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SSA 03-3 DIETARY SALT INTAKE AND HYPERTENSION IN SINGAPORE

Oh, Vernon Min Sen

doi: 10.1097/01.hjh.0000499883.00225.83
SPECIAL SATELLITE SYMPOSIUM 03
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According to the Singapore National Health Survey (NHS) of 2010, the population of the Republic of Singapore was 5.076,700, comprising four ethnic groups: Chinese (74.1%), Malays (13.4%), Indians (9.2%), and others (3.3%). The National Health Survey for 2016 is under way and due to be published in 2017. From the six-yearly national health surveys, the crude prevalence of clinical hypertension (HTn), defined as a sustained blood pressure ≥ 140/90 mmHg, in Singaporean residents aged 30 to 69 years rose from 22.2% in 1992 to 27.3% in 1998, but declined to 24.9% (2004) and to 23.5% in 2010.

The NHS of 2010 demonstrated that 18.9% of residents aged 18 to 69 years (an age band of 52 years) had hypertension. Its data show that the age-specific prevalence of HTn rises in a pronounced way from the age of 40 years (16.7%) onwards. The age-specific prevalence of the disorder was 53.4% in persons aged 60–69 years, compared with 7.6% in those aged 30–39 years.

The 2010 NHS showed, as have surveys in many other industrial nations, that more men (prevalence = 26.4%) than women (20.7%) have HTn. A disproportionate number of Malays and Indians have HTn, compared with Chinese persons.

A breakdown of the crude prevalence data from the 2010 NHS shows that the greatest prevalence of hypertension among persons aged 30–69 years occurred in Malay women (29.8%), Chinese men (27.2%), and Malay men (26.0%).

Information from a National Nutrition Survey (NNS) in 2010 conducted by the Health Promotion Board, a unit of the Ministry of Health (MoH) in Singapore, showed that the overall dietary intake of sodium chloride (common salt) was 8500 mg (8.5 grams) per day in residents aged 30–69 years (Table 1).

The dietary intake of sodium chloride was estimated by measuring the total amount of sodium in a 24-hour collection of urine. The urinary concentration of sodium was measured using a sodium-ion selective electrode. Men consumed significantly more salt than women. There was a noticeable reduction in the daily dietary salt intake between the age group 40–49 years (9000 mg) and those aged 60–69 years (7700 mg). The Indian and Chinese respondents in the Survey consumed more salt, 8700 and 8500 mg/day respectively, than the Malays (7600 mg/day).

A study led by Associate Professor Rob Martinus Van Dam in the School of Public Health, National University of Singapore, is addressing the validation and calibration of a technique to estimate the 24-hour urinary content of sodium from measurements of sodium concentration in a spot urine sample. The technique is intended to raise the accuracy of spot urine sampling.

It is difficult to interpret the significance of the decreased daily intake of salt in older persons, aged 50 to 69, in the face of the greater prevalence of hypertension in these age bands (31.9% in those aged 50–59 years, and 53.4% in those aged 60–69). For instance, some older persons might ingest less dietary sodium, or the net renal tubular excretion of sodium in the urine might decrease between age 50 and 69 years without any change in dietary sodium intake.

Moreover, the treatment of hypertension in persons aged 50 to 69 years is likely to include diuretic agents, both thiazide drugs, and loop diuretics such as furosemide and bumetanide. In most hypertensive patients, long-term treatment with diuretics exposes older persons to a risk of hyponatraemia, hypokalemia, or both. The latter conditions result from a net increase in the urinary excretion of sodium and potassium (apart from increased free water excretion). Such diuretic-related loss of sodium in the urine also needs to be factored into the estimation of dietary sodium intake by means of 24-hour urinary sodium output.

The increased prescription of renin-angiotensin-aldosterone blocking drugs to control HTn further confounds the estimation of dietary sodium intake from 24-hour urine collections. Angiotensin-converting enzyme inhibitors and angiotensin-receptor blockers, both reduce the sodium- and water-retaining effects of aldosterone. Their effects on the urinary sodium content of older hypertensive patients also need consideration.

The 2010 NHS showed a treated BP of < 140/90 mmHg was achieved in 69.1% of hypertensive patients. This was an improvement over the 52.9% of patients treated to target in 2004. However, as the 2010 NHS showed that about 26.3% of hypertensive persons were undiagnosed in the community, the ‘successfully treated’ proportion of hypertensive persons was actually 69.1% of 73.7%, i.e. 50.9% of the pool of known hypertensive persons.

The concerted program of prevention and management of HTn in Singapore applies the principles of early detection via BP screening in persons aged 18+ years, and of good treatment within the complex of cardiovascular risk factors (+ diabetes mellitus, obesity, physical inactivity, dyslipidaemia, smoking, obstructive sleep apnoea, etc). Medisave money from each working person's central provident fund may be used to help pay for outpatient costs of antihypertensive treatment.

Regardless of the confounded relation between dietary salt intake and HTn observed from NHS 2010 and NNS 2010, the MoH Clinical Practice Guidelines for Hypertension will contain advice to family physicians as well as hospital specialists to urge patients to cut their dietary intake of salt towards a moderate amount of 5000 to 6000 mg per day. Readily accessible methods of patient prompting and education include digital apps on smartphones and tablet computers, in a society in which mobile phones are owned by about 94% of the community.

National University of Singapore, Singapore

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