Salt is not a modern commodity and can be traced back to many ancient civilisations. In Roman times workers were often paid in salt and the root of the word ‘salary’ comes from the Latin word ‘salarium’ meaning ‘payment in salt’. Salt is an excellent preservative and hence smoked fish, salted and cured meats and so on, all very high in salt, were some of the first means of preserving food.
However, while humans have consumed salt for a very long time, modern diets contain far more than ever before. The food industry ubiquitously adds salts to our food, sometimes obviously as in salted chips, but often we may not even realise the salt content in unexpected foods such as breakfast cereals, bread and even sweet biscuits. Government health recommendations in almost all civilised countries, including Australia, lobby for a reduction in our salt intake to improve our health.
But once you delve beneath the surface, this seemingly straightforward advice is clouded with controversy. One of the key studies to ignite the call for salt reduction was the Intersalt study. This analysed data from 52 centres across 32 countries and included over 10,000 individuals. The researchers reported a significant, positive relationship between 24-hour urinary sodium excretion and systolic blood pressure. However, the study was subsequently criticised on the grounds that the results from four of the groups studied skewed the overall result. These groups were primitive societies (for example the Yanomami Indians in the rain forests of Brazil) with extremely low salt intakes and very low blood pressures. Since they also had less obesity, less stress and far more natural, unprocessed diets than centres in Western countries, the critics argued that they could not possibly be included in the same comparison study. Indeed when these four centres were removed from the analysis, the correlation between salt and blood pressure disappeared.
There were other discrepancies in the data – one of the Chinese groups recorded the highest salt intake, yet has extremely low levels of hypertension, while one of the centres recording the lowest salt intake, African American men in Chicago, had an above average incidence of hypertension. It seemed that what initially looked like a strong, predictable result was not what it first seemed. The researchers did re-analyse their data in an attempt to address these criticisms and successfully showed that a higher sodium intake was related to a higher rise in blood pressure with age.
So what have other epidemiological studies found?
There is enough fuel to stoke the fire on both sides of the argument. One of the major critics of the low salt campaign is Dr Michael Alderman, Professor of Medicine and Population Health at Albert Einstein College of Medicine in New York. Alderman’s group has published several papers that have controversially argued that reducing salt intake actually increases the risk of suffering a heart attack. These studies have been pounced upon by leading medics around the world and heavily criticised for the methods used, erroneous interpretation of results, and so on.
Two of these studies were analysis of the first and second National Health and Nutrition Examination Survey (NHANES I and II) conducted in several thousand individuals in the US. In both sodium intake was estimated from a single 24- hour recall of foods eaten. This is a notoriously inaccurate method of determining the intake of a nutrient and this has been the major criticism. Nevertheless data from both surveys show an inverse association with cardiovascular disease mortality. In other words contrary to mainstream thinking, those with lower salt intakes had more cardiovascular disease.
One other study, the Scottish Heart Study likewise found an inverse association with all cause mortality, but only in men. In women, the more salt they ate the more heart disease they got. Similarly, three others groups have shown the expected rise in cardiovascular disease risk and/or all cause mortality as salt intake increases: a subset of those who were overweight in the NHANES I study, a study from Finland and one from Japan.
Two further similar studies sit on the fence having found no statistically significant relationships. Of course observational studies such as these cannot conclude cause and effect.
In other words, a low or high salt intake may simply be a marker for some other factor that affects cardiovascular disease risk, and these confounding factors may explain the discrepancies. Nevertheless we cannot simply dismiss the fact that there are conflicting results.
