Dietary supplement use is increasing and widespread in many European countries and the USA( Reference Bailey, Gahche and Thomas 1 , Reference Skeie, Braaten and Hjartåker 2 ). As an example, 51·9 % of the US adult population and 40 % of the Dutch adult population takes dietary supplements regularly( Reference Briefel and Johnson 3 , Reference Van Rossum, Fransen and Verkaik-Kloosterman 4 ). Yet evidence-based recommendations are established only for certain population subgroups( Reference Van Rossum, Fransen and Verkaik-Kloosterman 4 – 6 ), such as vitamin D and calcium supplementation among the elderly, which lowers their risk of fractures( 5 ). There is no scientific consensus regarding the benefits of dietary supplement use at the general population level( Reference Harrison, Holt and Pattison 7 ). Therefore, it can be assumed that an individual’s decision to take dietary supplements may be mainly a result of factors other than physiological necessity. Yet knowledge on individuals’ motives for taking dietary supplements is fragmented and inconclusive. Furthermore, no research has yet addressed the question of why certain consumers consciously choose not to take them( Reference Okleshen Peters, Shelton and Sharma 8 ). Therefore, the aim of the present study was to gather in-depth knowledge on individuals’ beliefs concerning dietary supplements. In addition, users’ and non-users’ most salient beliefs would be compared.
Dietary supplement use has been shown to be associated with a healthy lifestyle: sufficient fruit and vegetable intake, adequate level of physical activity, low alcohol consumption and being less likely to smoke( Reference De Jong, Ocke and Branderhorst 9 – Reference Kirk, Cade and Barrett 11 ). Additionally, women with a higher education level, higher income and belonging to higher age groups are more likely to take dietary supplements( Reference Kirk, Cade and Barrett 11 – Reference Blendon, DesRoches and Benson 13 ).
Certain motivational factors are also associated with dietary supplement use. Women who have stronger intentions to take dietary supplements, attach more value to their health and are more convinced that dietary supplements make them less susceptible to illnesses, are significantly more inclined to take dietary supplements( Reference Conner, Kirk and Cade 14 ). Regarding one’s reasons for and beliefs towards dietary supplement use, individuals are convinced that dietary supplements are essential to health and can preserve or improve their overall health condition( Reference Bailey, Gahche and Thomas 1 , Reference Okleshen Peters, Shelton and Sharma 8 , Reference Blendon, DesRoches and Benson 13 , Reference Marinac, Buchinger and Godfrey 15 – Reference Nichter and Thompson 17 ). When comparing dietary supplement users with non-users, the following attitude beliefs differ significantly: dietary supplements ‘help me to be healthy’; ‘stop me getting ill’; ‘are the best I can do for myself’; and ‘don’t do any harm’( Reference Conner, Kirk and Cade 18 ). The studies mentioned above provided relevant but limited evidence since data were collected within the framework of larger projects, such as the National Health and Nutrition Examination Survey( Reference Bailey, Gahche and Thomas 1 ), or the study used a selective sample( Reference Conner, Kirk and Cade 14 , Reference Conner, Kirk and Cade 18 ).
Qualitative research on beliefs regarding dietary supplement use is limited to a couple of studies that did not use a specific theoretical framework. In-depth interviews with ten dietary supplement users revealed that curing an ailment, preventing chronic diseases, gaining ‘peace of mind’, supplementing a poor diet, saving money on medical care and achieving cosmetic benefits are the most salient motives( Reference Okleshen Peters, Shelton and Sharma 8 ). Taking an anthropological approach, Nichter and Thompson( Reference Nichter and Thompson 17 ) identified the following five main categories of reasons for dietary supplement use: health management/enhancement (e.g. postponing ageing); harm reduction (e.g. counterbalance unhealthy behaviours); resisting illness (e.g. boost immune system); illness management (e.g. slow progress of disease); and ideology (e.g. adoption of ‘natural health’ philosophy). Four focus group discussions held in Germany revealed the following motives among users: being afraid of having micronutrient deficiencies; relieving one’s mind (unhealthy eating habits); preventive reasons (boost immune system); curative reasons (reduce health complaints); and restoring subjective well-being (e.g. sufficient energy level)( Reference Rehaag, Tils and Röder 19 ). Studies have also shown that most dietary supplement users have multiple reasons at the same time( Reference Okleshen Peters, Shelton and Sharma 8 , Reference Nichter and Thompson 17 ).
