Micronutrients for bone health

As over half of an individual’s bone mass is laid down during the teenage years, sufficient intakes of calcium, vitamin D and magnesium are crucial to promote bone health due to the role of calcium and magnesium in bone mass accrual and the role that vitamin D plays in the absorption of calcium^{(Reference Cashman2,Reference Levine30)} . Intakes of vitamin D, calcium and magnesium are low amongst teenagers, with intakes substantially below recommended intakes. The prevalence of inadequate intakes among teenagers in the Western world ranges from 70–95 % for vitamin D, 45–73 % for calcium and 33–88 % for magnesium (Tables 4 and 6), which individually and combined may have negative implications for bone health. Typically a slightly lower prevalence of inadequate vitamin D intakes has been reported in the Nordic countries e.g. 70 % for teenagers in Sweden which is potentially due to fortification of fluid milk with vitamin D, in addition to different dietary patterns such as higher oily fish consumption, and the high population use of cod liver oil and vitamin D supplements^{(Reference Warensjö Lemming, Petrelius Sipinen and Nyberg25,Reference Lips, Cashman and Lamberg-Allardt31)} (Table 4). The low intakes of vitamin D globally are also reflected in biochemical status data (25-hydroxyvitamin D (25(OH)D) concentrations), with the prevalence of vitamin D deficiency ranging from 3 % in Australia to 21 and 22 % in the UK and Ireland, respectively, likely attributable to the higher dermal synthesis of vitamin D from ultraviolet B rays in Australia^{(Reference Bates, Collins and Jones23,32,Reference Cashman, Kehoe and Kearney33)} . Furthermore, while inadequate intakes of vitamin D are high in all teenagers, the prevalence of inadequate intakes of calcium is higher amongst teenage girls (61–88 %) than teenage boys (30–58 %)^{(Reference Walsh, Walton and Kearney13,Reference Lopes, Torres and Oliveira17,Reference Olza, Aranceta-Bartrina and González-Gross18,Reference Ng, Ahmed and L’abbe27,29)} , which may have implications further into adulthood with females more at risk of developing osteoporosis due to hormone changes during the menopause⁽³⁴⁾. The combination of very low intakes of calcium, vitamin D and magnesium may raise concern for the future bone health of teenagers and may increase the risk of osteoporosis and fractures^{(Reference Cashman2)}. Key sources of calcium in teenagers include dairy products and additionally, cereals and cereal products are an important contributor to calcium intakes in the UK and Ireland due to the mandatory fortification of wheat flour with calcium in the UK^{(Reference Bates, Lennox and Prentice24,Reference Flour Millers35)} . Milk (including vitamin D fortified milk), meat products and cereals and cereal products, including fortified ready-to-eat breakfast cereals (RTEBC), are key sources of vitamin D and magnesium^{(Reference Moyersoen, Devleesschauwer and Dekkers10,Reference Sette, Le Donne and Piccinelli15,Reference Lopes, Torres and Oliveira17,Reference Olza, Aranceta-Bartrina and González-Gross18,Reference van Rossum, Buurma-Rethans and Dinnissen22,Reference Bates, Collins and Jones23,28)} , highlighting the role of fortified foods such as RTEBC which are consumed by a high proportion of teenagers alongside natural sources of micronutrients^{(Reference Priebe and McMonagle36,Reference Fulgoni and Buckley37)} . While vitamin D supplementation is recommended in a number of countries, particularly during the winter months^{(Reference Moyersoen, Devleesschauwer and Dekkers10,34,Reference Weggemans, Kromhout and Van Weel38)} , data from nationally representative dietary surveys shows that nutritional supplements contributed just 6–18 % of vitamin D intakes in teenagers^{(Reference Moyersoen, Devleesschauwer and Dekkers10,Reference van Rossum, Buurma-Rethans and Dinnissen22)} .

