In Depth

Should you cut down on salt?

Sodium is an essential mineral that serves a range of purposes in the body. These include control of blood pressure and volumen, regulation of body temperature, cell membrane maintenance, nerve impulse transmission and uptake of nutrients in the small intestine. We consume sodium as salt, or sodium chloride.

Increased salt intake is often associated with high blood pressure (hypertension) and cardiovascular disease (CVD) [1]. For this reason, the NHS recommends a salt intake of less than 6 g per day (equivalent to 2.4 g of sodium) [2]. The World Health Organization (WHO) recommends even less [3]. But is restricting dietary sodium really an effective strategy for reducing CVD and improving health?

Indeed, countries with some of the highest death rates from coronary heart disease (CHD), such as Kazakhsta, Uzbekista, Turkmenista, etc., rank highly for salt consumption. However, countries such as Japan and South Korea, who also consume over twice the NHS recommended salt intake, have amongst the lowest rates of CHD and hypertension in the world [4].

In the typical western diet, much of the dietary salt comes from highly processed foods such as crisps and low fat deli meats, whereas in East Asia, it is usually added to cooking or present in fermented foods such as soy sauce or kimchi. The importance of the source of dietary sodium is a point of controversy in the field.

A systematic review and meta-analysis found that a reduction in dietary sodium decreased systolic blood pressure by (an arguably modest) 3.47 mmHg. However, the authors also concluded that “There were insufficient randomised controlled trials to assess the effects of reduced sodium intake on mortality and morbidity. The associations in cohort studies between sodium intake and all cause mortality, incident fatal and non-fatal cardiovascular disease, and coronary heart disease were non-significant” [5].

Analysis of data from the PURE study, 102,000 people from 17 countries, found a sodium excretion (as a surrogate for intake) between 3 – 6 g (equivalent to 7.5 – 15 g of salt) per day to be associated with a lower risk of death and cardiovascular events [6]. In addition, sodium intakes below this range were associated with higher risk than sodium intakes exceeding it, as shown in the graph below.

Source: O’Donnell et al (2014) https://www.nejm.org/doi/full/10.1056/NEJMoa1311889#t=article

Historically, salt consumption was much higher than it is today, as before refrigeratio, salt was the main method of preserving food. Over the past five decades, sodium intake has remained fairly constant [7], yet rates of hypertension have increased regardless [8]. This indicates that other factors may be responsible. Sugar intake, for example, has increased significantly over this time [9].

 

Sodium physiology

To appreciate the link between sodium, hypertension and CVD, we must explore the effects that sodium has on physiology.

Sodium intake is regulated by our sense of taste, which makes us crave salt when levels are low, whilst preventing overconsumption. This is called a negative feedback system.

Because sodium is so important, the kidney reduces its excretion when intake is low. This is achieved via increased activity of the renin-angiotensin-aldosterone system (RAAS), resulting in retention of sodium and water, and a rise in blood pressure. Chronically high aldosterone levels are associated with a range of ill effects including inflammation, oxidative stress and even insulin resistance (as in type 2 diabetes) [10].

Blood pressure often falls modestly on a low-sodium diet. However, blood pressure is just one marker of cardiovascular health, and one that naturally fluctuates at that. Low-sodium diets have also been shown to increase the levels of adrenaline and triglyceride, and to reduce heart rate variability [11] [12]. These are all effects that stress the cardiovascular system and impact negatively on health.

In contrast, a high-sodium diet stimulates the secretion of natriuretic (sodium-excreting) peptides, which act to relax blood vessels and improve heart function. These natriuretic peptides are known to be beneficial for the cardiovascular system and are used in the treatment of heart failure [13].

All in all, increasing sodium intake reduces RAAS activity and enhances the action of natriuretic peptides, acting to reduce blood pressure and improve cardiovascular health. Other markers such as heart rate and heart rate variability are improved by high-sodium diets which may have beneficial effects on longevity.

 

Sodium-potassium balance is important

Sodium is not the only essential mineral. Other minerals such as potassium also impact on cardiovascular health.

Potassium’s effects on blood pressure oppose those of sodium, hence, for proper cardiovascular health, an adequate balance between sodium and potassium should be maintained [14].

When salt intake is low, the body acts to excrete potassium, whilst retaining sodium. This means that a low-sodium diet may actually be worsening the sodium-potassium balance.

Increasing potassium intake, in order to maintain a healthy sodium-potassium balance, may be a good strategy for improving cardiovascular health. The aforementioned PURE data analysis showed potassium intake of over 2 g per day to be associated with a lower risk of death and cardiovascular events [6].

Good sources of potassium include nuts, seeds and leafy greens such as spinach, swiss chard and pak choi.

 

More salt is needed in certain circumstances

There are certain situations where the body requires more salt than normal to replace what is being lost.

People who have manual jobs, or do a large amount of exercise will need considerably more salt in their diet to replace what is lost through sweat. Professional footballers have been recorded to lose upwards of 6 grams of salt (the NHS daily allowance) over a 90-minute game [15].

People on a low carb or ketogenic diet may also need to up their salt intake. Insulin acts on the kidneys to retain sodium, therefore when dietary carbohydrate is restricted and insulin levels fall, the kidneys excrete more sodium [16]. This is called natriuresis. People who are not getting enough salt in their diet can experience symptoms of hyponatraemia (low blood sodium levels), which include light-headedness, fatigue and muscle cramps. This is the mechanism responsible for the ‘keto flu’, which is sometimes experienced when adapting to a low carb diet. Hyponatraemia can be avoided by simply adding more salt to food, or drinking some broth.

There are also other situations in which sodium loss is increased. Caffeinated drinks such as coffee can increase excretion of sodium, and also calcium and magnesium, through the kidneys [17]. In addition, gastro-intestinal disorders such as inflammatory bowel disease (IBD) result in decreased absorption of sodium [18].

 

Should we fear salt?

Lower sodium intakes have been shown to be associated with a higher risk of death and cardiovascular events, such as heart attack and stroke, than higher sodium intakes.

Given that a low-sodium diet results in only modest reductions in blood pressure, along with the possibility of a number of detrimental changes, reducing sodium intake may not be the best possible intervention for maintaining cardiovascular health in the average person.

As our sense of taste is a natural regulator of salt intake, relying on this may be the most logical way to determine how much we should consume.

Strategies such as increasing potassium intake, or reducing sugar intake, are likely more beneficial to cardiovascular health than reducing sodium.

People should focus on exercising more and eating real food.  And what’s better to help people exercise more and eat real food than a natural mineral such as salt?  Salt provides flavor and thus allows people to eat those high potassium foods such as bitter greens, nuts, and seeds.  Salt also cuts bitterness and thus in a way provides sweetness allowing people to use less of the more harmful white crystal (sugar).  Salt should be considered the antidote to sugar as it seems to reduce sugar cravings and helps to fix insulin resistance.

 

 

Expert’s note

James J. DiNicolantonio, PharmD

Author of ‘The Salt Fix’

 

 

 

 

 

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