Physical activity, BMI and body weight perception among 15-year-old boys and girls in Poland in the light of international comparisons

The body mass index is one of the most important indicators of human health. The first measurements of weight and length are made a few minutes after birth and for many years it is one of the most frequently examined features used for assessing the development of a child’s, teenager’s [1] and ultimately adult’s [2] health. For many years not only the objective body mass but also its subjective assessment has been frequently undertaken in Polish, as well as international studies [3]. Self-assessment of body weight is a multi-dimensional construct considered in its cognitive, affective and behavioural aspects [4]. Inadequate body weight assessment may indicate dissatisfaction with one’s appearance resulting from the discrepancy between the perception of one’s figure and the perceived ideal. Subjective body mass assessment is particularly important in adolescence, because at this age it affects one’s overall satisfaction with life [5] and the proper social interaction of teenagers [6]. Dissatisfaction with one’s body is associated with low self-esteem, occurrence of mental health problems and disorders (depression, nutritional disorders) and somatic complaints. Excessive preoccupation with one’s appearance and body weight lead to behaviours aimed at changing one’s appearance that are harmful for health, such as elimination or unconventional diets, induced vomiting or use of laxatives. The results of many studies indicate that gender determines objective body weight assessment. Boys tend to overestimate their body weight more frequently. They want to be seen as bigger and more muscular. Conversely, girls tend to underestimate their body weight during assessment [7]. Dissatisfaction with one’s appearance increases with age among both genders but lasts longer among girls and young women.

For many years the results of successive rounds of international HBSC (Health Behaviour in School-aged Children) studies point out Poland as the country where girls have the least favourable assessment of their body weight [8]. Detailed national [9] and international [10] studies have shown that the high percentage of girls dissatisfied with their appearance increases with age, which [11] is not confirmed by objective measurements of the prevalence of overweight and obesity. In fact excess body weight actually affects to a greater extent boys and younger children [12]. Analyses are usually performed by classifying BMI (Body Mass Index) groups, i.e. underweight, normal weight, overweight, obesity [13]. Less attention is paid to BMI variability on a continuous scale, including variability within a group of adolescents without excess weight, taking into account the differences between countries.

The level of physical activity may be a factor which modifies the perception of one’s figure; on the one hand it influences one’s self-esteem [14] and on the other the distribution of muscle and fat tissue [15, 16] affecting the image of one’s figure and level of satisfaction with one’s appearance [17].

The purpose of the study is to provide a detailed analysis of the discrepancies between objective and subjective assessment of body weight among 15-year-old boys and girls in Poland in the light of international statistics the paper aims to assess the impact of physical activity on the level and directions of these discrepancies.

Five research questions were formulated:

The analyses were performed on the basis of data collected during the last round of HBSC studies during the 2013/14 school year within the statutory activity of the Institute of Mother and Child. The data obtained from the international HBSC database in Bergen relate to 72 037 schoolchildren aged 14.5-16.5 years from 42 countries of Europe and North America. The average age of the respondents was 15.53±0.36 years. The size of the samples in individual countries varied from 320 in Greenland to 4973 in Canada. On average 1715±720 schoolchildren from this age group were covered by the poll in each country, including 1484 in Poland.

After taking into account the missing data for the key variables concerning objective assessment and subjective evaluation of the body weight of 61061 schoolchildren, they were enrolled for further analysis. In comparison with the original sample, 14.2% missing values were found concerning height and body weight (making it impossible to calculate BMI) and 2.1% concerning self-perception of body weight. The percentage of missing data concerning the BMI indicator in Poland was 6.0%, while in some other countries it reached 60% (Ireland, Scotland).

