2.5 Sodium (Na), chlorine (Cl) and potassium (K)
The element sodium is a soft silvery metal and the element chlorine is a greenish gas that is poisonous to humans and many other animals. Yet when these two elements are combined together in a compound called sodium chloride, the properties are quite different. Sodium chloride in its solid form is composed of white crystals and we call it salt. When salt dissolves in water, the constituent sodium ions, Na +, and chloride ions, Cl −, become separated. Both of these ions are common in the body, sometimes to the extent that body fluids such as sweat taste quite salty. It is important to normal body functioning that the concentrations of sodium and chloride ions, together with potassium ions (K + ), in the blood and in the fluid around cells, are regulated within quite tight limits, however much or little is present in the diet. The ions are also essential for transmission of impulses along nerves and for muscle contraction.
What is the name of the process by which the correct balance of ions is maintained within and around cells?
It is called homeostasis (see section on calcium).
In the UK, most people take in more sodium per day than is needed, mostly due to salt which is added to food either during the cooking process – particularly in manufactured foods, including bread – or at the table, to improve the taste of the food. There is no way of storing the ions and so the excess must be removed from the body by the kidneys in urine. It is particularly important that babies do not take in too much salt, since their kidneys are not fully developed and they are unable to remove excess from their bodies. If, when they start on solid food, they are given food with the normal adult quantity of salt, they can suffer kidney, liver and brain damage. As people get older, a small increase in salt intake has a greater effect on blood pressure than it does in younger people. The UK Government's guideline advice is that the intake of salt in adults should be no more than 5 g per day for women and 7 g for men. The average adult intake is currently around 9 g per day.
When levels of sodium are too high, the body retains too much water and the volume of body fluids increases, increasing the blood pressure (hypertension). High blood pressure is linked with a higher risk of cardiovascular disease and strokes. Reducing the salt intake does, over a number of weeks, lead to a blood pressure reduction. Since there appear to be no adverse consequences of a reduction in salt intake, such a reduction in the diet of all adults is to be recommended. However, this dietary change is not easy to achieve, since people become accustomed to the taste of a particular level of salt in food and taste buds need time to adapt to less. Additionally, food labels often give the sodium content of food, rather than the salt content.
The mass of a chlorine atom is about one and a half times that of a sodium atom. Knowing that salt is made up of equal numbers of sodium and chorine atoms, how many times greater would the salt content of a food be than the sodium content?
If you assume 1 unit for the mass of a sodium atom and 1.5 units for the mass of a chlorine atom, then the mass of sodium chloride is 2.5 units. So the salt content would be two and a half (2.5) times higher than the sodium content.
100 g of a particular breakfast cereal contains 0.3 g sodium while 100 g of a chicken curry ready-made meal contains 0.4 g sodium.
Calculate the amount of salt present in 100 g of each of these foods.
The average portion size of the cereal is 40 g while the ready-made meal, suitable for one person, is 450 g. How much salt would be taken in by eating a portion of each of them?
How does this total amount compare with the advised daily salt intake for a woman?
Since the salt content is 2.5 times higher than the sodium content, 100 g of the cereal contains 0.3 x 2.5 g salt = 0.75 g salt, while 100 g of the ready-made meal contains 0.4 x 2.5 g salt = 1 g of salt.
Since 100 g of the cereal contains 0.75 g salt, 1 g of cereal contains 0.75 divided by 100 g of salt and so a 40 g portion contains 40 x 0.75 divided by 100 g salt = 0.3 g salt. And similarly, since 100 g of the ready-made meal contains 1 g of salt, 1 g of the meal contains 1 divided by 100 g of salt and the whole of the 450 g portion would contain 450 x 1 divided by 100 g salt = 4.5 g salt.
The advised daily intake for a woman is 5 g per day. The ready-made meal contains 90% (4.5 g divided by 5 g x 100% = 90%) of this quantity, while a portion of the cereal contains only 6% (0.3 g divided by 5 g x 100% = 6%). However, a bowl of cereal for breakfast and the chicken curry later in the day, would provide almost all of the woman's recommended salt intake for the day.
Like sodium ions and chloride ions, potassium ions are also widely distributed in foods and intakes are thought to be similar to those of sodium. However, potassium appears to have quite the opposite effect on blood pressure to sodium; the higher the potassium intake, the lower the blood pressure. Studies indicate that higher potassium levels allow the body to deal more effectively with excess sodium. Since fruit such as bananas, and vegetables, are good sources of potassium, more fruit and vegetables in the diet can have a beneficial effect on blood pressure.