Salt cuts have 'saved lives,' says study
"Cutting back on salt does save lives," is the good news on the front page of the Daily Mail. The headline is based on a study of data obtained from the Health Survey for England, the National Diet and Nutrition Survey, and the Office for National Statistics between 2003 and 2011.
The researchers chose 2003 as the start date because this is when the Department of Health launched its salt reduction programme. This consisted of a range of measures, of which possibly the most influential was persuading food manufacturers to reduce the amount of salt put into processed foods.
Salt intake can increase blood pressure, and high blood pressure is linked to an increased risk of stroke and heart attacks.
The study looked at changes in average salt intake, blood pressure, and deaths from stroke and heart disease during this time. During this period, average blood pressure and salt intake fell significantly, and there was a reduction in the number of deaths from stroke and heart disease.
When the researchers only looked at people who were not taking blood pressure medications or other drugs that might affect blood pressure, there was still a significant decrease in blood pressure between 2003 and 2011, even after adjusting for some relevant confounders.
The researchers say it is likely that this decrease in blood pressure was the result of the reduction in salt intake during this period. However, although this is plausible, the study cannot prove this.
The reduction in blood pressure could be the result of other health and lifestyle changes that were not accounted for. There is also the possibility that improvements in medical care and treatments were also partially responsible for the reduced number of deaths.
Nevertheless, the study lends support to current health recommendations that we keep salt intake to no more than 6g per day for adults (around one teaspoon) to reduce the risk of high blood pressure.
Where did the story come from?
The study was carried out by researchers from the Wolfson Institute of Preventive Medicine, part of the Barts and The London School of Medicine and Dentistry, Queen Mary University of London. No funding was received specifically for this research.
It was published in the peer-reviewed BMJ Open, which is an open access journal. The article can be accessed free on the journal's website.
This research was covered well by the UK media, particularly by The Guardian, which included quotes from other experts outlining some of the inherent limitations of the study.
What kind of research was this?
This was a serial cross-sectional study. The study looked at three separate data sets:
- salt intake in a random sample of the English population
- blood pressure in another sample of the population
- deaths from stroke and heart disease at different time points to see whether these changed over time
The researchers tried to link changes in salt consumption with changes in blood pressure and deaths from stroke and heart disease. However, salt intake and blood pressure were measured in different people, and different people were surveyed at the different time points.
This type of study cannot show that changes in salt consumption directly caused changes in blood pressure and death. The changes seen are also likely to have been influenced by other various health and lifestyle factors.
What did the research involve?
The researchers analysed information on blood pressure and other cardiovascular disease risk factors from people aged 16 and over who had taken part in the Health Survey for England in 2003, 2006, 2008 and 2011. The Health Survey for England is an annual survey of a random sample of the English population.
During this survey, interviewers collected information on demographics (age, sex, ethnic group, education level and household income), smoking status, consumption of alcohol, and fruit and vegetable intake, and trained nurses measured participants' body weight, height and blood pressure.
There was information for 9,183 people in 2003, 8,762 people in 2006, 8,974 in 2008, and 4,753 people in 2011.
Salt intake was analysed in a separate random sample of the population aged between 19 and 64 in the National Diet and Nutrition Survey. This was measured by 24-hour urinary sodium excretion (how much salt was passed out during a day) and verified for accuracy using laboratory methods.
Information on salt intake was available for 1,147 people in 2000-01, 350 in 2005-06, 692 in 2008, and 547 in 2011.
Information on the number of deaths from heart disease and stroke was obtained from the Office for National Statistics using the cause of death on death certificates.
The researchers looked at how changes in salt intake had influenced changes in blood pressure over the decade. To do this, they compared blood pressure in 2011 with blood pressure in 2003 in people who were not taking blood pressure medications or other drugs that might affect blood pressure.
They assumed that the change in salt intake was responsible for the change in blood pressure seen after they adjusted for the following confounders:
- age
- sex
- ethnic group
- education level
- household income
- alcohol consumption
- fruit and vegetable intake
- body mass index (BMI)
They also looked how these changes were linked to the number of deaths from heart disease and stroke.
What were the basic results?
From 2003 to 2011:
- average blood pressure fell significantly – systolic (the upper blood pressure figure, showing the artery pressure when the heart contracts) fell by 3.0mmHg, and diastolic (the lower blood pressure figure, showing the artery pressure when the heart relaxes and fills with blood) fell by 1.4mmHg
- there were also significant reductions in total cholesterol and the number of people who smoked, and significant increases in fruit and vegetable consumption – but there were also increases in BMI and decreases in HLD ("good") cholesterol
- average salt intake also fell significantly, by 1.4g per day
- there was a 42% reduction in the number of deaths from stroke and a 40% reduction in the number of deaths from heart disease
The researchers suggest that the decrease in the number of deaths from stroke and heart disease could be influenced by several factors, including the decreases in blood pressure, total cholesterol, the number of people who smoke, and salt intake, as well as the increase in fruit and vegetable consumption. This could also be influenced by improvements in the medical treatment of blood pressure, cholesterol and cardiovascular disease.
The researchers then focused on people who were not on blood pressure medications or other drugs that might affect blood pressure. After adjusting for the confounders described above, there was still a significant decrease in blood pressure between 2003 and 2011 (systolic fell by 2.7mmHg and diastolic fell by 0.23mmHg). They say that it is likely that this decrease in blood pressure was a result of the reduction in salt intake that occurred during this period.
How did the researchers interpret the results?
The researchers concluded that, "The reduction in salt intake is likely to be an important contributor to the falls [in blood pressure] from 2003 to 2011 in England. As a result, it would have contributed substantially to the decreases in stroke and [heart disease] mortality."
Conclusion
This UK study used serial cross-sectional data collected as part of the Health Survey for England, the National Diet and Nutrition Survey, and the Office for National Statistics between 2003 and 2011. It found that average blood pressure and salt intake fell significantly, and there was a reduction in the number of deaths from stroke and heart disease.
The researchers only looked at people who were not taking blood pressure medications or other drugs that might affect blood pressure. After adjusting for some relevant confounders, there was still a significant decrease in blood pressure between 2003 and 2011 (systolic fell by 2.7mmHg and diastolic fell by 0.23mmHg). They say that it is likely that this decrease in blood pressure was thanks to the reduction in salt intake that occurred during this period.
However, although the changes in salt intake could have had an effect, this study cannot prove that this is the case. Salt intake and blood pressure were measured in different people, and in different people at the different time points.
There also could have been other factors that are responsible for the changes seen, such as differences in the people measured or other differences that occurred that were not observed by the researchers.
During this period, it is reported that the number of people who smoked fell, but this wasn't adjusted for in the analysis. The researchers didn't take into account other possible factors that could explain the changes seen, such as changes in physical activity, as no information was collected on this.
Overall, the changes could be the result of a complex mixture of various health and lifestyle changes in people over this time that the study has not been able to fully account for.
As the researchers acknowledge, it is also possible that the reduction in deaths from stroke and heart disease could be related to gradual improvements in medical care and treatments over the last decade. This may have had a greater influence than changes in salt intake, and – from this – changes in blood pressure.
Nevertheless, the study lends support to current health recommendations to keep salt intake to no more than 6g per day for adults (around one teaspoon) to reduce the risk of high blood pressure. This in turn may reduce the risk of other cardiovascular diseases, such as stroke and heart disease.
