One of the biggest debates in nutrition science is the seemingly simple question – “How much salt is safe in your diet?” We've come to believe that too much is bad but now it seems that too little may be just as bad.
Scientific dogma is always tested by erudite investigators but a result contrary to popular opinion and guidelines inevitably raises passionate debate, and in the process, more confusion for the general public.
Various guidelines from reputed scientific and medical organisations have set a safe limit of 1.5 grams to 2.4 grams of sodium per day. One teaspoon of salt contains 2.4 grams of sodium or 6 grams of salt. But most studies have shown that the majority of people consume 3-6 grams of sodium per day, which works out to 7.5-15 grams of salt. A high-salt diet is associated with high blood pressure, an important contributor to the burden of deaths from cardiovascular disease. Therefore, there is a public health campaign to reduce salt consumption at the population level.
The first major challenge to this dogma was raised in 2014 by a group of investigators led by Professor Salim Yusuf, a highly cited cardiologist based at the McMaster University in Hamilton, Canada. Yusuf is part of a research team conducting investigations in epidemiology across 17 countries in an ambitious study called Prospective Urban Rural Epidemiology.
Yusuf and his collaborators recruited 101,000 individuals from these 17 countries, including India, to provide fasting urine samples from which they assessed urine sodium excretion, which is a correlate of dietary sodium intake. The study, published in the New England Journal of Medicine, followed participants over a period of four years during which time 3,300 deaths or cardiovascular events such as heart attacks and strokes occurred.
The investigators found that persons who had high and low levels of urine sodium had a greater risk for dying or having cardiovascular events as compared to those persons with moderate urinary sodium excretion.
In a recently published study in the medical journal Lancet, the investigators further studied the effect of sodium on death or cardiovascular events, based on whether persons had high blood pressure or not. Among the 1,33,000 individuals from 49 countries assessed, 9,800 events occurred during four years of follow-up.
In persons with high blood pressure, both high and low sodium excretion in urine was associated with an increased risk for death or cardiovascular events compared to moderate levels of urine sodium. Surprisingly, in persons with normal blood pressure, a low urine sodium level was associated with an increased risk for death or cardiovascular events, whereas high sodium level was not associated with an elevated risk.
In this study, one in four individuals with high blood pressure was consuming a high sodium diet. The study results suggested that a low-salt diet intervention should be targeted to only the group comprising persons with high blood pressure consuming a high-salt diet, which formed only 10% of the entire study participants.
However, the negative effect of a low-salt diet noted in the study raises particular concern, especially in the light of national campaigns on dietary salt reduction. If the study findings are correct, such a strategy may prove to be counter-productive.
The strength of the Lancet study lies in the large number of participants recruited in a prospective manner, and the rigorous methodology followed by the investigators. One of the major limitations of the study is the fact that the use of salt in diet is assessed using a single criterion – the fasting urine sodium level.
Although, the investigators have validated such a test result against the more stringent 24-hour urine collection results in a previously published study, scientists from the United States Centres of Disease Control and Prevention have questioned the validity of using a single urine sample to measure sodium consumption.
On June 1, 2016, the Food and Drug Administration, issued a draft guidance statement to the food industry to reduce salt in processed and commercially prepared food.
The Lancet study result appears to throw the scientific dogma that a “low-salt diet is good for all persons” under the bus. National strategies for salt reduction may not be appropriate for all persons, and the latest research from Yusuf’s group suggests that such an intervention ought to be targeted to high-risk individuals. All of this inevitably leads to confusion for the general public.
What is the message for you?
If you have high blood pressure, too much salt is not good for health. It also raises the intriguing possibility that a very low-salt diet for a person with high blood pressure could be bad for health. Avoiding added salt and processed food makes better sense for a person with high blood pressure. For an individual without high blood pressure, moderate salt use is probably safe – for now.
Aju Mathew, MD, MPhil (Cambridge), American Board (internal medicine, medical oncology and hematology) is an assistant professor in medicine and oncology at the University of Kentucky Markey Cancer Centre.