Physical fitness among urban and rural Ecuadorian adolescents and its association with blood lipids: a cross sectional study

Non-communicable disease, predominantly cardiovascular disease and type II diabetes, have become leading causes of death and disability, accounting for 80% of total deaths in low- and middle-income countries worldwide [1]. Current evidence indicates that the development of non-communicable disease starts early in life [2] and is associated with poor physical fitness, low physical activity levels [3] and inadequate diet [4]. Physical fitness has a closer association to the occurrence of both cardiovascular disease, and cardiovascular risk factors, than do physical activity levels [3, 5]. Physical fitness, in contrast to physical activity, is stable over several months within an individual [6] and has therefore been proposed as a marker for cardiovascular risk in children and adolescents [7].

Recently, low- and middle-income countries have experienced a rapid increase in the development of risk factors for non-communicable disease among young people. Ecuador is no exception. A recent study in a group of urban and rural Ecuadorian adolescents [8] reported that dyslipidemia, abdominal obesity and overweight were prevalent in 34.2%, 19.7% and 18.0% of the population. Although elevated levels of dyslipidemia were found in both urban and rural populations, dyslipidemia was higher in the rural group. Unexpectedly, a previous analysis showed that dietary intake was weakly associated with plasma lipid (Ochoa–Aviles unpublished data). Therefore, it was hypothesized that an association of blood lipids with physical fitness is probable, and is a dimension of analysis that could further be explored.

There are few studies that have assessed physical fitness [9–13] and its association with cardiovascular risk factors in low- and middle- income countries [14]. In fact, only a single study in adolescents has investigated a comprehensive assortment of physical fitness components such as: speed, muscular endurance/strength, cardio-respiratory endurance and flexibility [11], and only one has assessed the association of cardiorespiratory fitness with dyslipidemia [14]. To the author’s knowledge no studies thus far have assessed associations of blood lipid levels with a similar variety of fitness components (speed, muscular strength endurance, cardio-respiratory endurance, flexibility and balance) according to residential location (rural vs. urban). This is surprising considering incidence of cardiovascular risk factors is known to vary along with environmental factors, such as location of residence (urban vs. rural areas) [15]. Rural areas differ considerably to urban areas, i.e. in terms of available health services, medical specialists [15], sport facilities or recreational areas [16], transportation (traffic and means of transport), safety issues [17], food availability [4] and formal education, among others [15].

This study has two objectives: i) to assess the physical fitness in a group of urban and rural Ecuadorian adolescents and ii) to analyze the associations of physical fitness and lipid profile in adolescents according to residential location.

In this study data from 648 adolescents were analyzed (83.3% of total sample). The average age was 13.6 ± 1.2 years and 52.3% of the population was male. In the rural area, more females (61.4%; n = 97) participated (p < 0.001) than in the urban area (43.3%; n = 212). According to the result of the aerobic capacity test, 59% of the adolescents (55.0% urban and 73.5% rural) fell below the Healthly Fitness Zone. Physical fitness with respect to the other EUROFIT tests was lower among adolescents whose aerobic capacity was below the Healthy Fitness Zone, with significant differences in all tests (p < 0.05) except for the plate tapping (p = 0.12).

There was no significant difference in mean age (p = 0.54), BMI (p = 0.35), cardiopulmonary fitness (p = 0.99), speed shuttle run (p = 0.44), plate tapping (p = 0.71), sit and reach test (p = 0.54), sit-up (p = 0.30), vertical jump (p = 0.89), bent arm hang (p = 0.11), handgrip (p = 0.55) and flamingo (p = 0.09) tests between the subsample providing blood samples and the total population that participated in physical fitness assessment. Only the gender balance (p = 0.03) was marginally different between the subsample who provided blood sample and the whole sample (52.8% girls in the subsample versus 47.7% girls in the total sample).

To our knowledge, this is the first study in a middle-income country that estimates physical fitness in urban and rural adolescents and explores its associations with blood lipid profiles. The findings show that more than half of the sample exhibits unhealthy levels of physical fitness. Furthermore, adolescents who had a low aerobic capacity as defined by the FITNESSGRAM had lower scores for physical tests, such as speed-agility, flexibility, muscle strength-endurance and balance. Our findings also show that urban adolescents were fitter than rural adolescents for five of the fitness test. Nevertheless, these differences in physical fitness did not explain those in lipid profile between urban and rural adolescents.

