This study assessed iron intake and the prevalence of ID among Swedish adolescents with varying dietary climate impact. Adolescents with higher dietary climate impact had higher intakes of total iron and heme iron, but not non-heme iron. The odds of ID were lower among girls with the highest compared to the lowest dietary climate impact, while no association was found between dietary climate impact and ID in boys. This study revealed large sex differences; boys had higher intakes of both iron and red meat, higher dietary climate impact, and lower prevalence of ID compared to the girls.

Iron intake and status in Swedish adolescents

In this study, large differences were observed between the sexes; 23% of the girls and 6% of the boys were classified as iron deficient, and the girls had lower ferritin concentrations than the boys in all age groups. Compared to the girls, the boys had, on average, 25% higher intake of total iron per day and a higher proportion from heme iron in all age groups. The mean intake levels of iron for girls (7.7 mg/d) were below dietary reference values for average requirements (9–10 mg/d), whereas the intake levels for boys (9.6 mg/d) exceeded the average requirements (9 mg/d) but not the recommended intake (11 mg/d) [6]. This suggests that boys may have some opportunity to reduce their intake of iron-rich foods, such as red meat, and the related climate impact, without risking ID.

Risk groups for iron deficiency

The risk of ID depends on iron requirements, which vary between girls and boys, as well as among different age groups of adolescents. In this study, ID was most prevalent among adolescents aged 14–15 years (30% of girls, 11% of boys) and among girls aged 17–18 years (26%). These results reflect the elevated iron requirement in girls to support growth and menstruation, and in boys for the pubertal growth spurt [8]. Boys, on average, mature later than girls and have shorter duration of their adolescent growth period, which is reflected in low prevalence of ID (2–3%) in 11–12- and 17–18-year-old boys in the current study. In contrast, 10% of the girls were iron deficient even in the youngest age group studied.

Girls with menstruation were indicated as a risk group for ID. Notably, the odds of ID were almost five times higher in girls compared to boys, and three times higher in menstruating girls compared to those not menstruating. Being born outside of Sweden was indicated as an additional risk factor for ID in both girls and boys; however, potential explanations for this observation could not be further explored in these data. Extra attention should be given to ensure adequate iron intake and status among these risk groups.

Further research is needed to investigate whether the increased risk of ID among individuals born outside of Sweden can be confirmed in other studies and in other age groups, as well as to explore underlying factors that may explain the increased risk of ID in this population.

Increased risk of iron deficiency in low-climate diets?

This study found a significant positive association between dietary climate impact and the intake of total and heme iron in both girls and boys. However, no such associations were observed for the intake of non-heme iron, highlighting the differences between the types of iron. These results indicate that diets with a lower climate impact in Swedish adolescents contain less total iron and, in particular, less heme iron, which has higher bioavailability compared to non-heme iron. To understand the health implications of this, it is necessary to evaluate whether the climate impact of diet also affects iron status and the risk of ID. Notably, the proportion of girls with ID differed across quartiles of dietary climate impact, while no association between dietary climate impact and ID was found in boys.

Boys had 22% higher energy-standardized dietary climate impact compared to girls, largely driven by higher meat intake. In fact, the highest quartile of red meat intake among the girls corresponded to the lowest quartile of intake among the boys. Our results highlight the importance of considering the risk of ID in interventions for climate-adapted diets, particularly for risk groups of low iron status, such as menstruating girls. However, the concern about ID is less justified among most boys, where efforts to reduce dietary climate impact are most needed. It should be noted that the mean dietary climate impact of both girls and boys in this study exceeds proposed per capita climate targets for the food system [2] by more than two-fold. Thus, for most Swedish adolescents, large reductions in dietary climate impact would be required to achieve this goal, which may have further implications for nutrient intake and status.

Role of specific food groups and types of iron

In this study, heme iron accounted for 10% and 14% of total iron intake in girls and boys, respectively. Both animal-based and plant-based food groups were identified as important sources of total iron intake, whereas the intake of heme iron is restricted to animal-based foods. For a correct interpretation of these results, it should be borne in mind that food categorization was based on the main ingredient in composite dishes. For example, a vegetable soup containing small amounts of meat was classified in the category of vegetables, which explains the contribution of heme iron from some plant-based food categories.

The bioavailability of heme iron is generally higher (about 25%) compared to non-heme iron, for which the absorption is more affected by dietary compounds and iron stores [8]. In this study, the odds of ID were halved in girls with the highest intake of heme iron compared to those with the lowest intake. In contrast, no association was observed between the intake of total iron and ID. Even in this perspective, the results differed between the sexes, as no associations were observed between quartiles of iron intake (total, heme iron, non-heme iron) and ID among the boys. These findings suggest that the intake of heme iron may indeed influence the risk of ID in girls, while, apart from this, other factors seem to have a greater impact on the risk of ID than iron intake.

Red meat’s high content of heme iron, with high bioavailability, and its simultaneously high climate impact per kilogram, compared to other food groups, is a main reason for concern of a goal conflict between nutrition and environmental perspectives in the pursuit of sustainable diets. Therefore, it is of interest to investigate how the risk of ID is affected by dietary composition. In this study, lower intake of red meat in girls and higher intake of dairy products in boys were associated with higher odds of ID. The odds of ID were 39% lower in girls with the highest compared to the lowest intake of red meat. Notably, only the girls in the lowest quartile of red meat intake had intake levels below the upper limit of 350 g per week (50 g/d) recommended by the Nordic Nutrition Recommendations 2023 [6]. This highlights that Swedish adolescents, especially boys, who consume more red meat than girls, are eating more red meat than currently recommended from an overall health perspective. The potential for reducing the risk of ID through increased intake of red meat therefore needs to be weighed against other health aspects and environmental considerations.

