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Journal of the American College of Nutrition, Vol. 17, No. 1, 71-74 (1998)
Published by the American College of Nutrition

Original Paper

Age- and Sex-Related Differences of Serum Carnitine in a Japanese Population

Nobuaki Takiyama, MD and Kojiro Matsumoto, PhD

Department of Pediatrics (N.T.), Keio University School of Medicine, Tokyo
Department of Clinical Chemistry (K.M.), Toho University, Chiba, JAPAN

Address reprint requests to: Nobuaki Takiyama, MD, Department of Human Genetics, University of Pittsburgh, E1650 Biomedical Science Tower, Pittsburgh, PA 15261.


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 REFERENCES
 
Objective: The objective of this study was to assess age- and sex-related differences of serum carnitine in a Japanese population.

Methods: Fasting blood samples were obtained from 296 Japanese males and 258 females 0 to 65 years of age. Serum free carnitine and acylcarnitine levels were determined by a fluorometric method using carnitine dehydrogenase. Of these, serum samples from 20 males 10 to 20 years of age and 23 females 40 to 60 years of age were assayed for testosterone and estradiol, respectively. Fasting blood samples and 24-hour urine samples were also obtained from 20 males and 23 females 0 to 50 years of age for the assessment of renal carnitine reabsorption.

Results: Serum free carnitine increased with age in children of both sexes, reaching adult value between 15 to 20 years of age in males and between 2 to 10 years of age in females. The mean free carnitine in males (50.3±7.5 µmol/L) was significantly higher than that in female (41.2±7.5 µmol/L) between 15–50 years of age (p<0.05). Serum acylcarnitine remained constant (14.7±3.3 µmol/L) in each subject of various ages in both sexes. A significant negative correlation was observed between serum free carnitine and estradiol in females (r=-0.55, p<0.01), but was not observed between serum free carnitine and testosterone in males (r=0.015). The percent renal reabsorption of free carnitine showed no age-related and sex-related differences.

Conclusions: Serum free carnitine level is related to age and sex, while serum acylcarnitine level remains constant. Our findings suggest that estrogen decreases serum free carnitine and causes sex-related differences.

Key words: aging, free carnitine, acylcarnitine, estrogen


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 REFERENCES
 
L-Carnitine ({gamma}-trimethylamino-ß-hydroxybutyric acid) is essential for the transfer of free fatty acids across the inner mitochondrial membrane before ß-oxidation. In addition, carnitine also clears excess acyl-CoA compounds, which are produced in some organic acidemias.

Carnitine in biological fluids is currently determined by radioisotopic or spectrophotometric methods using the enzyme carnitine acetyltransferase [1–3]. Several studies regarding reference values for serum carnitine reported an age-related increase [4–6] and sex-related differences [2,7,8]. However, the data available are based on small sample sizes.

Recently, we have developed an accurate and reliable fluorometric method, using the enzyme carnitine dehydrogenase (EC 1.1.1.108) for determination of carnitine [9]. In this study, we determined serum free carnitine and acylcarnitine in males and females in various age groups using this method, and clarified the age-related variation and the sex-related differences in detail. We also compared the percent renal reabsorption of carnitine in children and adults, and examined the relationship between serum free carnitine and sex steroids.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 REFERENCES
 
Subjects
Adult subjects were recruited from the general population, and children were either inpatients or outpatients who were recovering from mild diseases. All were Japanese and were apparently free from metabolic, neuromuscular, cardiac, hepatic and renal diseases. The subjects consumed ordinary diets and vegetarians were excluded. The infants were fed with formula containing 10 to 15 µmol of carnitine per 100 mL. The subjects were not taking any medication known to affect serum carnitine level. Blood samples were obtained for carnitine assay from 296 males and 258 females aged 0 day to 65 years of age and classified into 11 age groups (Table 1). Blood was drawn in the morning after an overnight fast in the older age group and before the morning feeding in neonates. Each sample was collected into a plain tube and centrifuged. Serum samples were stored at -20°C until analysis. Of these, samples from 20 males 10 to 20 years of age and samples from 23 females 40 to 60 years of age were also assayed for testosterone and estradiol, respectively. In addition, fasting blood samples and 24-hour urine samples were obtained for carnitine and creatinine assays from 20 males and 23 females 0 to 50 years of age, and stored at -20°C until analysis. The study was approved by the medical ethics committee of the hospital.


