The present paper studied the effects of implementation intentions to provide children (0–4 years) with 10 µg of vitamin D per day on (1) the number of parents who perform adequate supplementation behavior, (2) the number of days that the child was given vitamin D supplementation and (3) the number of parents who had vitamin D supplementation at home.
Our results confirm earlier studies indicating that, although almost all parents have a high intention to supply their child with vitamin D, only 50% of children receive adequate supplementation. Unfortunately, we have to conclude that the implementation intentions used here did not result in improvements on any of the outcome measures at traditional levels of significance (P
< 0.05), which contrasts with earlier findings that implementation intentions positively influenced various health behaviours [12
] including vitamin C pills [13
There may be several explanations for the lack of significant effects of the implementation intention intervention in this study. First, the intervention might have been too simple to be effective. We tested the effects of implementation intentions via a certified Internet panel before investigating their impact in toddler consultation clinics. This meant that parents had to form implementation intentions by themselves, and we could not control whether they took the time to think seriously about a plan or whether their plans were realistic and specific enough. Guiding participants with forming implementation intentions could have led to more realistic plans [17
], which could be a facilitated at toddler consultation clinics by training nurses in this domain.
Second, the study sample (n
= 171) could have been too small to detect differences between the groups. Although the Internet panel we used contains over 20,000 participants, there were only 900 participants in the target group. Of the participants, about 50% already performed the desired behaviour (conform [5
]) or did not have a positive intention toward vitamin D supplementation. The remaining group was probably too small to get significant results of the intervention. Moreover, it is possible that the intervention took participants by surprise, as questionnaires from this Internet panel do not usually include an intervention.
A third explanation could be a test effect of filling out the questionnaire. Participants in the control group may have been influenced by the extra attention for vitamin D supplementation or prompted to remember information about vitamin D that they had received in the past, which may explain why participants in the control group showed improvements on all outcome measures as well. So, parents are cued by filling in the questionnaire, and subsequently, there is limited additional value of forming implementation intentions. However, it is possible that only when implementation intentions are formulated, an automatic behavioural response is created due to the repeated responses in the same situation. If so, possibly, long term effects may only be achieved when implementation intentions are formulated.
The sample in our study consisted mainly of highly educated parents of Dutch origin. This does not fully represent the general Dutch population. Children of non-European parents are particularly vulnerable to vitamin D deficiency, due both to darker skin pigmentation that limits the conversion of vitamin D and (in some cultures) to the use of long sleeves and pants, and/or the use of oil instead of fortified margarine. We therefore recommend also studying the impact of implementation intentions on vitamin D supplementation among immigrant parents and parents with lower educational levels.
This study suggests that merely asking parents to formulate a plan towards giving their child daily vitamin D supplementation is insufficient to improve vitamin D intake among young children to desired levels. As mentioned above, we suspect that by using electronic questionnaires via an Internet panel to test our intervention we have not exploited the full potential of the strategy. Given the positive evaluation by the participants and better improvements in the experimental group on all outcome measures, we believe it is worth exploring the use of implementation intentions in the setting of toddler consultation clinics in order to stimulate habitual vitamin D supplementation. If nurses are trained to discuss implementation intentions with the parents, this could lead to more specific plans and probably to greater effects on vitamin D intake. However, pre-structured implementation intentions require less training of nurses. It has been stated that implementation intentions are most effective if the critical situation is clearly described. As a consequence, it is easier to recognize the situation and deliberation when encountering the situation is no longer necessary [7
]. Reminding participants of the plan they have made, for instance by sending a text message to their mobile phone or writing the plan on a fridge magnet, might also improve the impact of the implementation intention. It would therefore be worthwhile to investigate the willingness of child health clinics to include these simple strategies in their usual care.