Excessive salt consumption has been shown to increase the incidence of heart attacks, strokes and more recently, stomach cancer. Experts believe that reducing salt intake could save millions of lives worldwide.
The increased consumption of processed foods makes it extremely easy to exceed the daily salt allowance of 6 g/day. Many foods are loaded with salt to boost flavour and to make them last longer. Most people worldwide eat far too much salt without realising it, which can cause high blood pressure (hypertension), a risk factor for stroke and other serious conditions. A study in 2013 showed that the average salt consumption worldwide was roughly 10 g/day! Many studies have shown clear links between increased salt consumption and hypertension.
Salt, also known as sodium chloride (NaCl), is composed of sodium and chloride, two essential minerals for the body. Sodium is fundamental for the body to function, but we only need small amounts of this mineral. In large quantities, sodium is detrimental to health as it elevates blood pressure.
To understand how sodium elevates blood pressure it is important to understand the principle of osmosis. This is the movement of water from a solution of less concentration to one of higher concentration, through a semi-permeable membrane. When excess sodium is consumed, more water moves into the bloodstream by osmosis. This extra volume of water in our arteries is responsible for raising blood pressure.
Figure 1. When blood sodium levels are high, more water moves into arteries by osmosis. This causes fluid retention, increasing blood pressure.
Excessive salt consumption is associated with hypertension, a major risk factor for stroke, heart and kidney disease. Hypertension puts a large strain to the walls of arteries, which are damaged over time. To cope with the increase in blood pressure, a section of the artery, known as the smooth muscle, thickens. But in doing so the diameter of the artery becomes smaller, increasing the likelihood of developing atherosclerosis. This is the deposition of fatty substances to the walls of the arteries, narrowing them further. Blood clots can eventually occur, completely blocking the artery. A heart attack takes place when an artery supplying the heart gets blocked. Meanwhile if an artery supplying blood to the brain gets blocked, it will trigger a stroke.
Figure 2. This diagram shows how arteries change over time in individuals with hypertension. a) Shows a healthy artery. b) The smooth muscle thickens in hypertensive individuals, which leads to a reduced diameter (lumen) of the artery.
Recent studies have shown that very high salt intake can double stomach cancer risk. A Japanese study in 2004 investigated the relationship between increasing amounts of salt intake and stomach cancer in 40,000 middle aged subjects. The risk of stomach cancer in men that consumed low salt diets (3g) was 1 in a 1000. Meanwhile, men with very high salt intakes (10g) had twice the risk of stomach cancer: 1 in 500. It is clear that consuming too much salt makes us more likely to develop stomach cancer, but the biological mechanisms to explain why this happens are not clear. A bacterium known as Helicobacter pylori (H. Pylori) is the biggest risk factor for stomach cancer. Salt appears to increase the growth and action of this bacterium, which causes stomach ulcers and inflammation which could ultimately result in cancer. High dietary salt intake has also been shown to modify the mucous layer that protects the stomach. This appears to facilitate the exposure of carcinogens to the stomach layer, making it more likely to develop cancer.
Figure 3. Excessive salt consumption can damage the stomach directly by modifying the mucous membrane or cause indirect damage by potentiating the actions of H. pylori. These actions have been shown to increase the risk of stomach cancer.
Most studies suggest that eating above 6g of salt on a daily basis has many negative effects to our health. Hypertensive individuals and patients that have a family history of stomach cancer should be encouraged to reduce their dietary salt intake to less than 5g/day.
Cappuccio, F. P. (2013). Cardiovascular and other effects of salt consumption. Kidney International Supplements, 3(4), 312–315. http://doi.org/10.1038/kisup.2013.65.
Powles, J., Fahimi, S., Micha, R., Khatibzadeh, S., Shi, P., Ezzati, M., Engeli, R.E., Lim, S.S., Danaei, G. & Mozaffarian, D. (2013). Global, regional and national sodium intakes in 1990 and 2010: a systematic analysis of 24 h urinary sodium excretion and dietary surveys worldwide. British Medical Journal Open, 3:e003733 doi:10.1136.
Tsugane, S., Sasazuki, S., Kobayashi, M. & Sasaki, S. (2004). Salt and salted food intake and subsequent risk of gastric cancer among middle-aged Japanese men and women. British Journal of Cancer, 90, 128-134.
Wang, X.-Q., Terry, P. D. & Yan, H. (2009). Review of salt consumption and stomach cancer risk: Epidemiological and biological evidence. World Journal of Gastroenterology : WJG, 15(18), 2204–2213. http://doi.org/10.3748/wjg.15.2204.