The studies mentioned above explored users’ overall motives for taking dietary supplements, but no attention has been paid to why certain individuals do not use dietary supplements. To explore specific beliefs and to be able to compare users and non-users, a socio-cognitive model was used as the theoretical framework in the present study. One of the socio-cognitive theories that explains (health) behaviours is the Integrated Change Model (ICM)( Reference De Vries, Mudde and Leijs 20 ). The ICM distinguishes between distal, pre-motivational and motivational factors that may influence behaviour. Distal factors, such as behavioural, psychological and biological factors, have an indirect effect on behaviour that is mediated by pre-motivational and motivational factors. Knowledge, risk perception, awareness and cues to action belong to pre-motivational factors. These socio-cognitive determinants further influence motivational factors: attitude (pros and cons), social influence (social norm, social modelling, social support) and self-efficacy. Intentions and factors of the motivational phase together influence individuals’ health behaviour. The ICM has been applied previously in different qualitative studies on health-related behaviours, such as alcohol use among pregnant women, binge drinking among young people, and genetics and cancer( Reference van der Wulp, Hoving and de Vries 21 – Reference de Vries, Mesters and Van de Steeg 23 ).
In sum, the present study aimed to answer the following research questions: (i) What are the most salient beliefs derived from the socio-cognitive determinants of the ICM regarding dietary supplements among users and non-users? (ii) What are the differences between users’ and non-users’ beliefs?
Methods
To gather in-depth information on Dutch individuals’ beliefs towards dietary supplements, semi-structured focus group discussions were conducted. Focus group discussion is a widely used method in health sciences, since it can provide information about a range of ideas and feelings individuals have about a certain topic of interest( Reference Stewart and Shamdasani 24 , Reference Rabiee 25 ). Furthermore, focus group discussions can shed light on the differences in perspectives between groups of individuals( Reference Rabiee 25 ). The in-depth information generated was also meant as a preparation for a quantitative study on dietary supplement use. Systematic interpretative content analysis was used as the methodological approach of analysing the data( Reference Hijmans 26 ).
Recruitment and participants
The aim was to include both current dietary supplement users and non-users in the study sample. To make a distinction between users and non-users, inclusion and exclusion criteria were set up. Individuals were regarded as dietary supplement users if they took at least one pill per week of one or more types of dietary supplements prior to the recruitment period of the study. Individuals who consumed dietary supplements exclusively for sports-related purposes were excluded. Athletes’ needs, reasons and experiences regarding dietary supplement use may be mostly related to enhancing their sporting achievements, which may not be representative of the general population of dietary supplement users( Reference Rehaag, Tils and Röder 19 ). The only inclusion criterion applied for non-users was not having taken any type of dietary supplements for three months prior to the recruitment period.
Online and offline platforms were utilized for recruitment. Digital advertisements were posted on social media and Internet search engines, such as Google AdWords. Hard copies of brochures and posters were placed on the noticeboards of grocery shops, drugstores, community centres and general practices in Maastricht. Researchers recruited also through the handing out of flyers at local events, such as the Maastricht book fair.
Individuals who were interested in the study could contact the research team by telephone or email to receive more information about it. When potential participants applied, they were asked to fill in a brief screening questionnaire to assess eligibility. Based on the answers, participants were allocated either to the dietary supplement user group or to the non-user group. Focus group appointments were made according to respondents’ availability.
Convenience sampling was used for recruiting participants until theoretical saturation (e.g. little or no new information was discovered) was reached( Reference Glaser and Strauss 27 ). As suggested by Krueger and Casey( Reference Krueger and Casey 28 ), an initial analysis sample was set to at least four to five focus groups in order to adequately address the research objective (i.e. to explore the most prevalent beliefs)( Reference Francis, Johnston and Robertson 29 ). Additionally, after each session the research team discussed the main points participants made and to what extent those points differed from opinions expressed in previous sessions. Recruitment took place as long as potential participants kept replying to the call for participation.
