Carried out by scientists at a number of institutions, including Imperial College London and Northwestern University in the US state of Illinois, the team analyzed the diets of 4,680 people aged 40-59 from the USA, UK, Japan and China. 

Participants were followed over four days, with the team taking two urine samples during this time to assess concentrations of sodium and potassium. 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 assessed volunteers' intake of over 80 nutrients that may be linked to low blood pressure, including vitamin C, fiber, and omega-3 fatty acids, many of which are found in fruit, vegetables and whole grains.

They found that the average salt intake across the study was 10.7g a day. The average intake for the UK was 8.5g, while the intake for the USA, China and Japan were 9.6g, 13.4g and 11.7g respectively.

The recommended upper limit of adult salt intake in the UK is 6g a day -- around one teaspoon -- with the team finding that increasing salt intake above this average amount was linked to an increase in blood pressure, even if participants consumed a high level of potassium and other nutrients.

An increase of an additional 7g (1.2 teaspoons) of salt above the average intake was associated with an increase in systolic blood pressure of 3.7 mmHg.

Ideally, blood pressure should be between 90/60 and 120/80 mmHg.

Systolic pressure is the first number at the top of the ratio, which measures the force the heart pumps blood around the body. 

The second number, called diastolic pressure, measures the pressure in your blood vessels when your heart relaxes between beats.

High blood pressure can increases the risk of a number of conditions including heart attacks and stroke, however, reducing blood pressure even by just a small amount can also reduce the risk of these potentially fatal conditions.

High blood pressure affects more than one in four adults in the UK, and is thought to be brought about by a number of factors, including age and weight as well as eating too much salt.

The researchers are now advising people to monitor salt intake, with joint lead author Dr. Queenie Chan commenting that, "We currently have a global epidemic of high salt intake -- and high blood pressure. 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."

The results can be found published online in the journal Hypertension.