Vitamin D: role in growth and development and recommendations

Vitamin D is one of the key nutrients contributing to normal skeleton development from the very early life stages. A vitamin D level in the body (measured as 25-hydroxy vitamin D in blood serum) of ≥50nmol/L for young children has been defined by most scientific organizations as the optimum threshold for vitamin D status. Since these levels can be achieved with a vitamin D-intake of 15μg/day, this intake has recently in the US been defined as the Recommended Daily Allowance (RDA). Beyond sun exposure, there are limited natural sources of vitamin D. For this reason, in many parts of the world fortified foods and supplements are recommended for specific groups, like infants and toddlers.

Growing Child 6Vitamin D role in growth and development

At the beginning of the 20th century, vitamin D was identified as one of the key nutrients in skeleton development. Nowadays, its metabolic paths and role in bone development are well established.1,2  Vitamin D is primarily involved in intestinal calcium and phosphate absorption, renal calcium reabsorption and inhibition of parathyroid hormone secretion, leading to bone cell maturation and bone mineralization,1-3 thus supporting optimal skeletal growth and development from the very early life stages.4,5  Ensuring optimum vitamin D serum levels is particular crucial for toddlers, since there is a dramatic increase in bone mineral content (BMC) and bone mineral density (BMD) occurring in the first 36 months of life.6

Recommended serum Vitamin D-levels

Serum 25-hydroxyvitamin D (25OHD) is the best indicator of total body vitamin D-status. Although all scientific organizations agree on the importance of vitamin D for sustaining bone health, no consensus has been reached so far regarding the levels of serum 25OHD below which the adverse effect of vitamin D-deficiency or insufficiency occurs. Some scientific organizations have reported that children with poor bone development generally have serum 25OHD-levels lower than 25-30nmol/l,4,5 whereas the same insufficiency has been reported in breastfed colored skin infants, at serum 25OHD levels as high as 40-45nmol/l7,8 (Table 1).

Although it is clear that low levels of serum 25OHD impair growth, the question that remains is:

“What should the recommended serum 25OHD-threshold be, above which optimum growth and development in infants and toddlers is ensured?” Regarding sufficiency of vitamin D-status, most scientific organizations4,5,9 recommend serum 25OHD-concentration of ≥50nmol/l for infants and young children as a safe threshold, while certain scientific reports suggest that this recommended level should be even higher at 75nmol/l.10,11  Even though the optimum serum level of 25OHD has not yet been defined, there is no solid evidence indicating that serum 25OHD-levels above 75nmol/l are associated with greater benefit for bone health or other health outcomes.9

Vitamin D dietary intake recommendations

Most circulating vitamin D is synthesized endogenously from skin exposure to ultraviolet B (UVB) radiation. However, vitamin D cutaneous synthesis is determined from skin’s pigmentation (i.e. light skin populations produce much easier vitamin D), clothing, sunscreen use, latitude, season, time spent indoors, urbanization, air pollution, etc.3,5

The limited number of publications available worldwide on vitamin D-intake is pointing out that a significant proportion of the population, including toddlers, is suffering from vitamin D-insufficiency. Furthermore, populations living in higher latitudes, as well as immigrants living in developed countries, seem to be more strongly affected. Indicatively the prevalence of vitamin D-insufficiency (<50nmol/l) was found to vary from 11.3% in Jordan12 to 32 % in Canada,13 whereas it was reported to be 75% among two year old Asian children living in England.14  Low levels in children have also been observed in various Asian countries.

Taking into consideration the above, the US Institute Of Medicine (IOM) has in 2010 tripled the recommended dietary intake for children aged one year and older from 5μg/day to 15μg/day.9,15  The rationale behind this increased intake recommendation for vitamin D is that this has been reported to correspond to the desirable serum 25OHD-level of at least 50nmol/l.9 Previous recommendations, released until 2008, suggested a recommended dietary vitamin D-intake which varies from 5-10μg per day (Table 2).

Natural dietary sources of vitamin D include oily fish such as salmon, mackerel and sardines, and egg yolks (Table 3). As most of these foods are not commonly consumed in sufficient quantities by toddlers, they will not ensure their recommended dietary intake.4,5,16  For this reason, fortified food and supplements are important sources of dietary vitamin D to achieve the recommended intake for this age group. In some countries certain foods are fortified with vitamin D for this purpose, including milk, dairy products, margarine, breakfast cereals and fruit juices.4,5

Table 1 Vitamin D-status as defined by serum 25OHD-levels

Vitamin D-status 25OHD-levels nmol/l (ng/ml)
ESPGHAN1 AAP5 IOM9
Severe deficiency <25 (10) ≤ 12.5 (5)
Deficiency ≤ 37.5 (15)
Insufficiency 37.5-50 (15-20)
Sufficiency >50 (20) 50-250 (20-100) ≥50 (20)

*ESPGHAN: European Society of Paediatric Gastroenterology, Hepatology and Nutrition
AAP: American Academy of Pediatrics
IOM: Institute of Medicine

Table 2 Recommended vitamin D-intake for toddlers by various authorities in different regions

Recommended dietary intakes for toddlers

(1-3 years old)

published until 2008 published in 2010
IOM 1997/ 2010 (US)9,15 5μg/day (200IU) 15μg/day (600IU)
AAP 2008 (US)5 10μg/day (400IU)
UK 199117 7μg/day (280IU)
Department of Health 1998 (UK)18 7μg/day (280IU)
South East Asia 200819 5μg/day (200IU)
WHO 200420 5μg/day (200IU)

*AAP: American Academy of Pediatrics
IOM: Institute of Medicine

Table 3 Natural food sources of vitamin D*

Food Vitamin D-content, mcg
Egg yolk 0.5–0.6 per yolk
Canned tuna/sardines/salmon/mackerel in oil 5.6–8.3/100 g
Cooked salmon/mackerel 8.6–9/100 g
Atlantic mackerel (raw) 9/100 g
Shrimp 3.8/100 g

* Adapted from Misra M. et. al, 20085

References

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