Sodium is one of the minerals, which your body needs to keep it functioning properly. While we all need some sodium, most of us eat far more than we need and could easily cut down without in any way damaging our health. Sodium intake is measured in units called millimoles (mmol). The average intake in the United Kingdom is about 150 mmol a day – about 10 times the amount that is essential for health.
The main source of sodium in most foods is sodium chloride, which is the chemical name for ordinary cooking or table salt. This is present not only in cooking salt and table salt, but also in usually unsuspected foods such as milk, cheese and bread and in all all tinned or ready prepared foods such as most breakfast cereals, sausages, burgers, pizzas and soups. Because salt is such a major source of sodium in our diet, doctors often refer interchangeably to low-salt or low-sodium diets – reduce the amount of salt in your diet and you automatically reduce the amount of sodium. A diet insufficiently low in sodium to reduce blood pressure by up to 5 mmHg, with sodium intake reduced to about half normal at 60-70 mmol a day, must actually eliminate all these foods, as well as more obvious ones like kippers, bacon, olives, hummus, Marmite and snacks such as crisps and salted nuts. Sodium is also found in other substances used in cooking and food processing, such as carbonate of soda, sodium nitrate (a preservative), saccharin sodium (an artificial sweetener) and monosodium glutamate (used in Chinese food and many sauces).
High blood pressure does not exist at all, nor does average blood pressure rise with age in some tribes in Brazil and Papua New Guinea which sodium intake is at the bare minimum essential for life (less than 15 mmol per day). This is about one tenth of the present average intake in the United Kingdom (about 150 mmol a day) or rural northern Japan with more than two and a half times the United Kingdom intake (400 mmol a day) both high blood pressure and stroke are extremely common.
So, in general, evidence points towards salt intake being an important cause of the general rise in blood pressure with age in nearly all societies. This evidence is not absolutely conclusive, because people who consume natural rather than medically prescribed low-sodium diets are extremely poor, often hungry and generally can expect to live less than 40 years even if they survive infancy.
There are good biological reasons why sodium intake may affect blood pressure, mainly through its effects on the kidneys. The idea that salt overload is the cause of primary high blood pressure, with susceptibility to sodium overload genetically determined, seems quite quite and is currently fashionable. On the other hand, the evidence is not consistent and there are still many experts who doubted the truth of this theory. Evidence that substantive salt restriction is and effective or practical treatment for high blood pressure is much less convincing.