Getting a Handle on Hypertension

February 5, 2012

Dr. Frans Leenen is the Director of the Hypertension Clinic and Hypertension Research at the University of Ottawa Heart Institute. In 2008, he and fellow Heart Institute researcher Dr. George Fodor led the Ontario Survey on the Prevalence and Control of Hypertension, the most in-depth study of high blood pressure in Canada released in more than 15 years. Recent studies have confirmed their results that, although the control of hypertension in Canada has improved drastically over the past two decades, rates of high blood pressure have not improved and more than 20 per cent of the adult Canadian population has high blood pressure.

Dr. Leenen talked with The Beat about the impact of healthy diet, salt, and obesity on blood pressure, and the contribution of genetics to the field of hypertension research.

The Beat: A recent study projects that more than a quarter of Canadians will have hypertension by 2013. How does this number compare with your data from the Ontario Survey?

Dr. Leenen: That’s in line with our findings. The surprising fact is that there’s been no real change in the prevalence of hypertension in the past 20 or so years if you look at it by age categories. If you look at the whole Canadian population, the number with high blood pressure has gone up, because you have an aging population, and older people have a much greater chance of having high blood pressure.

The surprising finding is that hypertension rates are not getting worse considering that the prevalence of obesity has been going up. The higher a person’s body mass index, the more likely they are to have high blood pressure. So from that perspective, you would expect a much higher prevalence at each age for high blood pressure; but in the total population, somehow that doesn’t show up.

This suggests that while there’s one group of people that have higher blood pressure because of their weight, there’s another group that has less high blood pressure compared to the past. Researchers don’t know who that group is yet or what has changed. It’s not because of treatment. Up to 60 to 70 per cent of Canadians with high blood pressure are now well treated compared with only 20 per cent a decade or two ago. However, if someone is treated, they’re still counted as having high blood pressure in these large studies.

Click to enlarge

The Beat: Health Canada has reported that most Canadians don’t know how much salt they are consuming and how much they should consume. What should people know?

Dr. Leenen: In general, particularly for younger people, a high salt diet reflects an unhealthy diet. In other words, a high salt diet by and large reflects fast food and processed food intake and bad food habits, such as high caloric intake, high sugar intake through soft drinks, and poor nutrition.

You can highlight a particular food ingredient, like salt, but at the end of the day, if a food item is low in salt but other ingredients are unhealthy, it’s still a bad food. You have to look at the whole picture of promoting a healthy diet and lifestyle.

This brings us to what I think should really be the target for prevention of high blood pressure: obesity. In younger people, it doesn’t make too much of a difference, but at middle age, things start to change. At 40 to 60 years of age, for people with a normal body mass index, only 12 per cent have hypertension. For the obese, it’s up to 40 per cent. In people 60 to 80 years old, about a third with a normal weight have high blood pressure. For the obese, it’s nearly 70 per cent—a huge increase and just one impact on your health of being overweight.

Once people are obese, it becomes very difficult, if not impossible, to get back to a normal weight. So the crucial focus needs to be on children, on teenagers, to develop good diet and physical activity and exercise habits early. This requires parents and schools to participate but also the whole community. Public health messages alone are not enough. Like for smoking—the messages on packages of cigarettes aren’t the primary reason for the decline in smoking. A variety of other factors are largely responsible, such as that you can’t smoke in public anymore and that the price of cigarettes has gone up.

At this moment in our society, fast food, bad food and processed food are cheaper than healthy food. If we cut back on fast food, the salt intake would go down because the two are very much connected. To focus just on salt is not enough. It would be much better to focus on fast food per se. It shouldn’t be cool to eat fast food, just like it’s no longer cool to smoke. But that requires a change in our perceptions.

The Beat: What do we know about salt sensitivity in terms of controlling high blood pressure?

Dr. Leenen: For any factor you look at that influences high blood pressure, whether it’s salt or stress or alcohol or weight, the impact on your health is very much determined by the genes that you have. Some people can be obese and still have a normal blood pressure. Then there are other people who have only a little increase in their weight and their blood pressure shoots up. It’s the same for restricting salt: for some people, blood pressure goes down quite a bit; for some, it makes no difference, and for some people with hyperactive counter-regulatory systems, it actually goes up a little! It’s their physiology. It’s an interaction of genes and lifestyle.

The Beat: Is that a current focus in your own research?

Dr. Leenen: Right now, we’re looking for the genes and mechanisms through which salt can increase high blood pressure in one group of subjects and through which other people can be salt-resistant. If we know exactly how salt affects blood pressure, we can better target treatment against those mechanisms. And if we can better identify people who are salt-sensitive versus salt-resistant, we can help them focus on managing their risk factors. If you knew you had a genetic profile that predisposed you to coronary artery disease, that could give you an extra push in trying to stay healthy.

For our genome-wide association studies, Frédérique Tesson, Alexandre Stewart and myself are working with a number of European groups in Belgium, England and Switzerland. These studies require a large number of participants, and it’s better to collaborate than to take many years to do it all on your own.

If we had a simple genetic test—a marker for salt sensitivity versus salt resistance—that would help quite a bit. At the moment, the only way of really knowing whether someone’s blood pressure is salt-sensitive is to lower dietary salt. If after a number of weeks, you don’t see an effect, that could be for one of two reasons. Either their blood pressure is salt-resistant or they didn’t lower their salt enough. It’s very time consuming to determine the cause.

We also know that people who are salt-sensitive are likely to respond differently to certain medications. This offers targets for more specific drug treatment. For example, if someone’s blood pressure is salt-sensitive, there is evidence that they would also respond better to a diuretic. If someone’s blood pressure goes up on a low-salt diet, it would probably also go up on a diuretic. There are clearly practical consequences for not knowing what’s driving someone’s hypertension.