This study estimated UPF intake in toddlerhood and mid-childhood and explored stability and change in intakes between the two time points. UPF accounted for nearly half of total energy in toddlerhood (47%E), and 59%E in mid-childhood. Higher UPF consumers had higher intakes of free sugar and sodium, and UPF intake in toddlerhood was found to be predictive of mid-childhood intake.

Gemini toddlers’ energy and nutrient intake has been reported previously and is broadly similar to that of the much smaller sample of children aged 18–36 months in the UK national survey [21]. Intakes in mid-childhood have not been presented previously. In the current study, the contribution of UPF to total energy intake in toddlerhood (47%) was less than that of 2-5-year-olds in the UK national survey (61%) [12]. This is expected as although national survey data was collected at a comparable time point, the sample was more diverse in terms of ethnicity and SES, as well as covering a wider age range which spanned both toddlerhood and early childhood [12]. Likewise, UPF intake by Gemini children at age 7 years (59%) was lower than that reported in national survey data for a representative sample of UK children aged 6–11 years (67%) [12]. Some, although not all, studies have found a positive association between socioeconomic disadvantage and UPF intake, and the overrepresentation of children from higher SES households in the Gemini cohort compared to the UK population, along with age differences, may partially explain the lower observed UPF intakes [29, 30]. The higher dietary share of UPF in mid-childhood compared to toddlerhood is in line with cross-sectional observations described by Neri et al. comparing national survey data between pre-school, primary school and high school aged children in the UK, United States and Australia [12]. While an increase in UPF intake of 15.7% was observed in our longitudinal analysis, a smaller increase of 2% was observed among Portuguese children when they were aged 7 years compared to 4 years [14, 31]. A linear association between UPF intake and total energy intake was not found. This may partly reflect the association between toddlers’ UPF intake and consumption of CMF, which Gemini parents have previously reported giving toddlers due to concerns over poor appetite [18]. Further research, particularly longitudinal analysis is required to understand associations between UPF intake and adiposity [9].

In this paper UPF was used to characterise children’s diets. However, no evaluation was made of the effects of consuming any single food or UPF sub-group and it is recognised that these vary considerably in nutrient profile [32]. In line with previous research including children and adults in countries with varied income levels, higher UPF consumption at both time points was associated with increased intake of free sugar and sodium [24, 29, 33]. Free sugar intake among toddlers in the highest quintile for UPF was double that of those in the lowest quintile; in mid-childhood individuals in the highest UPF quintile consumed approximately 1.5 times more free sugar than those in the lowest quintile. Toddlers in all quintiles of UPF intake exceeded the UK free sugar maximum recommendation of 5%E, reflecting the high intake of UPF even among those in the lowest quintile (27.9%E) [1]. Toddlers in the two highest UPF quintiles also exceeded the more lenient UK population maximum recommendation of 10%E from free sugar [26]. Similarly in mid-childhood, the lowest UPF consumers received 40.9%E from UPF and individuals in all quintiles exceeded the maximum of 10%E from free sugar [26]. High free sugar intake increases the risks of dental caries and contributes to excess energy intake and is particularly concerning in toddlerhood when lifelong eating habits are becoming established [1, 6]. The negative association between UPF and protein intake in toddlerhood is consistent with findings in other populations but the diets of children in all UPF quintiles satisfied protein requirements [11, 27].

Many of the sugary UPFs consumed are easily recognisable as discretionary or noncore foods (puddings, ice cream, cookies and confectionery) which collectively contributed 4.1% energy to toddlers in Q1 and 12.6% to those Q5, and 11.0% and 23.2% in Q1 and Q5 respectively in mid-childhood (Table 2). Other UPFs, such as children’s yogurts and breakfast cereals, are often marketed as healthy, and while they may provide micronutrients and fibre, they are often a source of free sugar [1]. CMF is a particular concern as it contributes 50% of free sugar intake among UK consumers aged 1-1.5 years [1]. There is no evidence CMF offers any benefits over cows’ milk, which is much cheaper and is the recommended milk for toddlers from 12 months if they are not breastfed [18, 34].

Toddlers in Q5 consumed a third more sodium (32%) than their counterparts in Q1, while children in Q5 consumed a quarter more sodium (24%) than those in Q1. The positive associations between UPF and sodium intakes at both time points reflect observed associations between UPF intake and intake of savory snacks and processed meat. However, individuals in every UPF quintile exceeded the age-appropriate sodium Reference Nutrient Intake (RNI), reflecting high salt levels in UPFs which are UK staples such as bread and breakfast cereal as well as foods such as cheese [28]. Experimental studies with infants and young children suggest they learn to like salt, particularly in specific foods where it is usually found and once this liking is established, it is challenging to alter in later life [6].

In direct contrast to observations in populations including adults and children together, higher UPF consumption in toddlerhood was associated with lower intake of fat and saturated fat [11]. This negative association is largely explained by high intakes of cows’ milk among toddlers consuming less UPF. As population guidelines for fat and saturated fat don’t apply in full until children reach 5 years of age any health implications are unclear. No association was seen between UPF and fat or saturated fat in mid-childhood reflecting the UPF sub-groups commonly marketed for children, which are predominantly sweet foods, such as cookies, desserts and confectionery, rather than higher fat UPFs such as fast food and savoury snacks.

