Le calcium du lait : fonctions, intérêts, besoins, biodisponibilité

Abstract

More than 99% of body calcium are localised in the skeleton and this mineral is mainly known as a dietary factor in the prevention of osteoporosis. Nevertheless, its role is not limited to the constitution of bone and the low extra-bone part is indispensable for innumerable vital functions. Moreover, numerous recent studies tend toward a consensus on its interest in the prevention of arterial hypertension, cardiovascular diseases, kidney oxalic lithiasis, colorectal cancer, overweight and obesity. It is then important to follow the recommended nutritional intakes (RNI) which vary, in France, according to age and physiological status, from 800 to 1,200 mg per day. Such intakes are justified by the net requirements for growth, pregnancy or lactation, but also by the obligatory endogenous losses in the faeces, urine and sweat. Moreover, the efficiency of calcium intestinal absorption is limited and seldom exceeds 40% of the ingested amount. The usual dietary habits do not permit to reach the RNI, particularly in adolescents and old people who do not eat enough dairy products. Dairy calcium represents indeed about two-thirds of the dietary intake. All the other usual foods are very poor in calcium, except some green vegetables, dry fruits and calcium-rich mineral waters. However, plant calcium is often linked to oxalic or phytic acid which inhibit its intestinal absorption. When absorption is normal, calcium balance is mainly determined by the level of obligatory urinary loss. Calciuria is increased by acidogenic components of the diet like sulphates. Milk protein does not significantly increase urinary calcium because other associated ions, namely phosphorus and potassium, compensate for the opposite effect of sulphates. Considering the chemical speciation of milk calcium and the potential favourable effect of organic components on its bioavailability, milk calcium is, quantitatively and qualitatively, the reference in dietary calcium.