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The incidence of overweight and obesity determined from the BMI and the BF% was higher in public school than in private school children according to both methods. Categorization by BF% shows lower prevalence values compared to BMI. These discrepancies may be due to the same characteristics of body composition. In accordance with what was indicated by Widhalm et al. the BMI only explains 73% of the variance of body fat in children under 13 years of age. However, the BMI is still considered a practical tool for a simple calculation to detect excess weight in children [24].
The joint prevalence of overweight and obesity found in public school children based on BMI (49%) was higher than that reported in the ENSANUT 2020 for the northern region of the country (38%) [2]. It is evident that these values are increasing despite the numerous recommendations issued by the Ministry of Health. It has been mentioned that the incidence of overweight and obesity is accentuated more as children move up through elementary school. Olaíz-Fernandez et al. observed that upon entering elementary school, the incidence of overweight and obesity in six-year-old children was 24.3%. At 12 years old, when they were about to finish, there was an increase of 12.2%, reaching 32.5% [25]. The same behaviour is observed in the present study in the public school. Starting with the BF%, it is observed that the children with the lowest average (19.15%) have an age range of 6-7 years and those with the highest average (28.76%) are in the range of 10-11 years old. This is not observed in private school children (Table 3).
Table 3: Anthropometric characteristics of children from 6 to 11 years of age in public and private elementary schools of Hermosillo, Sonora, Mexico
Regarding their lifestyle, more than half of the private school children followed good eating habits and exercised enough to be in good health. On the contrary, in public school, a higher percentage of children with deficient eating habits and the highest incidence of overweight and obesity were observed. Little physical activity and poor diet are two of the most determining environmental factors for weight gain. Various authors have found an inversely proportional relationship between a high percentage of fat and vigorous or moderate physical activity in school-age children [26-28]. For some time now, the WHO has issued some recommendations mentioning that children should invest at least 60 minutes a day in physical activities of moderate or vigorous intensity, which consist of games, sports, displacements, recreational activities, physical education or programmed exercises in the context of family, school or community activities [29].
In general, the studied population showed some deficiencies in the consumption of fruit and vegetables, finding that only about half ate these on a daily basis. Within the recommendations of the WHO it is mentioned that at least 400 g or 5 servings of fruit and vegetables should be consumed per day to prevent non-communicable diseases and guarantee a sufficient intake of dietary fibre. The low intake of fruit and vegetables has already been observed in the child population from 6 to 12 years of age in Mexico, where only 34% meet the daily recommendations [30,31]. Additionally, it has been observed that the children with normal weight are those who consume fruit and vegetables more frequently [32].
The frequency of fish consumption was lower in public school, and less than half of the children in both schools ate meat or eggs daily. According to the Spanish Association of Paediatrics (AEP), in children the daily intake of lean meat, fish and eggs should be two servings (200 grams of meat or 250 grams of fish or one egg). The daily intake of meat in children is not necessary, since it can be alternated with other protein food groups such as legumes or dairy products [33]. However, in this study it was observed that 30% and 22% of the cases in the public and private schools, respectively, almost never ate meat, and this was accompanied, in both cases, by a low percentage of daily consumption of legumes, which could have otherwise, complemented the diet.
One very positive finding in the study population’s eating habits was the low frequency of consumption of sweets, fast food, and fats. In contrast, approximately half of the participants in both schools almost never ate nuts. These foods contain a high content of antioxidants, B vitamins and minerals. The low predilection for their consumption may be associated with outside influences. Although parents are the major decisionmakers for feeding, children can shape their preferences by the influence of their schoolmates and food advertisements [34]. Availability of, and repeated exposure to, healthy foods is key to developing preferences and can overcome food aversion.
