Friday February 22 2008
“Children who eat a lot of salt also consume more sugary drinks, increasing their risk of obesity”, The Daily Telegraph says today. BBC News also reports that British researchers claim to have found a link between a high salt intake and drinking large quantities of fizzy drinks. The researchers propose that reducing children’s salt intake by half (about three grams a day) would cut out two sugary drinks per week, a total of almost 250 calories.
As salt is known to increase thirst, and most children are more likely to reach for a can of sugary cola rather than a glass of water, it makes sense that there should be a link between the two. However, a healthy balanced diet with lower levels of salt, fat and sugar is the ideal, and should be the overall aim.
Where did the story come from?
This research was carried out by Feng J. He and colleagues from the Blood Pressure Unit, St George’s at the University of London. No sources of funding were reported. The study was published in the peer-reviewed medical journal: Hypertension.
What kind of scientific study was this?
The researchers report that sugary soft drinks have been linked to childhood obesity. The aim of this cross-sectional study was to investigate whether there is a link between the consumption of these drinks and salt intake. Previous research in adults has found that a reduction in salt intake reduces fluid intake, and therefore leads to a drop in soft drink consumption. Although the relationship is well documented in adults, it had not previously been examined in children.
The researchers used the 1997 National Diet and Nutrition Survey (NDNS) for young people in Great Britain. The survey aimed to investigate the dietary habits and nutritional status of young people in the UK aged between four and 18. The original survey was of 2,127 “nationally representative” individuals; of whom the authors included 1,688 for this analysis.
The participants completed a seven-day food and drink diary, which included the weight in grams of any food and drink they consumed. The amount of table salt consumed was not documented. For seven to 18 year olds, physical activity was also collected over the same seven-day period and defined as being of mild, moderate or physical intensity. The article does not report any other details related to the way the UK survey was conducted.
The researchers used statistical methods to examine whether there was any relationship between salt and fluid consumption (and sugary drinks in particular). They also took other factors into account (such as age, sex, body weight and physical activity) that could have had an impact on the results.
What were the results of the study?
The average age of the participants in the survey was 11. The researchers found that the average daily consumption of salt was 4.6 grams for four year olds, increasing to 6.8 grams for 18 year olds. Although daily fluid consumption varied highly across the age groups, about 31% of fluid intake in all age groups consisted of sugary soft drinks.
When the researchers compared daily fluid and salt consumption, they found that an increase in one was associated with an increase in the other. This relationship remained after factors that could have influenced the outcome were taken into account.
From this result, the researchers stated that reducing salt intake by one gram per day would result in a reduction in total fluid consumption of 100 grams per day. They then investigated whether the relationship would be the same if only sugary drinks were included, rather than total fluid. Once again, there was a statistically significant relationship between the two. Using this result, the authors predicted that reducing salt intake by one gram per day would result in a reduction in sugary soft drinks of 27 grams per day. The findings were similar when boys and girls were analysed separately.
What conclusions did the researchers draw from these results?
The authors conclude that “if salt intake in children in the UK was reduced by half (mean decrease: 3 grams per day), there would be an average reduction of [roughly] 2.3 sugar-sweetened soft drinks per week per child. A reduction in salt intake could, therefore, play a role in helping to reduce childhood obesity…This would have a beneficial effect on preventing cardiovascular disease independent of and additive to the effect of salt reduction on blood pressure.”
What does the NHS Knowledge Service make of this study?
Although it seems highly plausible that a diet high in salt will increase fluid consumption and that for children it is probable that this will include an increase in sugary soft drinks, care should be taken when drawing any implications from these findings.
- As this is a cross-sectional study it can only show a relationship between salt consumption, overall fluid intake or sugary drink consumption. It cannot prove that one is the cause of the other, or that by reducing children's salt intake they will drink fewer sugary drinks.
- As the participants reported their food and drink consumption themselves, they could have introduced inaccuracies (for example, some items may have not been recorded or weighed properly). There may also have been differences in the accuracy of recording between parents recording on behalf of their young children, and by older children who were able to record for themselves. It is also possible that people who overestimate salt consumption also overestimate their consumption of sugary drinks.
- It should also be noted that added table salt was not included in the food diary.
- Weight and height were not measured; therefore it cannot be assumed that those who had the higher salt diets were overweight. The study did not follow children over time to see whether they became obese. The news reports that cutting salt intake will fight obesity is only an assumption.
- The survey was carried out in the UK ten years ago and (particularly in view of the recent interest in healthy school meals for children) it is not clear if childhood diets are the same today.
- As the completed survey data was only available for 63% of those originally identified as being "nationally representative" the findings may not be completely representative of the whole group, as those who did not complete the survey may be different from those who did.
A healthy balanced diet with low levels of salt, sugar and saturated fat is the ideal, and should be the overall aim. The complete dietary pattern of children needs attention, rather than just focusing on modifying a single food ingredient in the hope that other healthy behaviours will follow.
Sir Muir Gray adds...
Simple messages keep reappearing: less salt, less sugar, more walking.