Why do women use dietary supplements? The use of the theory of planned behaviour to explore beliefs about their use.

Dietary supplements use is increasing, despite the lack of evidence to suggest they are needed to meet dietary deficiency in the majority of people. Reasons for consuming dietary supplements are likely to be complex, combining social, psychological, knowledge and economic factors. The Theory of Planned Behaviour (TPB) is a widely used model of social cognition, which has recently been applied to the nutrition field. It was used in a questionnaire, along with a number of additional measures, to explore dietary supplement use in a cohort of women. Data from 303 questionnaires were included in the analysis. The results showed that intentions were the major predictor of dietary supplement use. Health value and susceptibility to illness were also significant predictors of dietary supplement use (total of 82.9% of respondents correctly classified as users or non-users). Intentions themselves were most strongly predicted by attitude, with 70% of variance explained by attitude, subjective norms and perceived behavioural control. Other significant predictors of intentions were control beliefs, normative beliefs and health value. Beliefs underlying dietary supplement use revealed differences between supplement users and non-users in relation to the notion that taking dietary supplements acts as an insurance against possible ill-health, with supplement users believing more strongly than non-users that taking dietary supplements would stop them getting ill and help them to be healthy. Both users and non-users of supplements also perceived the media, in the form of books and magazines, to be a powerful influence on a person's decision to use supplements. The findings of this study highlight the potential of the TPB in exploring supplement-taking behaviour, while throwing light on the factors influencing an individual's motivations to use dietary supplements.