Focus group interview guide
Based on the ICM( Reference de Vries, Mesters and Van de Steeg 23 ), a semi-structured interview guide with open-ended questions was developed. The questions aimed to elicit data on participants’ definition of what a dietary supplement is, risk perception, attitudes (pros and cons), social environment (i.e. opinion of friends and family), self-efficacy of taking dietary supplements and response efficacy. Each topic was introduced by a central question, followed by additional prompt questions to stimulate the group discussion (see online supplementary material, Supplemental Tables 1 and 2). The semi-structured interview guide also gave individuals the opportunity to talk about additional relevant topics, such as (healthy) food.
The interview questions were pilot-tested both in an individual and a group setting. With the help of pilot-testing, researchers can develop and test the adequacy of research instruments( Reference Van Teijlingen and Hundley 30 ). Questions used for focus groups of users and non-users were as similar as possible, with slight changes made where needed (e.g. advantages of dietary supplements, users: ‘What are your most important reasons for taking dietary supplements?’; non-users: ‘Can you think of reasons why you would take dietary supplements?’).
Data collection and procedure
A standardized study protocol was established to keep the procedures of each session consistent. According to the protocol, after welcoming participants and providing coffee, tea and cake, a short introduction (research team, reason and aims of the study) was given. As stated in the invitation letter beforehand, it was emphasized that taking part in the discussion was confidential and voluntary and that withdrawal from participation was possible at any time. Participants were asked to write down their first name on a name badge to enable them to address each other by name. At the same time, individuals received a numerical identifier that was used in the notes and the transcripts. In this manner, it was impossible to trace back participants’ real identity based on the data. Before the actual discussion started, participants had the opportunity to ask questions. The focus group discussions were audio-recorded after participants gave permission to do so.
During the focus group discussions, the first author (E.M.P.) functioned as moderator and a research assistant took notes. The sessions took place in one of the buildings of Maastricht University and lasted 70 to 80 min on average. At the end of the sessions participants received vouchers to the value of 15€ and a travel reimbursement. The focus group audio materials were transcribed verbatim. In total thirteen focus group interviews were conducted; seven with dietary supplement users (twenty-eight individuals) and six with non-users (twenty-eight individuals). The focus group size varied between three and six participants.
Ethics approval
Ethics approval from an accredited Regional Medical Ethics Committee in the Netherlands was not needed, since the study does not fall within the scope of the Medical Research Involving Human Subjects Act (WMO): the present research is not of medical-scientific nature and participants were not subjected to procedures or required to follow certain rules of behaviour (http://www.ccmo.nl/en/your-research-does-it-fall-under-the-wmo). Prior to the focus group discussions, all participants received our brochure and invitation letter with important details about the study. This information was repeated verbally in the presence of participants before the actual group discussion took place. Verbal informed consent was obtained from all participants before each session.
Data analysis
Descriptive statistics (i.e. percentages, frequencies) were used to analyse the data of the screening questionnaire with the statistical software package IBM SPSS Statistics version 20. Transcripts of the focus group discussions were analysed by applying systematic interpretative content analysis( Reference Hijmans 26 ), using the qualitative data analysis software QSR NVivo version 9. The analysis process followed the stages of the framework approach involving familiarization, identifying a thematic framework, indexing, charting, mapping and interpretation( Reference Ritchie and Spencer 31 , Reference Lacey and Luff 32 ).
In the first stages, inductive coding was applied to the data independently by two coders. This step involved obtaining insight into participants’ salient beliefs belonging to the socio-cognitive constructs included in the topic guide (main themes). Coding was then compared and discussed to determine the final coding categories for users’ and non-users’ transcripts separately. Also, decision rules were made to standardize the coding process in the subsequent stages.