Two out of three Ecuadorian adolescents in our sample had early cardiovascular risk, defined by low aerobic capacity (20 m shuttle run). This proportion was higher than the proportion reported in Spanish [37] and Portuguese [38] adolescents. Furthermore, the group of adolescents who had a lower aerobic capacity also showed lower scores for other physical fitness components such as muscle strength and endurance. Previous research indicates that such low fitness levels can linger on into adulthood [39] where low cardiorespiratory fitness [40] or low muscular strength [41] is associated with increased mortality risk.

In general, the absolute physical fitness of our population was worse than estimates in the majority of previous studies. Adolescents from our sample had a lower cardiopulmonary performance (3.2 ± 1.3 laps) compared with Spanish [42] (6.1 ± 2.0 laps) and Belgian [43] (6.3 ± 2.3 laps) adolescents. The estimates from the speed agility components of the physical test (speed shuttle run 10 × 5 m, plate taping) were also lower compared with Spanish [42], Greek [44], Polish [45] and Belgian [43] adolescents [42–45]. The sit and reach scores were lower than those from Mexico [11], Spain [42], Poland [45] or Belgium [43]. However, the large variation between studies, when considering the results from muscle strength and endurance tests (sit-ups, vertical jump, bent-arm hang and handgrip), renders comparison to the present study difficult. For sit-ups we obtained lower absolute values compared to estimates from Spain [42], Poland [45], Turkey [10] or Belgium [43]. Also, the estimates from the handgrip test were lower than those from previous studies [10, 11, 42, 44, 45]. Conversely, for the sit and reach test, we obtained a higher score compared with Greek [44] and Turkish [10] adolescents. In our results for sit-ups our adolescents averaged higher scores than adolescents in a Mexican study [11]. The favorable fitness scores in European as compared to Ecuadorian adolescents may be a reflection of the favorable environmental conditions for physical activity found in Europe [46], as well as a longer tradition of health promotion programs [47], and genetic factors [48, 49]. This hypothesis may be reinforced by the fact that our results were similar when compared to those from Mexican [11] and Colombian [13] studies, which have similar environmental and genetic patterns to those of Ecuador [48].

Compared with rural adolescents, the urban participants in our sample had a significantly better performance for the cardiopulmonary, speed-agility, and muscle strength and endurance components of the fitness test. Although these findings are in line with measurements in Mexican [11] and Polish [45] adolescents, most literature consists of contradictory results with regard to comparison of performance between urban and rural adolescents [10, 50–52]. Therefore, explaining the difference between urban and rural adolescents remains speculative. Firstly, the urban adolescents in our sample were taller and heavier than rural adolescents. It has been reported that the physical fitness is influenced by body size. Taller and heavier (not necessarily overweight or obese) children may therefore have an advantage on strength, speed, power and endurance components [53]. Secondly, urbanization and better social conditions in urban areas may mean that urban adolescents have increased access to sport facilities compared to rural adolescents [54–56]. Organized sports facilities are more common in urban areas and might result in higher levels of cardio-respiratory and muscular fitness in urban adolescents [42]. Thirdly, we observed that urban schools had specialized physical education teachers in their physical education programs, while these kinds of specialized teachers were virtually absent in rural areas. In addition, a lower availability of sport facilities in rural schools might result in a lower variety of sport activities. The latter was confirmed during our observations in the schools themselves. As a point of potential bias, urban adolescents are possibly more familiar with physical fitness tests than rural adolescents [11, 44]. Fourthly, chronic undernutrition during childhood instigates mechanisms of adaptation such as growth stunting and reduced muscle mass. The latter are potentially related to the physical fitness impairment during adolescence and adulthood [25]. Indeed, chronic undernutrition mainly affects children in rural areas in Ecuador [18].

To our knowledge, only a few studies have analyzed the association of blood lipid profile with multiple components of physical fitness. These studies have reported that increased cardiorespiratory fitness and muscular strength are associated with favorable lipid profiles in adolescence [7, 24, 38, 57]. These associations were partially confirmed in our study. Total cholesterol and triglycerides were negatively associated with muscular strength in the urban area, whilst in the rural population these lipids were negatively associated with cardiorespiratory fitness.

