Nutrition Diet and blood pressure
Essential hypertension is a multifactorial disease. It is common in older people not only in urban and industrialised areas but also in a quiet in all areas.
Hypertension is not an inevitable accompaniment of ageing. Salt is the best known of the dietary factors affecting blood pressure.
The requirement for sodium in health is usually under 25 mmol Na/day (equivalent to 1.5 g NaCl). Normal kidneys can shut down sodium excretion almost to zero and sweat loss is reduced in people on low salt intakes or adapted to hot climates. Human milk contains only 7 mmol Na/litre, so young infants’ sodium intake per megajoule is only about one-sixth that of their parents.
Salt has been used since Neolithic times by most cultures as an important food preservative. Most of mankind has become used to the taste of more salt than we need now that canning, freezing, refrigeration, etc, are widely used to preserve our food.
Sodium accumulation and arterioles
The mechanism of action of sodium is undoubtedly complex and involves kidney tubules and several hormones. One aspect is that if sodium tends to accumulate in cells it interferes with calcium transport, and elevated free calcium in the cytosol of arteriolar smooth muscle cells increases their tone and consequently the arterial blood pressure.
How much reduction of blood pressure can be achieved with a low salt diet in people with hypertension?
Elevated blood pressure can usually be lowered by salt restriction. Diuretic drugs work by increasing urinary sodium excretion. Alternatively a sufficient reduction of dietary sodium can achieve the same degree of negative sodium balance. In mild to moderate hypertension, a reduction of sodium intake (which can be monitored with 24-hour urinary sodium) by 50 mmol/day will usually give a useful reduction of blood pressure, so that the patient may be able to come off the hypotensive drugs (or not start them) or reduce the dose (and with this the probability of side effects). Salt restriction increases sensitivity to all hypertensive drugs except slow channel calcium blockers, like nifedipine. Some people are more responsive than others. Older people may be more responsive to salt reduction and they are particularly susceptible to the side effects of drugs.
When people change to a lower salt diet their taste adjusts after a few weeks. Other flavours are perceived and appreciated more. The major obstacle to eating low salt is that most of the salt in food is put in during processing and is outside the individual’s control.
Sodium in foods
Most of the salt that we eat is not that added at the table or in cooking water (much of which goes down the sink). It is salt added in food processing, particularly of staple foods. Wheat flour contains 3 or 4 mg sodium/100 g but average breads have 520-550 mg/100 g. Oils like sunflower or olive oil contain only traces of sodium but butter averages 750 mg/100 g and margarines 800 mg/100 g. Many cereal products—biscuits, cakes and breakfast cereals (though not all)—are very high in sodium, which consumers cannot taste (being masked by the sugar content). Salted peanuts contain less sodium than breads; consumers can taste the salt because it is all on the surface.
Other sodium compounds in food, bicarbonate and glutamate, have less effect on blood pressure than sodium chloride.
Average percentages of sodium from different sources
Added at table = 9.0
Used in cooking = 6.0
Naturally occurring = 18.5
Added salt in processing = 58.7
Non-salt additives =7.2
Salt in water supply (average) 0.6
total == 100.0
Obese people are likely to have a higher blood pressure than lean people. Typically a 3 mmHg higher diastolic pressure may be expected for every 10 kg increase in body weight. Raised blood pressure and hyperlipidaemia are both major risk factors for cardiovascular disease, and effective weight reduction will improve both.
Alcohol intake is emerging as one of the important environmental factors associated with raised blood pressure. Heavy drinkers have higher blood pressure than light drinkers and abstainers. The effect starts above about three (stated) drinks a day. Systolic pressure is more affected than diastolic.
The pressor effect of alcohol can be demonstrated directly. It is found that when low alcohol beer (0.9% alcohol) was substituted for the same intake of regular beer (5% alcohol), their blood pressure fell 5/3mmHg. The mechanism(s) have not yet been established.
Acute ingestion of alcohol causes peripheral vasodilatation, but there are features of a hyperadrenergic state in the withdrawal syndrome. Plasma cortisol concentrations are sometimes raised in alcoholics. Increased red cell volume, and hence increased blood viscosity, is a possible mechanism.
Components in the diet that may lower blood pressure
In a placebo-controlled, crossover trial in mild to moderate hypertension, blood pressure fell by (average) 7/4 mmHg with a supplement of eight Slow-K tablets (64 mmol potassium) a day.
It is found that little or no effect in similar hypertensive patients who had managed to reduce their sodium intake (and urinary sodium) to around 70 mmol a day—potassium acts as a sodium antagonist and has little effect when sodium intake has been halved.
Potassium in foods
Moderate to high (mmol per usual serving)
Potatoes (12-26), pulses (19), dried fruits (5-12), fresh meat and fish (8-10), All Bran (8), fresh fruit (2-10), vegetables (2-10), orange juice (6), oatmeal (5), cows’ milk (5), nuts (2-6), wine (3-4), beer (3), coffee (2).
Low (1-3 mmol per usual serving)
Rice, chocolate, egg, biscuits, bread, cheese, flour, cornflakes.
Very low or absent
Sugar, jam, honey, butter, margarine, cream, oils, spirits.
Potassium is the major intracellular cation; the more concentrated the cells in a food, the higher the potassium is likely to be.
Analyses of a diet and health study suggested that people with low calcium intakes had more hypertension, and cardiovascular disease is reported in areas with hard water (which contains more calcium). Increased calcium, by diet or supplements, might be useful in a very small number of hypertensive patients who have a low calcium intake or increased plasma parathyroid levels.
Magnesium can sometimes lower blood pressure. In patients who had received long term diuretics (mostly for hypertension) and potassium supplements, half were also given magnesium aspartate hydrochloride for six months. Their blood pressure fell significantly. The diuretics had presumably led to subclinical magnesium depletion.
Magnesium distribution in foods
Magnesium is distributed in foods somewhat similarly to potassium. Bran, wholegrain cereals, and legumes are the richest sources. Most vegetables contain similar moderate amounts to meat.