Salt and the high blood pressure hypothesis

By Andy Blow | 7 Minute Read

Salt has a serious PR problem. Consumption of Sodium Chloride (NaCl) has been linked - in both scientific papers and in the general press - with a rise in cases of hypertension (high blood pressure) in the population at large.

Because hypertension is a huge risk factor in all manner of nasty cardiovascular diseases, salt has, by association, taken a great deal of flak for its alleged contribution to this growing issue.

In direct response, government guidelines in most countries, including the UK and USA, currently recommend very low daily salt intake levels (typically between 2000mg and 2500mg of sodium; salt is ~40% sodium so this equates to roughly 6g per day).

The American Heart Association even goes so far as to suggest a target intake of under 1500mg of sodium a day for optimal health (3.75g salt per day).

When taken at face value, there’s an appealingly simple reason why this association between salt intake and hypertension has been drawn.

It boils down to the fact that as you consume more and more salt, your body tends to retain more and more fluid in the blood to maintain acceptable blood sodium concentrations. This has the effect of expanding total blood volume and therefore acutely increasing blood pressure - i.e. elevating BP in the short term.

(As an aside, this is precisely why we recommend using a saltier drink in the immediate build up to hot or long races so that you start with increased blood volume before a period of heavy sweating -  a time when having a bit more blood on board offers a performance advantage).

Whilst there’s no debating that this acute effect of salt consumption on blood pressure is real, the link between sodium intake and chronic (long-term) hypertension is potentially a lot more complex and has been the subject of growing debate in recent years. 

Many people with high blood pressure don’t respond well to a low sodium diet and yet more people who eat a high salt diet don’t end up with hypertension either.

At the more extreme end of this emerging debate there are researchers such as Dr James DiNicolantonio, author of ‘The Salt Fix’ and whose opinions are summarized nicely in this academic paper.

DiNicolantonio and a number of his peers are starting to promote the idea that many other factors (such as high sugar consumption) are potentially more influential in the development of long-term high blood pressure than sodium intake alone.

They claim that salt has been demonized for long enough without sufficient evidence to actually charge it as the sole cause of high blood pressure..

In his writing, Dr DiNicolantonio goes so far as to argue that, for the vast majority of the population who are not ‘salt-sensitive’, increased salt consumption (or at least unrestricted consumption on the basis of personal taste and preference) is likely to be vastly more appropriate than following standard low-sodium guidelines, which is unlikely to adversely affect blood pressure in most healthy people.

He believes that it may be more unhealthy to under rather than over-salt – a claim that provoked a strong backlash in the media when his book initially came out as it definitely flies in the face of conventional thinking.

Whilst it has to be said that a few of his theories are a little 'OTT' for my liking, DiNicolantonio on the whole paints a very convincing, largely evidence-based picture about low sodium guidelines being potentially misguided and in need of a significant overhaul.  

One standout idea that he promotes heavily is that rather than simply being linear, the relationship between sodium intake and health is far more likely to be ‘J’ Shaped.

 

Sodium and health graph

Image source (©)


What a ‘J’ shape curve demonstrates is that, as is the case for just about all essential nutrients in the body, there are both lower and upper limits for what is optimally healthy.

In other words, consuming too little sodium is as likely to result in health issues as consuming too much, and this includes the specific effects it has on blood pressure.

Data published in 2017 from a 16-year study of blood pressure in a group of 2600 Americans seems to agree with DiNicolantonio, as they showed an INCREASE in blood pressure in those who routinely consumed less than 2500mg of sodium per day when compared with their peers who ate more.

Whilst the exact mechanisms that might cause these counterintuitive findings are likely to be pretty complex and require further investigation, the overall message seems both clear and plausible: too little sodium in the diet is likely to be as damaging as too much.

As a result, the conversation on sodium and blood pressure needs to start to shift to looking at what is optimal, rather than simply striving to promote arbitrary, universal upper limits for consumption, which has been the trend to this point in time.

Sodium intake and athletes

If we can accept that too little sodium is as (or more) problematic as too much for health and blood pressure, the conversation around necessary salt intake for athletes starts to get quite interesting.

The ‘average’ (read ‘non-exercising’) person may only require ~2000mg or less of sodium per day to meet their basic physiological needs - the average Western diet allegedly contains about 3,400mg/day, so it’s easy to see why most folk are able to meet and exceed this without even thinking about it.

It’s also easy to see why general guidelines to reduce sodium intake might not be completely misguided for a large number of people (if not taken to extremes) who are probably routinely consuming quite a bit more than they need.

