Thursday March 12 2009
Salt deficient rats shy away from activities they normally enjoy
“A taste for salt can keep you feeling chipper,” reads the headline in the Daily Mail. The newspaper said that researchers suggest that salt may act as a “natural antidepressant”. It said that while too much salt “can lead to high blood pressure and heart disease, not enough could trigger 'psychological depressions'”. The researchers found that rats deprived of salt “began to behave erratically and shun foods and activities they normally enjoyed”.
The review behind this news story does not suggest that people should use salt as an antidepressant. Instead, it discusses some studies which suggest that a possible reason why we consume so much salt is because our body “rewards” us for this behaviour. The authors give evolutionary reasons why this might be the case, and explore the biological and behavioural ways that our bodies promote and maintain this high salt intake.
As the authors state, most people on a modern Western diet consume more salt than they need. Too much salt can be harmful in the long term, and people should try to consume less salt than the recommended levels. The Food Standards Agency recommends adults should have no more than 6g per day, and 2g per day for children.
Where did the story come from?
Professor Alan Kim Johnson and colleagues from the University of Iowa carried out this research. The study was funded by the National Heart, Lung and Blood Institute, National Institute of Diabetes and Digestive and Kidney Diseases, and the American Heart Association. The study was published in the peer-reviewed journal Physiology and Behavior.
What kind of scientific study was this?
This was a non-systematic literature review in which the authors discussed the psychological and biological mechanisms that result in animals and humans consuming excessive amounts of salt (sodium chloride).
The researchers put forward their theories about salt intake, and discuss how their own and other studies in humans and animals have informed these theories. The specific methods of these studies are not presented in detail.
What were the results of the study?
The authors say that our ancestors, the hominids, evolved in hot and dry conditions, and ate diets which mainly consisted of plant material that contained only low levels of sodium salts. To survive these conditions, their bodies evolved complex ways of maintaining sodium levels.
Studies in mammals have shown that a lack of sodium in the body triggers physiological changes to retain the body’s sodium levels, as well as behavioural changes that lead to a higher sodium consumption. Under such conditions, laboratory animals will even drink very salty solutions that they previously avoided, suggesting that the nervous system alters the perceived taste of these substances.
The authors say that people eating modern Western diets and laboratory animals eating standard animal food are likely to consume more sodium than they need. They also say that some mammals lacking in sodium will consume much more sodium than is needed to achieve normal levels. They suggest that such behaviour in mammals is “out of step” with their actual need for sodium, and could be detrimental as excess sodium intake over an extended period can lead to adverse health effects, such as high blood pressure and heart failure.
The authors discuss studies in humans and animals which have suggested that persistent unsatisfied salt cravings can induce behaviours similar to those seen in depression. The cravings also cause changes in the regions of the brain that are involved in motivation, reward, drug sensitisation and withdrawal. They say that this raises questions about these affects on behaviour.
Such questions include whether animals that have been deprived of sodium consume excess amounts in case of future deprivation; whether sodium deprivation alters the sense of “reward” the animal’s brain feels when consuming it; and whether mood is affected by a reduction of sodium intake in animals expecting high-sodium diets. The authors subsequently discuss experiments in animals that look at brain changes associated with sodium deprivation, and studies in humans and animals that suggest that sodium deficiency can reduce the effect of usually pleasant and rewarding stimuli, and negatively affect mood.
The researchers then discuss the following areas:
The importance of sodium in mammals’ normal physiological function
They describe a case report from 1940 of a boy whose hormonal problems meant his body was unable to retain salt. This caused him to crave and consume very high quantities of salt from a very early age. At the time, it was not possible to properly diagnose the boy’s condition. When he was hospitalised and subsequently deprived of this high-salt diet, he died. This shows that insufficient sodium intake or inability to retain sodium can be fatal. The researchers then describe studies that show other effects of low salt intake in rats, such as growth restriction.
Daily sodium requirement
The authors say that the minimum sodium requirement for human health is debatable, but it is clear that in developed countries the average daily intake of sodium “far exceeds what is needed for survival”. They report that the worldwide average salt intake is about 10g daily, whereas the US Food and Drug Administration’s recommended intake is only 4g a day.
The researchers then discuss the history of salt consumption in humans, and cultural differences in salt consumption. They say that New Guinea Highlanders have low daily salt intake (about 0.5g per day), and they have less cardiovascular disease than groups who consume the worldwide average per day. When salt is introduced as a food additive to people from this group, they initially find it unpleasant, but some authors have claimed that after repeated exposures they develop an “addiction”, similar to caffeine or nicotine addiction. Similar results are reported for chimpanzees.