Cognitive function

Aside from bone health, the teenage years are a period of rapid cognitive growth with nutrients such as folate and iron playing important roles in cognitive health, yet intakes of these nutrients in teenagers are low with respect to recommendations. The prevalence of inadequate intakes of folate in teenagers ranges from 14–57 % and the prevalence of inadequate intakes of iron ranges from 7–44 % across countries in the Western world (Tables 5 and 6). In addition to the role of folate in cognitive health, the role of folate in pregnancy and infant health must also be considered in this cohort and during adulthood due to the possibility of pregnancy⁽³⁹⁾. Supplemental folic acid intake of 400 µg/d is recommended for all women of childbearing age (irrespective of pregnancy intention) to reduce the risk of neural tube defects in infants. As about half of pregnancies are unplanned, particularly in teenagers⁽⁴⁰⁾, low intakes of folate among teenage girls may have negative implications in the case of pregnancy⁽⁴⁰⁾. The high prevalence of inadequate intakes of folate among teenagers is of particular importance in this context with the prevalence of inadequate intakes of folate higher amongst girls (22–63 %) than boys (6–52 %)^{(Reference Walsh, Walton and Kearney13,Reference Lopes, Torres and Oliveira17,26,Reference Ng, Ahmed and L’abbe27)} .

Iron requirements increase significantly throughout the teenage years due to increased total blood volume and lean body composition, and increased iron requirements for girls due to the onset of menstruation^{(Reference Beard5)}. International bodies including the European Food Safety Authority and the US Institute of Medicine have acknowledged the difficulty in setting dietary reference values for iron due to large uncertainties in the rate and timing of pubertal growth and menarche, as well as substantial inter-individual variance in menstrual blood loss^(41,42) . Notwithstanding this, iron intakes are generally low in teenagers with respect to recommendations, and there is a higher prevalence of inadequate intakes of iron amongst girls (9–34 %) than boys (5–16 %)^{(Reference Tornaritis, Philippou and Hadjigeorgiou11,Reference Walsh, Walton and Kearney13,Reference Lopes, Torres and Oliveira17,Reference van Rossum, Buurma-Rethans and Dinnissen22,26,Reference Ng, Ahmed and L’abbe27,29)} . A higher prevalence of inadequate intakes of iron in girls in the Netherlands (77 %) may be due to a higher cut-off used to determine inadequacy reflecting increased requirements due to menstruation^{(Reference van Rossum, Buurma-Rethans and Dinnissen22)}. In the UK, 9 % of girls and 1 % of boys are classified as having iron deficiency anaemia^{(Reference Bates, Collins and Jones23)}, although manifestations of low iron intakes such as difficulty concentrating and tiredness will likely have manifested before iron deficiency anaemia is present^{(Reference Soppi43)}. Key sources of iron and folate in teenagers are a combination of animal sources (i.e. milk and meat products) alongside cereal products, while vegetables are an important source of folate^{(Reference Sette, Le Donne and Piccinelli15,Reference Lopes, Torres and Oliveira17,Reference Partearroyo, Samaniego-Vaesken and Ruiz19,Reference Samaniego-Vaesken, Partearroyo and Olza20,Reference van Rossum, Buurma-Rethans and Dinnissen22–Reference Bates, Lennox and Prentice24,28,29)} .