Boys in the highest quartile of dairy intake had four times higher odds of ID compared to those in the lowest quartile. Calcium is known to reduce both heme and non-heme iron absorption [23], which may partly explain the observed association. Alternatively, boys with high dairy intake may have a diet with lower iron content compared to those with lower intake of dairy products. Higher milk consumption has previously been associated with an increased risk of ID in Norwegian girls [29]. In this study, the association was found only in boys, which may be due to their higher intake of dairy products compared to girls. Dairy intake was highest among boys in grade 8, representing an age group with high pubertal growth and elevated iron needs, which may also have contributed to the observed association with higher risk of ID in boys in this study.

Results in relation to previous findings

According to a recent review [13], iron is the micronutrient most frequently studied in relation to dietary environmental impact. However, none of the studies identified in the review assessed the relationship between dietary environmental impact and iron status, which makes it difficult to compare our results with previous findings. The review identified six studies that analyzed iron intake in relation to dietary climate impact, three of which showed significantly lower iron intake in diets with lower climate impact, while no statistically significant differences were observed in the remaining three studies. While the overall results from the review suggest that diets aiming to reduce environmental impact may lower micronutrient intake, the results for iron differed between studies and by methodological approach [13]. A previous review found associations between increased iron intake and a reduction in dietary climate impact in 20 out of 38 diet studies assessed [5].

Other studies have assessed iron intake and status in relation to animal-based food intake but have not considered the environmental impact of diets. For example, a randomized controlled trial in Finland studied the effect of replacing animal-source proteins with plant-based proteins [30] and found that iron intake was higher in diets based on 30% animal-based protein compared to diets with higher proportions (50% or 70%), whereas no significant differences were observed in biomarkers of iron status (Hb, ferritin, transferrin receptor). In contrast, a meta-analysis of 24 cross-sectional studies showed that adult vegetarians had significantly lower serum ferritin levels compared to non-vegetarians [31]. Moreover, a study of European adolescents found no association between iron intake and biomarkers of iron status (ferritin, hematocrit, Hb, soluble transferrin receptor), apart from the concentration of red blood cells, which was negatively associated with total iron intake, heme iron, and non-heme iron [32].

In summary, there is a lack of knowledge about how iron status may be affected by a transition to more environmentally friendly diets, and existing studies show conflicting results. The complex regulation of the body’s iron stores means that changes in iron intake may not ultimately affect iron status, which is why more research is needed to explore the risk of ID based on biomarkers in relation to the environmental impact of diets.

Strengths and limitations

Strengths of this study include the availability of both dietary iron intake data and iron status based on plasma ferritin concentration. In addition, the intake of iron was decomposed into heme iron and non-heme iron, which enables the study of differences between types of iron as well as between iron intake and iron status. This approach provides a better understanding of the risk of ID in climate-adapted diets. Another strength is the focus on adolescents, a critical group due to their elevated iron needs. The use of a national survey, where detailed information on food and beverage intake was collected through a 24-hour recall method and where reported energy intake was judged to be plausible [15], further enhances the study´s reliability.

The main limitation of the study is the cross-sectional design, which hinders conclusions about causality. Furthermore, although blood sample data are available for more than 1000 participants in Riksmaten Adolescents 2016–17, the sample size still limits the possibility of performing more detailed subgroup analyses. However, the participants with available iron status data were shown to be similar to those without blood sample data, and the full Riksmaten Adolescents 2016–17 dataset has been shown to be nationally representative of Swedish adolescents [17]. Additionally, a two-day food record may not correctly capture habitual food intake. To overcome this, dietary intake was transformed from current intake to long-term intake [2, 27]; however, this could still attenuate the findings. Uncertainty due to recall-bias and dietary misreporting, such as underreporting of unhealthy foods, is a known challenge in self-reported dietary data [33] and may have also affected the estimated food intake levels and subsequent results for iron intake and dietary climate impact. The low prevalence of ID in boys makes the statistical analyses uncertain, particularly the logistic regression analysis, and hampers the ability to draw conclusions. On the other hand, since ID is more frequent among girls, where the analyses may be more robust due to higher numbers, this study still contributes valuable knowledge. Another limitation is the low variation in dietary climate impact in this sample, which may have contributed to the lack of an inverse dose-response relationship with ID. Uncertainties also exist in climate impact assessments of foods and diets [34]. In this study, efforts were made to reduce the uncertainty by using LCA data from a database that is annually updated and by applying LCA data representative of Swedish consumption to capture major variations in impact between production systems. The nutritional focus of this study is limited to iron, without any consideration of other nutrients. Previous studies have found that diets with lower climate impact often have lower intakes of saturated fat and salt, whereas the intake of several micronutrients may be compromised, including vitamin D, A, B12, calcium, iodine, and zinc [5, 13]. For a more holistic sustainability perspective, the scope of this research needs to be broadened to account for additional environmental and sustainability indicators.

Future research

Longitudinal studies based on repeated observations over longer time periods are needed to further investigate potential explanations for the observed differences in ID risk across population groups. Studies that include food intake data and blood samples from a large number of individuals are necessary to allow for detailed subgroup analyses to gain more knowledge about specific population groups and characteristics associated with more and less sustainable diets. More studies are warranted, especially focusing on identified risk groups for ID in the population (e.g., menstruating girls and individuals born outside of Sweden). To support the development of guidelines for sustainable diets, more studies are needed that examine the combined environmental impact and nutrient adequacy of diets, assessed based on both nutrient intake and nutrient status. Future studies should be based on a broad sustainability perspective, considering additional nutrients and environmental indicators beyond those analyzed in this study.