Salt and blood pressure in children

Children are eating so much salt that their health is at risk through raised blood pressure, reported The Guardian . This high blood pressure “increases the risk of heart disease and stroke in later life,” the newspaper explained.

The Daily Mail added “many children are thought to be regularly consuming 9-10g of salt a day, which is up to three times the recommended maximum”. The newspaper goes on to explain that for each 1 gram of salt eaten there is a small increase in blood pressure and this “puts youngsters at increased risk of hypertension in later life”.

This story is based on a cross-sectional study which has some limitations; this study design cannot prove causation, in other words, that the high blood pressure observed was caused by salt intake. Other studies, conducted in adults, strongly support a causal relationship between both salt and blood pressure, and blood pressure and cardiovascular disease. That this study in children demonstrates an association and suggests that more research is required in children. 

Where did the story come from?

Feng He and colleagues from St. George’s University of London conducted this research. It is unclear who funded this re-analysis of data, though the original survey, the National Diet and Nutrition Survey, from where these data come was funded by the UK government. The study was published in the Journal of Human Hypertension .

What kind of scientific study was this?

This is a cross-sectional survey. The researchers selected records from certain children (aged 4 to 18 years) who had previously had information collected as part of a larger survey – the National Diet and Nutrition Survey – in 1997. The original survey aimed to describe the dietary habits of young people in Great Britain. In this analysis, the researchers selected a nationally representative sample of over 2,000 children and looked to see who of these children had information available on their salt intake and blood pressure; 1,658 children were included using these criteria. The researchers estimated the children’s salt intake from seven-day food diaries that had been recorded (either by the child themselves or an adult if the child was young). Using these estimates of salt intake, the researchers assessed whether there was any association with blood pressure.

What were the results of the study?

The researchers found that there was an association between the level of salt intake and blood pressure. They found that this association remained even when they took into account other factors that may affect the relationship, including age, sex, body-mass index and levels of potassium in the diet. The researchers say that their study demonstrates that an increase in salt intake of 1g a day (less than a quarter of a teaspoon) was related to an increase of about 0.4mmHg in systolic blood pressure. These results are similar to the magnitude observed in other studies.

What interpretations did the researchers draw from these results? 

The researchers concluded that their results “provide further support for a reduction in salt intake in children and adolescents”. They say that their findings suggest a link that has relevance to public health and provide further support for a reduction in salt intake in childhood. The implications of such a reduction, they suggest, are a fall in blood pressure which, combined with other lifestyle changes may prevent the development of hypertension and therefore cardiovascular disease in the future.

What does the NHS Knowledge Service make of this study?

Though this is a well-conducted study, there are important limitations because of its design:

• Cross-sectional studies cannot establish causation, as the researchers themselves acknowledge: “no cause-effect relationship can be drawn from such a study”.
• The data that the researchers analysed were collected from children 10 years ago as part of a large, national survey. It is possible that children’s eating habits having changed during the last decade particularly considering the public and industry awareness of the potentially harmful effects of a high salt diet. More up-to-date research may give a different picture.
• Participants in the study kept a food diary (themselves or their carer if they were young) for seven days. From these diaries, researchers estimated the salt intake. It isn’t clear how they estimated salt intake or how accurate this was. However, the researchers state, “the 7-day record should be more accurate than one taken over 1-2 days”. 
• Salt intake may have been underestimated in the study, as the calculations did not quantify the amount of salt used in cooking.
• The researchers found that energy intake was linked to salt intake and blood pressure; this means that they could not be certain which, that is, salt intake or energy intake, was related to the raised blood pressure.

The interpretation that this study establishes that children are ‘ill’ because of salt intake is premature. Large studies and systematic reviews have demonstrated that adults benefit from reducing salt intake. It would not be surprising if this relationship between salt intake and blood pressure in children was confirmed in further studies. Any such studies should also look at whether regulation or parental education is the best way of reducing children’s salt intake.

Sir Muir Gray adds...

Salty snacks are often high in energy also, so parents should try to limit the amount that their children consume; the simplest way at home is only to have fruit available to snack.

All the evidence suggests it is sensible to have a light touch when adding salt to cooking. If tastiness is required try garlic or other spices instead and forget the salt when laying the table.