The object of the analysis was the assessment of the position of Polish 15-year-olds against the background of their peers from 42 countries of Europe and North America from the point of view of objective and subjective body weight. The authors indicated significant discrepancies between self-perception and objective measurements of body weight. Emphasis in the analyses was placed on the differences dependent on gender. The analyses indicated that the percentage of 15-year-old overweight boys in Poland is nearly three times higher than girls of the same age (18.7% compared with 6.5%). In the case of boys this is a result that is close to the international average (22^nd place in the ranking). Among girls the prevalence of overweight and obesity is evidently less frequent than the international average (5^th place). A similar difference in their observations of excess body weight depending on the respondent’s gender was recorded by scientists in Japan, who conducted a 6-year study on a group of 16 thousand students. It was demonstrated that Japanese boys are fatter than girls of the same age, the prevalence of overweight was approximately 23% for boys and approximately 18% for girls, and in the case of obesity approximately 7% for boys and 4% for girls [24]. On the basis of a systematic review of the literature embracing scientific tests with a global circulation published since mid-2012 concerning the prevalence of overweight and obesity among children and young people aged 10-19 years, Del Mar Bililioni et al. [25] confirmed that overweight is a frequent phenomenon in this population and to a greater degree affects teenage boys.

On the basis of the results obtained, it is to be concluded that the tendency to overestimate their body weight in their subjective assessment is present among adolescents all over the world and teenage girls are more inclined to do so. Among Polish and adolescents this correlation is even more evident. An analysis of the subjective assessment of body weight indicates that Polish 15-year-old girls rank first on the international list of girls who consider themselves too fat. In the group without excess body weight as many as 57.5% of the girls polled gave this answer. For the sake of comparison, in the Korean study Youth Risk Behavior Web-based Survey, 47.5% of teenage girls and 22.2% of teenage boys consider themselves too fat, while only 11.9% and 4.5%, respectively, are actually overweight or obese [26]. Studies concerning satisfaction with one’s appearance in relation to body weight among teenage boys and girls were also conducted by Kantanista et al. They demonstrated an important correlation between the BMI, gender and satisfaction with one’s body. Among boys satisfaction was greater in the group having normal body weight. Underweight girls expressed greater satisfaction with their appearance than those whose BMI was normal or showed overweight [27].

Another problem which must be noted is that of the failure to acknowledge excess body weight by boys, which appears among 7.8% of male teenagers in Poland (and as many as 26% of overweight boys). This is an aspect of the wrong self-perception of body weight that is essentially neglected in the literature, although it is important from the point of view of preventing obesity. It may be assumed that with regard to boys requirements are different and they do not feel stigmatized due to being slightly overweight [28]. More emphasis is also placed on the perception of overweight from the parents’ perspective and with regard to younger children [29].

If we compare the results obtained concerning the prevalence of overweight and obesity with the self-perception of body weight by adolescents, we observe that those girls who most frequently consider themselves too fat come from countries where the percentage of overweight or obese teenage girls is relatively low. Conversely, boys who fail to see their excess body weight, come from countries where the problem of overweight and obesity is most frequent (Malta, Canada). It may be supposed that the actual body appearance that dominates in those countries influences the shaping of opinions concerning a socially acceptable image, while the social norms determine how teenagers perceive their own body and its size.

Analyzing the incoherence between self-assessment of body weight by young people and objective indicators, we must also consider other reasons for the declared discrepancies. According to longitudinal studies by Dion et al. girls tend to be more dissatisfied with their body than boys. The level of satisfaction with their appearance may be projected on the basis of earlier measurements. A large number of girls whose body weight is normal or below norm wish to be slimmer, while some boys would like to be fatter [30]. This phenomenon is associated with internalized standards of a slim body for girls and a muscular body for boys propagated in the mass media [31] and the Internet, especially in the social media widely used by teenagers nowadays [32]. It is to be noted that girls are more prone to media influence, especially as the norms and messages are more frequently addressed to girls. Other risk factors for the development of problems with self-perception of the body by girls are associated among others with comparing oneself with others, low self-esteem, the peer group (teasing about appearance, modeling of peer behaviour, talking about appearance), the family (parents’ comments, modeling of parental behaviour, especially the mother). Among factors associated with experiencing violence, BMI values and genetic determinants are also to be noted. Moreover, in the case of boys depression and social roles associated with gender [33] are mentioned. When looking for the reasons why in some countries girls with a normal body weight are more likely to consider themselves too fat and boys with a higher body weight fail to see the excess, the standards of body appearance prevalent in various countries and cultures must definitely be taken into account [34].

