Excess salt intake is one of the great issues in preventative medicine. Last year I watched a bus go by, with an advert for reducing salt plastered all over the side. Some restaurants have taken salt cellars off their tables, to protect customers. Many foodstuffs now have their salt content clearly labelled, with high salt content given a red sticker.
Given all of this you would think, would you not, that the case for excess salt consumption causing cardiovascular disease had been made beyond even the slightest possibility of doubt. One of the arguments in support of the dangers of salt consumption (the one that I am looking at in this article), comes from the native peoples living in the Amazon
The Amazon is an extremely low salt environment, and the average salt consumption of those living there is at very low. Several studies have found that the tribes people living in the Amazon have very low blood pressure which does not increase with age. They also have very little in the way heart disease and strokes.
‘Primitive societies who ingest little or no salt have no hypertension1’
Proof, the anti-salt lobby cry, that it is excess salt intake that causes our blood pressure to rise dangerously.
Or is it? When presented with ‘proof’ like this I tend to look for contradiction, rather than confirmation. Are there, I wondered, other populations that fail to demonstrate a rise in blood pressure with age, that do not have a low salt consumption. My attention was drawn to nuns, living in Italy.
‘The powerful effect of psychosocial and acculturating influences on population blood pressure trends seems to be confirmed, through longitudinal observations, in the nuns in a secluded order. After initial observations had been made on culture, body form, blood pressure, diet, and other variables in 144 nuns and 138 lay women, included as a control group, a 30-year follow-up study was undertaken. Most striking were opposite trends noted between the two groups in blood pressure trend. During the follow-up period, blood pressure remained remarkably stable among the nuns. None showed an increase in diastolic blood pressure over 90 mm Hg.’
So, nuns do not develop high blood pressure as they age. What happened to the control women in this study?
‘By contrast, the control women showed the expected increase in blood pressure with age. This resulted in a gradually greater difference (delta>30/15 mm Hg) in systolic and diastolic blood pressure between the two groups, which was statistically significant.’
No difference in diet or salt consumption, yet one population developed the ‘normal’ Western rate of hypertension whilst the other did not. What did the authors of this thirty yearlong study think was the reason for this finding?
‘In conclusion, it seems reasonable to attribute much of the difference in blood pressure and cardiovascular events, to the different burden in psychosocial factor and to the preserved peaceful lifestyle of the nuns2.’
Now I do not know for sure if those living in the Brazilian rain forests have managed to preserve a traditional peaceful lifestyle – but it seems a reasonable assumption to make.
However, the main point I am trying to make here is that you do not need a low salt diet in order to prevent hypertension. You can find populations with a normal salt diet who do not develop hypertension either.
What factor, or factors, appears to link these two populations? The factor appears to be living a preserved peaceful lifestyle. This would suggest that stress is the cause of hypertension and cardiovascular disease, and not salt. Whilst association cannot prove causation, a lack of association disproves it.
1: Freis ED. The role of salt in hypertension. Blood Pressure 1992; 1: 196-200.
2: Timio M, et al: ‘Blood pressure in nuns in a secluded order: A 30-year follow-up.’ Miner Electrolyte Metab. 1999 Jan-Apr;25(1-2):73-9