To receive more information about up-to-date research on micronutrients, sign up for the free, semi-annual LPI Research Newsletter here.
Adolescence—the transitional stage of development between childhood and adulthood—is associated with marked physical growth, reproductive maturation, and cognitive transformations. Physical changes begin in early adolescence during puberty, when sexual maturity is reached and reproduction is possible (1). Girls generally begin their adolescent growth spurt at an earlier age (9 years of age) than boys (11 years of age); the pubertal growth spurt lasts between two to four years, with the average rate of linear growth being 5-6 cm/year (2-2.4 in/year). Boys experience greater gains in height compared to girls because of a higher rate of growth and a longer growth spurt (2). Gains in linear growth are accompanied by increases in body weight and changes in body composition. Weight gain in girls typically happens six months following the greatest gains in linear growth, whereas weight gain in boys is usually coincident with increases in height. Throughout adolescence, boys gain more lean (fat-free) mass than girls, and girls experience greater increases in adiposity, which is required for normal menstruation (3). Moreover, approximately half of adult bone mass is obtained during adolescence (4), with boys experiencing greater gains in bone size and bone mass compared to girls (5).
In addition to physical growth, reproductive maturation occurs during adolescence. Maturation of the reproductive organs and development of secondary sexual characteristics, including facial hair in males and breast development in females, take place during puberty. Girls also experience menarche—the onset of menstruation—during this developmental stage, typically following the peak period of gains in height and weight (6). Adolescence is further characterized by cognitive, emotional, and psychosocial development (2).
Good nutrition is needed to support the growth and developmental changes of adolescence. Undernutrition, in general, has been shown to delay the adolescent growth spurt (7). Overnutrition, a form of malnutrition where macronutrients (carbohydrates, fats, proteins) are supplied in excess of the body’s needs, can lead to obesity and is a concern in industrialized nations. In the developed world, adolescents are increasingly consuming energy-rich, nutrient poor diets comprised of fast food, processed foods, and sugar-sweetened beverages (8-10). Studies have also shown that many adolescents do not come close to meeting intake recommendations for nutrient-rich foods, such as fruits, vegetables, and milk (11, 12). Together, these dietary behaviors place adolescents at increased risk for micronutrient deficiencies. This article discusses micronutrient requirements of adolescents aged 14 to 18 years, an age range that is used by the Food and Nutrition Board (FNB) of the U.S. Institute of Medicine to establish dietary reference intakes. Due to limited data, many of the micronutrient intake recommendations for adolescents are extrapolated from recommendations for adults using a formula that accounts for metabolic body weight and growth (13), not unique physiological changes during adolescence. Metabolic body weight is determined by calculating the 0.75 power of body mass (body mass^0.75) (14). To account for growth, the equation used to derive a Recommended Dietary Allowance (RDA) or Adequate Intake (AI) involves an age group-specific growth factor (13). The FNB set different micronutrient intake recommendations for children 9 to 13 years, an age range that encompasses puberty and early stages of adolescence (15); discussion of the micronutrient requirements of younger children is included in a separate article (see Micronutrient Needs of Children Ages 9 to 13 Years).
Micronutrient Needs of Adolescents Aged 14 to 18 Years
For each micronutrient, the FNB sets an RDA or AI for adolescents aged 14 to 18 years. These recommendations are gender specific to account for the unique nutritional needs of males and females as they undergo the physiological changes of adolescence. The table below, RDA for Micronutrients During Adolescence, Ages 14 to 18 Years, lists the RDA for each micronutrient by gender. The RDA should be used in the planning of diets for individuals. A more detailed discussion of the requirements of certain micronutrients for adolescents can be found below.