The average litre of human sweat contains ~1000mg sodium (range ~200mg/l – 2000mg/l) and sweat rates easily reach 1.5-2 litres per hour in trained athletes, so it’s very easy for some of us doing multiple hours of exercise a day, on multiple days of the week, to rack up some significant sodium (and fluid) losses compared with the rest of the population. These losses can far exceed the 2500mg or even 3400mg/day that are recommended or consumed in a ‘normal’ diet.

This is especially true for anyone with exceptionally high rates of fluid and sodium loss or anyone doing sports that demand hours and hours of practice or training on back-to-back days.

As someone with both salty sweat (I lose around 1800mg/l) and a high sweat rate (2l per hour in hot conditions), I can lose 6-8g of sodium in a hard 2 hour run.  In the past I learned the hard way that taking insufficient sodium in and around exercise left me performing poorly and feeling terrible.

It’s clear to me that trying to get by on 2000mg/day if I’m training hard would be all but impossible and certainly far from optimal for health or performance. 

An excellent paper looking at the role of sodium in the diet of athletes summarizes the topic neatly. It points out that there’s been a widespread acceptance of the fact that athletes who sweat a lot need more fluid than the average person, but there’s been much less emphasis placed on them potentially needing a lot more sodium too – probably because it contradicts the ‘perceived wisdom’ that sodium is bad for you, so somehow it feels like it should not be promoted.

Something that backs this idea up is a fascinating case study of an athlete in Germany who suffered over-training type symptoms (and increased blood pressure) when undertaking regular endurance training, whilst following a low sodium diet that she perceived to be ‘healthy’.

The athlete was able to reverse her condition simply by increasing dietary sodium intake and this was without the usual decrease in training load that is normally required to correct symptoms of over-training syndrome.

This is similar to a host of anecdotal reports we’ve had from athletes over the years who’ve attempted to follow ‘best practice’ by eating low sodium whilst simultaneously training hard and losing a lot of sodium in their sweat.

It’s also interesting to note that despite advocating for a blanket target of 1500mg of sodium per day for all, the America Heart Association feel the need to add a clumsily worded, slightly self-contradictory caveat to their own website.

As the passage below states, they single out athletes and anyone working in environments that lead to large sweat losses, saying that the 1500mg/day limit might not be as universally applicable as the rest of their information claims:

“Insufficient sodium intake isn’t a public health problem in the United States. The guideline to reduce to 1,500mg doesn’t apply to people who lose big amounts of sodium in sweat, like competitive athletes, and workers exposed to major heat stress, such as foundry workers and fire fighters, or to those directed otherwise by their healthcare provider. If you have medical conditions or other special dietary needs or restrictions, you should follow the advice of a qualified healthcare professional.”

 

Zero-sum gain

The bottom line on sodium intake for athletes (or anyone who sweats a lot during the course of their average day) is that it almost certainly should strive to be roughly in-line with individual sodium losses as a minimum, rather than kept under some arbitrary limit that in no way takes into account the vast individual variance that can occur in this dimension of physiology.

To use an analogy from the field of Game Theory, sodium consumption should ideally be a zero-sum gain. Intake should essentially at least cancel out output and therefore, in theory, the effect on overall sodium balance (and as a result blood pressure and other homeostatic measurements) should be net zero.

If Dr DiNicolantonio is to be believed then even taking in a little bit more sodium than you need ought to be fine because the body can deal with a degree of excess relatively easily (assuming the kidneys are functioning well) and this avoids any risk of incurring a deficit over time.

In summary, it’s very important to point out that I am neither a medical doctor nor should anything written in this article be interpreted as prescriptive advice on what to do if you’re an athlete concerned about the effect of sodium consumption on blood pressure.

There’s evidence that some people are more ‘salt sensitive’ than others and any significant changes to dietary intake should always be discussed with your doctor if you’ve been diagnosed with hypertension or a related medical condition.

There are also complex interactions between sodium and fluid balance affected by blood pressure medications to take into account which have not been covered here.

All that said, the main hope is that the information in this blog might be useful in stimulating some thought by providing background information on what is a complex and controversial topic in health and performance.

It's a debating point that is currently dominated by rhetoric which basically only promotes low sodium consumption as being optimal for all.

 

Suggested further reading

The importance of salt in the athlete’s diet:  https://journals.lww.com/acsm-csmr/Fulltext/2007/08000/The_Importance_of_Salt_in_the_Athlete_s_Diet.9.aspx

Overtraining syndrome on low sodium diet:  https://www.ncbi.nlm.nih.gov/pubmed/23234732

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