Milk and blood serum Rubidium and Strontium concentrations in lactating donkeys

Donkey milk can be used as hypoallergenic food for infants suffering of cow milk protein allergy. Although a biological role has not been demonstrated, Rubidium (Rb) and Strontium (Sr) are potential essential elements in mammals nutrition. Aim of this study was to determine the concentrations of Rb and Sr in milk and serum of lactating donkeys. During the experimental period (3 month), individual milk and serum samples (n=112) were obtained from 16 lactating donkeys (averaging 205.4 kg body weight). Donkeys were fed meadow hay ad libitum, and 2.5 kg of mixed feed daily. Based on Rb and Sr concentrations in feeds, and assuming dry matter intake at 3.2% body weight, the estimated dietary intake of Rb and Sr were approximately 70 mg/d and 350 mg/d, respectively. Feeds, milk and serum samples were analyzed for Rb and Sr concentrations by inductively coupled plasma-Mass Spectrometry. Data were elaborated by analysis of variance for repeated measures. Average (±SD) Rb and Sr concentrations in donkey milk were respectively 339.0±81.8 μg/L and 880.3±269.3 μg/L. The concentrations of Rb and Sr in donkey milk were respectively 4.6 and 3.5 times higher than those in blood serum (Rb 74.1±15.2 μg/L; Sr 254.8±56.0 μg/L). Milk Rb and Sr concentrations were strongly correlated (P<0.001) with their serum counterpart (Rb r=0.85; Sr r=0.75). Rubidium was correlated (P<0.001) with Sr both in milk (r=0.66) and serum (r=0.50). The effect of stage of lactation was significant for Rb and Sr concentrations both in milk and serum, and lower concentrations were observed during the second part of the trial. Current results suggest that the mammary gland plays an active role in transferring Rb and Sr from blood to milk. Considering that Rb and Sr are in the same chemical group of Potassium and Calcium, respectively, it is possible that such elements share transport mechanisms.