Dietary supplements increase the risk of excessive micronutrient intakes in Danish children

In this study, the use of dietary supplements and the intake of 17 selected micronutrients were investigated in Danish children aged 4–10 years. The aim was to assess whether dietary supplement use leads to micronutrient intakes that exceed the respective ULs. Nearly two-thirds of children in the study had used a dietary supplement in the past year. This is similar to the proportion of Danish adults using dietary supplements (60%) [3, 15], but is a higher proportion compared to children in other European countries [1, 6,7,8, 31] and in many other developed countries (1.1–45.5%) [32,33,34,35,36,37,38]. However, differences in data collection methodology (e.g. duration of reference periods and types of dietary supplements recorded) makes it difficult to compare use of dietary supplements between countries. The reference period of 12 months in the present study and use of any type of dietary supplement (including herbal supplements, fish oil, yeast extracts, probiotics, etc.) may explain the relatively high percentage of supplement users in the present study.

MVMSs were the most popular type of dietary supplement in Danish children, similar to statistics in other Western countries [8, 11, 31, 32, 37]. The widespread use of dietary supplements in Danish children led to a considerable proportion of supplement users exceeding the ULs for retinol, zinc, iodine and TGL for iron. The Danish diet is rich in vitamin A, zinc, and iodine, and for a considerable proportion of the participants (11.5–30.1%), the diet alone provided retinol, zinc, iodine at levels that exceeded the ULs for these three micronutrients. The high iodine intake is mainly due to a high intake of milk which provides nearly half of the iodine in the diet of children. Furthermore, cereals and table salt are other significant sources of iodine (each about 15%). Main contributors of zinc are cereals, meat, and milk providing 73% of the total intake. The high retinol intake is mainly due to liver pâté and liver. About two thirds of vitamin A in the Danish diet is in the form of retinol. Adding a dietary supplement on top of a nutrient-adequate diet, led to an even greater proportion of the study population exceeding the ULs for these three micronutrients, although the proportions of users and non-users exceeding the UL were only significantly different in the case of zinc. However, in the case of retinol, the proportion exceeding the UL by 150% was significantly higher among users than non-users. Also, supplement use led to a significantly larger proportion of users exceeding the UL for iron compared with non-users.

In contrast to surveys of adults where former smokers were more likely to use dietary supplements [15, 37, 39,40,41], we found that supplement use was significantly higher in children whose parents were never-smokers, whereas the proportion of users among children of former smokers was not significantly different from that of children of smokers. This was confirmed in a logistic regression analysis where we found smoking status to be the only significant variable associated with dietary supplement use, with smokers being less likely to use dietary supplements than never-smokers. Similar associations between parental smoking status and supplement use in children have been reported in NHANES [42] and in a German survey [8]. In other surveys, use of dietary supplements in children were associated with a more prudent diet, healthier lifestyle, higher parental income and education, but also with chronic illness [8, 11, 32, 41, 43]. In adults, dietary supplement use has been linked with other healthy lifestyle choices [41, 44]. In studies on Danish adults other factors were also associated with dietary supplement use, including age, sex, self-perceived health status [15], intention to eat a healthy diet [3], health index and educational attainment [43]. However, in contrast to other studies [41, 45], we did not find significant differences in micronutrient intakes from the diet between users and non-users, nor did the diet quality score differ. The small sample size in the present study could be a reason why we did not find more factors associated with dietary supplement use or any significant differences in dietary intakes. However, this may also be due to differences in the factors associated with dietary supplement use between adults and children and in between countries. Studies show that in younger children, dietary supplement use often reflects the parents’ dietary supplement use [41, 46]. This could also explain why there was no difference in dietary supplement use between boys and girls in this survey, whereas surveys of adults showed that women are more likely to consume dietary supplements [2, 3, 15]. We observed a non-significant higher proportion of children using MVMSs in 4–6 year olds compared to 7–10 year olds which is in accordance with findings in other surveys [11, 33].

