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Why salt makes us hungry

As far as nutrition goes, salt does not fare very well. Consuming excess amounts of salt is associated first and foremost with the development of arterial hypertension and cardiovascular diseases. In spite of these fears, the daily salt intake of 5 grams recommended by WHO is exceeded by many people. Two simulated space missions (2009 and 2011) provided a surprising insight into how the body deals with high doses of salt.

 As part of the preparation for a mission to Mars, study participants spent 105 and 520 days respectively in a hermetically sealed habitat. The researchers under Dr Jens Titze from Vanderbilt University in Nashville used these strictly controlled conditions to investigate the impact of differing salt amounts on the body. They varied the daily salt intake between six, nine and twelve grams in 29-day cycles and examined the amount of water consumed as well as blood and urine samples.

Contrary to expectations, they found that participants on the salty diet drank less water and were reported to have a greater appetite, although they were given a constant amount of calories during the clinical trial.  

Urine tests showed that the participants released more of the stress hormone cortisol which, among other things, interferes with the energy balance and promotes the metabolic processes of catabolism. They concluded that participants on a salty diet began to break down body tissue in order to provide water.

Subsequent experiments carried out on mice confirmed the observations previously made. Where animals were fed a salty diet, they drank less and consumed about 25% more calories at the same time. Catabolism was noted in which the body increasingly broke down fat and muscles. In the same way as camels can survive for long periods without consuming water owing to their fat reserves, fat was broken down by the mice to provide water and energy. At the same time, their liver produced more urea. This is normally the by-product of proteins and is discarded from the body in the form or urine. On the other hand, it enables a greater concentration of urine to be collected in the kidneys and prevents excessive loss of water. The increased breakdown of proteins and the higher amount of urea thus counteract the suction effect of the high salt concentrations found in urine, meaning that salt was able to be discarded from the body without larger amounts of water loss. 

Nevertheless, this process consumes a large amount of energy and is only possible if the body uses up its own reserves or more food is eaten. This fact explains why participants had a greater appetite and why the mice consumed more food. 

 Even though a higher intake of salt causes the body to use up more energy, researchers warn against using this method in order to lose weight. As far as they are concerned, a greater appetite increases the danger that the number of calories consumed exceeds utilised energy. This is backed up by the fact that participants found items of food with a high salt content tastier, so that the person will most likely put on weight. 

The impact of a permanently increased glucocorticoid level should not be forgotten. It is seen to be a risk factor for arterial hypertension, type 2 diabetes, a distorted distribution of body fat and osteoporosis. 

Even though a lot of questions in this field remain unanswered and further research is required, current consensus is that a moderate intake of salt constitutes a healthy diet and is a way to keep the body in good health in the long term. 

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