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A team of Canadian scientists is hunting for the salt gene, hoping to unlock the mystery of why some ethnic groups are highly sensitive to salt and suffer from abnormally high rates of hypertension.

Blacks and South Asians are three times more likely than Caucasians to suffer from high blood pressure, a condition that is the leading cause of heart disease and stroke. Only cancer kills more Canadians every year.

The link between ethnicity and high blood pressure is well known, but until now, researchers have been unable to pinpoint the cause. Now, researchers at the University of Ottawa’s Heart Institute have launched an ambitious, multi-million-dollar, three-year study to uncover the genetic determinations of salt sensitivity. Their success could revolutionize the treatment and prevention of hypertension, which affects 20 per cent of Canadians, at an annual cost of $430-million.

Adding to the complexity, there is variation within these populations: some blacks and South Asians are not at all salt sensitive, and some Caucasians conserve salt.

Researchers at the University of Ottawa’s Heart Institute have launched an ambitious, multi-million-dollar three-year study to uncover the genetic determinations of salt sensitivity. This could revolutionize the treatment and prevention of hypertension, which affects 20 per cent of Canadians, at an annual cost of $430-million.

“Blacks are more susceptible to high blood pressure and to being salt sensitive than Caucasians. But looking at someone’s skin colour doesn’t tell you this, because some are not at risk at all, and vice versa for Caucasians,” said Frans Leenen, a cardiologist and director of the Heart Institute’s hypertension unit. “This is the first study in the world which will look at the genome sequence, and salt sensitivity.”

Dr. Leenen, the principal investigator, predicts there is not one, but 10 to 12 salt genes. Mapping them will pave the way for a salt sensitivity diagnostic test.

Salt sensitivity means people retain salt and water in their blood vessels, making the vessels more rigid and constricted. The heart then has to work harder to pump blood through the vessels, causing an elevation in blood pressure. Genetic variants play a role in determining how much blood vessels constrict.

If people knew they were salt-sensitive, they would know whether reducing dietary sodium would bring down their blood pressure.

The average Canadian takes in about 3,100 milligrams of sodium a day – more than double the recommended intake and significantly more than the upper limit most people can consume without compromising their health.

There are many environmental causes of high blood pressure, as well as genetic. But doctors say that by reducing dietary sodium, one fifth of the five million cases of hypertension in Canada could be eliminated.

Ethnic differences in hypertension rates have been well documented for many years in the United States. But in Canada, this health issue wasn’t studied until last year when the Heart and Stroke Foundation of Ontario commissioned a survey that found blacks and South Asians (Indians, Bangladeshis and Pakistanis) are three times more likely to have high blood pressure compared with the general population. They are also likely to develop it at a younger age.

One third of blacks and South Asians have high blood pressure, compared with about 21 per cent of the general population, and about 19 per cent of East Asians (Chinese, Japanese, Koreans).

The elderly, diabetics and people who are obese or have kidney disease are also salt sensitive.

“Knowing the hypertension rates are on the rise due to an aging and diverse population is an important wake-up call,” said Sheldon Tobe, a nephrologist and spokesman for the Heart and Stroke Foundation.

The elderly, diabetics and people who are obese or have kidney disease are also salt sensitive.

Researchers believe there are historical reasons why people of African heritage conserve salt. Their ancestors likely developed a genetic conservation gene so their bodies could conserve salt and ward off dehydration in a hot climate, said Sonia Anand, a professor of medicine at McMaster University in Hamilton.

South Asians are believed to have inherited what is known as the “thrift” gene, a propensity to store energy (fat) around their waist to survive feast-and-famine cycles. This abdominal fat, producing an apple-shaped body, increases the risk of heart disease and high blood pressure.

Hyacinth Solomon, 30, believes the search for the salt gene is long overdue. The 30-year-old native of St. Lucia had no idea that her race made her a target.

“When the doctor told me I had high blood pressure, and it was irreversible, I was shocked. I was only 29,” she recalled.

She was diagnosed last year, after she began suffering from fatigue and headaches. At the time she was working as a nanny, and her employer convinced her to go to the doctor. “I knew my mum had high blood pressure, but I never realized this could be me,” she said.

