توضیح مصرف میوه ها و سبزیجات: تئوری رفتار برنامه ریزی شده و تصور غلط سطوح مصرف شخصی

The influence of individuals' misconceptions in assessing fruit and vegetable consumption on the ability of the theory of planned behaviour to explain variance in the consumption of these foods was studied. Dutch women (mean age 41, n=159) completed a questionnaire assessing the theory's constructs with regard to the daily consumption of at least two pieces of fruit and 200 gram of vegetables. Consumption was assessed using a self-rated measure and more objectively with a food-frequency questionnaire. Both measures were combined to classify participants according to the accuracy of their self-assessed intake levels (‘realists’ vs. ‘overestimators’). The model explained variation in objective fruit and vegetable intake much better among realists (R2=45% for fruits and 39% for vegetables) than among overestimators (R2=18 and 5%, respectively). Perceived behavioural control was the strongest predictor of intentions and behaviour. When plasma vitamin C and carotenoid concentrations were used as objective indicators for fruit and vegetable intake, the explanatory value of the model was lower, but again more variance was explained among realists than among overestimators. We conclude that awareness of personal behaviour should be taken into account when applying the theory of planned behaviour to explain dietary behaviours as well as to design health education interventions.

Although fruits and vegetables (FV) are a major dietary source of vitamins and fibre, and results of many observational studies in humans suggest that ample consumption of FV may prevent cardiovascular disease (Ness & Powles, 1997) and cancer (Steinmetz & Potter, 1996), mean consumption of these foods is still below the intake levels recommended by public health authorities (200 g of vegetables and two pieces of fruit according to Dutch guidelines) (Hulshof, 1998). Many people seem not to be aware that their FV consumption is below the recommended level (Lechner, Brug, & De Vries, 1997). The present article deals with the influence of an optimistic bias in self-rated FV intake on the strength of the relations between FV consumption and its determinants. A model that has proved useful in explaining or predicting a variety of behaviours is the Theory of Planned Behaviour (TPB) (Ajzen, 1991). The TPB originated from the earlier theory of reasoned action (TRA), which states that the likelihood of someone engaging in a particular behaviour can be predicted by their intention to perform that behaviour, which in turn is predicted by attitudes and subjective norms. Attitude refers to the degree to which a person has a favourable or unfavourable evaluation or appraisal of the behaviour in question. Subjective norm refers to the perceived social pressure to perform or not perform a behaviour. In order to extend the scope of the TRA to behaviours that may not be under complete volitional control, the perceived behavioural control construct was introduced as a third predictor of behavioural intention. Perceived behavioural control reflects the perceived ease or difficulty of performing a particular behaviour, and is thought also to have a direct effect on behaviour, which is not mediated by intentions (Ajzen, 1991). In a meta-analysis including 185 independent studies on the predictive potential of the TPB for a variety of health-related behaviours, the TPB explained 27 and 39% of the variance in behaviour and intention, respectively (Armitage & Conner, 2001). For complex health-related behaviour such as diet and physical activity, the predictive value of the TPB or the related Attitude-Social influence-Efficacy (ASE) model (De Vries, Dijkstra, & Kuhlman, 1988) may be more limited (Brug, Lechner, & De Vries, 1995). Besides the fact that such behaviours could be partly habitual, this may be caused by the fact that what is studied as one behaviour (e.g. fruit consumption) in reality consists of many separate behaviours (e.g. buying, preparing and eating apples, pears, oranges) each of which may have its own determinants. The complexity of dietary behaviour probably also contributes to the fact that people tend to have an unrealistically optimistic view of their own intake levels. For example, a study by Lechner et al. (1997) showed that of those people who do not eat the recommended amounts of vegetables and fruits, 88 and 65%, respectively, think they actually do meet the recommendations. This misconception of their own consumption would be expected to reduce the explained variance in behaviour when an objective measure of behaviour is used instead of a self-rated measure. The above meta-analysis supports this hypothesis: studies using self-reported behavioural measures found an explained variance of 31% compared to 21% in studies using objective or observed behavioural measures (Armitage & Conner, 2001). Work by Brug et al. showed that psychosocial determinants (outcome beliefs, attitude, social pressure, self-efficacy, modelling) of fat intake were stronger correlates of self-rated fat intake than of fat intake as assessed by a food-frequency questionnaire (FFQ) (Brug, 1994 and Brug and Van Assema, 2001). Similar findings have also been reported concerning physical activity (Ronda, Van Assema, & Brug, 2001). Therefore, we hypothesized that in the TPB model a higher percentage of variance should be explained in participants having a realistic view of their FV consumption (in other words, individuals whose self-rated consumption approaches objective consumption) than in participants misjudging their FV intake. Due to the discrepancy between subjective and objective measures of FV intake, we cannot rely on self-rated measures if we want to know whether someone eats the recommended amounts of FV. A FFQ is a more objective instrument widely used for estimating habitual intake of foods (Willett, 1994). Nevertheless, because a FFQ is a self-administered assessment instrument, there remains a possibility of bias with respect to the recall of past diet, socially desirable answering, or the (conscious or subconscious) desire to match answers given in a FFQ to answers given in a questionnaire for measuring psychosocial determinants of consumption. Truly objective measures of dietary intake are biochemical markers of consumption (van 't Veer, Kardinaal, Bausch Goldbohm, & Kok, 1993), frequently used biomarkers for FV consumption being plasma carotenoids and vitamin C. Concentrations of these vitamins were found to correlate with the intake of FV as assessed by dietary questionnaires (Campbell et al., 1994 and Drewnowski et al., 1997), although correlations are moderate due to biological variation between persons in, for example, absorption or metabolism of vitamins. Plasma carotenoid and vitamin C concentrations respond to changes in dietary FV intake and were reported to stabilise within approximately two weeks of changing FV consumption to a new stable level (Yeum et al., 1996 and Zino et al., 1997). To our knowledge, biomarkers have not been used previously as an outcome measure in studies of psychosocial determinants of FV consumption. The objectives of the present study were to determine the effect of misconception of FV consumption on the explanatory value of the TPB, and to identify which TPB constructs predict intentions and FV consumption. Additionally, we tested the use of biomarkers as a measure of FV consumption in the TPB.

The mean age of the women was 41 (SD 4, range 29–50) years. Mean BMI was 24.1 (SD 3.5, range 18.7–36.1) kg/m2. Ninety-one percent were married or living together with their partner, 42% had a high educational level (polytechnic, university), 52% an intermediate level (high school) and 6% a low level (elementary school). Descriptive statistics for the TPB constructs and FV consumption are shown in Table 1. The distributions of attitude, perceived behavioural control and intention were skewed to the right for fruit and vegetables, indicating that most participants had positive scores for these variables. All scores were lower for fruit than for vegetable consumption. The percentage of misconception was higher for vegetable (68%) than for fruit consumption (40%). Misconception almost exclusively took the form of overestimation, except for two participants who underestimated their vegetable intake. Because of the small number of underestimators, we only show data for overestimators in the stratified analyses. With respect to the consumption of fruits, overestimators had more positive attitudes, a greater perceived control and a stronger intention, and higher self-rated and objective consumption. With respect to vegetable consumption, overestimators had a greater perceived control, a higher self-rated consumption and a lower objective consumption. The lower fruit consumption of realists compared to overestimators was a result of the fact that 86% of the realists ate less than two pieces of fruit a day (of whom 79% ate less than one piece), whereas for vegetable consumption 45% of the realists ate less than 200 g of vegetables a day.