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  		Table 1. Numbers of Subjects in 11 Age Groups

 
Biochemical Assays
Serum and urine samples were assayed for free carnitine and acylcarnitine by a fluorometric method using the enzyme carnitine dehydrogenase as previously reported [9]. Serum testosterone and estradiol were determined by RIA using Total Testosterone Kit and COAT-A-Count E2 (Nihon DPC, Chiba, Japan), respectively. Creatinine was determined spectrophotometrically, as described previously [10]. The percent renal reabsorption was calculated by the following formula [11]:

x100.

Statistics
Differences between mean values were analyzed using the Student’s test and correlations between two variables were analyzed using linear regression analysis. P<0.05 was considered significant.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
REFERENCES
 
Serum free carnitine levels related with age in males and females are shown in Fig. 1. Free carnitine was low in neonates (about 50 to 57% of adult value) and then increased with age. In males, free carnitine reached adult value between 15 to 20 years of age and remained constant after 20 years of age. On the other hand, in females, free carnitine reached adult value between 2 to 10 years of age, remained constant between 10 to 50 years of age and then further increased again after 50 years of age. Free carnitine levels in males were significantly higher than those in females between 15 to 50 years of age. The mean values±SD for serum free carnitine in adult males (15 to 65 years of age) and in adult females (15 to 50 years of age) were 50.3±7.5 µmol/L and 41.2±7.5 µmol/L, respectively.



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  		Fig. 1. Age-related variations in serum free carnitine. * Significant difference (p<0.05) between males and females.

 
Serum acylcarnitine levels related with age in males and females are shown in Fig. 2. Acylcarnitine levels remained relatively constant in the different age groups. No significant sex-related differences were observed. The mean values±SD for serum acylcarnitine in males and females were 14.7±3.2 µmol/L and 14.8±3.3 µmol/L, respectively.



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  		Fig. 2. Age-related variations in serum acylcarnitine. Acylcarnitine were relatively constant in the different age groups.

 
The mean value±SD for percent renal reabsorption of carnitine in the group between 0 to 15 years of age and in the group between 15 to 50 years of age are shown in Table 2. The percent renal reabsorption of free carnitine was higher than that of acylcarnitine. The values for percent renal reabsorption of free and acylcarnitine in the two groups were not significantly different in both sexes. No significant sex-related differences were observed.


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  		Table 2. Percent Renal Reabsorption of Free Carnitine and Acylcarnitine

 
Relationships of serum free carnitine to serum testosterone in males and to serum estradiol in females are shown in Fig. 3 and 4. No significant correlation was observed between free carnitine and testosterone in males aged 10 to 20 years (r=0.015). In contrast, a significant negative correlation was observed between free carnitine and estradiol in females aged 40 to 60 years (r=-0.55, p<0.01).



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  		Fig. 3. Relationship between serum free carnitine and testosterone. No significant correlation was observed.

 


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  		Fig. 4. Relationship between serum free carnitine and estradiol. A significant negative correlation was observed. The regression line is y=-0.079x+51.6.

 

   DISCUSSION
 
In this study, we clarified the following: a) serum free carnitine reached adult value between 15 to 20 years of age in males and between 2 to 10 years of age in females; b) serum free carnitine further increased after 50 years of age in females; c) significant sex difference in serum free carnitine was observed between 15 to 50 years of age; and d) serum acylcarnitine remained constant.

Battistella et al reported that adult value of serum carnitine was attained by the end of the first 6 months [4], and Deufel reported that adult value was attained during the first month of life [6]. However, our data revealed that adult value was reached between 15 to 20 years of age in males and between 2 to 10 years of age in females. The previous studies reported the time when adult value was reached irrespective of sex, which may cause the disagreement with our data.

It is known that renal handling of carnitine influences serum carnitine level [12,13]. More than 90% of filtrated carnitine is reabsorbed by the kidney under physiological conditions [14]. Our data showed that the percent renal reabsorption of free carnitine was higher than that of acylcarnitine as reported previously. Although the percent renal reabsorption of free carnitine in children was lower than that of adults and the percent renal reabsorption of free carnitine in females was lower than that of males, these differences were not significant. We could not demonstrate how renal handling of carnitine has an effect on age-dependent and sex-related differences in serum carnitine.

