Table of Contents
- What is salt
- How the human body uses salt
- What salt is used for
- Medical issues where salt can be a problem…and why
- How a high salt diet can impact health
- Recommendations for healthy salt intake
- Salt substitutes
- Different names for salt
Salt is a chemical compound made up of sodium (Na+) and chloride (Cl–). The molecule forms by the ionisation of sodium and chlorine atoms to from a cubic crystal structure that result from the attraction of the valence shell electrons. An atom of sodium has one electron in its valence shell whilst chlorine requires one electron to fill its outmost shell. This makes it favourable for the sodium atom to donate its electron the chlorine atom and results in ionic bonding to form the crystal. The larger chlorine atoms are arranged such that the smaller sodium atoms fill the gaps between them (Allan Blackman, Steve Bottle, Siegbert Schmid, Mauro Mocerina and Uta Wille, 2007), as shown in Figure 1.
Figure 1: Structure of sodium chloride crystal
The colour of salt varies from colourless (when pure) to white, grey or brownish when in different forms. Salt dissolves readily in water because water molecules are polar with two distinct ends, each with a partial negative and positive end. As opposites attract the partial negative end of the water molecule is attracted to the Na+ ions and the partial positive end attracted to the Cl– ions and pull them apart. Each ion becomes surrounded by water molecules and dissolves into the water, as shown in Figure 2:
Figure 2: The dissolution of salt in water
This dissolution of salt in water is important as it help to explain how salt is dissolved in the blood.
Where salt is found
Salt is found in nature in the oceans (salt water) and the foods that we eat (such as wholegrains, meat, dairy products and pre-packaged foods). Salt is also found naturally in the human body.
Salt provides the body with two elements (sodium and chloride) that it cannot manufacture itself and is necessary for the correct functioning of the body. Sodium is a mineral that plays a critical role in body physiology. It controls the volume of fluid in the body and helps the nervous system to fire impulses (to name a few).
Salt and blood pressure
The physiology of how salt influences blood pressure involves the rennin-angiotensin aldosterone system (RAAS). This is a cascade that begins with the biosynthesis of renin in the glomerulus of the kidney. This renin secretion is regulated by 4 independent pathways:
- A renal baroreceptor in the afferent arteriole senses changes in renal perfusion pressure, for example a reduction in circulating blood volume
- Changes in the delivery of NaCl to cells in the distal tubule
- Sympathetic nerve stimulation via beta-1 adrenergic receptors
- Negative feedback by direct action of angiotensin II (Ang II)
In the extracellular space, renin cleaves the angiotensin to form angiotensin I, which in turn is converted to Ang II by the removal of its C peptide by Angiotensin Converting Enzyme (ACE). Ang II stimulates the production and therefore release of aldosterone, whilst also promoting the constriction of renal and systemic arterioles and the reabsorption of sodium in proximal segments of the nephron. Aldosterone exerts an effect on the distal renal tubules, causing them to increase sodium resorption while secreting potassium (Atlas, 2007) . This is shown in Figure 3.
Figure 3: The Renin-Angiotensin Aldosterone System
This increase in sodium results in an increase in blood pressure and volume. How sodium increases blood pressure and volume relates back to its ability to dissolve when surrounded by water in the blood. The osmotic pressure increases when the salt dissolves in the blood and the kidneys react by the above mechanism. Water is then retained and the blood pressure increases.
Salt and the nervous system
Sodium and chlorine ions play a role in the nervous system by controlling the firing of impulses in neurons. This is achieved through the role they play in action potentials. Changes in the concentrations of sodium and chlorine ions cause an action potential to fire, allowing a neuron to send a signal to other cells which it’s connected to. This action potential requires a stimulus which in turn causes sodium channels to open. As there are more sodium ions on the outside of a cell, the sodium ions rush into the neuron, making it more positive and therefore depolarised. When the depolarisation reaches about -55mV a neuron will fire an action potential. This is referred to as the threshold. To reverse this depolarisation potassium channels will open and the sodium channels will start to close (Institute, 2011), as shown in Figure 4. This process is required to send signals throughout the nervous system.
