Nutri-score Labels on the Nutritional Quality of Out-of-home Food Selection

In line with published research into Nutri-score, the investigators will assess the healthiness of meals using UK Nutrient Profiling Model (NPM) scores(16). These scores are calculated in a similar way to Nutri-score, whereby the total value of positive nutrients is subtracted from the total value of negative nutrients. Lower scores represent healthier items. Scores range from -15 (healthiest) to 40 (least healthy).

To account for the likely event of multiple items being selected to form a meal, the investigators will calculate a mean NPM score, weighted by the energy content of individual food items. This will be done using the 'weighted.mean'(37) function in R grouped by participant so the weights are specific to individual orders. The following code will be used for this:

weighted_ppt<-combine %>% group_by(ppt) %>% summarise(score = weighted.mean(nutriscore, meal_kcal))

Nutri-score values of each menu item were calculated prior to creating the online ordering platform. To calculate overall scores, the total value of positive nutrients is subtracted from the total value of negative nutrients. Lower scores represent healthier items. Thresholds are used to dictate the Nutri-score value assigned to items, with A (healthiest) to E (least healthy)

When participants complete the food choice task, the Nutri-score labels (A-E) of each item selected by participants will be recorded by the online platform.

The total energy content (kcal) of food items selected by participants will be recorded by the online study platform. Nutritional information to inform this was taken from the food outlet websites.

The Food Choice Questionnaire(45) is comprised of 36 items across nine factors (health, mood, convenience, sensory appeal, natural content, price, weight control, familiarity & ethical concern). Participants are required to respond to the statement ''It is important to me that the food I eat on a typical day…" for each item (e.g. "Is nutritious", "tastes good") on a scale of 1 to 4 labelled not at all important, a little important, moderately important and very important. The average score for each factor is calculated. The health motive score will assess whether the value of health is associated with the impact of Nutri-score labelling on participant food choices. In the development of the food choice motives questionnaire, the Health Factor had a Cronbach's alpha value of 0.87, suggesting excellent internal consistency. Convergent validity of the health factor has also been shown by positive significant associations with health as a value and internal health-related locus

Participants will report their height and weight, which will be used to calculate participant BMI. In line with previous research(39), heights below 122cm or above 213cm, and weights below 34kg or above 227kg will be deemed implausible and excluded from descriptive statistics.

Participants will report their total annual household income on a 9-point scale: 1) ≤£5199; 2) £5,200 - £10,399; 3) £10,400 - £15,599; 4) £15,600 - £20,799; 5) £20,800 - £25,999; 6) £26,000 - £36,399; 7) £36,400 - £51.999; 8) £52,000 - £77,999; 9) ≥ £78,000.

Participants will also report their highest level of education achieved on an 8-point scale: 1) none; 2) GCSE Grade D or below; 3) GCSE Grade C or above; 4) A-level or equivalent; 5) University degree or equivalent; 6) Postgraduate qualification or equivalent; 7) Master's degree or equivalent; 8) PhD or equivalent.

Similar to previous research(40), a composite score made up of household income and education will be used as a measure of participant SEP. This will be done by summing the points scored for each measure. Higher scores indicate a higher level of socioeconomic position.

To identify whether our findings could be influenced by social desirability, the 13-item Marlowe-Crowne Social Desirability Scale will be included at the end of the follow-up questionnaire(41). Each participants will receive a Social desirability score from 0 (low social desirability bias) to 13 (high social desirability bias).