35 Clinica Chimica Acta, 87 (1978) 35-41 0 Elsevier/North-Holland Biomedical Press CGA 9378 SERUM MAGNESIUM CIRCADIAN RHYTHM IN HUMAN ADULTS WITH RESPECT TO AGE, SEX AND MENTAL STATUS YVAN TOUITOU *, CATHERINE TOUITOU, ANDRk BOGDAN, HERVI? BECK and ALAIN REINBERG Laboratoire de Biochimie, Facultd de Me’decine Pit%-Salpktrie‘re, 91, Boulevard de I’HGpital, 75634 Paris Ckdex 13 and E.R. de Chronobiologie humaine (CNRS 105), 75019 Paris (France) (Received December 13th, 1977) Summary 25 subjects volunteered to document circadian changes in serum magnesium. 4 groups were formed: 7 healthy young males (24.0 years ? 3.9), 6 elderly males (82.5 years 2 7.5), 6 elderly females 81.2 years + 10.7) and 6 elderly insane subjects of both sexes (SO.5 years ? 8.6). They were socially syn- chronized with a diurnal activity (07.00 to 21.00 for the old subjects; 07.00 to 23.00 for the young ones) and nocturnal rest. The subjects followed a spon- taneous diet. Venous blood was sampled at 4-h intervals and fixed clock hours (07.45, 11.45, 15.45, 19.45, 23.45, 03.45) during 24 h. The single cosinor method was used for the statistical analysis of the time series. A statistically significant circadian rhythm is detected in three of the groups: young males, elderly males and elderly females (no rhythm detection in elderly insane subjects). The 24-h mean is higher in elderly subjects than in the young ones. The rhythm amplitude is larger in elderly males than in young ones. The acrophase (peak time) location in the 24-h scale is 10.12 h for elderly females, 11.35 h for elderly males and 16.36 h for young males. Introduction Magnesium is the fourth most abundant cation in the human body as a whole, after calcium, potassium and sodium. The magnesium total body con- tent is 1000 mmol (2000 mequivalents); more than half of this amount is located in bones and the remainder is present, mainly intracellularly, in muscle and the non-muscular soft tissues [l-4]. * To whom correspondence should be addressed.