The aim of this study was to investigate the association between type of SDT motivation and diet and physical activity behaviours. Motivation partly explained the dietary and physical activity behaviour of vocational students. First, amotivation was associated with consuming more unhealthy products per week and consuming breakfast less often. This type of motivation is thus associated with unhealthy dietary behaviour. Second, high autonomous motivation scores showed a significant correlation with consuming less unhealthy products per week and consuming more water, breakfast and fruit and conducting more MVPA. Autonomous motivation thus seems to explain most of the healthy dietary and physical activity behaviour of vocational students. Third, controlled motivation did not show an association with any of the variables. It thus does not seem to explain the variation in diet and physical activity behaviour of vocational students. For moderate and vigorous physical activity separately, no motivation type seems to be of influence, since no type of motivation showed a significant association. Autonomous motivation is however significantly associated with MVPA, probably due to its large correlation with vigorous physical activity. For diet soda, no significant associations were found either. The most likely explanation for this is that it was not consumed enough in this target group to find any effect.
The average overall motivation scores show that autonomous motivation scored high for both diet and physical activity behaviour, while amotivation scored relatively low. This is interesting, because the vocational students generally did not make very healthy choices, as most dietary and PA guidelines were not met except for the MVPA guidelines in students 18 years and older. Socially desirable answers might have had an influence in this. They could have caused an overestimation of the autonomous motivation scores and an underestimation of the amotivation scores.
The type of VET education did not explain a large part of the variance of the variables. It was expected that type of education was an important factor of clustering in the data, because social norm is very important for emerging adults (33). However, the ICC for water consumption is smaller than 0.001. The other ICCs for dietary behaviour are larger and show that 2.1% to 8.2% of the variance in dietary variables was related to the type of VET education that participants followed. For PA, the ICCs explain 0.7 to 9.5% of the variation. The low intra-class correlation could be due to the fact that the group level variable that was used to adjust for clustering of the data might have been too large or too heterogeneous. Social norms may have more influence in smaller or homogeneous groups, like class level instead of type of VET education.
The associations found in this study are consistent with previous research in other age groups and educational levels. Amotivation shows a negative association with some of the dietary behaviours of vocational students. A possible explanation for this negative association is the indifferent attitude that vocational students have towards making healthy lifestyle choices. Giles and Brennan (33) found that British late adolescents (aged 18-25) are not willing to put much effort into adopting a healthy lifestyle and thus have a rather indifferent attitude to it. This seems to be the same in Dutch vocational students. If this indifferent attitude leads to amotivation, it could explain its negative effect on dietary behaviour. Amotivation however also showed a significant positive effect on consumption of diet soda. A possible explanation for this is the fact that students show no awareness of calorie content in beverages. It was found that the most important factors for choosing beverages for college students (mean age 19y) were taste and price(49). Health thus might not be an important factor for vocational students when consuming diet soda. Autonomous motivation is described as the type of motivation that facilitates persistence and sustainability of behaviour due to its high levels of autonomy, while controlled motivation does not lead to sustainable behaviour(23,50,51). This explains why autonomous motivation is important in healthy diet and physical activity behaviour in vocational students. Results of other studies follow the same pattern; in early and mid-adolescents, autonomous motivation was found to be associated with increased fruit intake and increased physical activity (27–30) and in vocational students autonomous motivation was found to be associated with MVPA (28). Controlled motivation apparently is not enough to maintain the healthy lifestyle behaviours investigated in this study. It often does not lead to sustainable behaviour, because controlled motivation is characterized by lower levels of autonomy compared to autonomous motivation. This can lead to a relapse to old behaviour, as the external factor that drove the motivation is likely to be removed at some point (52).
Limitations, strengths and recommendations
The first limitation of this study is the use of self-administered questionnaires. This may have caused recall bias. In case of diet and physical activity questions, participants tend to be too positive about their habits (53). In this study, recall bias could thus have led to an overestimation of the diet and physical activity behaviour of the study population. Additionally, the SQUASH questionnaire is known to overestimate the physical activity that participants conduct, which could have caused an overestimation of the physical activity of vocational students (54). The exact effect that these possible overestimations might have had on the found associations cannot be inferred. Second, the cross-sectional nature of this study is a limitation, as type of motivation and behaviour were measured at the same time, their interrelationship does not necessarily reflect a causal association.
Finally, the study population consisted of vocational students from three VET school locations in the urban region of the Netherlands. This makes the results not automatically applicable for VET education in general. Female students were overrepresented in the sample. This may have caused an overestimation of the diet and physical activity behaviour of the study population, because being female is related to having healthier lifestyle habits (40). Furthermore, the sample included a large number of Lifestyle & Sports students. This type of VET education attracts many students that are interested in sports and lifestyle. Therefore, this could have caused an overestimation of the diet and physical activity behaviour of the study population, especially in the amount of physical activity that vocational students engage in. The effects that the above-mentioned factors had on the associations cannot be inferred. The external validity of this study could thus be improved by obtaining a more representative sample of vocational students.
Despite the above-mentioned limitations, this study is one of the first study to our knowledge that reports associations between type of SDT motivation and dietary and physical activity behaviour of vocational students. Therefore, it provides new and much needed insights in their motivation and health behaviour. Moreover, the large sample size of the study increased its reliability. Furthermore, the use of multilevel analyses strengthened the conclusions of the study, because variability due to clustering of the data was accounted for.
For future research, we recommend diving deeper into the topic of SDT and self-directed health behaviours among vocational students. More insight is needed into the three basic psychological needs, autonomy, competence and relatedness and their relationship with autonomous and amotivation, in order to develop health promoting interventions for this group. In addition to diet and physical activity behaviour, more variables can be investigated to get a more complete picture of the determinants of vocational students’ health behaviour.
The results of this study show a strong association of autonomous motivation with dietary behaviour and MVPA in vocational students. Raising the mean autonomous score for diet with one point could lead to 10-20% increases in the mean scores for healthy dietary choices and a 15% decrease in the mean number of unhealthy week choices. For MVPA, an increase of 74 minutes a week can be accomplished by increasing autonomous motivation with one point. The associations found implicate that autonomous motivation is a reasonable target in the development of health promoting interventions.
A review by Ng et al. (21) showed that enhancing autonomous motivation led to beneficial health outcomes. Furthermore, satisfying basic psychological needs was found to be important. To enhance autonomous motivation, autonomy-supportive interventions must focus on four SDT components. First, they must increase the sense of competence of participants. Second, these feelings of competence must be coupled with feelings of autonomy. Third, interventions must make sure to give participants a sense of security or relatedness. Lastly, extrinsic rewards must be avoided, as they stimulate controlled motivation instead of autonomous motivation (52). One possible intervention to enhance autonomous motivation is motivational interviewing, as this is a method to adhere to behaviour change with many parallels with the mentioned SDT concepts (55). In adolescents, motivational interviewing was found to be effective in promoting several healthy behaviours (56). In addition, the peer relations have a positive effect on autonomous motivation. Gairns et al. (57) found that high school students showed stronger autonomous motivation for physical education class, when they felt a positive relatedness with their fellow classmates. Both motivational interviewing and enhancing positive peer relations could thus be effective in enhancing autonomous motivation among vocational students.