Pathophysiology of excess salt intake
The researchers describe human studies looking at the effect of salt intake on blood pressure. These studies found that groups with low salt intakes had lower blood pressure than groups with higher salt intakes, and that reducing salt intake can reduce blood pressure in people with high blood pressure. Studies in animals showed similar results. The authors report that it is difficult to voluntarily reduce our salt intake because of the high levels of salt in processed foods; they say that 77% of our salt intake comes from processed and restaurant foods.
The authors discuss studies that look at the nervous system and hormonal mechanisms by which the body regulates appetite for sodium.
They also discussed the relationship between taste and sodium appetite. They say that salt receptors on the tongue pass messages to areas of the brain that play a role in mood, reward, motivation and addiction. The authors report that salt becomes more palatable when sodium is lacking, and that in cases of severe sodium deficiency, this helps the body to identify and consume sources of sodium.
There have been reports that people who have cravings for salty foods lose large amounts of sodium in their urine because of hormonal problems or because they take diuretic drugs. They also say that people with high blood pressure who have been on a low sodium diet for a long time find salty tastes more pleasant, and this may affect how well they stick to their prescribed diets. Similar increases in the acceptability of salty solutions in sodium-deficient rats are reported, as well as changes in nerve cells involved in taste perception and reward. They also discuss sensitisation to sodium, and the changes in the hormonal and nervous system that may be related to this.
Mood and pleasure-related effects of sodium deficiency
The authors report that changes in mood are one of the first signs of an inadequate diet, and they discuss findings regarding various vitamins. They suggest that the effects of chemicals such as sodium, potassium, calcium, magnesium and phosphate on mood have largely not been studied. The authors say that people who lose large quantities of sodium through sweating while they work in extremely hot environments often experience fatigue, headache, difficulties concentrating and sleeping. These symptoms are often associated with depression.
They discuss a study from 1936 looking at the effects of sodium deficiency created by eating a no-sodium diet and inducing sweating for seven days. After being subjected to this, participants reported a loss of appetite, an inability to feel pleasure, difficulty concentrating, and a feeling of exhaustion. The authors also report a study in 21 people with chronic fatigue syndrome (CFS) and also with low blood pressure when they stood up suddenly (a condition known as postural hypotension).
These people were given a drug with sodium-retaining properties, and encouraged not to limit their sodium intake (about two-thirds of the people had been intentionally limiting their salt intake). This treatment improved CFS symptoms and low blood pressure in 16 of the participants, as well as improving scores on wellbeing and mood. They say that the increase in sodium intake and retention “may have contributed to the mood improvements” but that this was only speculative.
The authors also report on experiments in rats, including some studies from their laboratory. They say that their studies showed that treating rats with a specific drug that normally makes them ingest more sodium and removing their access to salt solutions reduced their sensitivity to activities that were usually rewarding, such as drinking a sugar solution, while the drug alone had little effect on these behaviours.
Rats that had been given another drug that made them urinate more (therefore depleting sodium) but had no salt solution to replenish their sodium levels, experienced a similar effect. This effect could be reversed by providing a salt solution. Depriving rats of sodium also showed lower heart rate variability, which is another sign often observed in people with depression.
They discuss the possibility that changes in levels of hormones connected with maintaining sodium levels in the body may be related to mood. For example, people with depression have been shown to have increased levels of a hormone that causes the body to retain sodium, and people with a disease that leads to high levels of this hormone sometimes show symptoms of depression. They also discuss studies which found that one particular drug for treating high blood pressure may also have mood-enhancing properties, but that other high blood pressure drugs were not found to have this effect.
What interpretations did the researchers draw from these results?
The researchers conclude that evidence from animal studies suggests that sodium may be “similar to other natural reinforcers (e.g. sex, voluntary exercise, fats, carbohydrates, chocolate) in [that it has] addictive qualities”. They say that large fluctuations in sodium levels in the body may effect mood and promote excessive sodium intake. They say that understanding sodium’s effects on the nervous system and its associated behavioural changes “are likely to increase our understanding of topics as diverse as homeostatic regulation, addiction, affective disorders, sensitisation, and learning and memory."
What does the NHS Knowledge Service make of this study?
This review was not systematic, which means it may not have included all relevant studies. Therefore, some studies may exist that do not support the authors’ hypotheses. The review looked at the biological reasons why we might continue to eat more salt than our bodies require, which can be harmful in the long term.
It is important to note that none of the studies cited directly suggest that salt deprivation causes clinical depression, or that people with clinical depression could improve their symptoms by eating more salt.
The review does not suggest that salt is an antidepressant. High salt intake over an extended period can lead to high blood pressure and a greater risk of heart problems. Accordingly, people should continue to keep their salt intake below the recommended levels. As the review itself notes, most people who eat a modern Western diet consume more than the amount needed to avoid a sodium deficiency.