|Dietary Reference Intakes Set by the FNB:
RDA for Micronutrients During Adolescence, Ages 14 to 18 Years, Per Day
|Biotin||25 mcg (AI)||25 mcg (AI)|
|Folate||400 mcga||400 mcga|
|Niacin||16 mgb||14 mgb|
|Pantothenic Acid||5 mg (AI)||5 mg (AI)|
|Riboflavin||1.3 mg||1.0 mg|
|Thiamin||1.2 mg||1.0 mg|
|Vitamin A||900 mcg (3,000 IU)c||700 mcg (2,333 IU)c|
|Vitamin B6||1.3 mg||1.2 mg|
|Vitamin B12||2.4 mcg||2.4 mcg|
|Vitamin C||75 mg||65 mg|
|Vitamin D||15 mcg (600 IU)||15 mcg (600 IU)|
|Vitamin E||15 mg (22.5 IU)d||15 mg (22.5 IU)d|
|Vitamin K||75 mcg (AI)||75 mcg (AI)|
|Calcium||1,300 mg||1,300 mg|
|Chromium||35 mcg (AI)||24 mcg (AI)|
|Copper||890 mcg||890 mcg|
|Fluoride||3 mg (AI)||3 mg (AI)|
|Iodine||150 mcg||150 mcg|
|Iron||11 mg||15 mg|
|Magnesium||410 mg||360 mg|
|Manganese||2.2 mg (AI)||1.6 mg (AI)|
|Molybdenum||43 mcg||43 mcg|
|Phosphorus||1,250 mg||1,250 mg|
|Potassium||4,700 mg (AI)||4,700 mg (AI)|
|Selenium||55 mcg||55 mcg|
|Sodium||1,500 mg (AI)||1,500 mg (AI)|
|Zinc||11 mg||9 mg|
|RDA, recommended dietary allowance; AI, adequate intake|
aDietary Folate Equivalents
bNE, niacin equivalent: 1 mg NE = 60 mg tryptophan = 1 mg niacin
cRetinol Activity Equivalents
Vitamin A:Vitamin A is a fat-soluble vitamin that is essential for growth and development, normal vision, the expression of selected genes, immunity, and reproduction (16). Vitamin A deficiency in children and adolescents is a major public health problem worldwide, especially in less developed countries (17, 18). Even marginal or subclinical deficiencies in vitamin A may have adverse effects on bone growth and sexual maturation of adolescents (19). Because of its role in immunity, inadequate intake of this vitamin also increases risk for infectious diseases (20).
Studies in industrialized countries have reported inadequate intakes of vitamin A among adolescents (21-23). Serum retinol binding protein (RBP) concentrations have been shown to increase throughout the stages of puberty, indicating that vitamin A is needed for adolescent development (24). However, few vitamin A supplementation studies have been done in adolescents; most supplementation studies have included younger children who are more susceptible to vitamin A deficiency.
The RDA for vitamin A is based on the amount needed to ensure adequate stores (four months) of vitamin A in the body to support normal reproductive function, immune function, vitamin A-dependent gene expression, and vision (16). Vitamin A intake recommendations for adolescents were derived by extrapolating the recommendation for adults using metabolic body weight, accounting for growth. The RDA for adolescent boys aged 14 to 18 years is 900 mcg per day of Retinol Activity Equivalents (RAE), which is 3,000 international units (IU); the RDA for adolescent girls aged 14 to 18 years is 700 mcg of RAE, which is equivalent to 2,333 IU. For information on vitamin A content in foods, see the separate article on Vitamin A.
Vitamin B6 is required for heme synthesis and in the synthesis and metabolism of amino acids—the building blocks of proteins. Thus, the vitamin has obvious relevance to adolescent growth and health. Dietary intake recommendations of vitamin B6 for adolescents were established by extrapolating data from adults, using metabolic body weight and accounting for growth. The RDA for boys aged 14 to 18 years is 1.3 mg/day, and the RDA for girls aged 14 to 18 years is 1.2 mg/day (25). Only a few studies have evaluated vitamin B6 status specifically in adolescents. In an analysis of American adolescent girls (aged 12-16 years), mean dietary intake of vitamin B6 was 1.2 mg/day; however, one-third of the girls not taking vitamin B6-containing supplements had either marginal or deficient vitamin B6 status (26). The same investigators found more than 40% vitamin B6 inadequacy when a group of 112 adolescent girls (12- and 14-year-old) were followed for two years (27). Results of more recent studies have suggested that most American and European adolescents are meeting current intake recommendations for vitamin B6 (28, 29), although a study in Canada found that more than half of adolescent males aged 14 to 18 years did not meet the Estimated Average Requirement (EAR) of 1.1 mg/day for vitamin B6 (22). For information on dietary sources of the vitamin, see the separate article on Vitamin B6.
The B vitamin, folate, is required as a coenzyme to mediate the transfer of one-carbon units. Folate coenzymes act as acceptors and donors of one-carbon units in a variety of reactions critical to the endogenous synthesis and metabolism of nucleic acids (DNA and RNA) and amino acids (30, 31). Thus, folate has obvious importance in growth and development. Moreover, higher intakes of folate in adolescents have been linked to better academic achievement (32). Like other B vitamins, adolescent intake recommendations for folate were extrapolated from adult recommendations, using metabolic body weight and accounting for growth. The RDA for adolescents aged 14 to 18 years is 400 mcg/day of dietary folate equivalents (33).
