To begin, we present a conceptual framework that describes the pathways from unhealthy diets to economic costs. Based on this framework, we define the three components of importance to estimating these costs (dietary exposure, nutrition and health outcomes, and economic impacts on individuals and society) and conduct a scoping literature review to examine the range and characteristics of the evidence on this topic. We assess the adequacy of the evidence by life-stage group with a focus on the most vulnerable periods, namely during pregnancy and lactation, early childhood (from birth to 23 months of age), school age and adolescence and older adulthood (beyond 65 years of age). As a result, we identify the knowledge gaps that remain. We then provide key findings that can inform modeling efforts to estimate the nutrition and health economic costs of unhealthy diets, highlighting limitations in the evidence that need to be considered when interpreting the results, with the aim of informing policy.

# Conceptual framework and definitions

**Figure 1** shows the conceptual framework that is guiding the review, describing the pathways leading from diet quality to various nutrition and health outcomes, that in turn have economic impacts on individuals and their society. There are different pathways for individuals in different stages in their life cycle, since the short- and longterm effects of diet quality may be different at each stage of development (Gernand et al. 2016). We next discuss in turn exposures, outcomes and impacts.

## **_Exposure of interest_**

**First, the starting point and exposure of interest is diet quality which can be defined as ‘healthy’ or ‘unhealthy’.** A ‘ **healthy diet** ’ is defined as one which promotes growth and development and prevents malnutrition in all its forms (FAO and WHO 2019). Dietary patterns tend to be highly contextual and depend on food availability, access, affordability, preferences, cultures and traditions. While there can be many healthful dietary patterns, the principles of what constitutes a healthy diet are universal: i) **adequacy** in amount of energy (calories) and essential nutrients, ii) **diversity** of food groups and within food groups, iii) **balance** of energy from protein, fats and carbohydrates; and iv) **moderation** of unhealthy foods and components (Verger et al. 2023). The State of Food and Agriculture flagship report (SOFA 2023) defines ‘ **unhealthy diets’** as those that do not meet one or more of the principles of healthy diets (adequacy, diversity, balance, and moderation) and are one of the primary drivers of all forms of malnutrition, and related morbidities (FAO et al. 2023).

The WHO dietary recommendations are global reference points for preventing undernutrition, including micronutrient deficiencies, and reducing NCD risks (WHO 2020). WHO recommends exclusive breastfeeding for infants during the first 6 months of their life, followed by continued breastfeeding with appropriate complementary foods for up to 2 years or beyond. The 2023 WHO guidelines for complementary feeding recommend that not breastfed infants 6–23 months can be fed with either animal milk or commercial formula milk up to 11 months while follow-up commercial milk is not recommended. The same guidelines recommend that infants and children 6-23 months should consume a diverse diet including daily consumption of animal source food, fruits and vegetables and frequent consumption of pulses, nuts and seeds, particularly when meat, fish or eggs and vegetables are limited in the diets. The new guidelines also provide specific recommendations on food that should be avoided or consumed in moderation. The new guidelines recommend that infant and young children 6-23 months should not consume foods high in sugar, salt and trans fats and sugar-sweetened beverages and should limit consumption of 100% fruit juice (WHO 2023a).

There are no specific WHO dietary recommendations for other age groups but the universally recognized guidelines for all age groups do emphasize increasing intakes of fruits, vegetables, legumes, nuts and whole grains; limiting energy intake from free sugars and total fats; consuming unsaturated rather than saturated or trans fats; and limiting salt intake. The Global Burden of Disease (GBD) study, which seeks to quantify the contribution of dietrelated risks to NCD burden, aligns its definition of a ‘healthy diet’ with WHO recommendations but includes additional risk factors like any consumption of processed meat and high consumption of red meat (Kumanyika et al. 2020). Characterization of healthy diets based on dietary patterns (rather than individual foods or nutrients) 3