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Pass the salt, please...

Adults are advised not to eat more than 6 g of salt a day, but the average intake remains higher, at around 9 g. Sara Stanner outlines the health implications of excess salt intake and gives practical advice on how to cut down

Sara Stanner
MSc RPHNutr
Nutrition Communications Manager
British Nutrition Foundation

Salt is the common name for sodium chloride and is the major source of sodium in our diet. Two and a half grams of salt provides one gram of sodium. Small amounts of sodium are essential for health as it plays a vital role in maintaining fluid balance within the body. It is also essential for the transmission of nerve impulses around the body, for contraction of muscles (including the heart) and in generating gradients across cells to enable uptake of nutrients.
Chloride, too, is essential to good health and is a fundamental element in the digestive process. It preserves acid-base balance in the body, aids potassium absorption, forms hydrochloric acid in the gastric juices used to break down and digest food, and controls the level of bacteria present in the stomach. It also enhances the ability of the blood to carry carbon dioxide from respiring tissues to the lungs.   

Intake and recommendations
In the UK, most people eat more salt than is good for their health. The government recommends that adults eat no more than 6 g of salt a day, which is equivalent to 2.5 g of sodium per day. This was recommended by the Committee on Medical Aspects of Food and Nutrition Policy (COMA) in 1994,(1) based on evidence of a link between high sodium intake and high blood pressure, and was supported as an achievable population goal (although not an ideal or optimal consumption level) by the Scientific Advisory Committee on Nutrition (SACN) in 2003. Population targets have also been set for children (see Table 1).(2)

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The Food Standards Agency (FSA) has been working with UK health departments and the food industry to reduce the average salt consumption of adults to the 6 g-a-day target by 2010. But despite a slight reduction in recent years, average intake remains high at around 9 g for adults (equivalent to 3.7 g of sodium) and exceeds the targets among children.(3,4)

The health implications of excess sodium intake
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 through 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. Too much sodium in the diet can lead to health problems. 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.
A high salt intake has also been linked to adverse effects such as failure to reach peak bone mass leading to an increased risk of osteoporosis in later life, an increased risk of stomach cancer and aggravation of asthma.(5)

Sodium and blood pressure
Accumulation of sodium and water in the body causes expansion of extracellular volume and can contribute towards hypertension, which substantially increases the risk of developing heart disease and stroke. Observational studies have shown a strong positive association between sodium intake and blood pressure within and between populations.(6,7) For example, the Intersalt study examined over 10,000 men and women in 32 countries and demonstrated a relationship between urinary sodium excretion (a good marker of sodium intake) and blood pressure.8 Those with low sodium excretion had lower blood pressure and a lower prevalence of hypertension. It was estimated that a reduction of 100 mmol/day of sodium (6 g of salt) corresponded to an average decrease of 3.5 mmg in systolic and 1.5 mmHg in diastolic blood pressure.
Most randomised trials of sodium reduction have supported these observational findings in individuals with hypertension (showing reductions that may reduce or obviate the need for antihypertensive therapy), as well as smaller reductions in normotensive individuals. Some trials have also shown sodium reduction among children to lead to falls in blood pressure.(9)
However, there remains controversy about the magnitude and the clinical significance of the fall in blood pressure in normotensive individuals and therefore the likely impact on morbidity and mortality outcomes of population-based salt reduction programmes. Some meta-analyses have concluded that intensive interventions have led to only minimal reductions in blood pressure and raised doubts about the benefits of a population approach to salt intake.(10-12) However, He and Macgregor concluded that modest and long-term reduction in salt intake for a duration of four or more weeks has a significant and important effect from a population viewpoint on blood pressure in both hypertensive and normotensive adults.(13) A reduction in salt intake of 6 g per day predicted a fall in systolic blood pressure of 7.1 mmHg in hypertensive and 3.6 mmHg in normotensive adults, and suggested that this may equate to a reduction in strokes and coronary deaths of approximately 14% and 9% respectively in hypertensives, and 6% and 4% respectively in normotensives.
Actual data on the effect of dietary sodium intake on subsequent cardiovascular events are limited and inconclusive, but follow-up of participants in the Trials of Hypertension Prevention (TOHP I and TOHP II) recently showed that people with "prehypertension" (high normal blood pressure) assigned to a sodium reduction intervention experienced a 25-30% lower risk of cardiovascular outcomes in 10-15 years after the trials.(14)
The Dietary Approaches to Stop Hypertension (DASH) Sodium Trial demonstrated that reducing salt intake lowered average blood pressure levels, but the greatest reductions were observed when combined with the DASH diet (one that is low in total fat and saturated fat and rich in fruit, vegetables and low-fat dairy foods).(15) The DASH diet and low salt intake (3 g) reduced blood pressure by an average of 8.9/4.5 mmHg below the control diet (representing a typical US diet) at the high salt (9 g) level. This highlights the importance of improving the diet as a whole rather than focusing on one nutrient alone. Other lifestyle factors, such as being physically active, not smoking and maintaining a healthy body weight, are also important in preventing hypertension.