When considering naturally occurring folate in foods, results of a national survey indicate that almost 80% of individuals aged 2-18 years in the U.S. have intakes below the EAR, which is 330 mcg/day of dietary folate equivalents for adolescents aged 14-18 years. However, when accounting for intake from fortified foods, less than 5% of individuals in that age group have intakes below the EAR (34). The U.S. Food and Drug Administration implemented legislation in 1998 requiring the fortification of all enriched grain products with folic acid (35). Globally, more than 50 countries have mandatory programs of wheat-flour fortification with folic acid, but flour fortification is not common in Europe (36). Dietary folate inadequacy is common among adolescents in European nations, especially girls (29).
Vitamin B12 is needed for two types of reactions in the human body. One is transmethylation (methyl transfer between two molecules) that leads to the synthesis of the amino acid methionine from homocysteine. Methionine, in turn, is required for the synthesis of S-adenosylmethionine, a methyl group donor used in many biological methylation reactions, such as the methylation of sites within DNA and RNA (37). The second sort of reaction is isomerization (rearrangement of a molecule). Vitamin B12 acts as a coenzyme for methylmalonyl-CoA mutase to convert methylmalonyl-CoA to succinyl-CoA, an important step for the metabolism of proteins and lipids. Both transmethylation and isomerization reactions are essential for the metabolism of components of the myelin sheath of nerve cells and for the metabolism of neurotransmitters. Accordingly, vitamin B12 deficiency damages the myelin sheath covering cranial, spinal, and peripheral nerves, resulting in neurological damage (38, 39). The myelin sheath is the insulating layer of tissue made up of lipids and proteins that surrounds nerve fibers. This sheath acts as a conduit in an electrical system, allowing rapid and efficient transmission of nerve impulses (40). In some cases, neurologic symptoms caused by vitamin B12 deficiency can be reversed by vitamin treatment (38), but reversibility seems to be dependent upon the duration of the associated neurologic complications (41).
Although myelination primarily occurs during fetal development and early infancy, it continues through childhood, adolescence, and stages of early adulthood (42, 43). Because of the role of vitamin B12 in myelination and other metabolic processes, it is important for adolescents to meet dietary intake recommendations. The RDA of vitamin B12 for adolescent boys and girls aged 14 to 18 years is 2.4 mcg/day (41), extrapolated from the recommendation for adults.
Vitamin B12 is naturally present only in animal products, such as meat, poultry, fish (including shellfish), and to a lesser extent in milk, but it is not generally present in plant products or yeast (44). Vitamin B12 deficiency has been reported in adolescents on very restricted or strict vegetarian diets (45, 46). Because vitamin B12 is stored in the liver, it may take three to six years for clinical symptoms to manifest (45). Thus, adolescents who have vegan diets need adequate intake from fortified foods or supplemental vitamin B12.
Vitamin C has a number of important roles during growth and development, including being required for the synthesis of collagen, carnitine, and neurotransmitters (47). Vitamin C is also a highly effective antioxidant and is important for immunity (see the separate article on Immunity). Further, vitamin C strongly enhances the absorption of nonheme iron by reducing dietary ferric iron (Fe3+) to ferrous iron (Fe2+). Specifically, iron absorption is 2- to 3-fold higher with co-ingestion of 25 to 75 mg of vitamin C (48). This has special relevance to adolescent health, considering the fact that iron deficiency is prevalent among adolescents, especially girls (see Iron below). The RDA for adolescents aged 14 to 18 years, which was extrapolated from recommendations for adults based on relative body weight, is 75 mg/day and 65 mg/day of vitamin C for boys and girls, respectively (49).
Data on vitamin C intake among adolescents are limited, but a recent U.S. national survey, the 2003-2004 National Health and Nutrition Examination Survey (NHANES), found that serum vitamin C concentrations of adolescents (aged 12-19 years) were lower in adolescents compared to younger children (6-11 years), and adolescent girls had higher levels than adolescent boys (50). In this analysis, 2.7% of adolescent boys and 3.9% of adolescent girls had overt vitamin C deficiency that could result in clinical symptoms of scurvy. A cross-sectional analysis of European adolescents (aged 12.5-17.5 years) also noted higher vitamin C status among adolescent girls compared to boys, and compared to the U.S. survey, the prevalence of overt vitamin C deficiency was lower in European adolescents (51). For information on food sources of vitamin C, see the separate article on Vitamin C.