Legumes have a low acceptance as favourite foods in private school children (49%). Almost a quarter of the study population said that they almost never ate legumes. Due to their low fat, sodium and glycaemic index content, in addition to being an excellent source of protein, iron, fibre, folate and potassium, the presence of legumes in a child’s diet should be highly encouraged. According to the Food and Agriculture Organization of the United Nations (FAO), endocrinologists and dietitians recommend eating legumes at least two to three times a week. Fish is another food that is unattractive to children [35]. Due to the benefits of its consumption, it has been proposed that its intake should be promoted in the school population through the creation of attractive products and an education programme for children and parents [36]. As for sweets, as expected these are favourites for the majority of the children population (75%). Children’s environment plays an important role in their food preferences. It has been estimated that, in Mexico, more than 80% of children who attend elementary school bring money to buy food, mainly sweets, fried foods or drinks, and to a lesser extent sugary drinks or foods such as cakes or sandwiches [37]. Morales-Ruan et al. when evaluating the incidence of school overweight and obesity among groups of different socio-economic levels, found a higher prevalence in children with a greater availability of food at school and a high socio-economic index compared to those with a high availability of food and a lower socio-economic level, or low availability of food and a medium socio-economic level [16]. Flores-Huerta et al. suggested that parents should prepare their children breakfast or lunch with healthy foods to combat the obesogenic environment that could exist in their school [38].
Furthermore, 75% in public school and 67% in private school reported doing some activity while eating, which increases the risk of being overweight or obese. This same eating behaviour was found in school children in Barcelona, Spain, where 70% of the respondents carried out activities during food intake, of which watching television was the most reported [39]. It is necessary that parents watch that their children do not watch television or do other activities during the meal, since this is distracting, and they do not have a good perception of the amount of food consumed. In addition, more than half of the children consume food in a very short time. Fogel et al. pointed out that eating quickly and not chewing food well can lead to a gradual increase in body weight. This is because more energy than necessary is consumed in a very short time, without giving the brain time to send a satiety signal [40].
Among the favourite drinks, the order of preference in both schools was water, fruit or flavoured juices, milk and cola. Drinking pure water instead of caloric drinks helps control body weight by reducing energy intake [41]. In Mexican children it has been reported that the daily total consumption of water derives from 24% of the intake of pure water, 19% from milk, 14% from natural or flavoured juices, 9% from soft drinks and 29% from food moisture [42]. Even though water is the favourite drink in the population of children, it was observed that its consumption is very low. Less than 10% of all children drink more than 6 glasses a day. The results show the existence of bad hydration habits, especially in public school since the consumption of beverages is associated with increased energy intake in children and the risk of being overweight or obese [43].
With regard to physical activity, it was found that a higher proportion of public school children go to school by bicycle or walking. Despite having a higher proportion of children who perform active transportation, this did not translate into a decrease in the incidence of overweight or obesity. According to the American College of Sports Medicine, moderate and vigorous-intensity activities 3-6 and 6-9 metabolic equivalents (METs), respectively) must be performed to obtain significant benefits in weight control [44]. However, walking is usually not enough since its metabolic equivalent is 2 to 3.5 METS [45].
In the activities during break time, most of the children in public school remained seated, while in private school the majority do physical activities. Due to the significant number of children who do not exercise during break time, it would be opportune to apply physical activity programmes to take advantage of this time to achieve an increase in energy expenditure. In some studies, it has been observed that exercise can be increased significantly by providing children with some simple items to play with, such as jump ropes, beach paddles, balls, or some inclusive games appropriate for their age [46].
The time that children spend in front of the television or electronic devices is relatively high, to such an extent that in both schools there were cases where some dedicated more than 4 h a day doing so. Reyes-Sepeda et al. pointed out that watching television for less than one hour a day can be considered a protective effect against the risk of obesity [47]. While another study carried out in ages 8 to 10 years showed that overweight or obese children were more sedentary and spend more time watching television (more than 2 h a day) than children with normal weight [48]. An interesting study carried out by Patriarca et al. observed that public or state school students whose parents were of a lower socio-economic level, spent more time in front of devices such as televisions, video games and computers, compared to their counterparts at private schools [49,50].
Regarding daily physical activity, a higher frequency was detected in private school students. Children are active by nature, however not all play or do sports outside of school. Reyes-Sepeda et al. reported some risk factors for being overweight or obese [47]. For example, staying at home without doing physical activities increases the risk 1.5 times and not having a set time to play or do physical activities increases the risk 1.3 times. Not all parents, especially those who work all day, have time to take their children to sports or activities in the afternoon. According to what has been observed, children from the private institution find it easier to travel by car. Particularly, the case of this school, it has a sports programme in which all students must return to sports sessions in the afternoon. This obligation seems to have an effect on the low incidence of overweight and obesity found in children who attend private school.