Imagine you want to investigate the effects of diet on mental health and behavior. You would need to collect a lot of data about what someone is eating, and about their mental health. However, such rich and complex data is not always available. We therefore sometimes use a ‘proxy’ of the variable (such as diet) that we want to study.
A ‘proxy’ is a variable used instead of the variable of interest when that variable of interest cannot be measured directly. For a variable to be a good ‘proxy’, it must have a close correlation with the variable of interest. As an example, years of education, occupation, and income are widely used in research as ‘proxies’ for socioeconomic status as they also are highly correlated with each other. Socioeconomic status has long been related to health, those higher in the social hierarchy typically enjoy better health than those below.
In my research, a part of the Eat2beNICE project, we wanted to study the effect of the mother’s diet quality during pregnancy and common genetic variants on the development of ADHD symptoms in offspring at different ages. We used data from The Norwegian Mother, Father and Child Cohort Study (MoBa) presented in a previous blog. Instead of constructing a composite of nutritional factors into a diet quality variable, we chose a variable of total fiber intake as a ‘proxy’ for overall diet quality. The total fiber intake was the calculated total amount of fiber from bread, cereals, fruits, vegetables, and legumes, measured in grams per day. This was justified through several other studies where the level of fiber contributed the most in effect from the composite diet (1-4). In addition, we chose to use maternal years of education when giving birth as a ‘proxy’ for socioeconomic status since we did not have information about income or occupation. The ‘proxy’ variables may not be perfect but were the best variables available for this study.
From our first analyses we found that higher intake of fiber during pregnancy was associated with lower level of ADHD symptoms in the child. We have also examined the contribution of genetic factors on the level of ADHD symptoms, and I will report more about that finding in another blogpost.
- Englund-Ogge L, Brantsaeter AL, Sengpiel V, Haugen M, Birgisdottir BE, Myhre R, et al. Maternal dietary patterns and preterm delivery: results from large prospective cohort study. BMJ. 2014;348:g1446.
- Borge TC, Aase H, Brantsaeter AL, Biele G. The importance of maternal diet quality during pregnancy on cognitive and behavioural outcomes in children: a systematic review and meta-analysis. BMJ Open. 2017;7(9):e016777.
- Borge TC, Brantsaeter AL, Caspersen IH, Meltzer HM, Brandlistuen RE, Aase H, et al. Estimating the Strength of Associations Between Prenatal Diet Quality and Child Developmental Outcomes: Results From a Large Prospective Pregnancy Cohort Study. Am J Epidemiol. 2019;188(11):1902-12.