Unravelling the link between salt, hypertension and chronic kidney disease

13 Mar 2024

Hypertension is a leading cause of kidney disease, and South Africa has one of the highest rates in sub-Saharan Africa at 35%, meaning that more than one in three South Africans are hypertensive.

Chronic kidney disease (CKD) is currently the 10th leading cause of death globally, with an estimated 850 million people affected, or one in 10 persons having some form of kidney disease, according to the World Health Organization (WHO). There are many causes of CKD, but one of the most common is hypertension (high blood pressure).

World Kidney Day is observed annually on the second Thursday in March (on 14 March in 2024) to raise awareness about the importance of our kidneys to our overall health. The theme for this year is “Kidney Health for All – Advancing equitable access to care and optimal medication practice”.

It is estimated that the prevalence of hypertension in Africa is 31%, with South Africa having one of the highest rates in sub-Saharan Africa at 35%, meaning that more than one in three South Africans are hypertensive.

The main factors that may be driving hypertension are increasing urbanisation, sedentary lifestyles with a lack of exercise, high-salt diets mainly from eating highly processed foods, obesity, smoking and excessive alcohol consumption.

In 95% of hypertensive patients, no identifiable cause can be found. When hypertension has an identifiable cause it is frequently due to underlying kidney disease or related to conditions that affect the blood vessels of the kidneys. Poorly treated hypertension may result in progressive kidney damage which in turn may worsen blood pressure control, and a vicious cycle ensues. Hence the saying “hypertension begets hypertension”.

According to a study published in 2017, only half of patients who were hypertensive had a blood pressure measured in their lifetime, and only 28% of these patients received a diagnosis of hypertension, with only one out of five of those diagnosed receiving treatment. Of even greater concern was that only 9% had controlled blood pressures.

The WHO points out that hypertension is a major cause of premature death worldwide. Uncontrolled hypertension is associated with the development of devastating cardiovascular diseases including stroke, heart failure and heart attacks, peripheral vascular disease (when narrowed blood vessels reduce blood flow to the limbs) requiring lower limb amputation and CKD.

A low-salt diet, weight loss and exercise, and stopping smoking and alcohol consumption… can be as good as drug treatment for hypertension.

According to the 2021 South African Renal Registry report, nearly 40% of patients receiving kidney replacement therapy (dialysis) had kidney failure due to hypertension. It is estimated that every 20/10 mmHg (millimetres of mercury) rise in blood pressure is associated with a doubling of cardiovascular disease-related death.

A common myth is that hypertension causes headaches. It may be associated with non-specific symptoms or, in the vast majority, no symptoms and therefore has been labelled the “silent killer”. Therefore screening is recommended, particularly for at-risk groups including the elderly, males, people with diabetes and a family history of hypertension, and overweight and obese individuals.

A diagnosis of hypertension requires at least three blood pressure measurements taken by a doctor which are at or above a systolic blood pressure (pressure when the heart pumps blood) of 140 mmHg and/or a diastolic blood pressure (pressure when the heart rests between beats) of 90 mmHg.

As the dictum goes, “prevention is better than cure”. Once a diagnosis of hypertension has been made, treatment should begin to prevent the development of, or to control existing cardiovascular and kidney diseases.

This includes lifestyle changes and/or the prescription of medication. Regarding lifestyle modification, a low-salt diet, weight loss and exercise, and stopping smoking and alcohol consumption are recommended. Although it is not so easy to make these changes, they can be as good as drug treatment for hypertension.

The effect of salt intake on the development of cardiovascular diseases is often underappreciated, yet it has a profound impact. In 2012, excess deaths attributed to hypertension through excessive salt intake for conditions such as ischaemic heart disease (heart damage caused by poor blood flow to the muscle of the heart), strokes, hypertensive heart disease and hypertensive kidney disease were 25%, 18%, 15% and 7%, respectively.

Studies have shown that South Africans eat too much salt. For example, a study in 2016 reported an average salt intake of 7.2 grams per day despite the guidelines limiting salt intake to less than 6g per day. This is the equivalent of a level teaspoon of table salt.

High salt intake can affect the effectiveness of certain drugs used to treat hypertension, especially diuretics.

In June 2016, South Africa implemented legislation that limited the maximum amount of salt in processed foods in order to reduce salt intake to under 5g per day. A study in 2021 reported that salt intake in the country has decreased since its implementation.

An important staple food in the South African diet is bread, second only to maize. The average bread consumption per capita by South Africans is three slices per day (or 100g), which contains roughly 1g of salt. Before the legislation, three slices of bread contained an average of 1.2g of salt. It is estimated that this small reduction in the salt content of bread may prevent 6,400 cardiovascular deaths alone.

Reducing salt intake can lower blood pressure by up to 8mmHg. Other lifestyle measures may also have a profound impact on reducing blood pressure.

Moderate exercise of at least 150 minutes per week or 30 minutes for five days per week can lower blood pressure by up to 9mmHg, while every 10kg of weight loss is associated with up to 20mmHg reduction in blood pressure. It is important to note that the effect of each lifestyle change has a cumulative effect on lowering blood pressure.

Regarding anti-hypertensive drugs, it is important to bear in mind that high salt intake can affect the effectiveness of certain drugs used to treat hypertension, especially diuretics, frequently referred to as “water pills”, such as hydrochlorothiazide and furosemide.

Another key consideration is the use of over-the-counter drugs which may also raise blood pressure. Common culprits include non-steroidal anti-inflammatory drugs such as ibuprofen and diclofenac sodium, and flu remedies including nasal decongestants and corticosteroids.

The intertwined relationship between CKD, hypertension and salt intake underscores the critical importance of proactive healthcare measures. With CKD ranking as the 10th leading cause of death globally and hypertension affecting a significant portion of the population, it is imperative to address these issues head-on.

Many individuals remain undiagnosed or untreated for hypertension, leading to a cascade of adverse health outcomes. Uncontrolled hypertension not only exacerbates CKD, but also significantly increases the risk of cardiovascular diseases.

Read more in Daily Maverick: 90-60-50 – can South Africa reach its hypertension targets?

However, there is hope in prevention and management. By prioritising lifestyle changes such as adopting a low-salt diet, exercising regularly and maintaining a healthy weight, individuals can significantly reduce their blood pressure and mitigate the risks associated with hypertension.

South Africa’s initiative to legislate salt content in processed foods is a step in the right direction, showcasing the potential impact of policy interventions on public health.

Through continued education, advocacy and concerted efforts at both individual and societal levels, we can strive towards a healthier future for all.

Remember, every small change matters, and together we can make a profound difference in combating the silent threats of hypertension and CKD. Let’s prioritise our health, starting today. DM

Professor Yazied Chothia is an associate professor and a medical specialist in the Division of Nephrology in the Faculty of Medicine and Health Sciences at Stellenbosch University.

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