Ms. Solomon unknowingly compounded the problem by indulging in her favourite foods: salty Caribbean chicken and Cheese Nips crackers.

Now she worries her 10-year-old son may also be genetically susceptible. She is thankful, though, that the diagnosis prompted a lifestyle change. No more packaged foods, chicken cubes, potato chips or table salt. She and her son check the sodium content on every grocery item.

“Even my son is rejecting some of the yummy foods because he knows they are unhealthy,” she said.

Unlike Ms. Solomon, most people with high blood pressure have no symptoms, making the disease a “silent killer.”

John Odai, a dentist in Ottawa, knew both his parents had hypertension, and that he could also develop it. He had no symptoms but four years ago, when he was 35, he asked his family doctor to refer him to a specialist. Sure enough, he already had elevated blood pressure. “Knowing this could kill you, it does make you scared,” he said. “But having it diagnosed early can prevent something worse from happening.”

If you have one or more of the so-called salt genes, then environmental factors such as salt, stress, obesity and alcohol will have a greater impact on your blood pressure.

To pinpoint the salt genes, the Ottawa research team is recruiting 400 patients with a family history of high blood pressure. For the first phase of the study, which is funded by the Canadian Institutes for Health Research, the patients will lower their salt intake to less than one teaspoon of salt a day for seven weeks. During three of the seven weeks, salt tablets will be added to bring their salt use closer to that of a typical person. The blood pressure effects of salt will be measured, using 24-hour urine samples, and 24-hour blood pressure monitoring.

Next, the patient will undergo a genome scan, to relate salt sensitivity to gene sequences. “It is possible that the people who are salt resistant have a certain protective variant in a gene, while others may have a genetic variant that means they conserve salt,” said Dr. Leenen, who is working with geneticists Frederique Tesson and Alex Stewart.

Their first group of research subjects are Caucasian, and they hope to replicate results with black patients.

The 2008 Ontario study on ethnicity and hypertension found that about 66 per cent of people with high blood pressure were being treated for it, a rate that did not differ significantly by ethnic group.

Nonetheless, the report signalled that more public education is needed in specific at-risk populations. Ms. Solomon said there is no buzz around the issue in Toronto’s Caribbean community. This underscores the fact that many people with hypertension have no reason to suspect they have the disease.

Now that she is on medication to control her blood pressure, Ms. Solomon feels much better. “I am calmer. My new lifestyle cools me down,” said Ms. Solomon, who has returned to university to become a registered nurse. However, if she stops taking her pills, her blood pressure shoots back up – a potentially deadly fate she hopes her son can escape.

Salt and iodine deficiency

While excess salt can lead to severe health problems, sodium is not all bad – a minimal amount is actually required for good health.

Sodium helps regulate blood pressure and the amount of fluid in the body; it also plays a role in the transmission of nerve impulses and muscle contractions.

Just as importantly, most table salt is fortified with iodine, a micronutrient that is essential to the health of two vital organs – the brain and thyroid.

Iodine deficiency is the “most common cause of preventable mental impairment worldwide,” according to the medical journal The Lancet. This despite the fact that it is one of the cheapest public health interventions, costing only about two cents per person reached.

Canada began fortifying salt with iodine in the 1920s to protect people from developing goitre, the swelling of the thyroid gland that can be caused by a deficiency of the mineral.

In the 1980s, scientists made the connection between iodine deficiency and mental retardation in babies and young children.

If a pregnant woman does not get sufficient iodine, in salt or otherwise, her child can suffer irreversible brain damage. Even a moderate deficiency in iodine in utero or in childhood can lower intelligence by 10-15 IQ points.

Iodine deficiency also causes growth impairments that result in dwarfism and cretinism.

Unicef, with the help of a number of charitable and non-governmental organizations, pushed to get iodine added to salt around the world.

Today, 70 per cent of the world's population gets a dose of iodine with their salt, in large part thanks to the initiative of groups like the Ottawa-based Micronutrient Initiative.

One of the reasons that iodized salt has not reached more people is resistance in the Muslim world. This is due to a persistent – but false – rumour that iodized salt is a contraceptive, and that iodine is added to salt as a way to keep Muslims from having babies.

Now there are Canadian-led efforts to also fortify salt with iron, which could help prevent anemia.

André Picard