WASHINGTON, March 5 (Xinhua) -- A study published on Monday in the journal Hypertension suggested that a healthy diet may not offset the effects of a high salt intake on blood pressure.

Researchers from Imperial College London and Northwestern University analyzed the diets of over 4,600 people aged 40 to 59 from the United States, the United Kingdom, Japan and China.

They concluded that people eating higher amounts of salt had higher blood pressure, no matter how healthy a person's overall diet.

High blood pressure is thought to have a number of causes, including age, weight and eating too much salt.

Previously, experts believed that eating high amounts of fruit and vegetables might help counteract the effect of high salt on blood pressure.

However, while these foods do tend to lower blood pressure, they do not counteract the adverse influence of salt intake, according to the research.

The volunteers were tracked over four days, and two urine samples were taken. Measurements of height, weight and blood pressure were also taken.

The team assessed concentrations of sodium and potassium in the urine samples. Sodium is the main component of salt, while potassium, which is found in green leafy vegetables, has been linked to lower blood pressure.

The team also used dietary data to assess the volunteers' intake of over 80 nutrients that may be linked to low blood pressure, including vitamin C, fibre, and omega-3 fatty acids. Many of these nutrients are found in fruit, vegetables and whole grains.

Researchers found a correlation between high blood pressure and higher salt intake, even in people who were eating a high amount of potassium and other nutrients.

The study found that the average salt intake for the United Kingdom was 8.5 grams, while the intake for the United States, China and Japan were 9.6 grams, 13.4 grams and 11.7 grams, respectively.

The recommended upper limit of adult salt intake in the United Kingdom is 6 grams a day.

Queenie Chan, joint lead author of the research from the Imperial College London said "this research shows there are no cheats when it comes to reducing blood pressure. Having a low salt diet is key -- even if your diet is otherwise healthy and balanced."

Researchers acknowledged that because the data was collected over four days, it provided information from a snapshot of time, so they hoped to focus on longer term studies, with a greater number of people.