A healthy diet with ups and downs throughout the life course

Although many public health actions have been implemented in recent years, a significant proportion of preventable deaths (1.6 million in 2019) and disabilities (30 million Disability-Adjusted Life Years) related to chronic diseases in Europe is still attributed to an unhealthy diet.1 Diet is determined by a combination of downstream and upstream factors that may interact.2 Downstream determinants operate at micro level, such as gender, age, socioeconomic status or migration status. Upstream determinants are at macro level and examples include the food environment and the social environment.

Disparities in diet have been documented to be partly responsible for the inequalities observed in health. As a matter of fact, social variations in diet may explain up to half of the social inequalities in health.3 In addition to being a problem of injustice and economy (estimated cost of €980 billion per year in Europe due to the reduction of economic and social productivity),4 inequalities are a major public health problem, as they prevent people from achieving their full health potential. Addressing the inequalities in diet is therefore crucial for reducing the risk of diet-related chronic diseases. To tackle inequalities in diet and in turn health, public health initiatives should include both downstream and upstream interventions. Indeed, as more than half of the general population carries an increased risk (e.g. due to overweight), only targeted or downstream approaches are not sufficient. In order to be able to implement such actions and for them to be effective, it is important to know what the upstream and downstream determinants of changes in diet are, the extent of their impact on diet and the interplay between these determinants. However, information is still lacking in this respect.

State-of-the-art. To reduce the incidence of food-related illness and death, populations must change their dietary habits, i.e. behavioral change is required. Following the “habit discontinuity hypothesis”, behavioral change is more likely to succeed when habits are strongly disrupted, such as during life-course changes. First job, departure from the family home, partnership and parenthood are examples of these life-course changes.5 Such changes usually occur during early adulthood, i.e. between adolescence and adulthood. For young adults (18-30 years old), these changes are accompanied by financial and residential responsibilities, as well as autonomy in decision-making, including regarding diet. Furthermore, beyond contributing towards immediate health risks, dietary behaviors acquired during early adulthood can last a lifetime6 and influence risk of chronic diseases in later life. Therefore, early adulthood provides one of the greatest opportunities for interventions on dietary behaviors. However, to take advantage of this window of opportunity and implement effective public health actions, there is a need to understand the changes in diet and the factors influencing dietary changes at this life stage.

However, very few studies have focused on early adulthood as a period disrupting acquired dietary habits and forming new dietary habits. In fact, few studies even addressed dietary changes among young adults. The paucity of studies together with the diversity of population age and diet outcome prevents conclusion on diet trajectories during early adulthood to be drawn. Nevertheless, changes in diet, such as in consumption of fruit or vegetable, are observed.7-10 However, the published studies on dietary changes are mostly limited to adolescence and early twenties.11 Yet, key life events and transitions that may modify dietary habits, such as leaving family home, usually occur beyond the early twenties.12 In addition, for studies through the late twenties, diet was almost exclusively assessed only at early and at late twenties, with mid-twenties not being assessed,11 which prevents a proper study of trajectories. Furthermore, most of them are from the U.S11  whose context, which may influence diet, differs greatly from Europe. 
Despite the several cohorts assessing diet in the Netherlands, early adulthood is also overlooked therein and knowledge still need to be deepened. As an example, diet during early adulthood have been addressed with the cohort Amsterdam Growth and Health Longitudinal Study, and dietary changes have been underlined.10 However, beyond the gap in diet assessment between early and late adulthood and the small sample size, the data analyzed date back to the 20th century whereas diet can be subject to secular trend.13

While studies investigating dietary changes in young adults are already scarce, especially in Europe, even fewer have examined the factors influencing dietary change among them. Gender differences in dietary changes were explored, with more pronounced and negative changes among men compared to women.9 Some studies have focused on economic factors. For instance, an increase in prices of fast food was found to be associated with a decreased in its consumption in the U.S.7 Others studied the associations with key life factors, such as leaving home, leaving education, beginning cohabitation and entering employment. While only limited association were found with the latter two, negative changes in diet were found with leaving education and leaving home. Indeed, leaving education was associated with an increase in confectionery and soda intakes whereas leaving family home was associated with a decrease in fruit and vegetable intakes in Norway.9 
Following the Dahlgren-Whitehead model of health determinants14 and the Glass-McAtee multilevel model,15 dietary changes of young adults may be determined by various interacting layers of factors surrounding the individual. These layers are age, sex and constitutional factors, individual lifestyle factors, social and community networks, living and working conditions, general socioeconomic, cultural and environmental conditions. However, to the best of our knowledge, no other factors from the models than those mentioned above nor the interrelations between them have been addressed. A few cross-sectional studies, although still too few in numbers, have emphasized their significant role in young adults’ dietary behaviors.16,17
Moreover, while moving from the family home was found to be associated with dietary changes, this association could be mediated by the changes in social factors that the move entails. In fact, the food and social environment of the new home may differ from that of their parents. In addition, as the young adult becomes responsible for many issues, their individual factors such as socioeconomic status may take precedence over their parents’ factors when it comes to diet. Following the literature on health,18 such changes in factors may have a major impact in diet, but this too remains to be determined. 
Thus, there are some opportunities to successfully change dietary behaviors during early adulthood, including when leaving the family home. However, to date, there is little or no knowledge of the diet trajectories and the factors influencing these trajectories at this time of life. It is therefore not yet possible to identify the levers that should be considered in the strategies to promote transitions towards healthier diet among young adults. Therefore, this project aims to fill the gaps in the literature regarding the dietary changes as well as the upstream and downstream determinants of dietary changes during this overlooked but critical period.