Figure 4: Action potential (picture from http://faculty.washington.edu/chudler/ap.html)
As already discussed the body has many uses for salt. From a food point of view, salt is typically used to flavour foods. This salt may be found naturally in a food or it may be as an added taste – i.e. salt has been added during the cooking or on the dish after it has been served. Salt is recognised by receptors on the tongue. Receptors (molecules specialised to bind a large number of chemical signals) are found on the apical microvilli of the taste cells and the transduction machinery involves ion channels on both the apical and basolateral membranes (Figure 5) (Purves D, Augustine GJ, Fitzpatrick D, 2001).
Figure 5: Transduction mechanisms in general taste cell
Voltage gated Na+, K+ and Ca2+ channels produce depolarising potentials when the taste cells interact with chemical stimuli. The receptor potentials raise Ca2+ to levels necessary for synaptic transmission. The greater the taste concentration, the greater the depolarisation of the taste cell. The receptor for salt is a Na+ channel on the apical membrane of some taste cells. In general, the larger the NaCl concentration applied to the tongue, the larger the depolarisation (Purves D, Augustine GJ, Fitzpatrick D, 2001) and the saltier the food tastes.
As discussed salt can raise blood pressure. This makes the heart work harder to pump the blood around the body and is seen as an increase in blood pressure (or hypertension). This in turn increases the risk of heart disease and stroke as well as other medical problems such as heart failure and kidney disease.
Meniere’s symptoms generally result from having excess fluid within the inner ear. A diet high in salt increases this fluid and makes the symptoms worse and can even cause vertigo attacks.
Could be due to Menieres Disease (as discussed above) or from poor circulation of blood to the brain. Chronic light headiness could be caused by ischaemia from atherosclerosis (hardening of the arteries). This can be a side effect of hypertension.
A diet high in salt can exacerbate the symptoms of vertigo in a similar mechanism to that of Menieres Disease. Vertigo can also be a symptoms of Menieres Disease.
Eating salt raises the amount of salt in the bloodstream and results in a higher blood pressure and discussed before. However the extra fluid places a large strain on the blood vessels leading to the kidneys, which over time can damage them (this is referred to as kidney disease). If the kidney disease is left untreated and blood pressure isn’t lowered, the damage can eventually lead to kidney failure.
As discussed earlier, salt plays a role in blood pressure. If a high salt intake continues it can lead to high blood pressure (hypertension) by the following mechanism. When the heart contracts, the blood inside the left ventricle is forced out into the aorta and arteries. The blood then enters small vessels with muscular walls, called arterioles. Blood pressure is a combination of two measurements:
Systolic – which is the highest pressure against the arteries as the heat pumps (normal systolic pressure is between 110 and 130 mmHg)
Diastolic – the pressure against the arteries as the heart relaxes and fills with blood (normal diastolic pressure is between 70 and 80 mmHg)
As salt has the ability to increase blood volume, this means that the heart works harder to pump the blood around the body and is seen as an increase in the systolic and/or diastolic measurements. The following table taken from the American Heart Association shows the healthy range for blood pressure and when hypertension is diagnosed:
A high salt intake may increase the urinary loss of calcium
Reasons for why a high salt intake may lead to an increased loss of calcium (hypercalciuria) are not yet fully understood. There are a two main theories that have been put forward to explain this.
The Renal Calcium Leak Hypothesis
This theory states that in patients with essential hypertension (persistent and pathological high blood pressure for which no specific cause can be found) renal calcium handling is altered in such a way that urinary calcium excretion is increased at each level of sodium output.
The Central Blood Volume Hypothesis
Evidence suggests that the increase in calcium excretion rather than a consequence of increasing salt intake may actually be secondary to the increase in extracellular volume that occurs. That is, sodium intake equals output but at an expanded extracellular volume. On a high salt intake, there would be a tendency for sodium retention, but compensatory mechanisms attempt to overcome the underlying effect. The suggestion is that these compensatory mechanisms are responsible for an increase in blood pressure and a shift of blood from the periphery to the centre. The expansion of central blood volume could cause the increase in urinary calcium excretion.
