In this 15-year population-based longitudinal study, gender-specific changes of weight and WC in the adult population of Tehran were evaluated. The rises in weight and WC were strongly age-dependent throughout the follow up period. Increases in weight and WC were more prominent in younger and older birth cohorts in men and women, respectively.
A large body of evidence suggests that weight, BMI, and WC increase throughout most of adult life (14–17), which is in line with our findings regarding the positive age effects observed in both genders. A prospective longitudinal study in Norway recently showed that mean weight in middle-aged men and women (aged 26–54 years) increased significantly during a 10-year follow-up while it reduced in elderly men and women (aged 60–69 years); however, mean WC increased throughout the follow up in all age groups (18). The recent findings were completely in accordance with those of the present study. We observed an overall positive age effect on weight in both genders which in combination with the observed inverse association of weight with age squared suggested an inverted U-shaped relationship between age and weight gain. This implies that the rapid weight gain seen in young and middle-aged men and women will most likely be reversed in the elderly. The weight reduction with aging reported in the elderly could be a result of decreased appetite and calorie intake, as well as substantial loss of muscle mass due to the aging process (19). However, in older men and women, despite a drop in weight, WC increased over years, which could be a result of body composition changes. Age has been shown to affect body composition with older people having higher proportions of the adipose tissue to the lean mass (20). Furthermore, age affects the distribution of the adipose tissue, with more intra-abdominal and visceral adipose tissues vs. peripheral subcutaneous fat mass being observed in the elderly (21). Besides, spinal degenerative changes and a consequent reduction in height during the aging process could explain the increase of WC despite decreased or sustained weight in this age group (22).
In this study, significant negative cohort effects were observed on weight and WC in adult men, indicating notable general and central weight gain in the youngest compared with older birth cohorts. This finding was consistent with the observations of previous studies (7, 9, 23–29). Iran, as a country located in the Middle East and north Africa region, is subjected to a dramatic epidemiological transition from a traditional to a modern nutrition and lifestyle (30). This modernization and associated lifestyle changes by encouraging sedentary habits and excess calorie intake have increased the risk of obesity and overweight in the new generations of Iranians, and as a consequence, the prevalence of obesity among children and adolescents has increased (31–33). Overweight or obese children and adolescents are likely to remain so in adulthood, and this may explain why people who were born in the most recent birth cohorts were more susceptible to weight and WC elevations than older individuals born in the years of economic and social instability. Moreover, in older men, the negative cohort effect could be partially explained by reduced calorie intake and aging-associated loss of appetite (19). In addition, exposure to stress might lead to unhealthy behaviors which could contribute to weight gain as well (34). Another explanation for the observed association of weight gain with younger birth cohorts could be more job commitments of younger generations and their less time for physical exercises and self-care, delivering them more prone to obesity. In contrast, the individuals born in earlier birth cohorts spent their youth in more active lifestyles and were less exposed to cumulative obesogenic health behaviors (35, 36). Another explanation for the negative impact of birth cohort on men could be related to the survival bias which might be a consequence of premature mortality among older obese individuals (37). In contrast to our findings, a study on Chinese adults found that being overweight was more common among middle-aged men born between 1950 and 1975 and revealed that the prevalence of overweight among Chinese adults decreased rapidly in the most recent birth cohorts (i.e., after 1975) (38). The observed difference between the two studies may be due to economic and social differences between the two populations. China has become one of the world's fastest growing economies since the 1980s and with increased access to health information and the implantation of new health policies, has somehow managed to overcome the epidemiological transition (39).
In women; however, we observed a positive cohort effect on weight. Regarding the difference between men and women in this area, both biological and social factors may be accountable for justifying the reported disparity between the two genders. The younger women born in recent birth cohorts are more likely to have higher education, have fewer pregnancies, and adhere to feminine beauty standards paying a premium for slimness as a desirable body form (40). Moreover, menopausal changes affect women in their later years, making elderly women more prone to obesity (41, 42). Another factor contributing to the observed gender disparity may be a higher health awareness level among young women rather than men (43).
We are aware that our research may have some limitations. First, since our subjects were only urban residents in Tehran, our findings cannot be extrapolated to the entire country's population. Second, in order to reach solid conclusions about the influence of the period effect on weight and WC, more measurements and longer follow-up periods are needed. Third, the body composition of the participants was not recorded in our study; therefore, we could not speculate to what degree the reported weight fluctuations might correspond to body fat percentage changes. Moreover, we did not take into account the effects of the participants’ dietary intake and nutritional habits on their weight and WC. However, the current study has several strengths worth mentioning. Anthropometric measurements were performed by trained staff instead of relying on self-reports, increasing the accuracy of measurements. Moreover, our research included participants from a wide adult age spectrum and birth cohorts, who were tracked for 15 years and underwent three measurement intervals.
In conclusion, this study demonstrated rising age-dependent trends in the weight and WC of Tehranian adult men and women. However, there were substantial gender-specific cohort effects, indicating that the men born in recent birth cohorts and the women born in earlier birth cohorts had the most alarming trends in gaining weight and WC. Given Iran’s epidemiological transition state, it seems that the nation has a heterogeneous tendency for weight gain, and our findings suggest that effective population-based obesity preventive measures should be targeted towards gender, age, and birth cohort, as the most potent determinants of obesity.