in livestock (Mills, 1987). A dietary deficiency of a mineral is sooner or later reflected in subnormal concentrations of the mineral in certain of the animal’s tissues and fluids, and a dietary excess of a mineral is similarly reflected in above-normal concentrations. In addition, both deficiencies and toxicities are usually accompanied by significant tissue or fluid changes in the concentra- tions of particular enzymes, metabolites or organic compounds with which the mineral in question is functionally associated. Many of these changes can be detected before the onset of clinically obvious signs of deficiency or excess in the animal.
A general model of pathophysiological events during mineral depletion is presented in Fig. 3.1 and it provides a rational basis for the differential diagnosis of disorders due to mineral imbalances. The model divides events into four phases: firstly ‘depletion’, during which storage pools of the mineral are reduced; secondly, ‘deficiency’, during which transport pools of the mineral are reduced; thirdly, ‘dysfunction’, when mineral-dependent functions become rate-limiting to particular metabolic pathways; and fourthly, ‘disease’, during which clinical abnormalities become apparent to the naked eye. There may be variable degrees of overlap between phases; for example, deficiency Depletion, deficiency, dysfunction and disease
Depletion Deficiency Dysfunction Disease
Time
4 Clinical signs 3
Function 1
Storage 2
Transport
Marginal supply 100
0
Nutrient pool
Fig. 3.1. Schematic sequence of pathophysiological changes in livestock given an inadequate mineral supply: for some elements (e.g. Ca and P), storage and function are linked by bone strength; for elements with small or slowly mobilized stores (Zn and Se), curves 1 and 2 are superimposed or interposed; there is usually a zone of marginal supply (shaded area) where mineral-dependent functions begin to fail but the animal remains outwardly healthy (upper limit on the y axis represents maximum attainable pool size).
may commence before reserves are entirely exhausted (e.g. iron; Chesters and Arthur, 1988). The above terminology will be adhered to throughout this book, to avoid the confusion which has arisen in the past from the varied use of words such as ‘deficient’ to describe diets low in minerals, healthy animals of subnormal mineral status and clinically sick animals. A diet is only ‘min- eral-deficient’ if it is likely to induce subnormal mineral status and, if fed, does not necessarily result in clinical disease. The precise sequence of events during the development of clinical disease varies widely from mineral to min- eral and slightly for a given mineral, depending on the magnitude of the deficit between daily mineral requirement and the daily supply from endoge- nous (e.g. body stores) and dietary sources. For example, there are limited body stores of zinc and reductions in albumin-bound, transport zinc in plasma occur soon after exposure to a diet inadequate in zinc, whereas with phosphorus mobilization of skeletal reserves can preserve serum phosphorus concentrations during long periods of depletion. Although similar mobiliza- tion of calcium will often maintain plasma calcium concentrations long after exposure to inadequate calcium supplies commenced, sudden increases in milk yield and hence calcium demand with the onset of lactation can lead to hypocalcaemia and acute clinical disease (milk-fever) while plentiful reserves remain. The rate of transition from deficiency to dysfunction and disease is therefore variable and depends partly upon the demand being placed on the critical pathway. A low selenium status may be tolerable until changes in diet and increased freedom of movement (e.g. turning out to spring pasture) pre- sent a twofold oxidative stress (Chapter 12); a low iodine status may be toler- able until a cold stress induces a thermogenic response (Chapter 11);
simultaneous deficiencies of selenium and iodine increase the likelihood that one or both become dysfunctional.
The investigator of a suspected mineral-responsive disorder is faced with a number of biochemical options when considering how to proceed with a differential diagnosis. The information provided varies with the analyte chosen but is rarely a direct measure of dysfunction and less so of disease.
Historically, sample sites and substances have been selected for conve- nience rather than diagnostic insight. Thus, for the six elements chosen in Table 3.1, the popular biochemical criteria generally convey information about depletion and deficiency, stores and transport forms, rather than dys- function or disease. Low values are not therefore synonymous with loss of health. The relationship with dysfunction may be tightened by lowering the threshold of normality (e.g. for vitamin B12) to one at which dysfunction is more likely to occur. Arguments as to whether a marker for depletion (e.g.
liver copper) or transport (serum or ceruloplasmin copper) is the superior measure of ‘status’ are often rehearsed but are largely irrelevant in terms of diagnosis, i.e. differentiating the diseased and dysfunctional animal from the healthy one (see Chapter 11). It follows that most commonly used biochem- ical criteria for animals, like those for soil and feeds, require to be
What message can a particular analyte convey?
interpreted using a three-tier system (Table 3.2), with a marginal band to separate the deficient from the dysfunctional individual (Fig. 3.1).
