Sunday, April 7, is World Health Day, and this year's theme is high blood pressure, also known as hypertension. The World Health Organization has recommended reducing salt or sodium intake to lower the risk of stroke, cardiovascular disease and kidney failure. But researchers say the benefits would be greater if dietary potassium intake was increased at the same time.

The WHO says high blood pressure affects one billion people worldwide. It leads to many deaths or permanent disabilities. Hypertension is called the silent killer because there are few obvious symptoms.

The good news is it's often preventable. There are many studies indicating that reducing salt or sodium intake can lower the risk of stroke and related illnesses.

Professor Graham MacGregor and his colleagues have reviewed past studies on salt intake and conducted their own. 

"When you eat more salt, the salt's absorbed into the body and then you get thirsty. You drink more water. As you know, salt makes you thirsty. That increases the amount of fluid around the cells because salt is the main regulator of the volume of fluid both in the circulation and the fluid around the cells - the so-called extra cellular volume. Now when that reaches a certain point, a message goes to the kidney that, hey, the body's got more salt in it. And then you start excreting in the urine more salt. So you come back into balance," he said.

MacGregor is a professor of cardiovascular medicine at the Barts and London School of Medicine and Dentistry.

"When you're on the high salt intake you always have some extra salt in you and a slightly greater volume of blood. And that's what puts up the blood pressure. I mean, if you wanted, an analogous thing would be really like a central heating system. If you put more water into closed system, the pressure will go up."

The body does need some salt, he said, about less than half a gram per day. However, people in developed nations are eating about eight to ten grams a day.

"So we're eating about 20 times more salt than we need, but no mammal normally adds salt to their food. We're the only mammals that do. We've only been doing it about 5,000 years because it had this magic property of preserving food and was very important to the development of civilization. But without that discovery, we wouldn't be eating salt," he said.

Much of the processed food today contains high levels of salt. That in combination with high sugar content can make so-called junk food delicious, but not nutritious.

MacGregor said that lowering salt can go a long way to reducing hypertension, but he says there's more than can be done, namely, increasing potassium intake. Studies show that higher potassium intake has been linked with a 24 percent lower risk of strokes in adults and may have a beneficial effect on blood pressure in children.

"It's in fruit and vegetables and also in unprocessed meat and fish. Probably during evolution, we were eating two of three times the amount of potassium we eat [now]. And the food industry, of course, when it processes food removes potassium and adds salt, which is the worst possible thing to do."

Potassium, he said, counteracts some of the effects of salt. It's also important for nerve function and muscle control. The general recommendation is to get it through food and not supplements. People in developed countries consume about three grams of potassium a day through diet.

"The recommendation is that we should eat about four grams. Now to increase your potassium by one gram is equivalent to two or three servings of fruits and vegetables. It's equivalent to two or three bananas or two or three oranges or an orange, an apple and a banana or a serving of a vegetable and two fruit servings," he said.

It sounds easy, but health officials say it can be difficult to get people to eat more fruits and vegetables. MacGregor says in Britain, despite spending millions of dollars on awareness campaigns, daily consumption of potassium has barely increased. But Britain has had some success in reducing dietary salt or sodium.

"Eighty percent of the salt we eat is courtesy of the food industry. We have no choice. It's already there. And what we've done in the U.K. is to get the food industry to slowly reduce the amount of salt they add to food. So salt intake in the U.K. has fallen from I think 9.5 grams a day to 8.1, which is about a 15 percent reduction, which will have saved I think 9,000 deaths a year from strokes and heart attacks," he said.

Professor MacGregor said that by gradually reducing salt in foods people are less likely to notice the taste difference.

Health officials are raising concerns about developing countries with growing economies. Those nations are adopting a western diet ¨C with its salty, sugary and fatty foods. Officials are forecasting a sharp rise in cardiovascular disease, along with obesity-related illnesses.