considered in this investigation . in this study , these data were collected by the national research center of medical sciences and are presented partially at the department of biostatistics and epidemiology / tehran university of medical sciences for research . this study was approved by the ethic committee of the university of social welfare and rehabilitation sciences . weight was measured to the nearest 0.1  kg with the subject in light indoor clothes , with emptied pockets and without shoes . bmi ( body mass index ) was calculated as weight in kilograms divided by square of height in meters , squared , and subjects were classified into underweight defined as bmi < 18.5 , normal weight as bmi 18.524.9 , overweight as bmi 25.029.9 , and obese as bmi  30 . ageinformation about the respondent age was based on their self - reported birth year , and subjects were stratified into five 10-year age groups ( 2029 , 3039 , 4049 , 5059 , and 6069 years ) . information about the respondent age was based on their self - reported birth year , and subjects were stratified into five 10-year age groups ( 2029 , 3039 , 4049 , 5059 , and 6069 years ) . years of schooling were divided into three groups : person with basic ( 08 years ) , moderate ( 912 years ) , or high ( more than 12 years ) education . years of schooling were divided into three groups : person with basic ( 08 years ) , moderate ( 912 years ) , or high ( more than 12 years ) education . economic indexdue to ethical considerations , we did not ask respondents about their income . because they were afraid of paying their taxes . economic index was defined as square meter of living place divided by number of household . participants were classified by their economy index status into four classes : ( 1 ) low ( economic index  quartile 1 ) , ( 2 ) lower - middle ( quartile 1 < economic index  quartile 2 ) , ( 3 ) upper - middle ( quartile 2 < economic index  quartile 3 ) , and ( 4 ) high ( economic index > quartile 3 ) . 
  due to ethical considerations , we did not ask respondents about their income . because they were afraid of paying their taxes . economic index was defined as square meter of living place divided by number of household . participants were classified by their economy index status into four classes : ( 1 ) low ( economic index  quartile 1 ) , ( 2 ) lower - middle ( quartile 1 < economic index  quartile 2 ) , ( 3 ) upper - middle ( quartile 2 < economic index  quartile 3 ) , and ( 4 ) high ( economic index > quartile 3 ) . place of residencethe subjects were grouped according to their place of residence as living in cities ( urban ) or villages ( rural ) . the subjects were grouped according to their place of residence as living in cities ( urban ) or villages ( rural ) . workforceactive workforce was defined as the part of the female population that belongs to the currently employed ( as employees ) or self - employed category as opposed to inactive workforce ( being a housewife / houseworker , pensioner , student , or unemployed ) . active workforce was defined as the part of the female population that belongs to the currently employed ( as employees ) or self - employed category as opposed to inactive workforce ( being a housewife / houseworker , pensioner , student , or unemployed ) . smokingsmoking status was dichotomized into smoker ( those who smoke every day and have smoked at least 100 cigarettes in their lives ) versus nonsmoker ( others ) . smoking status was dichotomized into smoker ( those who smoke every day and have smoked at least 100 cigarettes in their lives ) versus nonsmoker ( others ) . marital statusto make the marital status variable , it was dichotomized into legally married and nonmarried groups.note that we have no information on household income and physical activity , but economic index is surrogate for household income and we used workforce factor . due to ethical considerations , we did not ask respondents about their income , because they were afraid of paying their taxes . therefore , there were no information on alcohol consumption . 
