Title: Zinc Status Determination
Key words: Spectrophotometry, inductively coupled plasma, serum and plasma levels, enzymes, urine concentration
Date: Oct 1998
Category: 14. Measurement
Author: Dr Van Rhijn
Zinc Status Determination
Advantages and Disadvantages of Different Methods
In practice, assessment of zinc status has proved to be difficult. Various measurement methods have been suggested, but most fail to reflect the true status of total body zinc.
From all the various analytical instrumentation developed to measure trace elements, Atomic Absorption Spectrophotometry (AAS) and Inductively Coupled Plasma emission (ICP) techniques are the most widely used to measure zinc in biological samples.
Methods of zinc measurements
This has become standard and routine in practice, but it is unreliable and a poor index of zinc status. As plasma zinc is maintained at 12-18 m mol/L, numerous physiological changes, such as fasting and infection can quickly influence this concentration. Plasma zinc compromises only 0.1% of total body zinc, and albumin appears to be the main carrier protein for zinc.
All have been used as assessment measures, but proven to be poor indices of zinc status. They are relatively easy to do in practice, but these local measurements do not reflect true physiological zinc requirements. They are insensitive and poor markers.
White blood cell measurement may be a better parameter, especially if measured in conjunction with C-reactive protein (CRP), which is an acute phase protein, and a marker for infection. However, this has proved difficult to interpret in practice.
This is very easy and cost effective to execute in practice, and is based on the premise that individuals deficient in zinc are unable to taste a zinc solution. Experience, however, has shown it to be insensitive and non-specific.
These parameters have mainly been used to measure the dynamic activity of zinc either reflecting absorption (cysteine-rich intestinal protein - CRIP) or transcellular movement (metallothionein - MT). More than 80% of blood zinc is found mainly in erythrocytes, and reticulocytes MT in particular, may be a useful index for zinc intake. Some of these tests are radioimmunoassay based and very expensive. Plasma alkaline phosphatase and angiotensin-converting enzyme are zinc dependent and have been of some value when used in studies.
Plasma zinc concentrations increase within hours after consumption of 60 mg of oral zinc. This is a widely used test, but not recognised as a reliable method for status assessment. Urine excretion is more variable, and is the highest after 24 hours.
Differentiating between the exchangeable zinc pool size and tissue zinc distribution may be the best way to reflect true zinc status. Therefore, assessing plasma zinc together with plasma MT may prove to be the best parameter at present. Also, it may prove more accurate to measure several parameters together (for example: serum zinc together with CRP and Albumin) in order to gain a perspective and to account for some of the dynamic physiological factors.
Accurate measurements of zinc requires a proper understanding of the relevant physiology and metabolism before deciding which test to carry out. Physiological influences, diurnal variation and contamination affect measurements and no single test has proved satisfactory as yet. Simultaneous measurements of some parameters may aid interpretation and understanding, as well as providing a better reflection of total body zinc status.