Vitamin D is a fat-soluble vitamin that is essential for maintaining normal calcium metabolism and is therefore necessary for bone health. Severe vitamin D deficiency in infants and children results in the failure of bone to mineralize, leading to a condition known as rickets, but cases of rickets have also been reported during stages of puberty and adolescence (52-53). Rapidly growing bones are most severely affected by rickets. The growth plates of bones continue to enlarge, but in the absence of adequate mineralization, weight-bearing limbs (arms and legs) become bowed. Inadequate vitamin D during puberty and adolescence might prevent the attainment of peak bone mass and final height (54-56) and could possibly increase the risk of osteoporosis or other diseases in adulthood, but more studies on these associations are needed.
In 2010, the Food and Nutrition Board (FNB) of the Institute of Medicine set an RDA based on the amount of vitamin D needed for bone health and assuming minimal sun exposure; the RDA is 600 IU/day (15 mcg/day) for adolescents aged 14 to 18 years. In the U.S., milk is voluntarily fortified with 400 IU (10 mcg) of vitamin D per quart (946 mL); thus, adolescents would need to consume about 6 cups of milk daily to meet the RDA. Although fish is the best source of vitamin D in the diet, fortified foods and beverages are likely the major dietary source of vitamin D for U.S. adolescents. In Canada, fortification of milk and margarine is mandatory, with milk containing 35-45 IU per 100 mL (331-426 IU per quart) and margarine containing 530 IU per 100 grams (57), but vitamin D fortification of foods is less common in European nations (58). In addition to diet, vitamin D can be endogenously synthesized in the skin upon exposure to ultraviolet-B radiation from sunlight; however, sunscreens effectively block skin synthesis of vitamin D and vitamin D synthesis is diminished in northern latitudes during winter (see the separate article on Vitamin D).
Analysis of data from NHANES 2005-2006 found that average total vitamin D intakes (from diet and supplements combined) in U.S. adolescents (aged 14 to 18 years) were 6.9 mcg/day (276 IU/day) for boys and 5.0 mcg/day (200 IU/day) for girls—well below the current RDA. This analysis also found that 16% of adolescent boys and 27% of adolescent girls took vitamin D-containing supplements (59). Because sun exposure can substantially affect body vitamin D levels, measuring 25-hydroxyvitamin D—the major circulating form of vitamin D—is a more useful indicator of vitamin D status. However, studies assessing vitamin D status in adolescents have used various cutoffs to define vitamin D deficiency and insufficiency and there is no consensus of what level constitutes adequacy.
It is assumed that a dietary intake of 600 IU (15 mcg)/day results in a serum 25-hydroxyvitamin D level of 20 ng/mL (50 nmol/L), which the FNB considers as the cut-off point for vitamin D adequacy (60). However, many researchers believe that higher levels may benefit health. NHANES found that more than 25% of U.S. adolescents (12-19 years) had serum 25-hydroxyvitamin D concentrations lower than 20 ng/mL (50 nmol/L) and about 75% of adolescents had levels lower than 32 ng/mL (80 nmol/L) (61). Some studies have found a higher prevalence of vitamin D deficiency among European adolescents (19, 62). Ethnic and seasonal differences have also been reported, with higher levels in whites compared to blacks (61, 63) and in summertime compared to wintertime (62, 64, 65). Moreover, several studies have reported low vitamin D status among adolescents living in sunny climates (63, 66-68).
Oral vitamin D supplementation has been shown to improve vitamin D status among adolescents (69, 70), and one double-blind, placebo-controlled trial found that improvements in vitamin D status were accompanied by some musculoskeletal benefits in adolescent girls (70). Although more supplementation studies are needed, ensuring vitamin D adequacy throughout childhood and adolescence seems prudent. The Linus Pauling Institute recommends that adolescents aged 14 to 18 years should have a daily intake of 600 to 1,000 IU (15 to 25 mcg) of vitamin D, consistent with the recommendations of the Endocrine Society (71). According to the Endocrine Society, at least 600 IU/day may be required to maximize bone health, and 1,000 IU/day may be needed to increase serum levels above 30 ng/mL (75 nmol/L) (71). Given the average vitamin D content of the diets of adolescents, supplementation may be necessary to meet this recommendation. The American Academy of Pediatrics currently suggests that all adolescents who do not get 400 IU/day of vitamin D through dietary sources should take 400 IU of supplemental vitamin D daily (72)—an amount that is typically found in multivitamin supplements.