It is dificult to unequivocally determine the main reason for the disadvantageous situation of Polish girls in the international ranking. Based on theoretical models which describe the determinants of body self-perception by teenagers it would be necessary to examine the connection between potential factors affecting the negative self-assessment of appearance and their mutual interactions, and to analyze them in the international context.

The value added of the study is the analysis of the discrepancy between objective and subjective body weight in the context of the level of physical activity, which corresponds with the work of other authors. Golan M. et al. demonstrated that physical activity has an impact on the correct assessment of body weight by teenagers. However, this impact was most significant in the group of young people with appropriate body weight [35]. Sampasa-Kanyinga et al. came to similar conclusions in their study. The young people surveyed who fulfilled the physical activity recommendations were less frequently dissatisfied with their body weight than those who did not fulfill such recommendations (28.8% vs 37.5%). The first group was also more likely to perceive themselves as persons with appropriate body weight (71.2% vs 62.5%) [36].

On the basis of our investigations we may also conclude that physically active young people (with normal body weight) are less likely to consider themselves too fat, which can be linked with higher self-esteem among physically active adolescents in general [37].

The correlation between physical exercise and the perception of one’s body by young people was also studied by Czech scientists, although they adopted physical activity as an outcome variable. Their research confirmed the existence of such a correlation. They demonstrated, for example, that dissatisfaction with their own body among girls did not affect physical activity, while among boys it was associated with an insufficient dose of daily exercise. Conversely, studies of Spanish adolescents indicated that dissatisfaction with the appearance of one’s body negatively correlates with the level of physical activity, irrespective of gender [38].

Young people who perceive themselves as normal or slim more frequently follow the physical activity recommendations. It is likely that as a result of physical activity young people without excess body weight have a smaller tendency to put on weight. Conversely, excessively fat teenagers have a problem with being active, which contributes to maintaining excess weight. It may also be concluded that intervention programs aiming to promote physical activity and sport among young people ought to take into account the subjective assessment of their appearance and the factors which determine it. Only then will such programs have a chance of success.

significant discrepancies between the objective and subjective assessment of body weight by adolescents exist in all countries. International data indicate that about 15% of 15-year-olds have excess body weight in terms of the BMI, while twice as many persons of this age consider themselves too fat. In Poland this discrepancy is even more visible.

Polish 15-year-old girls have a major problem with the correct assessment of their body weight. They consider themselves too fat much more often than their peers from 42 other countries, although this is not confirmed in BMI categories. In the case of boys the situation is similar; only their peers from the Netherlands and Luxembourg had a less correct self-perception of body weight regarding overestimation.

However, the self-perception of body weight by adolescents strongly correlates with the BMI. It is to be noted that the average BMI value in homogeneous groups distinguished by self-perception of body weight shows great variation between countries. In Poland, boys who consider themselves too slim have higher BMI values than in many other countries. The reverse situation is observed among girls; significantly lower BMI values are recorded in the group which considers their body mass too high or ”just right”. This means that relatively slimmer Polish girls begin to consider themselves too fat.

Another point to be noted is the relatively large group on the international scale of boys who fail to notice their overweight. Their share in the population increases in countries where the phenomenon of overweight or obesity is more frequent. In the population of girls failure to notice actual overweight occurs very infrequently among 15-year-old girls, in Poland it is practically non-existent.

The level of physical activity is a factor which modifies the discrepancy between objective and subjective assessment of body weight by teenagers. Among less physically active young people the risk of belonging to the overweight group (those with a realistic assessment of their body weight) and to the group with normal body weight (wrongly identifying themselves as overweight) increases. In both cases the effect of physical activity is more strongly visible among boys.