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Influence of Diet

Urinary excretion of both Ca and Mg normally is primarily dependant on the nature of the diet and there is wide variation. Under usual dietary conditions, in normal healthy subjects, urinary excretion accounts for about 15-30% of the total Ca and about 20-50% of the total Mg elimination, the remainder being excreted in faeces. At low or moderate levels of Ca intake (0.1-0.5gm daily) about 30-50% is eliminated in the urine, whereas at high levels of intake (1.0gm), about 10-25% is eliminated. The renal threshold for excretion of Ca lies between 6.5-8.0mg/100ml of plasma, little being eliminated at lower plasma Ca concentration.

Clinical Significance of Urinary Calcium

There is an increased urinary Ca if the plasma ionised Ca is high.

Increase: Increased urinary excretion seen:

In hyperparathyroidism – 300-700mg per day may be lost. However, normal values are sometimes found, especially so if some impairment of renal function develops.

2. In hyperthyroidism

3. In hypervitaminosis D

4. In multiple myeloma, may be 300-750mg per day

5. Renak stones – an increase in urinary Ca occurs fairly frequently in patients with renal stones who do not hhave hyperparathyroidism

6. In renal tubular acidosis – in which neutral or alkaline urine low in NH3 but high in Na, K and Ca is passed

7. Hyperchloraemic acidosis – enhances loss of Ca from bone, can be seen in Fanconi’s syndrome, galactosaemia, alkaptonuria, Wilson’s desease, nephrogenic diabetes insipidus

8. Drugs – certain drugs e.g. steroids, diuretics, cholestyramine can increase excretion of urinary Ca.

ORGANIC CONSTITUENTS

I. Urea

In humans, it represents about 80-90% of total urinary nitrogen. Approximately 25-30gms of urea are excreted per 24 hours.

Clinical significance

Increase: urinary excretion is increased whenever Protein catabolism is increased e.g., in fever, Diabetes mellitus, and excess of adreno-cortical activity.

Decrease:

1. In certain Liver diseases such as cirrhosis Liver and acute yellow atrophy, in which capacity to form urea is decreased.

2. In cases of severe acidosis – the amount of urea excreted may be markedly reduced because of diversion of aminonitrogen to NH3 formation.

3. Nephritis – excretion of urea may also be decreased in nephritis when the ability of the kidneys to excrete it is severely impaired. This may cause increased concentration of urea in blood and other fluids (uraemia).

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