A flurry of recent headlines claimed that following a very low-salt diet could increase the risk of a heart attack or stroke. Don’t start freely shaking on the salt, however – the results are not what they seem.
The study, published in The Lancet, did not look at salt intakes, but assessed sodium excretion in over 133,000 people of whom around 63,500 had hypertension.
Salt and blood pressure
Around one in two people inherit genes which mean their kidneys are less efficient at excreting excess salt. In this widely reported study, however, involving people with hypertension, those with increased sodium excretion in the urine had a greater increase in systolic blood pressure. The link was equivalent to around a 2mmHg rise per additional gram of sodium excreted. For those without hypertension, each additional gram of sodium also increased systolic blood pressure, but by a lesser amount of around 1·22 mmHg.
In people with hypertension, the risk of experiencing a heart attack or stroke over the 4.2 years of follow-up also increased with sodium excretion, so there was a 23% increased risk where sodium excretion was 7 grams per day or greater. What caught the headlines, however, was that the researchers found an 11% increase in risk of cardiovascular disease when sodium excretion was less than 3g per day – whether or not hypertension was present.
The important thing to note is that the researchers did not measure dietary salt intake, but used the sodium concentration of urine to approximate dietary salt intakes. Although this method has been validated in Japanese populations, it may not be applicable to people following a more Western style of diet. Why? Because Western diets tend to contain significantly more carbohydrate and sugars which increase circulating levels of insulin.
Insulin has a little-known effect on the kidneys which causes sodium to be retained so, as your carbohydrate intake goes up, the amount of sodium you excrete in your urine goes down. This effect also helps to explain a recent study suggesting that eating too many potatoes increases blood pressure risk.
In the current study, a low sodium urinary excretion might therefore reflect a high sugar diet rather than just a low dietary sodium intake.
More importantly, a low sodium excretion might also reflect worsening kidney disease in people with hypertension, for whom chronic kidney disease is a relatively common complication. In fact, chronic kidney disease is common whether or not you have hypertension, with an estimated one in ten people affected in the United States alone. Reduced kidney funciton is especially likely in those who also follow a high sugar diet and who have metabolic syndrome, type 2 diabetes or who are obese.
Whatever the explanation for the associations the researchers found, please ignore the headlines. When you have hypertension, it is important to continue to follow a low salt regime, and to eat more of the delicious foods that feature on the blood pressure friendly DASH Diet.
If your blood pressure is raised, self-monitoring is key to maintaining good control.
Click here for advice on choosing a blood pressure monitor to use at home.
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