References
1.    Institute for Health Metrics and Evaluation (IHME). Global Burden Disease (GBD) Compare. 2019. https://vizhub.healthdata.org/gbd-compare/ (accessed 07 October 2022).
2.    Mackenbach JD, Nelissen KGM, Dijkstra SC, et al. A Systematic Review on Socioeconomic Differences in the Association between the Food Environment and Dietary Behaviors. Nutrients 2019; 11(9).
3.    Petrovic D, de Mestral C, Bochud M, et al. The contribution of health behaviors to socioeconomic inequalities in health: A systematic review. Prev Med 2018; 113: 15-31.
4.    EuroHealthNet. Health Inequalities in Europe. 2019. https://eurohealthnet.eu/wp-content/uploads/documents/2019/191023_Factsheet_HealthEquityEU_WebLayout.pdf (accessed 07 October 2022).
5.    Verplanken B, Roy D. Empowering interventions to promote sustainable lifestyles: Testing the habit discontinuity hypothesis in a field experiment. J Environ Psychol 2016; 45: 127-34.
6.    Sawyer SM, Azzopardi PS, Wickremarathne D, Patton GC. The age of adolescence. Lancet Child Adolesc Health 2018; 2(3): 223-8.
7.    Gordon-Larsen P, Guilkey DK, Popkin BM. An economic analysis of community-level fast food prices and individual-level fast food intake: a longitudinal study. Health Place 2011; 17(6): 1235-41.
8.    Wiium N, Breivik K, Wold B. Growth Trajectories of Health Behaviors from Adolescence through Young Adulthood. Int J Env Res Pub He 2015; 12(11): 13711-29.
9.    Winpenny EM, van Sluijs EMF, White M, Klepp KI, Wold B, Lien N. Changes in diet through adolescence and early adulthood: longitudinal trajectories and association with key life transitions. Int J Behav Nutr Phys Act 2018; 15(1): 86.
10.    van de Laar RJJ, Stehouwer CDA, van Bussel BCT, et al. Lower lifetime dietary fiber intake is associated with carotid artery stiffness: the Amsterdam Growth and Health Longitudinal Study. Am J Clin Nutr 2012; 96(1): 14-23.
11.    Winpenny EM, Penney TL, Corder K, White M, van Sluijs EMF. Change in diet in the period from adolescence to early adulthood: a systematic scoping review of longitudinal studies. Int J Behav Nutr Phys Act 2017; 14(1): 60.
12.    Eurostat. Leaving home: Young Europeans spread their wings. 2022. https://ec.europa.eu/eurostat/web/products-eurostat-news/-/ddn-20220901-1#:~:text=In%20the%20EU%2C%20on%20average,average%20earlier%20than%20young%20men. (accessed 07 October 2022).
13.    Vereecken C, Pedersen TP, Ojala K, et al. Fruit and vegetable consumption trends among adolescents from 2002 to 2010 in 33 countries. Eur J Public Health 2015; 25: 16-9.
14.    Dahlgren G, Whitehead M. The Dahlgren-Whitehead model of health determinants: 30 years on and still chasing rainbows. Public Health 2021; 199: 20-4.
15.    Glass TA, McAtee MJ. Behavioral science at the crossroads in public health: Extending horizons, envisioning the future. Soc Sci Med 2006; 62(7): 1650-71.
16.    Desbouys L, Mejean C, De Henauw S, Castetbon K. Socio-economic and cultural disparities in diet among adolescents and young adults: a systematic review. Public Health Nutr 2020; 23(5): 843-60.
17.    Rummo PE, Guilkey DK, Ng SW, et al. Understanding bias in relationships between the food environment and diet quality: the Coronary Artery Risk Development in Young Adults (CARDIA) study. J Epidemiol Commun H 2017; 71(12): 1185-90.
18.    Levesque AR, MacDonald S, Berg SA, Reka R. Assessing the Impact of Changes in Household Socioeconomic Status on the Health of Children and Adolescents: A Systematic Review. Adolesc Res Rev 2021; 6(2): 91-123.