We clarified that sex difference was significant between 15 years and 50 years of age, which approximately correspond to puberty and menopause, respectively. Considering variations of sexual hormones with age and sex [15], these facts suggest that serum carnitine is regulated by sexual hormones. Borum showed that serum carnitine levels in rats were influenced by androgens and estrogens [16]. Rats injected with androgens developed higher serum carnitine concentrations, while rats injected with estrogens developed lower concentrations. Cederblad reported that pregnant women showed low plasma carnitine concentrations [13] and Buchta reported a negative correlation of plasma free carnitine concentrations with pubertal stage in females [17], suggesting that estrogen has an effect on serum carnitine concentration.

Actually, we showed a significant negative correlation existed between serum free carnitine and serum estradiol in females. In contrast, there were no correlations between serum free carnitine and serum testosterone in males. It has been suggested that estrogen decreases serum carnitine concentration, though the mechanism is not known well. Our results support this concept.

Received February 1, 1997. Accepted July 1, 1997.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 REFERENCES
 

  1. McGarry JD, Foster DW: An improved and simplified radioisotopic assay for the determination of free and esterified carnitine. J Lipid Res 17: 277–281, 1976.[Abstract]
  2. Roessle C, Kohse KP, Franz H-E, Fruest P: An improved method for the determination of free and esterified carnitine. Clin Chim Acta 149: 263–268, 1985.[Medline]
  3. Duefel T, Wieland OH: Sensitive assay of carnitine palmitoyl transferase activity in tissue homogenates with a modified spectrophotometric method for enzymatic carnitine determination. Clin Chim Acta 135: 247–251, 1983.[Medline]
  4. Battistella P, Vergani L, Donzelli F, Rubaltelli FF, Angelini C: Plasma and urine carnitine levels during development. Pediatr Res 14: 1379–1381, 1980.[Medline]
  5. Warshaw JB, Curry E: Comparison of serum carnitine and ketone body concentrations in breast- and in formula-fed newborn infant. J Pediatr 97: 122–125, 1980.[Medline]
  6. Deufel T: Determination of L-carnitine in biological fluids and tissues. J Clin Chem Clin Biochem 28: 307–311, 1990.[Medline]
  7. Cederblad G: Plasma carnitine and body composition. Clin Chim Acta 67: 207–212, 1976.[Medline]
  8. Rebouche CJ, Engel AG: Carnitine metabolism and deficiency syndromes. Mayo Clin Proc 58: 533–540, 1983.[Medline]
  9. Matsumoto K, Yamada Y, Takahashi M, Todoroki T, Mizoguchi K, Misaki H, Yuki H: Fluorometric determination of carnitine in serum with immobilized carnitine dehydrogenase and diaphorase. Clin Chem 36: 2072–2076, 1990.[Abstract/Free Full Text]
  10. Haeckel R: Assay of creatinine in serum, with use of Fuller’s earth to remove interferents. Clin Chem 27: 179–183, 1981.[Free Full Text]
  11. Li BUK, Lloyd ML, Gudjonsson H, Shug AL, Olson WA: The effect of enteral carnitine administration in humans. Am J Clin Nutr 55: 838–845, 1992.[Abstract/Free Full Text]
  12. Frohlich J, Seccombe DW, Hahn P, Dodek P, Hynie I: Effect of fasting on free and esterified carnitine levels in human serum and urine: correlation with serum levels of free fatty acids and ß-hydroxybutylate. Metabolism 27: 555–561, 1978.[Medline]
  13. Cederblad G, Fahraeus L, Lindgren K: Plasma carnitine and renal-carnitine clearance during pregnancy. Am J Clin Nutr 44: 379–383, 1986.[Abstract/Free Full Text]
  14. Rebouche CJ, Mack DL: Sodium gradient-stimulated transport of L-carnitine into renal brush border membrane vesicles: Kinetics, specificity and regulation by dietary carnitine. Arch Biochem Biophys 235: 393–402, 1984.[Medline]
  15. Richard SD: Physiology of sex and tests of function. In Richard SD (ed): "Handbook of Endocrinology." Philadelphia: Lea & Febiger, pp 620–623, 1980.
  16. Borum PR: Regulation of the carnitine concentration in plasma. In Frenkel RA, McGarry JD (eds): "Carnitine Biosynthesis, Metabolism, and Function." New York: Academic Press, pp 115–126, 1980.
  17. Buchta R, Nyhan WL, Broock R, Schragg P: Carnitine in adolescents. J Adolesc Health 14: 440–441, 1993.[Medline]




This Article
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Right arrow Articles by Takiyama, N.
Right arrow Articles by Matsumoto, K.

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