Compared to other European countries, Danish children have a high dietary iodine intake [1]. This is despite a relatively low iodine fortification level (13 µg iodine/kg salt until mid-2019) in Denmark and fortification of only table salt and salt added to bread and bakery products [47, 48]. High intakes of dairy products, bread, cereals and, to a lesser extent, marine foods may explain the high iodine content of the diet of Danish children [17, 48]. Tap water is also an important source of iodine in Denmark, although the iodine content of tap water varies with geography [47]. In the present survey, a considerable proportion of children had a dietary iodine intake that exceeded the UL. When adding the iodine from supplements on top of a high dietary intake, about a third of 4–6-year-olds and 26% of 7–10-year-olds exceeded the UL for iodine. Following reporting of the DANSDA survey results [17], the Danish Veterinary and Food Administration recommended removing iodine from MVMSs marketed for children. Several manufacturers have subsequently revised the formulation of their MVMSs products for children. The Danish Health Authority recommends parents avoid giving MVMSs containing iodine to 3–10-year-old children who drink milk [12]. However, the mandatory iodine fortification programme was revised in 2019 and the iodine content of salt changed from 13 to 20 mg/kg. Thus, many children may still be at risk of exceeding the UL for iodine, although use of MVMSs may contribute less to excess iodine intake now.

It has previously been reported that Danish children have a high zinc intake (95% percentile > UL) compared with children in other European countries [1]. In the present survey, 22% of the children aged 4–6 years and 14–15% aged 7–10 years had a dietary zinc intake that exceeded the UL for the respective age groups. With even more zinc provided by dietary supplements, 73% of 4–6-year-old users and 45% of 7–10-year-old users exceeded the UL. Similar findings have been reported children in other high-income countries [33, 49, 50]. However, exceedance of the UL for zinc does not appear to be associated with adverse effects and is therefore not considered an issue [24, 51].

Iron intake exceeded TGL among supplement users in both age groups. The higher prevalence of excess iron intake in 4–6-year-olds may be due to provision of the same vitamin/mineral supplements to 4–6 and 7–10 years-olds. However, a significant proportion of children in the present study concurrently exceeded the TGL due to the small margin between the RI and TGL. Chronic excess iron intake could be problematic as there is no biological mechanism for excreting excess iron [52] and the consequence can be iron overload. Iron overload can lead to health issues such as gastrointestinal bleeding, diarrhoea and nausea [26]. As no UL for iron has been established for children in Europe, we chose to use the TGL suggested by Rasmussen et al. [26]. We are aware that the US Institute of Medicine (IoM) has published a UL for iron of 40 mg/day for children. No children in this study exceeded the IoM UL for iron. However, as discussed by Rasmussen et al. [26] the IoM UL is based on observations of acute adverse gastrointestinal effects and the UL does not consider population groups vulnerable to iron overload. Rasmussen et al. extrapolated the provisional maximum tolerable daily intake (PMTDI) established at 0.8 mg/kg bodyweight by FAO/WHO Joint Expert Group on Food Additives in 2003 to children based on surface area [26].

We also found that a high proportion (12–30%) of children exceeded the UL for retinol. When looking at the diet alone this was especially true among 4–6-year-olds. Among supplement users, 43.1% of 4–6-year-olds exceeded the UL for retinol, whereas 20.7% of 7–10-year-olds exceeded the UL. The diet of Danish children is generally rich in vitamin A due to frequent consumption of food items such a pork liver pâté, liver, and carrots. Especially the youngest children have a vitamin A-rich diet [17]. Limiting further retinol intake from dietary supplements would help prevent toxic effects of prolonged excessive vitamin A intake. Long term hypervitaminosis A can lead to hepatotoxicity, hypercalcemia, and skin and bone changes [53]; however, data on the long-term effects of excessive retinol intake in children is scarce at present [33]. To our knowledge, no adverse effects from excessive retinol intake in Danish children have been reported. However, adverse events related to dietary supplement use are not systematically recorded in Denmark. The 5th percentile for vitamin A intake in this study population (4–10 years old children) is approx. 400 RE, which is the RI for 6–9-year-olds, and the median intake is 2.5-fold the RI. Thus, reducing the amount of vitamin A in dietary supplements is unlikely to lead to widespread vitamin A insufficiency in Danish children.

