We are inundated with information about what increases or decreases our risk of heart disease. In this post I will talk about what risk really means, how we calculate risk, and some things you can do to reduce risk of heart disease. OK, here we go.
Risk: What does it mean?
Although we often talk about individual risk (or luck, which is the same thing) risk really only applies to populations. We know, for example, that in the population of people buying lottery tickets, one person will win the lottery. We know that for certain, but we have absolutely no way of predicting who that person will be. The millions of other people who buy lottery tickets will not win the lottery. The population chance of winning the lottery ranges from one in 42 million to one in 176 million depending on the lottery. Any individual’s chance of winning the lottery though is either 0% or 100%. You either win it or you don’t. The very low population chance just means that there will be millions and millions more people who don’t win.
The same thing is true for risk of heart disease or any other disease. We can define a population that has a certain risk of heart disease, let’s say 15% over ten years. We can be very sure that 15% of that population will develop heart disease. but 85% of those people will not develop heart disease. For any individual, we have absolutely no way of knowing or predicting whether they will or will not develop heart disease. Individual risk is either 0% (you won’t get heart disease) or 100% (you will get heart disease). Even though a population risk of 15% is considered high risk of heart disease, the vast majority of those people will not get heart disease. Their risk is 0%. If you are one of the 15% who gets heart disease then your risk is 100%.
Doctors and other health professionals talk about individual risk, even though there is really no such thing. They do this to try to get people to change their behavior or to convince them to take a medicine. If you successfully change your behavior in a healthier direction or you take a medicine that you were not taking before, you then belong to a different population that has a lower risk of disease. Even though you belong to a lower risk population, your individual risk is still either 0% or 100%.
Relative Risk Reduction vs Absolute Risk Reduction
When research is done on some medicine or other intervention to see if it works, the medicine or intervention is given to one population and a placebo (inactive medicine) to another very similar population and the results are compared. A common way to report how much the intervention reduces the risk of a particular disease compared to the risk in the control group is to report it as relative risk reduction.
Here is a real world example. Eating a handful of nuts per day reduces your risk of heart disease by 20%. That sounds like a lot for a very simple intervention. The problem is that number does not tell you anything about the baseline risk of the population. What we want to know is 20% of what?
Suppose we have determined by a risk calculator that you belong to a population that has a 15% risk of heart disease over ten years. 20% x 15% = 3%. The absolute risk reduction is 3%. That doesn’t sound quite as impressive, does it? What is really important for you to know about a treatment is how much it reduces the risk compared to the baseline risk of the control population; that is, you want to know the absolute risk reduction for the population of people treated compared to the people who are not treated. Often results are not reported that way and you have to calculate it yourself. More about this later on.
How do we calculate the population risk of heart disease?
There are several heart disease calculators that let you know what heart disease risk population you belong to. Although each of them talk about calculating your individual risk of heart disease over the next ten years, what they really mean is that you belong to a population that has that risk. Remember that there is really no way to calculate risk for an individual. Here is a link to a page from Harvard Health Publishing that lets you try out each of three different risk calculators: Heart Attack Risk Calculators.
How to put yourself in a lower risk population for heart disease
In order to calculate absolute risk reduction of any medicine or change in behavior, you first have to calculate the risk of the population you are currently in. Multiplying the relative risk reduction of any change times your current population risk gives you the absolute risk reduction of that change. The higher your current population risk, the greater the absolute risk reduction of any change. That means that the first thing you have to do is to use one the risk calculators in the link above to calculate your current population risk of getting heart disease in the next ten years.
In the examples below I’m going to assume a moderate ten year population risk of heart disease of 9%. Remember that if you are in a population whose risk is higher than that, the absolute risk reduction of each change will be larger.
Things that don’t involve taking medicine
In this post I’m going to focus on things you can do to reduce your population risk of heart disease that don’t involve taking medicines prescribed by your doctor. Medicines that treat high blood pressure or high cholesterol can also lower your population risk of heart disease. That is a discussion better left to you and your doctor, however.
Exercise
Regular exercise, depending on the amount, decreases 10 year risk of heart disease by 31 to 45%. For a moderate population risk of 9%, that translates to an absolute risk reduction of 2.8% (.09 x .31) to 4% (.09 x .45). To get the higher level of absolute risk reduction you have to exercise moderately for 150 minutes a week and do activities that enhance muscle strength twice a week. Simply being active most of the time as opposed to sitting most of the time puts you in a population that has a significant absolute risk reduction, on the order of 2-3%. Exercise level is included in many of the risk calculators. If your population risk includes exercise then you should not count that separately.
Saturated vs unsaturated fat
Despite what we have all been told over the years, the evidence is pretty weak that eating saturated fat increases your population risk of heart disease. On the other hand, eating more unsaturated fats does decrease your absolute risk of heart disease. Most studies show that people who ate the most polyunsaturated fats (vegetable oils) had about a 25% relative risk reduction for heart disease. Using our 9% population example the absolute risk reduction is about 2% (.09 x .25).
Eating fish
Eating fish twice a week reduces your relative population risk by 16%. Using our 9% population risk example, that translates to an absolute risk reduction of a little over 1% (.09 x .16).
Eating nuts
As I mentioned at the beginning of this post, eating a handful of tree nuts (cashews, pecans, almonds) reduces the relative population risk by about 20%. In our 9% population risk example, the absolute risk reduction is about 2% (.09 x .20).
Increasing dietary fiber
Fiber in the diet can be increased by eating lots of whole fruits, vegetables and whole grains. Observational studies show this causes a population relative risk reduction of 16%. In our 9% population risk example, the absolute risk reduction is a little over 1% (.09 x .16).
Not Smoking
Smoking cigarettes doubles the relative population risk of heart disease. Using our 9% example, the population absolute risk increase is 9% (1 x .09). Conversely, if you quit smoking your absolute population risk would be cut in half.
Putting it all together
If you make all the exercise and diet changes outlined above, they add up. Lets do the math.
9% ten year risk of heart disease -3% for exercise – 2% for eating more polyunsaturated fats – 1% for eating fish twice a week – 2% for eating a handful of nuts per day – 1% for increasing dietary fiber = 0! Obviously no one has zero risk of heart disease over ten years but this calculation makes the point that doing multiple things that reduce your absolute population risk of heart disease by small amounts add up to a large reduction in your population risk of heart disease. Doing all these things puts you in a population that has less than 1% ten year risk of heart disease. Does that mean you won’t get heart disease if you are a member of this population? No it doesn’t. Remember that it is impossible to calculate the risk for an individual. What being in this population does mean is that only a very few people will get heart disease over the next ten years.
Bottom Line
Risk refers to populations, not individuals. Relative risk reductions are reported in the medical literature most of the time because they are larger and look more impressive. Absolute risk reductions are important to know, because they take into account baseline population risks. If you calculate your baseline population risk by using one of the risk calculators, you can easily calculate absolute risk reduction by multiplying the baseline population risk times the relative risk reduction reported in a study or magazine article. Even though one behavior change produces only a small absolute risk reduction, multiple small absolute risk reductions add up to a big absolute risk reduction.