Volume 91, Issue 1, January (2004), pp. 113-120 © The Author 2004
doi:10.1079/BJN20031023

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Comparison of urinary monitoring, faecal monitoring and erythrocyte analysis of stable isotope labels to determine magnesium absorption in human subjects

Torsten Bohn¹, Thomas Walczyk¹, Lena Davidsson¹, Wolfgang Pritzkow², Patrick Klingbeil², Jochen Vogl², Richard F. Hurrell^1
1Swiss Federal Institute of Technology (ETH), Institute of Food Science and Nutrition, Laboratory for Human Nutrition, 8803 , Ru¨schlikon, Switzerland
²German Federal Institute for Materials Research and Testing (BAM), 12205 , Berlin, Germany

 (Received 5 May 2003–Revised 2 September 2003–Accepted 12 September 2003)

We have evaluated urinary monitoring and erythrocyte analysis to determine Mg absorption in human subjects as alternatives to the conventional technique of faecal monitoring by stable-isotope techniques. Ten healthy adults received 2·2 mmol ²⁵Mg in water, together with wheat bread, followed 15 min later by intravenous injection of 0·6 mmol ²⁶Mg (day 1). Brilliant blue and Yb (given on day 0 and day 1 respectively) served as qualitative and quantitative faecal markers. Urine was collected for 6 d after test meal intake. Complete collections of faeces were made until excretion of the second brilliant blue marker (given on day 7). Mg isotope ratios were determined by thermal ionisation-MS in urine and faeces and by inductively coupled plasma-MS in erythrocytes. Absorption was determined based on: (1) 6 d urine pools; (2) 24 h urine pools (collected 22–46 h after test meal intake); (3) erythrocytes from a blood sample drawn on day 14; (4) complete 6 d faecal pools; (5) faecal pools based on the first three consecutive stools after excretion of the first brilliant blue marker. Differences in mean Mg absorption (42 44 %) were statistically insignificant between techniques, except when based on 6 d urine pools for which the value was significantly lower (33 (sd 7) %, P=0·0003, ANOVA). The results indicate that Mg absorption can be determined from 24 h urine pools or erythrocytes obtained 14 d after test meal intake, an alternative method to the more time-consuming and labour-intense faecal monitoring. The choice of technique depends on practical and financial considerations.

Keywords:
Magnesium absorption, Stable isotopes, Faecal monitoring, Urinary monitoring, Erythrocytes

Abbreviations:
AA, apparent absorption, ICP, inductively coupled plasma, TA, true absorption