In the last stages, through deductive coding, the definite version of the thematic framework was applied to the data by two coders, using NVivo version 9 (2010). This step involved regrouping participants’ statements according to the index reference and, if needed, new coding categories were established( Reference Ritchie and Spencer 31 ).
In the interpretation phase data were reduced to present results at a reasonable level of abstraction. Through grouping data, the number of (coding) categories was reduced by collapsing those that were closely related into broader categories( Reference Elo and Kyngäs 33 ). In this manner, the most concrete and specific beliefs were at the lowest level of abstraction, and were then brought under integrative categories (sub-themes). At the highest level of abstraction are socio-cognitive constructs which are regarded as the main themes and consist of several sub-themes (Fig. 1). During data interpretation, users’ and non-users’ complete thematic frameworks were put against one another and were searched for differences and similarities at the sub-dimension level.
Fig. 1 An example of the abstraction process (positive attitude beliefs, users)
Results
Participants
In total, fifty-six individuals participated in the study. The majority were female (75 % of users; 61 % of non-users). Users were slightly younger (mean age 41·6 years, range 20–78 years) than non-users (mean age 44·2 years, range 19–68 years). More than the half of non-users (57·1 %) had past experience with using dietary supplements (see Table 1).
Table 1 Demographic characteristics of participants (n 56) in focus group discussions on dietary supplement use, Maastricht, the Netherlands, 2014–2015
NA, not applicable.
A priori and emergent themes from the qualitative data
The following themes were a priori included in the topic guide: (i) definition of what is a dietary supplement; (ii) positive attitude beliefs towards dietary supplements (advantages, reasons for use); (iii) negative attitude beliefs towards dietary supplements (disadvantages, reasons against use); (iv) risk perception (chance and severity of adverse effects); (v) social environment; (vi) self-efficacy regarding taking dietary supplements; and (vii) response efficacy (noticing product efficacy). Emerging themes from the focus group discussions were: (viii) attitude towards health professional and/or dietitian; and (ix) quality/safety of food, practices of food industry. In Tables 2, 3 and 4 direct quotations are given for illustrating each of these topics. Results are presented according to the overarching topics outlined above.
Table 2 Qualitative results from focus group discussions regarding pre-motivational factors among users and non-users of dietary supplements, Maastricht, the Netherlands, 2014–2015
Words in capitals are the main themes of the group discussions. A–F are sub-themes of the main themes. Words in bold font are dimensions of the sub-themes.
Table 3 Qualitative results from focus group discussions regarding motivational factors among users and non-users of dietary supplements, Maastricht, the Netherlands, 2014–2015
NA, not applicable.
Words in capitals are the main themes of the group discussions. A–Z are sub-themes of the main themes. Words in bold font are dimensions of the sub-themes.
Table 4 Qualitative results from focus group discussions’ newly identified themes among users and non-users of dietary supplements, Maastricht, the Netherlands, 2014–2015
NA, not applicable.
Words in capitals are the main themes of the group discussions. A–D are sub-themes of the main themes. Words in bold font are dimensions of the sub-themes.
A priori themes
Pre-motivational factors
Definition of dietary supplements
Users and non-users held very similar opinions about what a dietary supplement is and what its main functions are (Table 2, row A–B). Dietary supplements were described along several product property dimensions, such as their aggregate (e.g. liquid), form (e.g. powders, pills, drops), and type and nature of ingredients (e.g. vitamins, minerals, synthetic, natural).
Regarding functions that dietary supplements could fulfil, both users and non-users agreed that dietary supplements are by no means a replacement for food, and they should not be used out of pure laziness. The way dietary supplements may help to maintain health and treat micronutrient deficiencies in one’s body was also extensively discussed across several groups of users and non-users.
Risk perception
When speaking about commonly used products (e.g. multivitamins) risks were regarded as negligible by both groups (Table 2, row C–D) and participants gave a few examples of possible side-effects (Table 2, row E). Users and non-users were convinced that most dietary supplements may only cause adverse effects when they are excessively used, when the consumer’s body is not functioning properly, or when someone is using inappropriate products (Table 2, row D). Users and non-users also believed that dietary supplements undergo rigorous safety checks before being sold, otherwise – they argued – it would not be permitted to sell those products (Table 2, row F).