In mid-childhood, mean fibre intake was below the RNI [26]. A negative association was found between UPF and fiber intakes in mid-childhood, in line with observations reported in other child and adult populations [11, 12]. However, in toddlerhood, a negative association was only observed when toddlers consuming CMF were excluded (supplementary material 2, Table S2). This and the closer association between UPF intake in toddlerhood and mid-childhood among non-CMF consumers, compared to CMF consumers, suggests that CMF inclusion should be considered carefully when characterising toddlers’ dietary patterns using UPF, depending on the research question. Likewise, higher intakes of cows’ milk among young children with low intakes of other unprocessed or minimally processed foods, such as fruit and vegetables is also pertinent and may explain the lack of an association between UPF and fibre intakes shown here and previously reported for UK pre-school children [12].

The association between UPF intakes in toddlerhood and mid-childhood aligns with previous evidence suggesting dietary trajectories are set early in life. For example, a dietary pattern characterized by ‘processed and fast foods’ was shown to track moderately between the ages of 2 and 5 years in the French EDEN cohort (r = 0.35, p < 0.001, n = 989) [35]. Our analyses provide a comprehensive insight into UPF intake trends from toddlerhood to mid-childhood. As children matured a rise in dietary share in UPF was seen. Furthermore, an increase in dietary share of UPF was accompanied by a reduction in intake of unprocessed and minimally processed foods in favour of ready-to-eat and ready-to-heat products, including pizza, processed meat, savoury snacks, puddings and sweet cereal products, desserts, and confectionery. It has been suggested that the hyperpalatable nature of some UPF may partly drive continued consumption of these foods, which goes beyond habit formation [32, 36]. A Brazilian study found UPF intake at 4 years of age was associated with higher food responsiveness (eating in response to external food cues) at 7 years, however, intake and appetitive traits may be bidirectional and therefore this requires further investigation [14].

UPFs classified as infant foods and drinks provided less than 2%E for toddlers, compared to UK national survey data showing commercial infant foods and drinks provided 6%E among younger toddlers (12–18 months) [1]. When applying Nova classification to foods consumed by toddlers in the current study, a range of commercial products were identified that didn’t meet criteria for UPF classification but mimicked UPFs, including infant ready meals and infant snacks resembling cookies and flavoured maize (corn) puffs. Early exposure to these processed foods is unlikely to encourage consumption of vegetables, which are generally less popular although liking may be increased by repeated early exposure [1, 37, 38].

Foods marketed for children in the UK, for example breakfast cereals with cartoon characters, are often less healthy options, with many meeting the criteria for classification as high fat, salt or sugar (HFSS) [39]. Across Europe, UPFs marketed for children typically have a worse nutrient profile – i.e. more fat, saturated fat, sugar and sodium – compared with less processed products [17]. Restricting promotion of HFSS products is currently proposed in the UK to address childhood obesity, partly by prompting reformulation [40, 41]. However, foods that fall just below the threshold for HFSS but comprise a large proportion of children’s diets, such as some breakfast cereals, are a concern. Integrated, far reaching policies would be needed to redress the balance of children’s diets toward a lower proportion of UPF, such as adding warning labels to products, inclusive school food policies and subsidies on fresh and minimally processed food [29, 42, 43]. While an increasing number of countries recognize UPF in national guidelines and are taking a policy approach to limit intake, the UK’s Scientific Advisory Committee on Nutrition (SACN) are adopting a more cautious approach, as indicated in a recent statement, and plan to reconsider existing and new evidence in 2024 [44].

The results of the present study should be considered in the light of methodological strengths and limitations. Firstly, in Gemini, as in many cohort studies, there is an over-representation of parents of white ethnicity with a higher SES, particularly in mid-childhood, compared with the UK population. Parent reports of children’s diets come with inherent limitations, for example, foods not consumed in the presence of the diary keeper, such as in school, may be less accurately reported. The lower energy intake recorded when children were 7-years-of-age, compared to the EAR (7 to10-years-of-age), may reflect underreporting, which is common in dietary surveys, as well as age, as children were at the lower end of the EAR age range [25]. Furthermore, data were collected in 2009/10 and 2014/15 so may underestimate current UPF intakes nationally. The Nova classification system has been criticized for grouping together foods with differing nutritional attributes and because some research groups report lack of inter-assessor agreement, which was minimized in the current study by utilizing multiple coders [45, 46]. The dietary assessment software used (DINO) was not specifically designed for Nova classification and a pragmatic approach was necessary, which may have resulted in systematic errors. Some simple composite items, such as fried mushrooms, were coded in DINO as a single item and the decision was made to classify these as Nova 1, resulting in the small amount of cooking oil not being accounted for in the estimation of culinary ingredients (Nova 2) and energy from Nova 1 being overestimated. Another issue arose with composite dishes such as lasagne, which were generally reported with brand information (if commercially produced) or recipe details (if homemade), but a small number of entries lacked these details, so these were classified as Nova 4. The results presented in Tables 2 and 3 don’t represent a direct comparison between diets at each age as only 592 diet records were completed in mid-childhood compared with 2,591 in toddlerhood. As there is a dearth of data published for very young children, this was considered more appropriate than disregarding data for 1,999 toddlers. Analyses of dietary change (Tables 4 and 5) included only the 570 children with diet records at both ages.

In conclusion, UPF intake was high in toddlerhood and mid-childhood in the UK and recommended free sugar and sodium limits were exceeded. In toddlerhood the principal UPFs consumed were those typically marketed as healthy options, including flavoured yogurts and higher-fiber breakfast cereals, whereas in mid-childhood puddings and sweet cereal products and white breads were most common. Higher UPF intake at both time points was associated with higher free sugar and sodium intakes, and in mid-childhood, a negative association with fiber was observed. UPF intake in toddlerhood was predictive of UPF intake in mid-childhood. Integrated policies are needed to provide children with diets with fewer ultra-processed foods in order to lay the foundations for a healthy diet in adulthood.