Artificially fluoridated drinking water is supplied throughout the world as a mass-prophylactic against dental caries. No data are available, however, on the transfer of fluoride from maternal plasma to breast milk. We have therefore compared the fluoride concentration in the plasma and breast milk of nursing mothers after an oral dose of fluoride.

Subjects and Methods

Five mothers aged 27-36 years were given fluoride by mouth on the third postpartum day. A dose of 1.5 mg was given as an aqueous solution of sodium fluoride in the morning after fasting for 10 hours. Blood and breast milk were sampled simultaneously in a non-glass system before and 30, 60, 90, and 120 minutes after the dose. Fluoride concentrations in plasma was measured with a fluoride-sensitive electrode (1), and in milk was determined using a modified microdiffusion technique. (2) Recovery experiments showed no evidence that fluoride binds to the constituents of breast milk. Hence the figures given for breast milk represent exclusively free fluoride.

The plasma fluoride concentration in all subjects rapidly increased after fluoride intake (figure). The highest plasma concentrations appeared 30 minutes after dosing, when they varied between 3.6 and 4.5 umol/L (70 and 86 ng/ml). This rapid absorption was in line with our results from oral single-dose studies in man. (3) There was no corresponding increase in the fluoride concentration in the breast milk, the values varying between 0.1 and 0.4 umol/L (2 and 8 ng/ml).

Comment

These findings show that plasma fluoride is poorly transferred to breast milk and infants thus receive almost no fluoride during breast feeding. This is in contrast to another halogen, bromide, which reportedly accumulates in breast milk. (4)

In this study plasma and milk were sampled for a period of two hours after a single dose had been taken by the mother. The possibility of delayed transfer, not detected during the period of observation, was ruled out by other results from our laboratory, which showed that fluoride concentrations in breast milk remain steady, even when fluoride is given repeatedly and despite considerable variations in the maternal plasma fluoride concentration.

The reason for the poor transfer of fluoride into the cells of the breast is obscure. Such a phenomenon has not been observed in other mammalian gland — for example, the salivary glands.  The concentration in saliva follows that in plasma, and the ratio between salivary and plasma fluoride is around 0.65. (3) Fluoride is excluded from certain cultured cells, (5) but whether the same mechanisms are concerned in the mammary gland remains to be clarified.

Infants fed on milk substances receive higher fluoride doses, especially in areas where the drinking water is rich in fluoride. Thus in an area where the fluoride concentration is one part per million the daily fluoride dose in the newborn infant will be about 800-1000 ug when a milk substitute is used, whereas the fluoride dose for breast-fed children in the same area will not exceed 10 ug/day. The existence of a physiological plasma-milk barrier against fluoride suggests that the newborn is actively protected from this halogen. Hence the recommendation made in several countries to give breast-fed infants fluoride supplementation should be reconsidered.

1.  Ekstrand J. A micromethod for determination of fluoride in blood plasma and saliva. Californian Tissue Research 1977; 23:225-8.

2.  Taves DR. Determination of submicromolar concentrations of fluoride in biological samples. Talanta 1968;15:1015-23.

3.  Ekstrand J, Alvan G, Boreus LO, Norlin A. Pharmacokinetics of fluoride in man after single and multiple oral doses. Eur J Clin Pharmacol 1977;12:311-7.

4.  Kwit NT, Hatcher RA. Excretion of drugs in milk. Am J Dis Child 1935;49:900-40.

5.  Holland RI, Hongslo JK. Fluoride, fluoride resistance and glycolysis in cultured cells. Acta Pharmacol Toxicol 1978;43:240-5.