Children whose diet provided vitamin and/or mineral intakes above ULs had a higher energy intake and consumed more foods rich in iodine (dairy products, bread, cereals, salt), retinol (pork liver pâté, liver) and zinc (meat, bread, cereals, dairy products) than children whose diets did not provide excessive levels of micronutrients (data not shown). Excess micronutrient intakes obtained through the diet is less concerning due to variation in diet over time.

Children are particularly vulnerable to excess intake of micronutrients because they are growing and because of the small margin between optimal intake and excess intake [35]. The ULs for children are extrapolated from adult values on the basis of metabolic body weight [24, 54] and the margins between the Estimated Average Requirements and ULs are smaller for young children than for adults due to the use of a greater uncertainty factor in the calculation of ULs for children [54]. This could partly explain why a larger proportion of children than adults exceed ULs through the diet and why a relatively larger proportion of dietary supplement users amongst children exceed ULs [1]. Another factor contributing to excess intakes is consumption of fortified foods. As fortified foods become more common in Denmark, these foods also need to be regarded as significant sources of micronutrients.

The strength of the Danish National Survey of Diet and Physical Activity is that it is based on a random sample of the general population, it has a high response rate among children, the reference period for dietary supplement use was 12 months, and the habitual diet was recorded for 7 days. Whilst a reference period of 12 months may be a strength in that it accounts for supplements consumed episodically, it may also be susceptible to difficulties in recalling supplements used occasionally or many months ago. It may also be considered a limitation that any supplement use in the past 12 months categorizes a person as a user. This may partly explain the high proportion of users in Denmark compared to other countries. Measuring dietary intake in children is difficult and often prone to reporting error with under-reporting of energy being frequent, but some over-reporting also occurs [55, 56]. However, in this study, 96.5% were classified as plausible reporters of energy making misreporting a minor issue. The contents of micronutrients such as iodine and selenium are highly variable in some foods. However, the national food composition database in Denmark is updated regularly, the vitamin and mineral contents of many foods have been analysed and seasonal variation is taken into account when collecting samples for analyses. Missing data in databases can lead to underestimation of the intake of micronutrients, however, there are few missing values in the food composition data used in DANSDA.

Another limitation is the calculation using a generic MVMS instead of the specific MVMS taken by participants. Two studies found greater accuracy with using more specific types of MVMSs in calculations of micronutrient intakes from supplements compared to using a generic MVMS [57, 58]. Furthermore, due to the small sample size, we did not distinguish between daily, frequent and occasional users of dietary supplements, nor did we exclude mis-reporters of dietary intake from the analyses. Improvements in methodology will make the estimation of micronutrient intakes from dietary supplements more accurate in future surveys compared to the present survey.

In conclusion, MVMSs are the most commonly consumed dietary supplement. Most dietary supplements users consumed only one dietary supplement. Only smoking status of the parent was significantly associated with supplement use, with children of non-smoking parents more likely to be supplement users. There were no differences in dietary intakes of micronutrients between dietary supplement users and non-users. However, a considerable proportion of children consumed excess amounts of retinol, iodine, and zinc through their diet. With dietary supplements further increasing intake of retinol, iodine, and zinc, some children reached very high intakes of these three micronutrients. The consequences of chronic excessive intake of retinol, iodine and zinc in children is unknown. Excess zinc and iodine may be less of a problem than excess iron and retinol, which can cause longer-term health issues in the form of hepatic fibrosis [53, 59]. Supplement use also led to excess iron intake in some children, particular in the youngest children. We acknowledge that the diet of some children may provide insufficient amounts of zinc, iodine, and iron (data not shown); thus, for those consuming a suboptimal diet, a dietary supplement may be beneficial. However, median intakes of zinc and iodine were above the RIs, indicating that the prevalence of inadequate zinc and iodine intakes is low. For most Danish 4–10-year-olds, the diet seems to provide enough micronutrients with the exception of vitamin D. Thus, most Danish children do not need dietary supplements.

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