Motivational factors
Positive attitude beliefs
Regarding reasons for dietary supplement use, reducing health complaints and preventing illnesses were frequently mentioned aspects both by users and non-users (Table 3, row A). Users also reported how dietary supplements help them enhance their mental performance. Also, users indicated enjoying the taste and use of dietary supplements (Table 3, row B).
Only non-users discussed micronutrient deficiency and evidence-based effectiveness of the products as important reasons for considering taking dietary supplements (Table 3, row B). In addition, only non-users indicated that, in general, the persuasiveness of commercials or individuals’ laziness could also lead to dietary supplement use (Table 3, row C–D).
Negative attitude beliefs
Both users and non-users regarded the commercial nature of the dietary supplement industry as a disadvantage (Table 3, row E). Also, in both groups concerns were expressed towards product properties, such as high-dosage pills, uncertainty about ingredients and lack of sufficient evidence of safety (Table 3, row E–I). Regarding eating habits, users and non-users agreed that individuals’ nutritional intake may decline as a consequence of using dietary supplements (Table 3, row F).
While users raised the issue that getting micronutrients from dietary supplements is not natural, non-users went further and regarded dietary supplements as an intervention in the normal functioning of the body (Table 3, row F). Concerning necessity (Table 3, row G), users emphasized the need to think thoroughly about the aims one will achieve with dietary supplements and then decide whether to (continue) to take them. Non-users approached necessity differently: supplementation is only necessary when suffering from deficiencies and there is no need for dietary supplements if individuals already eat healthily (Table 3, row G).
In several groups of users an extensively discussed topic was whether micronutrients from dietary supplements are properly absorbed through the digestive system (Table 3, row I).
Social environment
Both users and non-users reported being exposed to different social influences at the same time. Individuals in their social environment held different opinions about dietary supplement use, ranging from being positive and encouraging to being negative and discouraging (Table 3, row J–N). Besides approvers and disapprovers, there were also people in participants’ social environment who consider regular food as a sufficient source of nutrients and therefore taking dietary supplements is not necessary (Table 3, row M). Furthermore, users and non-users often heard from others that dietary supplements are harmless (Table 3, row K).
Both users and non-users indicated that their decision regarding dietary supplement use did not depend on others’ opinions. At the same time, both groups admitted that ‘dietary supplement use’ is not a commonly talked about topic.
Self-efficacy
Users and non-users mentioned situations in which dietary supplement use would be difficult (Table 3, row P). Some of the situations mentioned by both groups were: having a different (daily) routine than usual (e.g. being on holiday) or not liking the taste of the product. Users also frequently mentioned forgetfulness as a barrier to taking dietary supplements.
Additionally, users and non-users mentioned a few situations in which they would never stop using dietary supplements: when suffering from micronutrient deficiencies and in the case of medical prescription (Table 3, row O).
Response efficacy
Only users were asked whether they actually experience the beneficial health effects that dietary supplements are intended for (Table 3, row Q–Z). Users agreed that taking dietary supplements can help people improve their health. Yet users’ opinions differed regarding their ability to experience directly those beneficial health effects. Only some of them could precisely indicate whether and how they notice the advantageous effects, and a large number of participants gave a detailed explanation as to why they have doubts about an immediate experience (Table 3, row W–Z). Users’ doubts were related to the preventive use of the products and the difficulty in determining effectiveness. Furthermore, users were aware of possible placebo effects through which dietary supplements may seem to be effective.
Emergent themes
Attitude towards health professional and/or dietitian
One of the unprompted discussion topics raised both by users and non-users was their experience with and opinion towards physicians (Table 4, row A–B). Several users had had unfavourable experiences with their physician, such as not being able to diagnose a health problem. Users also did not confide in dietitians and labelled their recommendations as ‘old-fashioned’ or ‘inappropriate’. However, several users consulted regularly with so-called anthroposophical general practitioners (adherents of a spiritual philosophy aiming to optimize physical and mental health, and well-being) who gave them advice about lifestyle and diet, including recommendations for dietary supplements. In contrast, the majority of non-users held a positive attitude towards their physician (Table 4, row A).