There are numerous intervention studies to show that reducing salt intake lowers blood pressure. The greatest reductions are seen in those who already have high blood pressure and the elderly. For those who are normotensive any reduction is far smaller and may be nonexistent. Indeed it seems that some of us are ‘salt-sensitive’ while others will find little effect on their blood pressure from manipulating salt in their diet. The salt-guideline critics argue that any advice to restrict salt intake should be directed at those groups likely to benefit rather than the blanket population-wide recommendations currently made. However further support for reducing salt comes from the Dietary Approaches to Stop Hypertension (DASH), a multicentre trial in the US. The DASH diet, rich in fruits, vegetables and low-fat dairy products, was first shown to improve blood pressure when compared to an average American diet. The investigators then repeated the study but additionally randomised participants to eat foods with high, moderate or low levels of sodium for 30 days. The result – lowering sodium intake lowered blood pressure and the lowest blood pressure was seen in those following the DASH diet and the lowest intake of sodium.
Since so many of the positive studies have been criticised for being too short, in 2005 the highly regarded Cochrane Collaboration published a review of trials lasting at least 4 weeks. The conclusions were that lowering salt intake lowered blood pressure in both hyper- and normotensives. Even so, the debate continues as others have shown that after six months or so the benefits to blood pressure are lost. What is not clear is whether this occurs as a result of physiological adaptations or as a result of increasing non-compliance with a low salt diet over time.
The bottom line is, as well we know from weight loss diets, it is incredibly difficult to change what we habitually eat over the long term. The focus of scientific arguments and in messages to the public has centred on the relationships between salt, blood pressure and cardiovascular disease. However, a high salt intake has several other detrimental effects in the body:
- The more salt you eat, the more calcium you excrete in your urine. This puts you at risk of both kidney stones and osteoporosis.
- Many Asian countries, including Japan, have a high incidence of stomach cancer and this is thought to relate to their high salt intake. (Soy, oyster and fish sauces are all extremely high in salt.)
- A high salt intake can exacerbate fluid retention since increased cellular sodium results in an increased uptake of fluid.
- Finally a high salt intake is thought to aggravate, although not cause, asthma.
So can a low salt intake be harmful?
The critics of the anti-salt message claim that it can be. When sodium intakes come down, what’s called the reninangiotensin system kicks in, in part to preserve sodium levels in the blood. High renin (and therefore angiotensin II) in the blood has been linked to cardiovascular disease. So the question has been raised that if we lower blood pressure through lowering our salt intake, are there other detrimental factors that outweigh this benefit? The anti-salt supporters say that increased renin is not a concern in the context of modest salt reduction recommendations and very low sodium intakes would almost never occur in people eating varied diets.
So how do we interpret this ongoing debate?
Well if we strip away the complicated scientific arguments and use our common sense to think instead about how humans would have eaten before modern industrialised life, this may provide answers. In the average Western diet, some 75 per cent of the salt we eat comes from processed, pre-packaged foods that we buy. None of these foods were available a few generations ago,
never mind back in the days of hunter gatherer man. Surely this alone tells the tale.
Unravelling and understanding the complex relations between diet and health is supremely difficult and no matter which dietary aspect we care to look at, there are almost always conflicting results, if only because as soon as you change one aspect of a diet, other changes are unavoidable.
However the one resounding message is that eating more fresh whole foods and fewer processed food is better for our health. The fact that this lowers salt intake may or may not explain part of the benefit. For example, compare two meals; a popular fast food outlet burger or a grilled fillet steak with steamed veggies. The former has an estimated 1,284mg of sodium, 13 times the 98mg found in the fresh food meal! While we do need some sodium in our diet, it is quite clear that modern diets provide way in excess of our requirements and I for one find it exceedingly hard to believe that encouraging people to eat less can do us anything but good.
So what are my recommendations?
If you know you have high blood pressure the overwhelming weight of evidence suggests you would do well to cut down on salt. In addition, all of us should eat fewer processed foods and eat more fresh whole foods. But if you enjoy a sprinkle of salt on your poached egg, then go ahead and enjoy it without guilt. This way you control how much salt is in your diet, and not the food manufacturers, and you will reap the rewards of all the many other benefits to be found in such a diet. And remember, maintaining a healthy weight, stopping smoking and taking up regular exercise remain the big three in the fight against chronic disease. Don’t let the arguments for and against salt divert you from the big picture.
Information by Dr. Joanna McMillan, article from OH! Magazine.