The RDA of vitamin E for adolescents, expressed as an amount of the alpha-tocopherol form of the vitamin, was based on extrapolations from intake recommendations for adults, accounting for differences in lean body mass and increased needs of growth during adolescence. The RDA is 15 mg/day (22.5 IU/day) for boys and girls ages 14 to 18 years (73). A U.S. national survey, NHANES 1999-2000, found that adolescent boys and girls aged 14 to 18 years had average intakes of 7.5 mg/day and 5.7 mg/day of alpha-tocopherol, respectively. Moreover, 92% of adolescent boys and more than 99% of adolescent girls of this age group had daily intakes below the EAR of 12 mg/day (74). Vitamin E intake has been reported to be similarly low in adolescents in Spain (21), Switzerland (23), Brazil (75), France (76), and Germany (77). However, true vitamin E deficiency is rare and has been observed only in cases of severe malnutrition, genetic defects affecting the alpha-tocopherol protein, and fat malabsorption syndromes; see the separate article on Vitamin E.
About 99% of calcium in the body is found in bones and teeth (78). Adequate intake of calcium throughout childhood and adolescence is important for proper mineralization of growing bones, attainment of peak bone mass, and reduction of risk of bone fracture and osteoporosis in adulthood. Dietary intake recommendations for calcium in adolescents were established using a factorial method that summed average calcium accretion and calcium losses to urine, feces, and sweat and also adjusted for calcium absorption (60). Specifically, data used by the FNB to determine calcium accretion came from a recent longitudinal study in 642 Caucasian adolescents aged 14 to 18 years (79). The authors of this study estimated that the daily calcium requirement is higher in boys than girls; however, the FNB concluded that the differences were relatively small and it would be more practical to establish a single recommendation for all adolescents. Thus, the RDA was set at 1,300 mg/day; this level of calcium intake is expected to cover the needs of 97.5% of adolescents.
Many U.S. adolescents have dietary calcium intakes below the RDA, with girls having lower intakes than boys. A recent analysis of data from NHANES 2003-2006, a U.S. national survey, found that 42% of adolescent boys and only 10% of adolescent girls (14-18 years) had dietary calcium intakes above 1,300 mg/day. When accounting for use of calcium-containing supplements (19% and 24% of boys and girls, respectively; average supplemental intake of 142 and 182 mg/day of calcium in boys and girls, respectively), 42% of adolescent boys and only 13% of adolescent girls had total daily intakes above the current RDA (59). A recent publication that reviewed average calcium intake among adolescents in 23 nations found that boys generally have intakes of ~100-200 mg/day higher than girls and that many adolescents do not meet intake recommendations (80).
Dairy products, which provide about 72% of the calcium in the American diet (78), represent rich and absorbable sources of calcium. Milk contains 300 mg of calcium per cup; therefore, adolescents could meet the RDA for calcium by drinking 4.3 cups of low-fat milk daily. However, NHANES data show that U.S. adolescents (12-19 years) on average consume only about 1 cup of milk daily (81). Lactose intolerance may prevent some adolescents from consuming milk, and consumption of soft drinks and other sweetened beverages might displace milk consumption in adolescents (82).
Certain vegetables and grains also provide calcium, but their bioavailability is lower compared with dairy. For more information on dietary sources of calcium and calcium bioavailability, see the separate article on Calcium. The Nutrition Facts label of packaged foods lists calcium content in one serving as a percent of the Daily Value (DV), with the DV being 1,000 mg. Since the RDA for adolescents is 1,300 mg/day, the percentage of the DV listed on the food label would be an overestimation of the percentage of the RDA. If adolescents do not meet the RDA through diet alone, LPI recommends supplemental calcium. Multivitamin/mineral supplements generally provide no more than 200 mg of calcium.
Iron is an essential component of hundreds of proteins and enzymes involved in various aspects of metabolism, including oxygen transport and storage, electron transport and energy metabolism, antioxidant and beneficial pro-oxidant functions, oxygen sensing, and DNA synthesis (44, 83-85); see the separate article on Iron. Iron deficiency, which is the most common nutritional deficiency in the world, is a major public health problem, especially in developing nations, but it is also prevalent in industrialized nations, notably in women of childbearing age. Severe iron deficiency leads to iron-deficiency anemia; anemia affects more than 30% of the global population (2 billion people) (81). Adolescents have increased requirements for iron due to rapid growth. In particular, adolescent girls are at a heightened risk of iron deficiency due to inadequate intake of dietary iron, especially heme iron; increased demands of growth; and iron loss that occurs with menstruation. Following puberty, adolescent girls have lower iron stores compared to adolescent boys (87).