Quality/safety of food, practices of food industry
An additional frequently discussed topic by users and non-users was food quality (Table 4, row C–D). Very often, a comparison was made between food nowadays and 50–100 years ago regarding quality, supply and food industry practices. The majority of users were convinced that the quality of food products has gone downhill recently and food contains considerably lower amounts of vitamins and minerals (Table 4, row D). As users further argued, quality deterioration is a result of poor practices by the food industry.
Non-users acknowledged that the quality of food products may have declined a little recently. Yet food products still may contain sufficient amounts of vitamins, minerals and other important nutrients. Moreover, compared with 100 years ago, there is more opportunity to eat healthily as a result of the enriched variety of supply of fruits and vegetables (Table 4, row C).
Discussion
The present study sought to explore and compare users’ and non-users’ salient beliefs related to dietary supplement use, applying the ICM as its theoretical background. Users’ and non-users’ beliefs were comparable regarding the risks of dietary supplements, how they define dietary supplements, and their decision regarding whether or not to take dietary supplements being independent from their social environment. Users’ and non-users’ beliefs differed substantially regarding their attitude beliefs towards dietary supplements, trust in health professionals and judgement on the quality of food products.
The majority of users and non-users defined dietary supplements as products that are intended to supplement the normal diet. Both groups considered the risks of dietary supplements as low and assigned the occurrence of possible adverse effects more to the consumer (e.g. excessive use, being ill) than to the product. How dietary supplements are judged by significant others (e.g. family, friends) did not seem to influence participants’ own decision about whether to use dietary supplements or not.
Users considered dietary supplements as a potential tool to improve their health. Users’ most frequently mentioned reasons for taking dietary supplements can be grouped into three overarching categories: preventive reasons (e.g. to avert getting the flu); mental and physical health enhancement (e.g. to enhance mental performance); and curative benefits (e.g. to reduce health complaints). Users’ motivations were most commonly related to certain (health) complaints that they suffered from. This finding is in line with previous research that suggests that health problems such as joint pain, muscle cramp, osteoporosis or arthritis are frequently mentioned motives for taking dietary supplements, especially among the elderly( Reference Bailey, Gahche and Thomas 1 , Reference Brownie and Rolfe 34 ).
In contrast, non-users put emphasis on investigating the necessity of use first when discussing reasons for taking dietary supplements. As they argued, nutritional deficiencies can be detected, for example with the help of a blood test. Non-users indicated that a healthy, balanced and varied diet would be of sufficient quality for most people to cover their nutritional needs. Additionally, most non-users were convinced that the human body is able to regulate all the different biological processes properly on its own. Therefore, they considered supporting the normal functioning of the body with dietary supplements as unnecessary. Hence, non-users did not feel the need to improve their health as much as users did.
Most dietary supplement users were critical of general practitioners’ and dietitians’ knowledge on nutrition, whereas the majority of non-users did not have such a critical view of their physician. Research has shown that 44 % of frequent dietary supplement users believed that their own physician knows little to nothing about nutrition( Reference Blendon, DesRoches and Benson 13 ). The distrust towards health-care providers was also revealed by previous research showing that dietary supplement users often took control of their own health by seeking alternative forms of medicine, including dietary supplements( Reference Greger 10 ).
Users’ and non-users’ beliefs regarding food quality and safety also differed considerably. Whereas users estimated food quality as poor, non-users still considered food products as a sufficient source of nutrients. Evidence suggests that dietary supplement users tend to have a healthy lifestyle and sufficient nutritional intake( Reference Kirk, Cade and Barrett 11 , Reference Bailey, Gahche and Lentino 12 , Reference Bailey, Fulgoni and Keast 35 ). The fact that dietary supplement users feel the need to supplement their nutritious diet might be explained by their distrust in food quality.
