Clin Chem. 1989 Aug;35(8):1802-3.
Normal reference values for creatine, creatinine, and carnitine are lower in vegetarians.
Delanghe J, De Slypere JP, De Buyzere M, Robbrecht J, Wieme R, Vermeulen A.
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Because creatinine forms in vivo, by hydrolysis of creatine and phosphocreatine, at a constant rate, creatinine determinations in serum and urine are useful for evaluating kidney function. As an indicator of kidney function, creatinine clearance is often used as a factor determining the administered dose of many drugs. Moreover, elimination rates of many endogenous compounds (hormones, toxic metabolites) are compared with those of creatinine (1). In humans, creatine is in part synthesized endogenously, but it is also present in the diet, mostly originating from meat (2). Thus, dietary intake of creatine is extremely limited in vegetarians.

To test the influence of dietary creatine intake on serum and urine creatinine values and creatinine clearance, we studied creatine metabolism in a group of healthy vegetarians. We also studied the effect of vegetarian diet on the concentrations in serum of free carnitine, another low-molecular mass compound, which stimulates beta-oxidation of long-chain fatty acids in muscle. Although carnitine is present in normal human diet (mainly in red meat), carnitine can also be synthesized endogenously (3).

Creatine was assayed according to Delanghe et al. (4) and carnitine according to Rodriguez-Segade et al. (5). Creatinine was determined by a Jaffe-type method (6). To estimate intracellular creatine concentration, we assayed creatine in erythrocytes by an enzymatic procedure (7).

Table 1 compares results for creatine, creatinine, and carnitine in vegetarians [n = 99: 55 males, age 36.3 (SD 9.1) y; 44 females, age 35.3 (SD 9.8) y], and in a reference population En= 60: 25 males, age 37.4 (SD 10.5) y; 35 females, age 34.5 (SD 11.2) y]. In the reference population on a standard European diet, mean daily meat consumption was 146 g for males and 107 g for females. In vegetarians, creatine values in serum were very low for both sexes ( P <0.01). In erythrocytes, creatine content was shown to be significantly (P <0.01) less in vegetarians than in the reference group. In male vegetarians, serum creatine concentrations were significantly (P <0.05) lower than those in female vegetarians. Also, values were lower for serum creatinine and creatinine clearance. Daily creatinine losses were significantly lower than those obtained for the age-matched reference population (P <0.05). Because body creatine content is directly proportional to daily creatinine loss, we could estimate mean total creatine (creatine + phosphocreatine) content in the vegetarians as 80 g per 1.73 m2 of body surface (vs 120 g per 1.73 m2 for the reference population) (2). Similarly, free carnitine concentrations in serum of vegetarians are low.

As shown, measured serum and erythrocyte creatine content, and estimated muscle creatine content, are lower in vegetarians than in the reference population. As can be calculated from the data on creatine, the vegetarians have a considerable decrease in creatinine production rate. Because the turnover rate of creatine is rather slow (daily loss, about 2% of body reserves), sustained creatine-poor diets result in a lower creatinine production rate, an effect that will continue for several weeks even after a normal diet is resumed. Earlier, Addis et al. (8) found creatinine excretion to be independent of protein consumption. However, more-recent studies have revealed short-time effects of meat intake on creatinine excretion (9, 10). Our study confirms that prolonged depletion of dietary creatine results in decreased creatinine production, owing to insufficient endogenous compensatory creatine synthesis. Similarly, decreased dietary intake of carnitine results in lower reference values for serum. Therefore, for vegetarians, we recommend that urinary analyte quantities expressed per mass unit of excreted creatinine preferably be replaced by the analyte quantity expressed per time unit.

Categories: 1989, Creatine, Carnitine, Creatinine, Nutrition and diet, Vegetarian