This is a two-group parallel randomized controlled trial testing whether a gamified healthy living smartphone app for youth aged 10-16 representative of the Canadian population and one of their parents is more effective at improving a composite of health behaviours (diet, physical activity, sleep and screen time) than a simple app providing links to healthy living websites.

Living Green and Healthy for Teens (LiGHT) is a Canadian smartphone app-based program that combines health promotion (healthy eating, active living, screen time and sleep) with additional novel motivators such as environmental stewardship (e.g. reduce prepackaged foods, walk rather than drive) and cost-savings (e.g., eat at home rather than restaurants), that may further increase the likelihood of behaviour change.

The primary aim of the trial is to determine, among youth aged 10-16 years, if randomization of their family to the use of the interactive gamified Living Green and Healthy for Teens (LiGHT) app over a 6 month period, compared to a control app, increases the number of healthy active living behaviours engaged in by youth. Additional outcomes will include youth anthropometrics, diet quality, fitness, quality of life, and parental health behaviours and cardiovascular risk.

Using a multi-centre randomized, parallel, controlled single-blind design, 376 eligible youth-parent dyads stratified by youth (gender, and age <13.5/≥13.5y) will be allocated 1:1 to receive intervention or control apps that provide information and tips on healthy eating and activity in different ways. Participants will be followed for 1 year, with the primary outcome assessed at 6 months.

Participants will be recruited from the community in Hamilton, Ontario, and Surrey, British Columbia, Canada. The target population is the general population of youth and their families. Therefore, entry criteria will be broad. Youth aged 10-16 years of age who identify a need or potential to improve health behaviours will be included if one parent is willing to attend all study visits and use the app, there is at least one internet enabled device in the household, and there are no contraindications to healthy eating and activity, and no family member is participating in a weight management program or other clinical trial.

Participants will be blinded to treatment group. Both apps have appealing interfaces, and both provide information, tips and interactivity (though to different degrees). Every effort will be made to keep research staff who help participants with the app separate from staff who collect data, and the latter will be trained to avoid discussing the intervention.

Evaluations at baseline, 3 months and 6 months by virtual or in-person appointments will include a youth questionnaire including questions about food intake , screen time, physical activity, health related quality of life, and resilience, and measurement of height, weight, and fitness using a push-up test and standing long jump test. Participants completing in-person visits will also complete the following: body fat percentage using bioelectrical impedance assessment, blood pressure, hand grip strength using a dynamometer, and cardiorespiratory fitness using a step test. Youth participants will also provide a urine sample and wear an accelerometer for 7 days. Parents will complete a questionnaire including questions about sociodemographics, their own food intake, screentime, physical activity, parental role modelling and logistic support. Parents will also complete the non-laboratory based INTERHEART risk score questionnaire and have their waist and hip circumference measured. Participants and parents will complete questionnaires about youth experience of online bullying and access to undesirable online content. 12 months after randomization, participants and parents may be invited to complete the same study questionnaires online.

This trial will demonstrate whether use of a rigorously-designed, evidence-based, user-friendly app is able to help families with teenagers improve health behaviours of youth and have any early impact on CV risk factors. Conducting an RCT in the context of the full range of youth in the community has the potential to inform population-scale implementation as a public health tool for sustainable obesity prevention across Canada.

Inclusion Criteria:

1. Living in a home setting with at least one smartphone or tablet and internet access in the household, and
2. One parent or guardian (the "primary parent") who is able to attend all study visits.
3. Youth or parent identifying a need or potential to improve health behaviours

Exclusion Criteria:

1. Inability of the youth and/or primary parent to read English at a grade 5 level
2. A health condition in the youth that precludes eating a variety of foods or engaging in physical activity
3. Current participation of any family member in a weight management program
4. Current participation in another clinical trial
5. Current or prior participation of another member of the household/family in this trial