12 January - 20 June 2016

A “healthy diet” may not offset the effects of a high salt diet

Scientists are reporting in the journal Hypertension that healthy eating may not offset the harmful effects of a high salt diet. A high salt diet is linked to high blood pressure but some studies however have suggested that body weight and other nutrients in the diet may modify or offset the effects of sodium on blood pressure.

Scientists are reporting in the journal Hypertension that healthy eating may not offset the harmful effects of a high salt diet.  A high salt diet is linked to high blood pressure but some studies however have suggested that body weight and other nutrients in the diet may modify or offset the effects of sodium on blood pressure.  In the UK high blood pressure affects more than one in four adults.  It increases the risk of a number of conditions including heart attacks and stroke.  The recommended upper limit of adult salt intake in the UK is 6g a day. 

This current study by a team of scientists from universities in the US, UK, China and Japan including Imperial College London and Northwestern University, analysed data from the INTERMAP study, an international study of 4680 people aged 40 to 59 years of age, carried out from 1996 to 1999.   During four clinical visits the participants were asked a number of questions about their daily alcohol consumption, educational level, and use of dietary supplement, amongst others. Two 24-hour urine samples and four 24-hour dietary recalls were collected from each participant. The urine samples were analysed for sodium and potassium and this was used to measure how much salt was being consumed in the diet.  Measurements of height, weight, and blood pressure were also taken. Chan et al. used the dietary data to assess the volunteers' intake of over 80 nutrients that may be linked to low blood pressure, including vitamin C, fibre, and omega-3 fatty acids.

The study found that on average the participants were consuming around 10.7g of salt per day, with those in the UK consuming around 8.5g, while those in the USA, China and Japan consuming 9.6g, 13.4g and 11.7g respectively.  Chan et al found that an increase of an additional 7g (1.2 teaspoons) of salt above the average intake was associated with an increase in systolic blood pressure of 3.7 mmHg. They report that those who had higher levels of sodium in their urine also had high blood pressure, even when they took into account the influence of 12 nutrients (including saturated fat and unsaturated fat, sugar, starch, dietary fibre and protein), 12 vitamins and 7 minerals.  They note that apart from potassium, which weakens the link between dietary salt and elevated blood pressure but only for people who had low sodium levels in their urine, none of the nutrients appeared to influence the link between eating a high-sodium diet and having higher average blood pressure readings.  The connection between sodium and blood pressure was similarly strong for both normal weight and obese people in the study, however the connection was weaker for overweight individuals who weren’t obese.

In discussion, Chan et al. report the limitations of the study include that the data was collected over four days, and only provide a “snapshot of time”, noting that longer term studies are needed.  NHS Choices also note that as the INTERMAP study collected data from 1996 to 1999, over 20 years ago, public health campaign warning about the effects of a high salt diet may have changed consumer’s consumption, for better or worse, in the meantime. Chan et al conclude by stating that “to prevent and control the ongoing epidemic of prehypertension and hypertension major reductions are needed in the salt content of the food supply.”  

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