Vitamins in human, cow, and donkey milk: a comparison of nutritional properties.

Donkey milk (DM) has been used since ancient times in the feeding of infants who, for various reasons, could not be breastfed. Furthermore, the health benefits of DM have been tested in several clinical trials for the treatment of debilitated and convalescent subjects [1]. Today we know that DM is a valuable source of nutraceutical substances beneficial to human health, including bioactive peptides, essential fatty acids, and vitamins [2]. The present work focuses precisely on this last aspect, the identification and determination of the water- and fat-soluble vitamins in DM and their comparison with the vitamins content in human milk and cow's milk. Donkey and cow’s milk water-soluble vitamins were determined by a chromatographic method [3], using an HPLC Agilent 1260 Infinity LC System (Agilent Technologies), connected to a C18 HiQSil HS column (5 m, 4.6 mm i.d, 250 mm, Kya Tech Corporation, Tokyo, Japan). The obtained results in cows’ and donkey milk were compared to the vitamins content of human milk taken from the published literature. DM is a good source of nicotinic acid (18.75 ± 1.92 M), this value is higher compared to human milk (4.64 M, [4]) but lower than the one found in cow’s milk in the present work (27.52±4.91). Nicotinic acid (niacin) is a member of the B3 vitamin group that reduces the release of fatty acid from the adipocytes, thus showing a lipid-lowering effect, this molecule is used to treat the progression of atherosclerotic cardiovascular disease. Folic acid (vitamin B9) is present in DM with a concentration (0.83 ± 0.08 M) higher than human milk (0.365 M) and about half of that found in cow's milk in our determinations (1.93 ±0.53 M). Milk represents an excellent source of folic acid that is involved in nucleic acid synthesis, accelerates cell division, and for this reason, very important for children’s growth. Pyridoxine (vitamin B6) content is 5.38 ±3.35 M, this value is very similar to that found in bovine milk both from our determinations (2.5 ±0.61M) and in the literature (1.0-11.23 M; [5]). Vitamin B6 is involved in several metabolic processes such as lipid and amino acid metabolism, gluconeogenesis, the regulation of the immune system, and the development of the nervous system. Donkey milk also contains thiamine (0.66 ±0.26 M), riboflavin (0.168 ± 0.04 M), with levels very close to those found in the literature for bovine milk [5] but higher compared to human milk [4]. Very interesting is the content of vitamin C, a vitamin with well-known antioxidant properties which in donkey milk has a value of 0.32 mM, similar to the value found in human milk (0.28 mM) but much higher than the quantity found in cow's milk (28.4 -170.3 M). Among fat-soluble vitamins, it is known by the literature that the vitamin D2 and D3 content in donkey milk (0.042 M and 0.016 M, respectively) is higher than the values found for this vitamin in human and bovine milk (0.008 M) [6]. Furthermore, it has been shown that vitamin D levels in donkey milk are related to seasonality since they show the highest value in summer due to the greater sun exposure of the animals [6]. Donkey milk vitamin E amount is about 3.4 M [6], higher than the value found in the literature [5] in bovine milk (1.4 M, respectively), but lower concerning the amount found in the human milk (8.0±3.8 µM). Vitamin E is a potent antioxidant that neutralizes free radicals, protecting tissues and organs from oxidative damage. These results show that DM, besides the presence of the well-known nutraceutical compounds, is also a valuable source of vitamins, indicating that this milk can represent a functional food for infants but also for elderly consumers.