Our study also found that, as expected, there are few studies for children in LMICs that assess the contribution of diet to risk factors for NCDs. This is partly due to the limited research on older children and adolescents, when the most critical metabolic risk factors for a range of diseases are most likely to occur compared with younger children. Another reason for the limited research in this area might be due to the emerging nature of the nutrition transition, which is currently under explored also for the adult population in LMICs. It is also likely that there is less recognition of how excess consumption of unhealthy foods such as UPF contributes to nutrient deficiencies in LMICs, since there is a tendency to associate UPF consumption with overweight/obesity and NCDs (Scrinis 2020). In young children, unhealthy snack food and beverages can account for a significant proportion of total energy intake and the micronutrient dilution that results from high consumption of energy-dense but nutrient-poor foods may contribute to micronutrient deficiencies and poor growth outcomes (Pries, Filteau, and Ferguson 2019).

More research is needed to describe how dietary patterns interact with undernutrition (stunting, wasting and micronutrient deficiencies) and overweight/obesity and NCDs at individual, household and country level and in which direction they influence the economic effects. For example, recent evidence from a national survey in South Africa highlights the complexity of malnutrition and diet quality within households, where 18% of children were stunted and 70% of these children lived with an overweight or obese adult (Harper et al. 2022). While children <5 years and adolescents living in households with medium dietary diversity were more likely to be stunted than children in households with high dietary diversity, increased household dietary diversity was associated with increased risk of adult overweight and obesity. The contribution of diet to the growing double and triple burden of malnutrition in LMIC populations is not well-tested in the research available to date, with most SRM looking primarily at the role of diet in contributing to either undernutrition or overweight, obesity and diet-related NCDs.

The widespread use of diet diversity metrics in young children and women of reproductive age along with emerging metrics of diet quality has potential to better capture the link between diet and overweight/obesity and NCDs. Complementary studies are needed to expand the scope of these efforts to include older children, adolescents, men and older adults. This remains challenging given the limited availability, quality and representativeness of dietary data for such groups in LMICs (Demmler et al. 2024).

While no SRM was found that summarized the effect size for the association of ASF intake with nutritional outcomes, there are several observational studies that have used multi-country datasets to explore this (Zaharia et al. 2021; Headey, Hirvonen, and Hoddinott 2018; Krasevec et al. 2017). Four SRM have estimated the risk of child overweight/obesity associated with consumption of dairy products (Babio et al. 2022; Lu et al. 2016; Vanderhout et al. 2020; W. Wang, Wu, and Zhang 2016).

Where SRM looked at the associations between diet diversity and nutrition and health outcomes, it appears that low diet diversity impacts GDM and poor neonatal birth outcomes in a similar way as unhealthy diets. Similarly, low diet diversity was associated with overweight and obesity among older children in one meta-analysis of only two studies (Gezaw et al. 2023b). We could not find a SRM that looked at the relationship between diet diversity and overweight among children <5 years of age. One cross sectional study in India has looked at this, reporting that a dietary diversity score of 7-9 food items for children 0-59 months of age was associated with increased risk of overweight and obesity (OR 1.22; 95% CI 1.12, 1.34) (Saha et al. 2022).

Limitations of this review The inclusion criteria were designed to identify research studies that were of high relevance to the theoretical framework we had developed to estimate the nutrition and health economic costs of unhealthy diets. This resulted in excluding several groups of literature, including fortified foods and micronutrient supplementation. Many studies examined individual dietary risks and these have limited usefulness since it is not possible to add them up to obtain overall estimates of the costs of unhealthy diets, since the risk factors may be correlated and/or interact. We also limited our review to studies in English, which likely excluded the emerging body of evidence from studies conducted in LMICs and published in other languages. For diet-related NCD outcomes, the search focused on 14