In addition to the negative effects of iron deficiency on physical growth, iron deficiency during adolescence may impair immunity (see the separate article on Nutrition and Immunity) as well as cognition. Iron is needed for proper development of oligodendrocytes (the brain cells that produce myelin) (88), and the mineral is also a required cofactor for several enzymes that synthesize neurotransmitters (89). Iron deficiency—even levels not associated with anemia—during important stages of brain development, such as adolescence, may have detrimental consequences. A double-blind, placebo-controlled trial in 73 adolescent girls (aged 13-18 years) with non-anemic iron deficiency found that high-dose iron supplementation (260 mg/day of elemental iron) for eight weeks resulted in greater improvement in verbal learning but not in other cognitive domains (90). Another study reported that one-month iron supplementation beyond that included in a prenatal vitamin improved some measures of attention and short-term memory in young pregnant women (aged 14-24 years) without severe iron deficiency (91). Clinical trials of iron supplementation to date have been mostly done in other age groups; large, well-designed trials in adolescents are needed to determine the effects of iron supplementation on cognition.
Dietary intake recommendations for adolescents were based on a factorial modeling approach that accounts for the amount of iron needed to replace basal losses (losses in urine, feces, and sweat), iron requirements associated with growth (increases in hemoglobin and iron content of tissues), and iron losses associated with menstruation in girls. The intake recommendations also account for average bioavailability (the fraction of iron retained and used by the body) of dietary iron for this age group (92). The RDA of iron is 11 mg/day for adolescent boys and 15 mg/day for adolescent girls. A U.S. national survey, NHANES 2001-2002, found that average dietary intake of iron was 19.1 mg/day in adolescent boys and 13.3 mg/day in adolescent girls; however, 16% of adolescent girls had intakes below the EAR of 7.9 mg/day . Because several different criteria have been used to identify iron deficiency, it is difficult to report the prevalence of iron deficiency among adolescents.
The amount of bioavailable iron in food (or supplements) is influenced by the iron nutritional status of the individual and also by the form of iron (heme or nonheme). Individuals who are anemic or iron deficient absorb a larger percentage of the iron they consume (especially nonheme iron) than individuals who are not anemic and have sufficient iron stores (94, 95). Heme iron, found in meat, poultry, and fish, is more readily absorbed, and its absorption is less affected by other dietary factors than nonheme iron—the form found in plants, dairy products, fortified foods, and supplements. Although heme iron generally accounts for only 10-15% of the iron found in the diet, it may provide up to one third of total absorbed dietary iron (83, 95). The absorption of nonheme iron is strongly influenced by enhancers and inhibitors present in the same meal. For instance, vitamin C strongly enhances the absorption of nonheme iron by reducing dietary ferric iron (Fe3+) to ferrous iron (Fe2+) and forming an absorbable, iron-ascorbic acid complex. Organic acids, such as citric, malic, tartaric, and lactic acids, also enhance nonheme iron absorption. Further, consumption of meat, poultry, and fish enhance nonheme iron absorption, but the mechanism for this increase in absorption is not clear (92, 94). Inhibitors of nonheme iron absorption include phytic acid, which is present in legumes, grains, and rice. Polyphenols found in some fruits, vegetables, coffee, tea, wines, and spices can also markedly inhibit the absorption of nonheme iron, but this effect is reduced by the presence of vitamin C (92, 96). Soy protein, such as that found in tofu, has an inhibitory effect on iron absorption that is independent of its phytic acid content (92).
The mineral magnesium is involved in more than 300 essential metabolic reactions that are generally involved in energy production and the synthesis of nucleic acids (DNA and RNA), proteins, carbohydrates, and lipids (97). Magnesium also plays structural roles in bone, cell membranes, and chromosomes and is also required for various cellular processes, including ion transport across cell membranes, cell signaling, and cell migration (98).
