Sodium restriction is one of the most frequently prescribed lifestyle measures taken in everyday clinical activity. However, as frequently as this easy and effective preventative therapy is recommended by doctors, as often it is neglected by patients. One possible way to increase adherence to salt restriction – mainly for patients insisting that they actually do so- would be to objectively demonstrate how large their daily salt intake actually is. To that end, urinary sodium content measurement seems to be the preferred method. Considerations regarding the practical utility of this method from the point of view of a practical cardiologist will follow.
- Both excessive salt consumption and individual salt sensitivity are risk factors for hypertension and cardiovascular disease.
- Salt consumption in many European countries at a population level is excessive. Few European individuals comply with the recommended daily salt intake of less than 5 g per day. Patients with hypertension or heart failure do not perform much better.
- Quantifying salt intake is not as difficult as some doctors consider.
- Measuring salt intake by urinary sodium excretion would theoretically be useful in some scenarios: resistant or pseudo-resistant hypertension, close relatives of hypertensive individuals, salt-sensitive subjects, non-adherent patients or unwilling to comply with a low-sodium diet, or in whom thiazide diuretics would be less advisable (e.g., diabetics, metabolic syndrome).
1 - Sodium and cardiovascular health and disease
Evidence relating salt intake and cardiovascular health and disease come from numerous animal experiments, epidemiological studies, dietary trials, and clinical observations.(1) Dietary sodium content is directly related not only with blood pressure (INTERSALT Study), but also with total mortality (NHANES 1988-2006), coronary risk (CARDIA Study), heart failure outcomes (2) and other cardiovascular outcomes, despite a recent controversy related mainly to patients with advanced heart failure. (3, 4)
2 - Sodium restriction as a therapeutic goal at a population level
Unfavorable high sodium intakes remain prevalent around the world (INTERMAP Study). (5) Sources of dietary sodium come from processed foods, table salt voluntarily added and in some cases soluble over-the-counter drugs (Fig.1).
International guidelines on nutrition unanimously recommend sodium restriction, ideally to less than 5 g/day.(6) It is estimated that decreasing dietary intake from 10 grams to 5 grams per day would reduce the overall stroke rate by 23% and cardiovascular disease rates by 17%.(7) Thus, a number of scientific organisations and some countries have accepted this challenge and have implemented some strategies in that direction at the community level, by means of affordable and cost-effective public health interventions (involving multi-stakeholders, from family doctors to food manufacturers or restaurant industries). For example, the American Heart Association has launched the improvement of cardiovascular health of all Americans by 20% as one of its goals for the year 2020; two parameters included in the “ideal cardiovascular health” are a blood pressure of less than 120/80 mm Hg and a sodium consumption of less than 1500 mg/day (1,5 grams).
3 - Sodium restriction as an individual goal
But we as doctors have to deal with individual patients and have the responsibility of guaranteeing that our patients who would benefit from sodium restriction (most of them) or have a compulsory indication for that (heart failure, hypertension) actually manage to comply with it. A useless way to achieve that goal is general advice “try to use as less salt as possible” briefly offered among other dietary recommendations, allowing more time and giving more importance to prescribing drugs. On the contrary, as important this objective is, salt restriction needs a greater effort and a continuing support in order to be achieved. (8)
4 - Quantifying sodium intake
One of the tools which could help to succeed in reducing salt intake by individuals is to measure directly how much salt (sodium) they eat, and to use this information to reinforce the necessity of reducing it. Self-reported food consumption in questionnaires or surveys is usually inaccurate. Measuring the 24-hour urinary sodium excretion is the reference method, but has been used up to now almost exclusively as a research tool. Why not apply this method to clinical needs? The main reason not to has probably been the perceived complexity of collecting urine in the course of one day. Other drawbacks are the need to assure that all 24-hour urine has been adequately and completely collected (detailed instructions, excretion of para-aminobenzoic taken simultaneously by mouth or creatinine excretion in relation to body weight), the underestimation or true sodium intake by a 10-15% (due to electrolyte loss by skin and lungs), and the interindividual variability of sodium excretion. For clinical purposes, it is possible to simplify the technique by measuring sodium excretion only once in a single morning urine sample simultaneously with creatinine and total daily urine volume (measured or inferred).
Daily salt intake based on 24-hour urinary sodium excretion (assuming that all sodium ingested was in the form of sodium chloride) with a formula: figure 2 shows a practical method to estimate salt or sodium intake.
Figure 2: Calculation for estimation of salt or sodium intake
Na (mg/day) = Na (mmol/day) x 23; NaCl = Na (g/day) x 100/ 39,3
1 gram salt (NaCl) = 393,4 mg Na = 17,1 mmol Na
5 - Practical applications
This method to quantify sodium intake, although simple, easy and inexpensive, is probably not worth applying to the whole cohort of cardiologist’s outpatients. But perhaps it could help in the management of some cases, summarised as follows:
- Heart failure needing diuretics,
- Resistant or pseudo-resistant hypertension,
- First-degree relatives of hypertensive individuals,
- Salt-sensitive subjects, (9, 10)
- Non-adherence to dietary recommendations
- Unwillingness to comply with low-sodium diet, or
- Patients In whom thiazide diuretics are not recommended (e.g., diabetics, metabolic syndrome), or
- Unwilling to take diuretics for adverse effects (e.g. impotence).