Centenary controversies of salt

Salt, or sodium chloride, plays several roles on metabolic steps such as excitation of muscle and nerve cell, control of the acid-base balance, regulation of water balance and control the absorption of some nutrients.

It is also important for the taste of a food and it is used as food conservant.

There are many available types of salt that vary their characteristics in the amounts of sodium and other minerals in the composition, color, taste, texture and how to use it.

Besides the benefits salt can have negative effects and there are recommendations of maximum consumption and it could be restricted in some diseases.

The link between salt and blood pressure remains controversial even though this topic has been investigated for more than 100 years.

• History

The hypothesis that salt is responsible for increasing blood pressure raised on 1904 by Ambard and Beauchard. They observed a blood pressure tendency to downward in low salt intake (3g/day) in hypertensive patients, although salt restriction did not completely normalize blood pressure.

In the following decades until the mid-1950s many studies were developed on the prevention of hypertension and kidney disease and in 1929 Berger and Fineberg found that low-salt diets were not so effective in reducing blood pressure.

However the evidences were fragile cause all these trials were not randomized controlled and had a low number of participants

• Nowadays

Currently is already known that excessive dietary salt intake is associated with an increased risk of hypertension, cardiovascular disease and kidney disease. Then it is recommended consumption until 5g/day of salt (or 2000mg of sodium) (equal to 5 tablespoons of coffee) for cook, addition salt and processed food intake. 4g/day is recommended to Black and Asians, middle-aged and elderly, hypertensives, diabetes and chronic kidney disease as they are considered sensitive to salt.

The concept of salt sensitive and insensitive people comes from the fact that when there is high salt intake some people have changes in blood pressure while others do not. Genetic polymorphisms, age, ethnicity and morbidities such as metabolic syndrome and obesity are involved in this mechanism.

• The controversy

Among hypertensives there is a considerable reduction in systolic and diastolic blood pressure by reducing dietary salt intake while the same effect is not found in normotensive people, as observed in the 2017 Cochrane review.

PURE and INTERSALT, two large randomized controlled trials, failed to prove the causality between salt intake and blood pressure, they are suggestive.

This small effect can be explained by the activation of the hormonal salt conserving system, the renin-angiotensin-aldosterone system. If plasma sodium is low this system is activated and its final effect is keeping the blood pressure by retention of sodium and water on kidneys; there are also the action of counterregulatory hormones such as adrenaline and noradrenaline in increasing heart rate in order to maintain homeostasis. Therefore, there is no significant reduction in blood pressure.

Important changes were also shown in the same Cochrane review, such as increased renin, aldosterone, adrenaline, noradrenaline as a compensatory mechanism for reducing salt intake, as well an increasing in cholesterol and triglycerides in normotensive individuals.

• Conclusion

1. There is no evidence so far of benefits of reducing sodium consumption below 2000mg so there is no need for severe restrictions regardless of salt type.

2. It also does not mean there is a free dietary salt consumption since the excessive intake has negative effects.

3. The reduction of consumption must be made gradually to the good adaptation of the body and to don’t generate exacerbated compensatory mechanisms.

4. To control the blood pressure mechanisms may be associated such as increased potassium consumption and an adequate weight maintenance, for example.

5. In some diseases please follow the medical and nutritional individualized guidance.

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DiNicolantonio JJ, O’Keefe JH, The History of The Salt Wars. The American Journal of Medicine. 2017. doi: 10.1016/j.amjmed.2017.04.040.

Rust P, Ekmekcioglu C. Impact of Salt Intake on the Pathogenesis and Treatment of Hypertension. Adv Exp Med Biol. 2017;956:61-84. doi: 10.1007/5584_2016_147.

Garfinkle MA. Salt and Essential Hypertension: Pathophysiology and Implications for Treatment. Journal of the American Society of Hypertension. 2017. doi: 10.1016/ j.jash.2017.04.006.

Graudal NA, Hubeck-Graudal T, Jurgens G. Effects of low sodium diet versus high sodium diet on blood pressure, renin, aldosterone, catecholamines, cholesterol, and triglyceride. Cochrane Database of Systematic Reviews. 2017. DOI: 10.1002/14651858.CD004022.pub4.