General health and immunity

Maintaining a healthy immune system and overall general health is important throughout the lifecycle and during the teenage years. Nutrients important for general health including vitamins A, C, E, zinc and potassium are low with respect to recommendations among teenagers and may result in increased risk of impaired immunity and susceptibility to infections^{(44,Reference Carr and Maggini45)} (Tables 2–6). Intakes of vitamin C in teenagers are generally below recommendations with the prevalence of inadequate intakes ranging from 21–49 % (Table 5). Although zinc intakes are generally in line with recommendations, it is of note that between 12–25 % of teenagers (and up to 68 % in Spain) have inadequate intakes of zinc (Table 6). Similar to other micronutrients, there is a higher prevalence of inadequate intakes of zinc in girls (7–57 %) compared to boys (16–36 %)^{(Reference Walsh, Walton and Kearney13,Reference Olza, Aranceta-Bartrina and González-Gross21,26,Reference Ng, Ahmed and L’abbe27,29)} . As zinc is necessary for the synthesis of tissue, these low intakes may have implications for optimal growth⁽⁶⁾. Intakes of vitamin A are also low in the diets of teenagers with respect to recommendations which may impact functions such as normal vision, growth and development, gene expression and reproduction⁽⁴⁾. Although clinical signs of vitamin A deficiency are not common in developed countries^{(Reference Zhao, Liu and Zhang46)}, there is a high prevalence of inadequate intakes of vitamin A (24–56 %) in the teenage population (Table 4). Intakes of vitamin E and potassium are also generally below recommendations (Tables 2–4), which may impact functions such as cell metabolism and synthesis of protein and glycogen⁽⁴⁾. Meat products and milk are important sources of vitamin A and zinc which contribute to general health in teenagers while fruit, vegetables and potatoes are key sources of vitamin C intakes. Vegetables, cereal products, meat and dairy are key sources of potassium, and fats and oils are an important source of vitamin E^{(Reference Sette, Le Donne and Piccinelli15,Reference Lopes, Torres and Oliveira17,Reference Olza, Aranceta-Bartrina and González-Gross21,Reference van Rossum, Buurma-Rethans and Dinnissen22,Reference Bates, Lennox and Prentice24,28,29)} .

Risk of excessive micronutrient intakes in teenagers

Due to negative health consequences from high intakes for some micronutrients, it is important to consider the balance between the benefit of addressing low intakes with the risk of excessive intakes. Although few data are available for teenagers, there is little to no risk of excess intakes, with minimal excess intakes of zinc (0–1 %), vitamin D (0–0·5 %), vitamin A (0–1 %), folic acid (4 % Ireland only) and magnesium (0–4 %) (Table 7). As there is little risk of excess intakes based on current dietary patterns, strategies should focus on increasing micronutrient intakes in order to decrease the risk of negative impacts resulting from low intakes.

Table 7.

Prevalence of excess intakes (%>UL) of micronutrients in teenagers from national dietary surveys in Europe, the US, Canada, Australia and New Zealand

* UL (Tolerable upper intake level) as indicated in individual studies.

† <3 % in the US indicates negligible amounts, ‘-’ Data not available or reported.

Considerations for the future

While the available evidence shows that intakes of vitamin B₆, vitamin B₁₂, vitamin K, thiamine, riboflavin, niacin, biotin, pantothenate, iodine, copper, phosphorus and selenium in teenagers are currently in line with generally accepted recommendations, ongoing monitoring of all micronutrient intakes at both the lower and upper ends of the distribution is important due to ongoing changes in the food supply, as well as potentially different food consumption patterns due to changing dietary recommendations. A sustainable, nutritious diet is a global objective, and it is well recognised that changes to dietary patterns may improve planetary health^{(Reference Birt, Buzeti and Grosso47)}. As a result, dietary guidelines are now recommending a more sustainable diet, with reduced consumption of animal foods including dairy products⁽⁴⁸⁾, while the recent EAT Lancet report suggests a need for a 50 % reduction in meat intake for environmental benefit^{(Reference Willett, Rockström and Loken49)}. Considering the importance of animal sources, particularly meat and dairy, to intakes of vitamins and minerals in all population groups but especially teenagers, adherence to these recommendations may have unintended effects on micronutrient intakes. A decline in red meat intake in teenagers has been shown in the UK⁽⁵⁰⁾, while a decrease in dairy consumption in teenagers has been reported in Ireland⁽⁵¹⁾, the Netherlands^{(Reference van Rossum, Buurma-Rethans and Dinnissen22)} and the UK^{(Reference Bates, Collins and Jones23,Reference Bates, Lennox and Prentice24)} over the last 15 years, with a concurrent decrease seen in calcium intakes in these countries. Additionally, due to the reduced bioavailability of iron from fortified foods and non-haem iron from plant-based foods, more research on the impact of these dietary changes on biochemical status of iron is warranted⁽⁴²⁾. Although plant sources such as grains are good sources of zinc, it is important to consider that grains also contain phytates, which can reduce the absorption of a number of minerals including zinc⁽⁶⁾. While most recommendations account for this in a mixed diet, the efficiency of mineral absorption is likely to be lower in a predominantly plant-based diet⁽⁶⁾.