Blood is pumped through our circulatory systems under pressure. When blood pressure is persistently higher than normal, the condition is called hypertension.
Hypertension appears to be both a disease itself and a risk factor for other diseases. It is sufficiently common to warrant being classified as a public health problem.
High blood pressure can be controlled. An important aspect of control is dietary restriction of salt (sodium chloride) intake.
The evidence that salt plays a critical role in blood pressure regulation is compelling and it is clear a reduction of salt intake by the general population would significantly improve public health. I am indebted to Dr Ivan Perry, professor of public health at UCC, for information on the connection between salt and hypertension.
Blood pressure is the pressure exerted by the blood against the walls of the arteries in the circulation system. Most of us have had our blood pressure measured.
Blood pressure is expressed in two numbers, each expressed in millimetres (mm) of mercury (Hg).
The first number is called the systolic blood pressure and is the pressure exerted while the heart is contracting. The second number is called the diastolic pressure and is the pressure between contractions while the heart is relaxing. Usually blood pressure is quoted as the ratio of the systolic over the diastolic, e.g., 140/80.
In healthy people, blood pressure varies from about 80/45 in infants, to about 120/80 at age 30, to about 140/85 at age 40 and over.
The increase with age occurs as the arteries lose the elasticity that absorbs the shock of heart contractions in younger people.
Blood pressure varies in individuals and in the same individual at different times. A reliable indication of blood pressure requires several measurements at different times.
A range of blood pressures is seen in the adult population. At the highest end of blood pressure distribution there is an increased probability of premature death secondary to stroke, heart disease, or kidney failure.
Lower on the distribution curve (for example diastolic pressure of 90-104, which is called mild hypertension) the risk of premature death is lower and continues to drop with further decrease in blood pressure. Blood pressure is considered high when it is persistently greater than 140/90 mm Hg.
Most hypertension is caused by a combination of genetic predisposition and environmental factors such as excessive salt intake, lack of exercise and stress.
In virtually all mammals, a high salt intake causes or aggravates high blood pressure. A large INTERSALT study of over 10,000 people in 32 countries found a positive association between salt intake and blood pressure.
A Dutch trial involving 500 newborn infants found babies fed on formula milk and solids with reduced salt content had lower blood pressure aged six months compared with a control group of infants fed a standard diet. A follow-up study at 15 years showed the beneficial blood pressure effects of early salt restriction persisted into adolescence.
A community intervention trial in Portugal reduced the salt intake of an entire village by reducing salt in cooking and in processed food, including bread.
At the end of the trial period the blood pressures of people in the village were significantly lower than in a control village where no dietary modifications were made.
The good news is that we can easily put this knowledge to work to achieve very impressive public health improvements. It is estimated that in people aged 50-59 a cut in daily salt intake of three grams - a moderate reduction - lowers systolic blood pressure by about 5mm Hg. Such a reduction in salt intake in Ireland would reduce the incidence of stroke by 26 per cent and the incidence of ischaemic heart disease by 15 per cent.
Reducing salt intake is particularly beneficial in older people. A recent UK study of 60- to 78-year-olds showed reducing daily salt intake over one month from 10 grams (usual UK intake) to five grams, reduced systolic blood pressure by an average of 7mm Hg. This reduction would reduce the risk of stroke by 36 per cent over a five-year period. Since the underlying incidence of stroke is high in the elderly, such a reduction would represent a marvellous improvement in public health.
We should all watch our salt intake, but particularly those of us who suffer from hypertension, have blood pressure in the high-normal range, and older people. It is a straightforward matter to discipline use of the salt-shaker and intake of highly salted products, such as crisps. But most dietary salt is hidden in processed foods such as bread, biscuits and breakfast cereals.
The food industry is very fond of salt because it is an easy way to transform the bland, or even the inedible, into tasty products.
It is time for food-processors to begin gradually reducing the salt content of their products. This is particularly important for bread, the single largest source of dietary salt. Medical doctors and the Department of Health should vigorously raise public consciousness about dietary salt, and politicians should pursue the matter.
William Reville is a senior lecturer in biochemistry at UCC.