Effects of health literacy, screening, and participant choice on action plans for reducing unhealthy snacking in Australia: A randomised controlled trial

Background Low health literacy is associated with poorer health outcomes. A key strategy to address health literacy is a universal precautions approach, which recommends using health-literate design for all health interventions, not just those targeting people with low health literacy. This approach has advantages: Health literacy assessment and tailoring are not required. However, action plans may be more effective when tailored by health literacy. This study evaluated the impact of health literacy and action plan type on unhealthy snacking for people who have high BMI or type 2 diabetes (Aim 1) and the most effective method of action plan allocation (Aim 2). Methods and findings We performed a 2-stage randomised controlled trial in Australia between 14 February and 6 June 2019. In total, 1,769 participants (mean age: 49.8 years [SD = 11.7]; 56.1% female [n = 992]; mean BMI: 32.9 kg/m2 [SD = 8.7]; 29.6% self-reported type 2 diabetes [n = 523]) were randomised to 1 of 3 allocation methods (random, health literacy screening, or participant selection) and 1 of 2 action plans to reduce unhealthy snacking (standard versus literacy-sensitive). Regression analysis evaluated the impact of health literacy (Newest Vital Sign [NVS]), allocation method, and action plan on reduction in self-reported serves of unhealthy snacks (primary outcome) at 4-week follow-up. Secondary outcomes were perceived extent of unhealthy snacking, difficulty using the plans, habit strength, and action control. Analyses controlled for age, level of education, language spoken at home, diabetes status, baseline habit strength, and baseline self-reported serves of unhealthy snacks. Average NVS score was 3.6 out of 6 (SD = 2.0). Participants reported consuming 25.0 serves of snacks on average per week at baseline (SD = 28.0). Regarding Aim 1, 398 participants in the random allocation arm completed follow-up (67.7%). On average, people scoring 1 SD below the mean for health literacy consumed 10.0 fewer serves per week using the literacy-sensitive action plan compared to the standard action plan (95% CI: 0.05 to 19.5; p = 0.039), whereas those scoring 1 SD above the mean consumed 3.0 fewer serves using the standard action plan compared to the literacy-sensitive action plan (95% CI: −6.3 to 12.2; p = 0.529), although this difference did not reach statistical significance. In addition, we observed a non-significant action plan × health literacy (NVS) interaction (b = −3.25; 95% CI: −6.55 to 0.05; p = 0.054). Regarding Aim 2, 1,177 participants across the 3 allocation method arms completed follow-up (66.5%). There was no effect of allocation method on reduction of unhealthy snacking, including no effect of health literacy screening compared to participant selection (b = 1.79; 95% CI: −0.16 to 3.73; p = 0.067). Key limitations include low–moderate retention, use of a single-occasion self-reported primary outcome, and reporting of a number of extreme, yet plausible, snacking scores, which rendered interpretation more challenging. Adverse events were not assessed. Conclusions In our study we observed nominal improvements in effectiveness of action plans tailored to health literacy; however, these improvements did not reach statistical significance, and the costs associated with such strategies compared with universal precautions need further investigation. This study highlights the importance of considering differential effects of health literacy on intervention effectiveness. Trial registration Australia and New Zealand Clinical Trial Registry ACTRN12618001409268.


Discussion (p27-28, lines 607-650):
This study found that the effectiveness of an action plan to reduce unhealthy snacking may depend on the individual's health literacy level. In doing so, this study replicated a novel effect that has received little attention in the literature, in a sample of people with diabetes or overweight/obese BMI, for whom reducing unhealthy snacking is of clinical benefit. On average, people with lower health literacy reported less unhealthy snacking using an action plan that employed health literacy principles, whereas people with higher health literacy appeared to benefit more from using a 'standard' version of the action plan. Although we could not rule out the possibility of no true interaction effect in the full sample, this effect was robust in our sensitivity analysis. Results also suggested that the intervention overall was less effective for people with diabetes, and that screening for health literacy could be a more effective method of allocating an action plan than allowing the participant to select their preferred plan (however the confidence intervals around the effect estimate for the latter observation do not preclude the possibility of not true difference). These last two observations should be interpreted with caution as they were influenced by a small number of participants with extreme (yet plausible) values.
This study raises questions about health literacy theory and intervention implementation. On the one hand, tailoring an action plan to a person's health literacy level may improve plan effectiveness. If this is the case, this directly challenges the assumptions of a universal precautions approach to health literacy [7] (that is, that everyone benefits from simplified health materials), at least as it applies to a behaviour change action plan intervention. On the other hand, tailoring appeared to provide a substantial benefit for people with low to moderate health literacy but only a comparatively modest benefit for those with high health literacy. Given the aim of reducing health inequality, a universal precautions approach (i.e. only providing the literacy-sensitive action plan) may be more appropriate in this context than tailoring. A practical compromise may be to present the literacy-sensitive plan as the 'default' option, particularly as the benefits of different allocation methods (screening tool and allowing participants to choose) remains unclear.
Please also note, in the analysis section of the methods (p15 lines 339-340) we have now explicitly identified the process of considering influential observations as a 'sensitivity analysis' so that the revised text in the above section of the discussion is clear: "We conducted a sensitivity analysis by sequentially removing influential observations individually from the model to check the robustness of the model."

Conclusion (p30, lines 707-711):
In conclusion, this study found that for people with diabetes or overweight/obese BMI, action plans to reduce unhealthy snacking may be effective for people with lower health literacy when they employed health literacy principles. In contrast, 'standard' action plans that offered greater flexibility to personalise plans may be more effective for people with higher health literacy.
The text below outlines further changes made in relation to Aim 2 (allocation method). Table S12)

Discussion (p30, lines 699-706)
Lastly, the findings from this study highlight the potential limitations of conventional preference trial analyses. Overall, we found limited evidence that screening may be a more effective method of allocating an action plan than participant selection, but our exploratory findings suggest that a person's preference for an action plan may also play a role in action plan effectiveness. As such, further (a priori) analysis is warranted to estimate the independent contributions of a person receiving their preferred action plan and the effects of self-selection. An analysis that addresses this issue is outlined in the study protocol [38] and is in progress.

Conclusion (p30, lines 713-718)
In addition, it remains unclear whether the intervention is more effective when plans are allocated through health literacy screening or allowing the individual to choose. Given the lack of a clear benefit of either allocation method, and a relatively modest benefit of the standard action plan for people with higher health literacy, it may be more practical (and more cost-effective) to present the literacy-sensitive action plan as the 'default choice' rather than tailor to health literacy.

Please trim the background subsection of the abstract
In previous revisions, feedback requested that we describe the universal precautions approach and 'who signs up to it.' To balance this previous request with the current comment, we have retained the description of the universal precautions approach in the abstract but removed the statement that "this approach is widely endorsed internationally (eg by the World Health Organisation), and nationally (e.g. by the CDC and the Australian Commission for Safety and Quality of Healthcare)", along with some additional small editorial changes to keep language concise. The content of the deleted statement is still retained in the introduction.
The abstract background (p1, lines 18-26) now reads: Background: Low health literacy is prevalent worldwide and is associated with poorer chronic disease outcomes. A key strategy to address health literacy is a universal precautions approach, which recommends using health-literate design (e.g. simple language and images) for all health interventions, not just those that target people with low health literacy. This approach is advantageous because there is no need to assess a person's health literacy or tailor interventions. However, action plans for behaviour change may be more effective when tailored by health literacy. This study evaluated: the impact of health literacy and action plan type on unhealthy snacking, for people who have type 2 diabetes or high BMI (Aim 1); and the most effective method of action plan allocation (Aim 2).