  to make the marital status variable , it was dichotomized into legally married and nonmarried groups . note that we have no information on household income and physical activity , but economic index is surrogate for household income and we used workforce factor . due to ethical considerations , we did not ask respondents about their income , because they were afraid of paying their taxes . we used proportional odds model to assess the influence of the independent variables listed previously on the probability of obesity and overweight . we carried out score tests for the proportional odds assumption , which was found to hold . in addition , we tested the interaction terms using reduced models excluding nonsignificant terms . for obesity and overweight , odds ratios and 95% confidence intervals were calculated . for all covariates , we calculated the probability of obesity and overweight across age groups . the mean bmi of women was 25.33  kg  m ( 95% ci : 25.2525.41 ) . prevalence ( % ) of the body mass index levels according to independent variables was assessed ( table 1 ) . table 1 shows that obesity is much more prevalent among women aged 4060 years , less educated , high economic index , inactive workforce , nonsmoker , married , and resident in city . we started by fitting a preliminary proportional odds model including only age to observe the influence of the potential confounders on overweight and obesity . unadjusted odds ratios were 2.50 , 3.41 , 3.52 , and 2.45 for age groups 3040 , 4050 , 5060 , and 6069 years , respectively . to assess effects of factors in obesity and overweight across age groups , adjustment for age was performed by using proportional odds model . with four response categories , the model had three intercepts . figures 16 show that age was directly associated with overweight and obesity before 60 years of age . in other words , the probabilities of overweight and obesity increase from 20 to about 50 years of age and we used proportional odds model including age , economic index , workforce status , education level , place of residence , smoking status , and marital status . table 2 shows that among women , being inactive , with high economic index , married , being nonsmoker , in an urban residence , with lower educational attainment , all increased the probability of overweight and obesity . the odds ratios were 2.16 ( 95% ci : 1.962.35 ) , 2.93 ( 95% ci : 2.673.23 ) , 2.96 ( 95% ci : 2.633.32 ) , and 1.99 ( 95% ci : 1.762.25 ) for age groups 3040 , 4050 , 5060 , and 6069 years , respectively .   for overweight and obese participants with moderate and high education , odds ratios were 0.90 ( 95% ci : 0.830.98 ) and 0.57 ( 95% ci : 0.470.69 ) , respectively . the odds ratios for overweight and obese participants with lower - middle , upper - middle , and high levels were 1.37 ( 95% ci : 1.261.50 ) , 1.38 ( 95% ci : 1.271.51 ) , and 1.49 ( 95% ci : 1.361.63 ) , respectively . we observed an association between workforce level and bmi groups . odds ratio was 0.68 ( 95% ci : 0.540.85 ) . among women , the odds for urban was 1.97 times that for rural ( 95% ci : 1.842.11 ) . concern about the increased prevalence of overweight and obesity has heightened interest in the association between some factors with weight gain across age groups . in this cross - sectional study we found probabilities of overweight and obesity by identifying a variety of factors that are associated with weight gain . the probabilities of overweight and obesity initially increased , and these probabilities decreased for women aged more than 60 years . it is possible that weight loss among older women is result of medical advice to control or prevent obesity - related chronic diseases . for all covariates , the highest probabilities of overweight and obesity were among women aged 5060 years . among women aged 2040 years , the rates of change in probabilities of overweight and obesity were highest . the mechanism of weight gain in women aged 2040 years is likely multifactorial ; that is , younger people love fast food and their spouses have to follow them . an increased consumption of fast foods by young adults has been repeatedly shown to be associated with obesity and excess weight gain . although some studies showed that any association between number of children and weight gain is a result of lifestyle and behaviors associated with family life rather than being as result of the biological impact of pregnancy in women [ 31 , 33 ] , it may include physiological mechanisms in the women , especially after their pregnancy . prevention of weight gain among women aged 2040 years may have a significant public health impact   and further work is needed to understand these relationships . educationwomen with moderate education had higher probabilities of overweight and obesity than high educated women . note that the women with moderate education had higher probabilities of overweight and obesity than basic educated women . the differences in probabilities of overweight and obesity between high educated and two other levels were noticeable . women with moderate education had higher probabilities of overweight and obesity than high educated women . note that the women with moderate education had higher probabilities of overweight and obesity than basic educated women . the differences in probabilities of overweight and obesity between high educated and two other levels were noticeable . higher education may provide knowledge or resource influences on weight loss . 
 economic indexmany studies have found an inverse relation between socioeconomic level and weight [ 3945 ] . it is not straightforward matter to compare those results with ours , because of the different study designs , time span , different region , and method of analysis . in our study , subjects with high level had higher probabilities of overweight and obesity than the other level . these results are consistent with the findings of some study in developing country . in iran , economy , business , social affair , and so forth are controlled by some people named bazarry . many studies have found an inverse relation between socioeconomic level and weight [ 3945 ] . it is not straightforward matter to compare those results with ours , because of the different study designs , time span , different region , and method of analysis . in our study , subjects with high level had higher probabilities of overweight and obesity than the other level . these results are consistent with the findings of some study in developing country . in iran , economy , business , social affair , and so forth are controlled by some people named bazarry . residenceurban women had higher probabilities of overweight and obesity than rural women . among urban women , the probability of overweight initially increased and then changes were fairly slow but this decreasing was sharply in rural women aged > 60 years . among women aged 5060 years , the probability of obesity for urban was approximately 2 times that for rural . urban women had higher probabilities of overweight and obesity than rural women . among urban women , the probability of overweight initially increased and then changes were fairly slow but this decreasing was sharply in rural women aged > 60 years . among women aged 5060 years , the probability of obesity for urban was approximately 2 times that for rural . our results are consistent with some studies . for example , swedish women who returned to work soon after pregnancy also retained less weight than women who stayed at home [ 47 , 48 ] . it is also possible that there is more discrimination against the obese , or obese women may end up in lower status jobs through stronger selective processes in iran . another explanation for the effect of the workforce may involve energy expenditure at work or the structured lifestyle that active woman imposes . for example , swedish women who returned to work soon after pregnancy also retained less weight than women who stayed at home [ 47 , 48 ] . it is also possible that there is more discrimination against the obese , or obese women may end up in lower status jobs through stronger selective processes in iran . another explanation for the effect of the workforce may involve energy expenditure at work or the structured lifestyle that active woman imposes . 
