-- Although salt is the major source of sodium in our food, sodium is also a component of other ingredients, such as sodium bicarbonate used in baking and monosodium glutamate used as a flavour enhancer.

-- Too much sodium in the diet can lead to health problems. It is one of the risk factors that contribute towards high blood pressure (hypertension), which substantially increases the risk of developing heart disease or stroke.

-- In the UK, most people are eating more salt than is good for their health and a reduction in average intake to 6 grams (which is equivalent to 2.5 g of sodium) per day has been recommended. Population targets have also been set for children.

-- By law, when information is provided on food labels, it has to be given as sodium. However, some manufacturers also express this information as a salt equivalent. Either way, labels can be used to gauge the amount of sodium present in various foods and to select lower sodium options.

Salt is the common name for sodium chloride. There are about 2.5 grams of sodium in 6 grams of salt. Whilst sodium and chloride are essential in the diet in small amounts, there is rarely any likelihood of shortage but in the case of sodium, too much can lead to health problems.

Functions in the body

Small amounts of sodium are essential for health. All body fluids contain sodium, including blood, and it has an important function in maintaining fluid balance within the body. In this context, it is important that the body is able to regulate the level of sodium in the blood. Sodium is also necessary in generating electrical impulses in nerve and muscle and in generating gradients across cells to enable uptake of nutrients.

As excess salt in the diet is readily absorbed; control of sodium in the blood is achieved by excretion through the kidneys into the urine. There is also variable loss though sweat. Salt requirements are closely related to water requirements, and in extreme circumstances too low an intake results in muscular cramps. This can occur after strenuous exercise or in hot climates. People suffering from kidney disease and very young infants cannot tolerate high sodium intakes because their kidneys cannot excrete the excess. For this reason, salt should never be added to any foods for young babies.

Sources of sodium

Sodium is present in additives such as monosodium glutamate (a flavour enhancer), sodium saccharin (a sweetener), sodium nitrite (a preservative), sodium ascorbate (an antioxidant) and sodium bicarbonate (baking soda), and in some medicinal products e.g. antacids. But most sodium in the diet comes from salt.

Sodium and chloride levels are comparatively low in all foods which have not been processed. However, salt has been used as a preservative and a flavouring agent for centuries. It is also used as a colour developer, binder, texturiser and fermentation control agent (e.g. in bread making). For these reasons, it is added to foods such as ham, sausages, bacon and other meat products, smoked fish and meats, canned vegetables, most butter, margarine and spreads, cheese, bread, savoury snack foods and some breakfast cereals.

In the UK, about 75% of the salt in the diet comes from processed foods. The salt added when cooking or at the table contributes a further 10-15% and naturally occurring salt (it is found naturally in most foods) represents the remaining 10-15%.

The health implications of excess sodium intake

The average sodium intake amongst adults in the UK is 3.7 grams per day (equivalent to 9.5 grams or around 2 tsp of salt) and is well above the amount needed to maintain normal functions in a temperate climate. The RNI (recommended nutrient intake) for sodium is only 1.6 grams per day.

Too much sodium in the diet has been associated with an increased risk of developing stomach cancer and adverse effects on the kidney if there is some underlying abnormality. It is also one of the dietary and lifestyle factors that have been linked to high blood pressure or hypertension. Whilst hypertension is often symptomless, it increases the risk of conditions such as heart disease and stroke. The Dietary Approaches to Stop Hypertension (DASH) Study has shown the most effective diet to prevent or treat high blood pressure to be one that is low in fat and sodium and includes low fat dairy products (a source of calcium), as well as fruit and vegetables (a source of potassium). This emphasises the importance of improving the whole diet rather than focusing on any individual nutrient. Other lifestyle factors, such as being physically active, not smoking and maintaining a healthy body weight are also important in preventing hypertension.