It is important to note that many users were uncertain whether they actually experienced any of the advantageous health effects of dietary supplements. Despite these doubts, users still believed that dietary supplements help improve their health. High levels of response efficacy may explain a contradictory result of a previous study: about 70 % of dietary supplement users would still take dietary supplements, even if the government or the Food and Drug Administration announced that dietary supplements are ineffective( Reference Blendon, DesRoches and Benson 13 ).
The findings mentioned above may imply that both users and non-users base their decision regarding dietary supplement use mainly on their beliefs. To facilitate consumers’ informed decision-making regarding dietary supplement use, users’ and non-users’ understanding of dietary supplements’ health effects should be improved.
Limitations
As with many qualitative studies, the present sample may be biased since convenience sampling was used to recruit participants. Therefore, future research should assess individuals’ motives towards dietary supplement use quantitatively, using a representative sample. The semi-structured topic guide made it possible to standardize the study procedure and topics discussed. But due to the predefined nature of the discussions, participants had limited opportunities to discuss relevant topics that were not included in the topic guide. With regard to theoretical saturation, the research team evaluated each session in terms of differences and similarities in beliefs compared with previous sessions, but some aspects of dietary supplement use may be still undiscovered. As an example, in contrast to previous studies, compensation for unhealthy eating habits was not raised as a reason for taking dietary supplements by users( Reference Nichter and Thompson 17 , Reference Rehaag, Tils and Röder 19 ).
Implications for future research
Data were collected among a selective sample of Dutch individuals in the region of Maastricht. Therefore, the study should be extended to more areas in the Netherlands as well as to other countries.
Several salient beliefs regarding dietary supplements, food and health have been discovered in the present study. However, it is impossible to investigate the predictive value of those beliefs regarding dietary supplement use based on qualitative research. Therefore, future quantitative longitudinal studies should assess to what extent socio-cognitive factors determine dietary supplement use and non-use, and address the question of, for example, whether decreased trust in physicians and negative judgement on food quality may indeed lead to dietary supplement use.
Some salient beliefs identified by the current study can be related to existing theoretical constructs, such as mental and physical health enhancement and illness prevention, which correspond to the two motivational needs distinguished by the regulatory focus theory( Reference Higgins 36 ). In addition, the differing needs of users and non-users for managing their own health can be related to Rotter’s locus of control theory( Reference Rotter 37 ). Therefore, the predictive value of relevant socio-cognitive and psychosocial factors of such theories should be tested statistically in a representative sample of the Dutch population.
Conclusion
The aim of the present study was to explore and compare dietary supplement users’ and non-users’ beliefs towards dietary supplement use through applying the ICM as its theoretical background. Users’ and non-users’ beliefs corresponded to a large extent regarding their definition of dietary supplements, the evaluation of the risks of dietary supplements and the influence of the social environment. Users’ and non-users’ beliefs differed considerably regarding their attitude beliefs towards dietary supplements, their trust in health professionals and their trust in the quality of food products.
Acknowledgements
Acknowledgements: The authors would like to thank the participants for sharing their opinions on dietary supplement use, and the research assistants who performed different tasks. Financial support: This study was funded by the Netherlands Food and Consumer Safety Authority. The Netherlands Food and Consumer Safety Authority had no role in the design, analysis or writing of this article. Conflict of interest: The authors have no conflict of interest to declare. Authorship: Each author of this article contributed significantly to the study. All authors read and approved the final manuscript. Ethics of human subject participation: Ethics approval from an accredited Regional Medical Ethics Committee in the Netherlands was not needed, since the study does not fall within the scope of the Medical Research Involving Human Subjects Act (WMO): the present research is not of medical-scientific nature and participants were not subjected to procedures or required to follow certain rules of behaviour (http://www.ccmo.nl/en/your-research-does-it-fall-under-the-wmo). Prior to the focus group discussions, all participants received our brochure and invitation letter with important details about the study. This information was repeated verbally in the presence of participants before the actual group discussion took place. Verbal informed consent was obtained from all participants.
Supplementary Material
To view supplementary material for this article, please visit https://doi.org/10.1017/S1368980017001707