The RDA of magnesium for those aged 14 to 18 years, 410 mg/day for boys and 360 mg/day for girls, was derived from results of balance studies in adolescents. Good dietary sources of magnesium include nuts, and green leafy vegetables because magnesium is part of chlorophyll—the green pigment in plants. Meats and milk have an intermediate magnesium content, with milk providing 24-39 mg per cup (97). Refined foods generally have the lowest magnesium content. Although data are limited, some studies have found that a large percentage of adolescents have magnesium intakes below recommended levels (100-102). Data on magnesium intake among adolescents are lacking. In an analysis of NHANES data, U.S. adolescents who consumed milk (plain or flavored) had higher daily magnesium intakes than adolescents who did not drink milk (103). However, NHANES data show that U.S. adolescents (12-19 years) on average only consume about 1 cup of milk daily (81). Low-fat milk, nuts, whole grains, and green leafy vegetables are important sources of magnesium for adolescents. If adolescents do not meet the RDA through dietary sources, LPI recommends a combined magnesium-calcium supplement.
Potassium is required for maintenance of cellular membrane potential and thus for nerve impulse transmission, muscle contraction, and heart function. The AI recommendation, set by the FNB in 2004, is based on potassium intake needed to lower blood pressure, reduce salt sensitivity, and minimize the risk of kidney stones. The AI for adolescents, 4.7 g/day for ages 14-18 years, was extrapolated from the adult recommendation, adjusting for energy intake. According to a U.S. national survey, only 3% of American adolescents (aged 14-18 years) meet the AI. In general, adolescents have low intakes of fruits, vegetables, and dairy products (104-106)—foods that are rich in potassium. Low intakes of potassium, coupled with high intakes of sodium (see section on Sodium below), have been linked to elevations in blood pressure and a heightened risk of hypertension and stroke later in life (see the separate article on Potassium). In a study that followed 2,368 adolescent girls for nine years, lower intakes of potassium were associated with a higher incidence of hypertension (107). Fruits, vegetables, low-fat milk, and nuts are all good sources of potassium, and increasing intake of these foods during adolescence should help regulate blood pressure and may decrease risk of chronic disease during adulthood.
In 2004, the FNB set the AI for adolescents by extrapolating from the adult AI using relative energy intakes; however, since energy intakes of adolescents are similar to that of adults, the recommendations are identical: 1,500 mg/day of sodium (108). The 2010 Dietary Guidelines for Americans recommend limiting sodium intake to 1,500 mg/day to lower blood pressure and thus reduce risk of cardiovascular and kidney diseases in adulthood. However, daily sodium intake in U.S. adolescents (aged 12-19 years) is 3,000 mg in girls and 4,000 mg/day in boys (109). These intake levels are well above the tolerable upper intake level of 2,300 mg/day. Low-sodium interventions in adolescents have shown some improvement in blood pressure, but compliance to such a diet is problematic (110, 111).
The mineral zinc is essential for growth and development, immune function, neurological function, and reproduction. Zinc plays a number of catalytic, structural, and regulatory roles in cellular metabolism (see the separate article on Zinc). Zinc deficiency, which is estimated to affect more than 2 billion people in less developed nations (112), can retard normal growth, impair cognitive development, and delay sexual maturation (113, 114). Adolescents are at increased risk of zinc deficiency due to the demands of growth (115). Mild zinc deficiency, which is common in both the developing and developed world, may also have negative effects on growth and development (47, 116); however, the lack of a sensitive indicator of mild zinc deficiency hinders the scientific study of its health implications.
Because a sensitive indicator of zinc nutritional status is not readily available, the RDA for zinc was based on a number of different indicators of zinc nutritional status and represents the daily intake likely to prevent deficiency in nearly all individuals in a specific age and gender group. The RDA for adolescent boys and girls, aged 14 to 18 years, is 11 mg/day and 9 mg/day, respectively (113). A U.S. national survey, NHANES 2001-2002, found that average dietary intake of zinc was 15.1 mg/day in adolescent boys and 9.5 mg/day in adolescent girls; only 4% of adolescent boys had intakes less than the EAR (8.5 mg/day), but 26% of adolescent girls had intakes less than the EAR (7.3 mg/day).
Shellfish, beef, and other red meats are rich sources of zinc; nuts and legumes are relatively good plant sources of zinc. Zinc bioavailability (the fraction of zinc retained and used by the body) is relatively high in meat, eggs, and seafood because of the relative absence of compounds that inhibit zinc absorption and the presence of certain amino acids (cysteine and methionine) that enhance zinc absorption. However, the zinc in whole-grain products and plant proteins is less bioavailable due to their relatively high content of phytic acid, a compound that inhibits zinc absorption (117). The enzymatic action of yeast reduces the level of phytic acid in foods. Therefore, leavened whole-grain breads have more bioavailable zinc than unleavened whole-grain breads (113).
