Background Worldwide, an estimated 10% of school-aged children were overweight or obese in 2004. Intervention studies reveal promising obesity prevention strategies but there are gaps in published research on childhood diet and associated obesity risk. Small associations between dietary patterns and adverse changes in fat mass during mid-childhood, found in European cohorts, indicate that longitudinal studies are a useful way to investigate dietary risk factors for future obesity. This systematic review identifies childhood and adolescent cohorts with a minimum follow up of 2 years, to address the research question:

To what extent do dietary factors during childhood or adolescence influence future indicators of overweight or obesity?

Methods The PRISMA-P 2015 statement guided protocol development. A PICO (Patients, Interventions, Comparators, Outcomes) framework was used to generate search terms, (Children or Adolescents) and (Diet or Nutrition or Food intake or Diet quality) and (Body composition or Height and Weight).

Ovid Medline, Embase, Cochrane CENTRAL, Web of Science and Scopus were searched for articles published from January 1990, with no limit on language. Records were included if:

• a full article, available in English AND

• a cohort study or trial with an untreated control arm AND

• over half of participants were healthy children/adolescents aged from 8 to 19 years at baseline AND

• whole diet was measured by diet diary, 24 hour dietary recall or food frequency questionnaire and reported as partial or total intake of foods and drink AND

• adiposity outcomes (measured at least 2 years after diet) were reported.

Data from included studies will be extracted into Excel and imported into Stata for meta-analysis of individual dietary factors where appropriate. Internal validity will be assessed using the NICE quality appraisal checklist.

Results Literature searches initially identified 11,604 records. After removing duplicates and irrelevant titles 3,431 records remained to screen on title and abstract. 390 potentially relevant records were screened in duplicate on full-text. After the third review, 57 papers were included, from 33 studies (24 observational, 9 intervention). Sixteen studies are from North America, eight from Northern Europe and six from Australasia. Over half the papers report a follow-up of five years or less and sample sizes range from 100 to over 6,500 participants. Dietary factors identified include sweetened drinks, energy dense foods, fruit and vegetables, dairy foods and breakfast frequency.

Conclusion High quality cohorts exist from which the influence of dietary factors on childhood/adolescent obesity can be assessed.