Outcome measurements

Participant body weight was measured at CID1 and CID2.

European Social Survey and International Social Survey38 was used to collect information on gender, year of birth, citizenship, spoken language, highest educational level, marital status, number of people living in the household, employment status, work status, and households’ financial income.

The instrument to measure participants’ intention was adapted from Renner and Schwarzer39 and included three separate questions such as “I intend to lose weight” to assess intentions to eat as healthily as possible, to exercise regularly, and to lose weight. Response options ranged from “1” (don’t intend at all) to “7” (strongly intend). High scores indicated strong intention to perform the behavior, e.g., lose weight.

Participants were asked how certain they are to overcome barriers for both diet (5 items) and physical activity (5 items). Participants were asked to answer questions such as “I can manage to stick to a healthy diet, even if I have to rethink my entire diet”. Response options ranged from “1” (very uncertain) to “4” (very certain).39 For both dimensions, a mean value was calculated ranging between 1 and 4, with low scores reflecting low self-efficacy and high scores reflecting high self-efficacy. Internal consistencies were calculated using Cronbach’s alpha, which for CID1 and CID2 for diet were α = 0.89 and for exercise α = 0.91, indicating sufficient reliability.

The instrument assessing participants’ confidence in staying physically active (11 items) and adhering to a healthy (3 items) diet despite upcoming barriers was adapted from Renner and Schwarzer.39 Participants were asked to respond to statements like “I am sure I can keep being physically active regularly, even if I am tired”. Responses ranged from “1” (not at all true) to “4” (exactly true). For both dimensions, a mean value was calculated ranging between 1 and 4, with low scores reflecting low coping self-efficacy and high scores reflecting high coping self-efficacy. Data for coping self-efficacy was collected at CID2. Cronbach’s alpha for CID2 for diet was α = 0.89 and for exercise α = 0.94.

Outcome expectancy of behavior change was assessed for a healthy diet with 12 items and exercising regularly with 13 items.39 Participants rated expected benefits and disadvantages of behavioral change from “1” (not at all true) to “4” (exactly true). Participants were asked to rate statements such as “If I eat as recommended in PREVIEW, food won’t taste as good”. For both dimensions, a mean value for benefits and disadvantages was calculated, ranging between 1 and 4. Low scores reflected fewer expected benefits/disadvantages and high scores reflect more expected benefits/disadvantages. Cronbach’s alpha for CID1 and CID2 for benefits of diet were α = 0.70 and α = 0.73 and for exercise α = 0.79 and α = 0.85. For disadvantages, Cronbach’s alpha at CID1 and CID2 for diet were α = 0.67 and α = 0.65, and for exercise α = 0.61 and α = 0.63.