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Salt and the Kidneys

Kidney Disease 

Your body removes excess water from your body by filtering your blood through your kidney. This requires a balance of sodium and potassium in the body to pull the water across the wall from the bloodstream into a collecting channel in the kidney. A high salt diet will alter this sodium balance, causing the kidneys to have reduced function and remove less water resulting in higher blood pressure. This puts strain on the kidneys and can lead to kidney disease.

In addition, a high salt intake has been shown to increase the amount of protein in the urine which is a major risk factor for a decline in kidney function. There is also increasing evidence that a high salt intake may increase deterioration of kidney disease in people already suffering from kidney problems.  

Over 3 million people in the UK have chronic kidney disease (CKD), with 61,000 people being treated for kidney failure.  3% of the NHS budget is spent treating kidney failures and it is believed that over 3000 kidney replacements are carried out each year, with over 5000 patients on a waiting list. There are 40,000 - 45,000 premature deaths in the UK each year due to CKD.

Who is at risk?

People of Black African and South Asian descent are 3-5 times more likely to suffer from kidney failure (requiring dialysis) compared to white Caucasians. South Asian patients with diabetes are 10 times more likely to go on to have kidney failure. High blood pressure also puts the kidney under excess stress leading to deterioration of function.

How does salt contribute?

Salt intake increases the amount of urinary protein which is a major risk factor for developing kidney disease and cardiovascular disease.

Kidney (Renal) Stones

Kidney stones are relatively common. Over a lifetime, 6% of women and 11% of men will have kidney stones at least once. Although common, kidney stones are painful and can cause nausea, difficulty passing urine and may progress to kidney disease if there is a blockage.

As well as being a risk factor for kidney disease, a high salt diet has been associated with kidney stones. A high salt diet can increase the amount of calcium lost in urine, which can cause kidney stones. A number of studies have successfully shown that a reduction in salt consumption can reduce calcium excretion, and reduce reoccurrence of kidney stones. 

Losing too much calcium in the urine (known as Hypercalciuria) is present in 80% of kidney stone patients and it has also been found that individuals with raised blood pressure are more likely to develop kidney stones. A reduction in salt intake may therefore be of particular benefit to these people as it not only lowers blood pressure but can also reduce urinary calcium excretion. A diet designed to reduce hypertension (the DASH diet) has been found to be associated with a marked decreases in kidney stone risk. Please visit the Canadian Heart and Stroke Foundation's website for more information on the DASH diet  

Who is most at risk of kidney stones?

People with high blood pressure, persistent urinary infections and Crohns disease are at greater risk of kidney stones. Also, white British men between the ages of 30-60, and those with a family history of kidney stones are at greater risk.

Dietary Advice

People with or considered at risk of kidney disease or kidney failure should ensure that they keep their salt intake below the recommended maximum of 6g. This can be achieved by simple changes, such as consuming less processed foods and checking product labels before purchase.

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