This page is written for the doctors who check this leaflet

A 24-hour urine collection gives valuable information without which you and your patient are working in the dark. When patients get their own baseline and follow-up urine results, they can see from the figure below how they compare with 194 people who were recruited from the Hobart electoral roll [1]. Except for one woman on the Pritikin Diet, they all said their diet was 'normal', yet 24-hour urine results varied between 26 and 337 mmol/day (from 598 to 7751 mg). Medical reports use international (SI) units while food labels still use milligrams (mg) for sodium.

Reference Range: Lab reference ranges are based on the middle 95% of the distribution. For urinary sodium excretion - as for serum cholesterol - the usual distribution in an industrial society has no clinical relevance. Full compliance with the salt guideline (described on page 1) may result in sodium excretion as low as 10 to 35 mmol and patients who exceed 50mmol need individual dietary counselling. The appearance of good control (under 50mmol) for six people in the above picture was accidental. Sodium excretion varies daily over a wide range in people who ignore food labels, and low results are ephemeral. Permanent control requires the daily and exclusive choice of low salt foods (sodium <120mg/100g).

The Upper Intake Limit (UL) for sodium intake in Australia - 100mmol/day - is based on presumed feasibility. It should help to prevent hypertension but is far from ideal because fluid retention and some other salt-related health problems begin at about 50mmol/day [3,4].

Diuretics. In life-threatening conditions, such as congestive cardiac failure, the justification for prescribing diuretics at very low salt intakes is that the close supervision needed by these patients permits constant checking of serum electrolytes at the same time. For out-patients with other salt-related health problems the risk of diuretics is greater than the clinical benefit. Extracellular fluid (ECF) volume is expanded only at higher salt intakes [3] and below 50mmol/day diuretics have no effect on either ECF volume or blood pressure [3,5]. The risk of iatrogenic hyponatraemia is greater the low the sodium intake, especially for elderly women [6], and greater with combination diuretics such as Moduretic and Dyazide [6,7].

Conditions affecting sodium metabolism. Change in salt intake after the onset of pregnancy should be avoided except on medical advice. This, and the need for extra sodium in rare conditions such as Addison's disease, are discussed in Salt Matters: a consumer guide with full bibliographic references [8].

References:

1. Beard TC, Woodward DR, Ball PJ, Hornsby H, von Witt RJ, Dwyer T. The Hobart Salt Study 1995: few meet national sodium intake target. Medical Journal of Australia 1997;166:404-07.

2. Hames WPT, Ralph A, Sanchez-Castillo CP. The dominance of salt in manufactured food in the sodium intake of affluent societies. Lancet 1987;1:426-29.

3. Freis ED. Salt, volume and the prevention of hypertension. Circulation 1976;53:561-63.

4. Antonios FT, MacGregor GA. Salt - more adverse effects. Lancet 1996;348:250-51.

5. Morgan T,, Myers J. Diuretics. Curr Therapeutics 1981;22:93-97.

6. Matthew TH, Boyd JW, Rohan AP. Hyponatraemia due to the combination of hydrchlorothiazide and amiloride (Moduretic). Australian spontaneous reports. Med J Aust 1990;152:208-09.

7. Brown MA, Whitworth JA. Think again about combination diuretics. Aust. Prescriber 1993;16(1):4-5.

8. Beard TC. Salt matters: a consumer guide. Melbourne. Lothian Books, 2004;227-32.