Pregnancy during adolescence—a time when the girl is still growing herself—has been associated with increased risk of miscarriage, prematurity, low birth weight infants (<2,500 grams), and increased maternal and neonatal mortality (2, (118-119). Pregnant adolescents are also at a heightened risk for pregnancy-related complications, including pregnancy-induced hypertension and anemia (119). Because they are growing themselves, it is extremely important for pregnant adolescents to meet dietary intake recommendations. Recommendations for some key micronutrients needed for adolescent growth, including calcium, magnesium, phosphorus, and zinc, are higher than those for older pregnant women; see the discussion of Micronutrient Requirements During Pregnancy in a separate article. Pregnant adolescents are at increased risk for select micronutrient inadequacies, especially iron, zinc, calcium, magnesium, folate, vitamin B6, vitamin D, and vitamin E (2, 120-121). Adequate nutrition is important not only for a healthy pregnancy outcome but also for the overall and skeletal health of the adolescent. A recent cross-sectional study of 719 postmenopausal women associated their pregnancy during adolescence with lower bone mineral density at several sites and a 2-fold higher risk of osteoporosis compared to women without a history of adolescent pregnancy (122). However, it is not known whether adequate calcium intake during adolescent pregnancy might prevent age-related declines in bone mineral density or osteoporosis. For more information on nutrition and adolescent pregnancy, see the University of Minnesota Website; note that the RDA for vitamin D has been increased to 15 mcg (600 IU)/day since the publication of this book.
The FNB establishes a tolerable upper intake level (UL) for most micronutrients. The UL is the highest level of daily nutrient intake likely to pose no risk of adverse health effects in almost all individuals of a specified age group. This level applies to total daily intake from food, water, and supplements. Due to the potential for adverse effects, it is recommended that individuals not exceed the UL. Thus, individuals should use the UL as a guide to limit daily micronutrient intake, not as a recommended level of intake (123). There is no evidence that consumption of micronutrients at or above the UL results in any health benefits for adolescents, and the UL should not be exceeded except under medical supervision. See the table below, which lists the UL for Micronutrients During Adolescence, Ages 14 to 18 years.
|Dietary Reference Intakes Set by the FNB:
UL for Micronutrients During Adolescence, Ages 14 to 18 Years, Per Day
|Micronutrient||Males and Females|
|Vitamin A||2,800 mcg (9,333 IU)c|
|Vitamin B6||80 mg|
|Vitamin C||1,800 mg|
|Vitamin D||100 mcg (4,000 IU)|
|Vitamin E||800 mg (1,200 IU)d|
|aND, not determinable|
bApplies to the synthetic form in fortified foods and supplements
cApplies only to preformed retinol
dApplies to any form of supplemental alpha-tocopherol
eApplies only to the supplemental form
A healthy diet throughout puberty and adolescence is important to provide nutrients that support optimal physical growth and cognitive development. Although it is generally advised that micronutrients should be obtained from food, many adolescents do not reach daily intake recommendations for select micronutrients from diet alone. Therefore, the Linus Pauling Institute recommends that adolescents aged 14 to 18 years take a daily multivitamin/mineral supplement with 100% of the daily value (DV) for most vitamins and essential minerals, keeping the following suggestions in mind:
Written in July 2012 by:
Victoria J. Drake, Ph.D.
Linus Pauling Institute
Oregon State University
Reviewed in July 2012 by:
Pamela S. Hinton, Ph.D.
Department of Nutrition and Exercise Physiology
University of Missouri
This article was underwritten, in part, by a grant from
Bayer Consumer Care AG, Basel, Switzerland.
Copyright 2012-2015 Linus Pauling Institute
The Linus Pauling Institute Micronutrient Information Center provides scientific information on the health aspects of dietary factors and supplements, foods, and beverages for the general public. The information is made available with the understanding that the author and publisher are not providing medical, psychological, or nutritional counseling services on this site. The information should not be used in place of a consultation with a competent health care or nutrition professional.
The information on dietary factors and supplements, foods, and beverages contained on this Web site does not cover all possible uses, actions, precautions, side effects, and interactions. It is not intended as nutritional or medical advice for individual problems. Liability for individual actions or omissions based upon the contents of this site is expressly disclaimed.
Thank you for signing up for the LPI Research Newsletter; this newsletter is available at: http://lpi.oregonstate.edu/nswltrmain.html