Throughout this book, all biochemical criteria will be interpreted using a three-tier system, rather than a diagnostically unhelpful reference range.
This system is by no means the ultimate and can be improved upon by treating an indicator of dysfunction as a continuous variable and predicting the probability of a given response (e.g. minimum economic) occurring (see Chapter 19 and Clark et al., 1989).
The use of clinical biochemistry to diagnose or predict mineral-responsive disease is likely to remain an imprecise science. What is needed are detailed studies of the pathophysiological events associated with all economically important mineral imbalances of the type conducted by White (1996) on pooled data for zinc-deficient lambs (Fig. 3.2). The relationship between plasma or serum zinc and growth rate was fitted by a Mitscherlich equation and used to define a threshold value (6.7 µmol Zn l21 plasma), 95% of the critical value or asymptote below which growth impairment became likely.
This indicates the lower limit of the ‘marginal’ band. Once again, a familiar complication arises: the chosen value depends on the index of adequacy and, with optimal wool growth as the goal, the lower limit of marginality rises to 7.7 µmol l21 (White et al., 1994). Sadly, worldwide reductions in the funding of studies pertaining to the diagnosis of nutritional diseases of livestock in developed countries means that many uncertainties are unlikely to be resolved.
There are two approaches which potentially offer a way out of the morass and they both relate to the recognition of dysfunction. Biochemical markers, such as methylmalonic acid (MMA) and creatine kinase (CK) (Table 3.1), indicate when particular functions have become rate-limited, MMA coenzyme A (CoA) isomerase activity in cobalt or vitamin B12 deficiency (Chapter 10)
Table 3.2. Interpretation of biochemical and other indices of mineral deprivation (or excess) in livestock is enhanced by the use of a marginal band between values consistent with health and those consistent with ill health.
Responsiveness to
Label Phasea supplement (or antidote)
Normal Equilibrium, depletion (or accretion) Unlikely
Marginal Deficiency (or excess) Possible
Dysfunction
Abnormal Disorder Probable
aSee Fig. 3.1 for illustration of terminology.
Delineation of marginal values
Criteria of dysfunction
and muscle membrane permeability in selenium deficiency (Chapter 15).
However, a rise in plasma MMA may precede the important clinical sign of cobalt deficiency, loss of appetite (see Chapter 10). By studying relationships between markers of depletion or deficiency and dysfunction, the interpreta- tion of each one can be clarified (Mills, 1987). It is becoming apparent that animals have internal biochemical systems for recognizing low mineral status and correcting it; mRNAs for serum ferritin – the storage protein – are switched off in iron deficiency and those for glutathione peroxidase (GPX) switched off in selenium deficiency, while those for cholecystokinin are induced in zinc deficiency (see appropriate chapters). The use of mRNAs for routine diagnosis is likely to remain prohibitively expensive, but their experi- mental use could resolve some long-standing arguments by letting the animal decide how mineral-deficient it is.
When using the mineral composition of animal tissues and fluids to indicate the quality of mineral nutrition, it is important to avoid the confounding effects of physiological and developmental changes. The normal newborn animal often has ‘abnormal’ tissue or blood mineral composition when judged inappropriately by adult standards (see Chapter 11 for liver and blood copper). Most blood mineral concentrations change abruptly and briefly around parturition, and sampling at this time is only appropriate for acute disorders of calcium and magnesium metabolism (see Chapters 4 and 6).
The choice of tissue or fluid for analysis varies with the mineral under investigation. Blood, urine, saliva and hair have obvious advantages because of their accessibility without harm to the animal.
Fig. 3.2. Relationship between growth rate and plasma or serum zinc in experimentally deprived lambs (White, 1996).
Physiological and developmental changes