The main sources of sodium in the diet
Salt is the major source of sodium in our food, but sodium is also a component of other ingredients, such as sodium bicarbonate (used in baking), sodium nitrite (a preservative), monosodium glutamate (used as a flavour enhancer) and in some medicinal products (eg, antacids).
Sodium and chloride levels are comparatively low in all foods that 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 (eg, in breadmaking). 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%. In March 2006, the FSA published salt targets for 2010 for a range of processed food categories, as part of its work aimed at reducing the population average salt intake to 6 g/day (these targets will be reviewed in 2008). Many food and drink manufacturers are working hard to optimise the levels of sodium in their products and have successfully reduced the sodium content of foods while maintaining their palatability, or have produced reduced-sodium alternatives. Many supermarkets also now offer "healthy-eating" brands that contain less sodium. The catering industry, working with the FSA, has also developed practical advice for caterers on ways they can contribute to reducing salt intakes, and several of the largest catering companies have committed to make substantial salt reductions.

Understanding food labels
Food labels can be used to select lower salt options. It is not currently mandatory to give nutrition information on foods unless a nutrient claim is being made, although most manufacturers provide nutritional information voluntarily. By law, when information is provided on food labels, it has to be given as sodium; it is sodium intake that impacts on health and not all sodium in the diet is in the form of salt. However, some manufacturers also usefully express this information as a salt equivalent and the FSA is pressing the European Commission for mandatory salt equivalent labelling on the back of packs.
As salt is only 40% sodium by weight, grams of sodium can be converted to grams of salt by multiplying by 2.5 (ie, 1 g of sodium per 100 g is equivalent to 2.5 g of salt per 100 g).
Foods produced by some supermarkets and manufacturers have "traffic light" colours on the front of the pack that can be a useful guide to establishing if a food is high or low in salt. Those labelled green contain 0.3 g or less per 100 g and are the healthier choice; foods with a red traffic light label for salt will contain more than 1.5 g per 100 g and should be eaten in small amounts or just occasionally. Other retailers and manufacturers display the amount of salt in comparison to the guideline daily amount. The FSA has suggested levels of salt in foods that indicate high or low amounts (see Table 2). However, these values are given per 100 g and any judgment must consider portion size.

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Foods labelled as "reduced salt/sodium" or "low salt/sodium" may indicate a lower salt content, but it is still important to check food labels. While the law currently does not state how much less salt or sodium these foods should contain, the FSA has recommended guidelines for the use of these terms (see Table 3).

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References

  1. Department of Health.Nutritional aspects of cardiovascular disease. Report on health and social subjects, No 46. London: HMSO; 1994.
  2. Scientific Advisory Committee on Nutrition. Salt and health. London, The Stationery Office; 2003.
  3. Food Standards Agency.Dietary sodium levels surveys. London: FSA; 2007. Available from: http://www.food.gov.uk/science/dietarysurveys/urinary
  4. Gregory J, Lowe S, Bates CJ, et al. National diet and nutrition survey: young people aged 4 to 18 years. Volume 1: Report of the diet and nutrition survey. London: HMSO; 2000.
  5. MacGregor GA. Salt - more adverse effects. Am J Hypertens 1997;10:37S-41S.
  6. Law MR, Frost CD, Wald NJ. By how much does dietary salt reduction lower blood pressure? I—Analysis of observation data among populations. BMJ 1991;302:811-5.
  7. Frost CD, Law MR, Wald NJ. By how much does dietary salt reduction lower blood pressure? II—Analysis of observation data within populations. BMJ 1991;302:815-8.
  8. Intersalt Cooperative Research Group. Intersalt: an international study of electrolyte excretion and blood pressure. Results for 24 hour urinary sodium and potassium excretion. BMJ 1988;297:319-28.
  9. He FJ, MacGregor GA. Importance of salt in determining blood pressure in children: meta-analysis of controlled trials. Hypertension 2006;48:861-9.
  10. Midgley JP, Matthew AG, Greenwood CM, et al. Effect of reduced dietary sodium on blood pressure: a meta-analysis of randomized controlled trials. JAMA 1996;275:1590-7.
  11. Graudal NA, Galløe AM, Garred P. Effects of sodium restriction on blood pressure, renin, aldosterone, catecholamines, cholesterols, and triglyceride: a meta-analysis. JAMA 1998;279:1383-91.
  12. Hooper L, Bartlett C, Davey SG, et al. Advice to reduce dietary salt for prevention of cardiovascular disease. Cochrane Database Syst Rev 2004;(1):CD003656.
  13. He FJ, MacGregor GA. Effect of modest salt reduction on blood pressure: a meta-analysis of randomized trials. Implications for public health. J Human Hypertens 2002;16:761-70.
  14. Cook NR, Cutler JA, Obarzanek E, et al. Long-term effects of dietary sodium reduction on cardiovascular disease outcomes: observational follow-up of the trials of hypertension prevention (TOHP). BMJ 2007;334:885.
  15. Sacks FM, Svetkey LP, Vollmer WM, et al. DASH-Sodium Collaborative Research Group. Effects on blood pressure of reduced dietary sodium and the Dietary Approaches to Stop Hypertension (DASH) diet. DASH-Sodium Collaborative Research Group. N Engl J Med 2001;344:3-10.