The study, published in Hypertension Journal, found that a 1 g increase in daily salt intake was associated with a 28% increased risk of becoming overweight or obese for children and a 26% increase for adults.
This was the first study to make a direct link between the two with consistent and significant results, wrote the authors.
Previous research had drawn similar but indirect associations between salt and obesity, suggesting that a high salt intake increased thirst and, consequently, raised consumption of sugar-sweetened beverages, while other scientists have placed blame with excessive consumption of processed foods that are high in calories and salt.
Although the researchers did not yet fully understand the mechanism behind this, they suggested it may be because high salt consumption increased the volume of extracellular water, which led to increased weight. Another possible mechanism was that higher salt intake altered the body’s fat metabolism resulting in more fat deposits.
Victoria Taylor, senior dietitian at the British Heart Foundation, called for more research on the underlying mechanism but said the take-home message was to cut salt.
“Most of the salt we eat is already in the foods we buy, which is why checking nutritional information on packs to make sure we are making the healthiest choice is important to help limit the amount of salt we are eating,” she said.
Meanwhile campaigners said the findings had important implications for the food industry and public health.
Sonia Pombo, campaign manager at Consensus Action on Salt and Health (CASH), said: “The responsibility lies predominantly with the food industry, where most of our salt intake comes from, but people can also make a difference to their health by reading the labels and opting for foods lower in salt, saturated fat and sugars.”
But other health experts have said the study's conclusions should be taken with a pinch of salt.
Catherine Collins, dietitian at St George’s Hospital NHS Trust, said her interpretation was that a high calorie diet in general – in this case most likely from savoury foods due to the high salt intake – was predisposed to obesity.
"Obesity per se increases your risk of hypertension, which is made worse with higher salt food choices. [But] it’s impossible to select out one aspect of a whole diet to critique," she said.
Meanwhile Susan Jebb, professor of diet and population health at Oxford University, said the findings could be misinterpreted. "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.”
But co-author and CASH chairman, Graham MacGregor, nonetheless said the findings demanded decisive action from government and industry to achieve a 30% reduction in population salt intake.
“The government and the food industry now need to take much stronger action. Unfortunately the previous government handed power back to the food industry with the Responsibility Deal which has completely failed to tackle these issues in the way that it needs to be,” he said.
The Conservative government’s Responsibility Deal was criticised for derailing a successful, nation-wide salt reduction programme at a purported cost of 6000 preventable deaths.
Data was obtained from 458 children and 785 adults taking part in the UK National Diet and Nutrition Survey (NDNS), a cross-sectional study.
Researchers used data for weight, height, diet reporting and salt intake using 24-hour urinary sodium samples to calculate the increased obesity risk. They found that a 1-g/d increase in salt intake was associated with an increase in the risk of obesity by 28% in children and 26% in adults. Results were adjusted for age, sex, ethnic group, household income, physical activity, energy intake and diet misreporting.
Published 2 September 2015, doi: 10.1161/HYPERTENSIONAHA.115.05948
‘High salt intake: independent risk factor for obesity?’
Authors: Y. Ma, F. J. He, G. A. MacGregor