 smokingthe increase in body weight by age an increase of energy expenditure while smoking , both in resting and in light physical activity conditions , may relate to weight loss in smokers . the increase in body weight by age was found to be lower among smokers than among nonsmokers . an increase of energy expenditure while smoking , both in resting and in light physical activity conditions , may relate to weight loss in smokers . marital statusmost studies showed that marriage is associated with weight gain [ 54 , 55 ] . the finding in this study showed that women who were married tended to gain more weight across the age than those who were not married . the rate of change in probability of overweight in aged < 40 years was noticeable . it is possible that the presence of a spouse may operate as a social factor on weight gain.in iran , it is commonly believed that overweight and obese people are lazy and gluttonous and they lack self - control . many obese people do not go out in public because the devices are too uncomfortable . for example , they can not go to the movies because the seats are too small . obese people are also more likely to lose the benefits of exercise and it may cause further weight gain . they often feel inferior to others because many people would not to be friends with an obese person . some people believe that an obese person is taking up more space than he or she should and a job is often denied because of their weights . most studies showed that marriage is associated with weight gain [ 54 , 55 ] . the finding in this study showed that women who were married tended to gain more weight across the age than those who were not married . the rate of change in probability of overweight in aged < 40 years was noticeable . it is possible that the presence of a spouse may operate as a social factor on weight gain . in iran , it is commonly believed that overweight and obese people are lazy and gluttonous and they lack self - control . many obese people do not go out in public because the devices are too uncomfortable . for example , they can not go to the movies because the seats are too small . obese people are also more likely to lose the benefits of exercise and it may cause further weight gain . they often feel inferior to others because many people would not to be friends with an obese person . some people believe that an obese person is taking up more space than he or she should and a job is often denied because of their weights . 
 limitations and strengthsome limitations of this study should be noted . cause - and - effect can not be inferred from our cross - sectional data . it is a limitation that in this study marital status could be categorized into legally married and nonmarried only . nonmarried people are also a very heterogeneous group and should be more closely examined in further studies . unfortunately , income and physical activity were not used in our investigation.strengths of this study include the national random sample with a considerable age range and a bmi measurement that has been shown to be more valid than self - report measures on body weight and height . obese people tend to underreport their bmi whereas thin people do the reverse [ 56 , 57 ] cause - and - effect can not be inferred from our cross - sectional data . it is a limitation that in this study marital status could be categorized into legally married and nonmarried only . nonmarried people are also a very heterogeneous group and should be more closely examined in further studies . unfortunately , income and physical activity were not used in our investigation . strengths of this study include the national random sample with a considerable age range and a bmi measurement that has been shown to be more valid than self - report measures on body weight and height . obese people tend to underreport their bmi whereas thin people do the reverse [ 56 , 57 ] . increases in response were observed through the 2060 years ; however , all age beyond 60 years result in a decrease in probabilities of overweight and obesity . they may need additional information and more support on how to reduce their risk for weight gain through positive health behaviors .
background . to investigate the associations between some factors with weight gain across age groups in iranian women . methods . 
 proportional odds model was used to estimate the probability of bmi categorized as a function of education , economic index , workforce , smoking , marital status , and place of residence adjusted for age , using data from the  national health survey in iran  database . 
 it included 14176 women aged 2069 years . results . for all covariates , age 
 was directly associated with overweight and obesity before 60 years of age . among women aged 2040 years , the rates of change in probabilities of overweight and obesity were highest . among women , being inactive , with high economic index , married , being nonsmoker , in an urban residence , with lower educational attainment , all increased the probabilities of overweight and obesity . conclusions . 
 women aged 2040 years gained weight faster than other groups . 
 they may need additional information and more support on how to reduce their risk for weight gain through positive health behaviors .