Creatine synthesis: the origin of creatine in rat milk
It is concluded that there is only a modest increase in the need to provide creatine during lactation and that this may be largely provided by hyperphagia when creatine is present in the food.
Abstract
Creatine serves as an important energy buffer and shuttle system in cells. Approximately 1.7%/day of the body's creatine pool is spontaneously converted to creatinine and excreted in the urine. This must be replaced by a combination of diet and synthesis. During lactation there may be an additional requirement due to the provision of creatine to the milk. Our objectives were (1) to quantify milk creatine in lactating rats, (2) to determine the origin of milk creatine, and (3) to determine the activities of the enzymes of creatine synthesis in lactating rats. We determined the origin of milk creatine by administering 14C‐creatine to lactating rats, followed by determination of its isotopic enrichment in milk and plasma. Since these isotopic enrichments were identical, we conclude that milk creatine is extracted from the blood. Supporting this is our failure to find significant activities of the enzymes of creatine synthesis in mammary glands. Milk creatine amounted to about 13 μmol/day, which is equivalent to 25% of the creatine lost by conversion to creatinine. The first and limiting enzyme in creatine synthesis was not increased in activity during lactation; neither was the renal output of guanidinoacetate. We conclude that there is only a modest increase in the need to provide creatine during lactation and that this may be largely provided by hyperphagia when creatine is present in the food. Supported by CIHR grant 97851.