Regardless of the exact mechanism, a high salt intake, with its effects on urinary calcium excretion, is related to reduced peak bone bass in adolescence and accelerated bone mineral loss in post-menopausal women as well as increased movement of calcium from bone (Francesco Cappuccio, Rigas Kalaitzidis, Stuart Duneclift, John Eastwood, 2000).
Currently there are guidelines designed to help individuals manage their salt intake. As sodium can play a large role in health it is recommended that:
- The adequate intake (AI) for males and females (19 years+) is between 460-920mg/day
- The upper level (UL) for males and females (19 years+) is 2300mg/day
- The suggested dietary target (SDT) to reduce chronic disease risk recommend an intake of 1600mg/day for men and women (Government, 2005)
Salt substitutes typically contain potassium chloride in place of sodium chloride. As potassium is involved in many of the pathways in the body (such as in the nervous and cardiac systems) salt substitutes can be inappropriate for some people. It is best to check with your doctor before taking salt substitutes. Additionally, to reduce your salt intake you could try some of the following:
- Don’t add salt at the table or in cooking
- Replace salt with herbs and spices for example dried herbs, ground pepper, lemon juice, onion and garlic
- Choose salt reduced products – for example tomato paste and cheese
- Limit takeaway and processed foods – these tend to be high in salt and fat
- Avoid highly processed meats and deli meats such as salami, bacon and sausages
- Choose unsalted products where possible – for example nuts
- Look for products that have < 120mg/100g of salt. These are considered to be low salt options
- Chose salt reduced breads and breakfast cereals – bread is a major source of sodium in the diet
It’s important to remember that taste receptors grow used to a certain level of salt and as such, when salt intake is reduced, foods can often taste bland. In this case it is important to give taste buds time to change and reducing salt by increments can help.
There are many different names for salt which may appear on food labels – this often makes it confusing and difficult. Any of the following names mean salt and care should be taken when choosing products that have high levels of salt in them:
- MSG (monosodium glutamate)
- Vegetable salt
- Sodium lactate
- Sodium bicarbonate
- Sodium ascorbate
- Yeast extracts
- Celery salt
- Garlic salt
- Sea salt
- Rock salt
- Onion salt
- Sodium nitrate Sodium nitrite
- Stock cubes
An important note: Salt can come in many different forms. It may referred to as rock salt or sea salt or even the new and popular himalyan pink salt. It is important to remember that salt is salt and should be reduced/avoided as part of a healthy balanced diet.
If you have specific dietary requirements and need advice about a salt reduction diet contact ENT Wellbeing on 1300 123 0368 to make an appointment to see our dietitian and nutritionist Rhiannon today!!
Allan Blackman, Steve Bottle, Siegbert Schmid, Mauro Mocerina and Uta Wille, 2007. Chemistry. s.l.:John Wiley and Sons LTD.
Association, A. H., 2012. Understanding Blood Pressure Readings. [Online] Available at: http://www.heart.org/HEARTORG/Conditions/HighBloodPressure/AboutHighBloodPressure/Understanding-Blood-Pressure-Readings_UCM_301764_Article.jsp
[Accessed 24 May 2013].
Atlas, S., 2007. The Renin-Angiotensin Aldosterone System:Pathophysiological Role and Pharmacologic Inhibition. Journal Managing Care Pharmacy, 13(8), pp. 9-20.
Francesco Cappuccio, Rigas Kalaitzidis, Stuart Duneclift, John Eastwood, 2000. Unravelling the links between calcium excrection, salt intake, hypertension, kidney stones and bone metabolism. Journal Nephrology, Volume 13, pp. 169-177.
Government, A., 2005. Nutrient Reference Values. [Online] Available at: http://www.nrv.gov.au/ [Accessed 24 May 2013].
Institute, S., 2011. How the human body handles salt. [Online] Available at: http://www.saltinstitute.org/Issues-in-focus/Food-salt-health/How-the-body-handles-salt [Accessed 23 May 2013].
Purves D, Augustine GJ, Fitzpatrick D, 2001. Taste Receptors and the Transduction of Taste Signals. In: Neuroscienc 2nd Edition . s.l.:Sinauer Associates.
For tips to ensure a healthy salt intake click here.