September 2, 2015

expert reaction to study reporting that a high salt intake could directly increase risk of obesity

A paper in the journal Hypertension reported that there could be a direct link between high salt intake and obesity risk.

Ms Catherine Collins, Principal Dietitian at St George’s Hospital NHS Trust, St George’s University Hospital Foundation Trust, said:

“Previous claims have linked salt intake with obesity, attributing the additional calorie intake to an increased consumption of sugar sweetened beverages to assuage the thirst induced with a high salt diet.

“This paper sought to define a potential link between salt and obesity, and examined salt intake in relation to BMI, and also the consumption of sugar-sweetened beverages (SSBs). The study derived salt intake data from both adults and children by using the NDNS survey methods (potentially unreliable) and a 24-hour urinary salt collection (highly reliable marker).

“Despite statistical modelling, we can’t ignore that adults with the highest salt intake and highest BMI had the highest calorie intake (at an average 2480kcal a day, 25% more than the average 2000 kcal daily consumed by those with the lowest salt intake and borderline healthy BMI).

“It was also unsurprising that consumption of SSBs appeared unrelated to overall diet or salt intake – increasing salt intake was associated with a higher calorie intake and BMI in both adults and children, however, in children there was a non-linear increase in SSB intake with increasing BMI and age, and in adults there was a reverse correlation between SSB intake and obesity, albeit a minute one. We can’t read too much into this as the absolute difference between groups was small, but we can say that in older adults in this study, SSBs don’t contribute significantly to obesity risk – rather overall calorie intake does (which includes the calorie contribution from any amount of SSB consumed). This has been shown in the NDNS rolling survey data over successive years that pre-empt this publication. As an aside, this review showed that those with the lowest salt intake and the lowest BMI were 40% more likely to be smokers, an unhealthy habit which increases resting metabolic rate and in some smokers is used as a substitute for controlling appetite.

“One area that needs to be addressed is the way in which the data on children, salt and BMI was interpreted. Using the BMI calculation to decide if a child is overweight or obese is not, by itself, a useful marker. From birth to 17 years of age, BMI is usually used alongside other predictors including the Child Growth Foundation standard growth charts to ensure a child is developing healthily.

“The data on children in this study appears to show that as salt intake increased, both BMI and waist circumference (an indicator of central body fat stores in adults) increased, too (table 1). The immediate ‘headline message’ suggests that a higher salt diet increases body fat stores and overall BMI in children – but this interpretation needs qualifying: a closer look (table 1) shows that in reality the lowest daily salt and calorie intake were in younger children (mean age 8½, range 4½ to 12½), with oldest children (mean age of 12½, range 9-16 years old) consuming the most salt and the highest calorie intake of the three groups.

“Parents of teenagers will already know just how much food a pre-teen/teenager can tuck away, and how difficult it can be to control food eaten away from the home. Parental control of food intake and amount consumed is far different when their child is at primary school compared with when they are at secondary school, and puberty-related growth spurts increase dietary and calorie intake, as was shown in this study. To suggest that salt, rather than food intake and age-related body changes were responsible for changing weight and shape, is in my opinion overly simplistic.

“Secondly, parents recognise that children experience growth spurts at different ages. Using growth charts commonly used to plot obesity risk in children, the youngest children had the smallest waist size by measurement (70.8cm), but for their age their average waist size categorises them as borderline obese (at 99.6th centile). By the time they reached the ‘highest salt intake/ oldest children’ group their waist size of 76.7cm – the highest of all three groups – placed them into the ‘overweight’ category (around 95th centile). Put simply, as children aged and as their salt intake increased, their waist sizes increased too – but it was in proportion to their predicted changes in height, and if an interpretation was needed, it suggested that older children were ‘remodelling’ their body shape as their height increased.

“What about the increase in children’s weight with increasing salt intake? Again, using Child Growth Foundation charts to plot their BMI at the average ages of each group, we find that the youngest children (lowest salt intake) and middle children group (middling salt intake) had BMIs within the acceptable healthy range for their age. Only in the older children with the highest salt intake did BMI increase to a mean BMI which in this age range would put the child into a borderline overweight category. Given what is known about the diet and lifestyle of high school children a higher calorie and salt intake would not be unremarkable.

“Bottom line? A high calorie diet predisposes to obesity if calories consumed exceed daily needs. Where those calories come from is immaterial. NDNS survey data confirms that our weight increases as we age but our intake of sugar reduces as a proportion of calories consumed. I’d interpret the findings to be that in this study a high calorie diet, most likely from savoury rather than sweet foods given the salt intake, predisposes to obesity. Obesity per se increases your risk of hypertension, which is made worse with higher salt food choices. It’s impossible to select out one aspect of a whole diet to critique. It appears that salt is a marker of calorie intake and obesity risk, not necessarily a cause of it.

“The role of SSB as a contributor is difficult to interpret, and is not a clear risk factor for obesity in this study. Taken simply, it appears that SSBs did not increase with increasing salt intake or body weight. This is likely too simplistic a statement, as those with the highest BMI may well have swapped to sugar-free beverages in an attempt to prevent further weight gain (‘reverse correlation’).”

Prof. Susan Jebb, Professor of Diet and Population Health, University of Oxford, said:

“This is a small cross-sectional study which observes that people who consume more salt are more likely to be overweight than those who eat less salt. This would not be surprising. In general a higher salt intake often occurs with higher energy intake and so would be expected to be associated with a higher risk of obesity. Here the authors claim the effect is independent of energy intake, but since energy intake was self-reported there is no way to be confident in this assertion. They have attempted to statistically adjust for mis-reporting of intake, but simply classifying people as plausible or mis-reporters is completely inadequate to examine detailed associations between energy intake and weight.

“Cross-sectional studies can only show associations, they do not demonstrate causality and the authors acknowledge the mechanism of any putative association is unclear. Salt reduction is important to reduce cardiovascular risk but the combination of a weak study design and lack of any strong mechanistic basis for the association between salt and fatness means that this study should not detract from the main cause of weight gain which is consuming too many calories. I would not want to see the public misled by the publicity around this paper into thinking that cutting salt alone will reduce their risk of obesity or help them to lose weight.”

‘High Salt Intake. Independent Risk Factor for Obesity?’ by Yuan Ma et al. was published in Hypertension on Wednesday 2 September 2015.

Declared interests

Ms Catherine Collins and Prof. Susan Jebb: No interests to declare.