Dietary salt and health
Is lower better?
Health authorities urge us to reduce our salt intake and mostly we have accepted that we should. However, the science behind this recommendation has been shaky (sorry) from the start. Even the authorities are unsure how dangerous it is. In the US, the American Heart Association (AHA) sets the recommended daily intake (RDI) of sodium to 1,500mg (1.5g). The Centres for Disease Control and Prevention (CDC) sets it to 2,300mg (2.3g) for the general population but 1,500mg for people over 50 or with high blood pressure. Note that these figures are for total daily sodium intake from all sources, not just what we add to our food.
Is sodium different to salt?
Table salt is sodium chloride. Sodium makes up ~40% of salt (by weight) because the chloride atoms are heavier than the sodium atoms. This means that 2.3g of sodium (the CDC RDI) is ~6 g of table salt — about a teaspoon (a teaspoon = 5g of water). The AHA limit (1.5g sodium) is ~4g of salt.
Where is the salt in our diets?
About 75% of dietary sodium (in the US population) comes from processed food (which includes bread, cheese, ham etc), packaged food or eating out. Of the remaining 25%, about half comes from salt we add to our cooking and the other half is naturally-occurring sodium in fresh (unprocessed) foods. Bread consumption may be the biggest source of sodium in western diets (simply because bread consumption is significant). There are sources other than table salt, for example baking soda (sodium bicarbonate) in baked goods, sodium glutamate in processed food (eg. vegemite, fish sauce) and fresh food (eg. tomatoes, mushrooms), and curing salts (sodium nitrite/nitrate) used in preserved meats.
To reduce dietary sodium, a strategy might be to replace processed/packaged food with fresh food. No doubt sodium intake would be reduced, and health improved, but salt may have nothing to do with that. The health benefit came from replacing processed food with fresh food.
What would it mean to follow the RDI?
The United States Department of Agriculture (USDA) maintains a database of foods and ingredients. They attempt the herculean task of determining the composition of the foods that we eat in all their complexity. According to this database, a seemingly healthy sandwich wrap for lunch weighing just 270g (containing meat, poultry, or fish, vegetables and cheese) contains 1.1g of sodium. If following the AHA recommendation then all the food consumed for the rest of the day can contain no more than 0.4g of sodium. Two slices of toast at breakfast could do it. So much for the evening meal. Be careful of mineral water.
In reality, it is impossible to adhere to the RDIs for salt and live a normal life (or a liveable one). The RDIs are unrealistic and ill-advised. The dietary restriction they require ignores the wider health implications and risks of malnutrition and of electrolyte imbalance (eg. sodium/potassium). I suggest demanding evidence that the people setting these RDIs have adhered to them for decades and lived happy and healthy lives.
What is the science of salt and health?
There are two ways to address this in the human — use a biomarker of health or use actual health outcome. Biomarker studies are quick and relatively easy. Health-outcome ones are time consuming and expensive and the results come out much later. However, the health-outcome studies are what matter the most.
In the case of salt, the biomarker is blood pressure. These studies, mostly in laboratory animals, show that increasing salt intake is associated with increasing blood pressure. The studies are replicable and definitive. The RDI limits were set according to these studies.
However, these studies do not determine whether salt increases the risk of heart disease (i.e. health outcome). Their logic goes: salt causes an increase in blood pressure; high blood pressure is associated with a greater risk of heart disease; therefore salt increases the risk of heart disease.
This logic is flawed. It assumes that salt increases the risk of heart disease because both are associated with high blood pressure. But, heart disease may arise from other non-salt related causes that also happen to increase blood pressure (e.g. by atherosclerosis). So, the ‘health-outcome’ studies address the direct question: is salt associated with heart disease?
In 2013 the CDC commissioned the Institute of Medicine (IOM) to review all studies that used health outcome (not blood pressure) to determine whether there was a case for lowering their 2.3g RDI for sodium down to the AHA’s 1.5g level. The IOM first drew attention to the heterogeneity of the studies they reviewed, but decided that it was possible on balance to answer the question. The answer was no.
In 2014, a large study (17 countries and over 100,000 people), that also measured health outcome rather than biomarkers, concluded “sodium intake between 3g per day and 6g per day was associated with a lower risk of death and cardiovascular events than was either a higher or lower estimated level of intake” (my bold).
In other words, a sodium intake equal to or less than the CDC or the AHA levels might increase the risk of heart disease. Fortunately for our health, most of us don’t follow the rules. It is estimated that we currently consume about 3–4g of sodium daily — comfortably at the lower end of the 3–6g range. While low sodium intake is indeed associated with low blood pressure, this does not translate into lower risk of heart disease. Presumably, low-salt has other consequences that increase the risk.
If reading reports about salt and health, look for whether they are biomarker studies or health outcome studies. Pay attention to the health outcome studies.
Summary
Health authorities continue with their message — it is possible they are in too deep to back out now. However, their RDIs are unachievable and likely to be incompatible with good health — if they were to be followed the diet would almost certainly have adverse secondary effects. This is rarely mentioned or addressed. Salt added in home cooking or at the table has a minor part to play in total daily sodium consumption. In the context of heart disease, the current evidence indicates that our salt intake is fine as it is.
Addendum:
In October 2021, the US Food and Drug Administration (FDA) sought to encourage manufacturers to decrease sodium content for various processed and take-out foods (it would have made more sense to discourage the consumption of processed foods). As the FDA has no authority over processed food, the code is voluntary.