We report that differences in blood lipid profile among urban and rural adolescents are not explained by differences in physical fitness, even after adjusting for BMI and total energy intake. The association found in this study between blood lipids and fitness was adjusted for BMI and total energy intake, as these factors have previously been found associated with blood lipids [4, 7]. Mean energy intake was not significantly different (P = 0.08) between urban (1863 ± 181 kcal/day) and rural (1766 ± 153 kcal/day) adolescents. (Ochoa-Avilés unpublished data). In our sample, the relationship of different blood lipid parameters with each of the EUROFIT tests according to residential location was generally weak and non-significant.

Another possible explanation for the differences in blood lipid profile among urban and rural adolescents may be the differences in moderate to vigorous physical activity [58], or body fat distribution [59]. Physical activity and fitness have been found independently associated with certain blood lipid levels among children and adolescents [6]. For example, the favorable TG and HDL levels are inversely associated with moderate to vigorous physical activity, independent of time spent sedentary [58] and fitness [6]. In our sample, the time spent on moderate to vigorous physical activity could be longer in urban adolescents compared to rural adolescents because of differences in the availability of sport facilities and organized group sports, detailed earlier in this discussion. In addition, qualitative research performed in adolescents from Cuenca and Nabón has shown that rural adolescents felt an inability to perform physical activity in contrast to the urban adolescents (Van Royen unpublished data). This fact could lead to differences in physical activity levels between urban and rural adolescents, as self-efficacy is an important determinant of physical activity in adolescence [60]. On the other hand, total cholesterol, LDL, HDL and TG also have been associated with fat distribution measured by skin-fold thickness. Lean adolescents, as determined using the skin-fold system, have been found to have healthier blood lipid profiles compared to their heavier peers [61]. However, skin-fold thickness was not a parameter measured in the present study.

There are a few limitations of this study. Firstly, its cross-sectional nature of only allows us to establish associations and not causality. Secondly, we did not measure important variables associated with blood lipids such as physical activity, pubertal stage, sex hormone level, skin-fold thickness and familial health background. Third, the blood lipid determinations were conducted only in a subsample. Nevertheless, there were no differences in physical fitness and BMI between the subsample and the total sample. Fourth, reliability and validity of EUROIFIT were not done in our sample. Although, EUROFIT has shown good validity in previous studies performed in the region [11]. We followed the EUROFIT guidelines in order to avoid source of bias, such as learning effect, or low motivation of adolescents to do their best performance during each test [20]. Measurements of the 20 m shuttle run could be influenced by the temperature and weather conditions during the test. In Cuenca and Nabón, however, the average temperature and altitude are similar. In addition, the estimation of VO_2max from the FITNESSGRAM standards of the 20 m shuttle run is known to vary with the equation used. A previous study [28] has tested the degree of agreement between various equations used to estimate VO_2max and the actual the VO_2max. In the present study, we used the equation that shows the highest agreement. Finally, our results could be compared with only one other similar study in a low- and middle- income country, which hinders comparison of our findings with previous data in similar populations. The trial included adolescents from high altitude urban and rural areas of Ecuador that are characterized by mixed mestizo (in urban area) and Amerindian (in rural area) ethnicities [49]. The external validity of our findings is hence limited to urban and rural schools in the regions that share these characteristics [62].

The results from our study suggest that 59% of Ecuadorian adolescents have poor physical fitness. Even though urban participants showed better scores in the majority of EUROFIT tests, physical fitness of the total population was lower compared to that of adolescents from other countries. These findings call for specific health promotion programs aimed to improve physical fitness among Ecuadorian adolescents. Differences in fitness did not explain differences in blood lipid profile between urban and rural adolescents. We only found a weak association between physical fitness and blood lipid profile, even after adjustment for energy intake. Additional studies are needed to clarify the frequent occurrence of unfavorable blood lipid profiles among rural participants. Such studies might explore associations with physical activity levels, body fat distribution, risk factors at early ages, familial hypercholesterolemia and ethnic differences.