In sufficient concentration, vitamin A supports cell growth and the immune system, and is therefore very important for healthy child development. If deficiencies arise, these may lead to severe complications such as a weakened immune system and night blindness. Children in developing countries are particularly susceptible to suffering these effects.
Although the body is generally able to produce vitamin D itself with the help of UVB light, vitamin D deficiency is still a widespread problem. Babies therefore often receive supplementation – particularly directly after birth, but also commonly beyond.
Vitamin C is involved in many essential processes in the human body. These include cellular reactions, iron absorption, and the metabolism of cholesterol, as well as important antioxidant functions.
Vitamin A plays a crucial role during cell and tissue growth, and is especially important for a strong immune system. In the form of its active metabolite – retinoic acid – it regulates growth and development of skin and mucosa; the aldehyde of the vitamin – retinal – is important for eyesight; and the vitamin A alcohol – retinol – is involved in spermatogenesis.
Vitamin A levels in plasma are subject to strict homeostatic regulation: a decrease in vitamin A levels only occurs if the vitamin pool in the liver is fully depleted. The clinical consequences specific to severe vitamin A deficiency are xerophthalmia (abnormal dryness of eyes) and night blindness (an impairment of the visual system's ability to adjust to the dark). Bitot spots in the eye, which appear as a result of xerophthalmia, are a classic initial indication of a severe vitamin A deficiency. They are followed by keratomalacia and ultimately blindness. Due to the simultaneous weakening of the immune system, even normal infections can lead to death. While such critically low levels of vitamin A are seldom seen in industrialised countries, they are widespread in developing countries. On a global scale, vitamin A deficit is one of the most prevalent micronutrient deficits (together with iron and zinc). Dietary sources with the highest levels of vitamin A are liver followed by vegetables with a naturally high content such as spinach, carrots, cabbage and kale.
Children are particularly sensitive to vitamin A, with daily intakes of about 450µg RE /kg bodyweight per day leading to signs of toxicity. Signs of chronic hypervitaminosis A reported in infants are loss of appetite, dermal dryness, loss of hair, fissuring of the corners of the mouth, bone pain, hepatomegaly, increased intracranial pressure and failure to thrive (Fomon, 1993). Recommended dosages are:
There is an interaction between iron and vitamin A. Vitamin A deficiency impairs iron mobilisation and vitamin A supplementation improves haemoglobin concentrations (Michelazzo et al, 2013). A similar relationship between vitamin A and zinc has not been unequivocally proven.back
Vitamin D is an important nutrient involved in the regulation of calcium and phosphate homeostasis, consequently supporting bone health (SACN, 2016; EFSA, 2016). It comprises two subtypes of vitamins - vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol) – which the body can obtain via foods such as eggs, oily fish, and fortified foods. However, vitamin D is primarily associated with sunlight as its major source: skin exposure to ultraviolet-B light (UVB light) triggers the synthesis of vitamin D (EFSA, 2016).
The absorption rate of vitamin D from food (which is absorbed with fat) is around 80%, which the body is unable to store efficiently. This creates a considerable risk of developing vitamin D deficiency. Geographical, climate, and cultural factors, combined with exposure to sunlight – latitude, time of year and day, length of sunshine hours, clothing and sun protection, skin pigmentation and thickness – increase the risk of low vitamin D reserves. Accordingly, vitamin D deficiency is particularly common in the northern hemisphere where sunlight is not sufficient for an adequate supply of vitamin D during winter. Vitamin D deficiency (or hypovitaminosis D) can have severe consequences for the body: absorption of minerals like calcium and phosphorus is negatively affected, leading to impaired bone mineralisation (rickets in children and osteomalacia in adults), seizures, and breathing difficulty (EFSA, 2016; Holick et al, 2012).
To avoid the aforementioned health problems, particularly in some countries in the northern hemisphere (e.g. Germany), prophylactic supplementation of vitamin D is common for children younger than one year. The recommended dosage by DGE (2015), Institute of Medicine (US) (2011), and Yu et al. (2017) is:
Although vitamin D supplementation is very common, dosages should be handled with care. Overdoses of vitamin D may lead to hypercalcaemia with soft tissue calcification, and renal and cardiovascular damage (Vieth, 1999; Zitterman and Koerfer, 2008).back
Vitamin C is a water-soluble antioxidant that is essential for humans. The term vitamin C is used as a synonym for ascorbic acid and encompasses semi dehydro-L-ascorbic acid and the oxidised form, dehydro-L-ascorbic acid, which together form a reversible redox system. It is involved in the biosynthesis of collagen, catecholamines, L-carnitine, amino acids, and certain peptide hormones, and is needed for healthy metabolic function within the body. Vitamin C is a highly effective reductive substance involved in many intra- and extra-cellular reactions. It is a cofactor of copper- and iron-dependent enzymes and, among other things, is involved in the function of oxytocin and other hormones (EFSA, 2013).
In addition, vitamin C is a strong antioxidant; active against free radicals and other reactive forms of oxygen and nitrogen such as superoxide, hydroxyl radicals and hypochlorous acid. It protects fats – and possibly also DNA – against oxidative damage. Among its extracellular functions are the protection of LDL particles against oxidation, and the regeneration of tocopherol and glutathione from their oxidised forms.
Vitamin C is also important in reducing plant-bound alimentary iron, thus improving the intestinal absorption of iron. Additionally, vitamin C is involved in hydroxylation reactions – which lead to inactivation of xenobiotics in the liver – and plays a role in the metabolism of cholesterol to bile acids (DGE, 2015).
The best sources of vitamin C are fruits and vegetables together with related products such as juices and smoothies made from berries, citrus fruits, papaya, kiwi, cauliflower, cabbage, or sweet pepper. Products of animal origin also contain vitamin C in lower quantities. Supplying the body with vitamin C is important in order to avoid weakening of collagenous structures, connective tissue defects, and even some diseases (including scurvy in adults). For children, bone tissue defects, impaired bone growth, and ossification are possible consequences of vitamin C deficiencies (Shenkin, 2008). If it is not possible to reach proposed daily requirements via a balanced diet, supplementation is recommended. Although suggested dosages vary with the source, under normal circumstances, an overdose of vitamin C cannot occur. This is confirmed by EFSA (2013) who state that no tolerable upper intake levels exist. Generally, recommended dosages are the following:
US Institute of Medicine (2000):