Diets containing too much salt and too little potassium significantly increase the risk of dying from heart disease, say researchers.
The combined effect is greater than either factor on its own, a study suggests.
Sodium in salt is known to narrow arteries and raise blood pressure, whereas potassium is believed to counteract these effects.
The association may also be a reflection of dietary habits.
Someone with a high salt and low potassium intake is likely to consume a generally less healthy diet. Processed food contains added salt, while fruits and vegetables are good sources of potassium.
Scientists analysed data on more than 12,000 people enrolled into a major US health and nutrition study.
After an average follow-up period of around 15 years, 2,270 participants had died. Of these deaths, 1,258 were linked to heart and artery disease.
The study found that all-cause risk of death for people with the highest sodium-potassium ratios – those with the highest sodium and lowest potassium intake – was 46 per cent greater than for those with the lowest ratios.
Deaths specifically attributed to cardiovascular (heart and artery) disease were equally more likely for people at the high end of the ratio scale.
In the case of ischaemic heart disease – damage caused by poor blood supply to the heart muscle – having a high sodium-potassium ratio doubled the chances of dying.
The findings, from a US team led by Quanhe Yang, from the Centres of Disease Control and Prevention in Atlanta, Georgia, appear in the journal Archives of Internal Medicine.
The authors wrote: “In summary, our findings indicate that higher sodium-potassium ratio is associated with significantly increased risk of CVD (cardiovascular disease) and all-cause mortality in the general US population.
“Public health recommendations should emphasise simultaneous reduction in sodium intake and increase in potassium intake.”
Victoria Taylor, senior heart health dietician at the British Heart Foundation, said: “The importance of reducing sodium to help lower our risk of cardiovascular disease through reductions in blood pressure is well known, but the role of potassium is less so.
“We need to understand this relationship better to make the dietary advice we give people about reducing their blood pressure as effective as possible.
“What this means for our diets, is that it’s important to continue to reduce the amount of salt we eat by checking food labels for the lowest salt choices and avoid adding salt to our meals. But we also need to make sure that we are including enough foods that contain potassium in our diet. So it’s yet another reason to make sure we’re getting our five-a-day with fruit and vegetables being the obvious choice to provide us with potassium as part of a heart healthy diet.”
The findings appear to be at odds with a claim last week that reducing salt intake does little to reduce the risk of heart attacks and strokes.
British and US researchers examined data on 6,489 people taking part in studies looking at the effect of cutting down on salt.
Although blood pressure was slightly reduced, there was no evidence of a significant impact on the chances of suffering a major cardiovascular event.
The number of heart attacks and strokes recorded was too small to allow any meaningful conclusion to be drawn, said the authors. However, this did not mean cutting down on salt had no benefit.
The results were published in the Cochrane Library, which collects evidence to aid future health policy.