It is well established and widely known that obesity (even at low levels) is associated with an increased risk of heart disease. The more interesting question is why. I find few of my patients know why. The word Doctor comes from the latin word doctoris which means teacher; but somewhere along the road we stopped teaching our patients. 

When I ask patients why obesity causes heart disease, the most common answer patients share with me is that “the fat makes our heart work too hard” or that “the fat clogs up the arteries” and this is just not the case.

Obesity causes heart disease because the fat cells in the abdomen release chemicals, which inflame the heart’s vessels until they block. How and why this happens is explained below….

They say a picture is worth a thousand words and this MRI shows the story well. Fat on an MRI looks beige. As you can see in the MRI, fat lies in two places in our bodies.  One is a blanket under our skin called subcutaneous fat or under the skin fat. You can see it all around the guy in the MRI. Subcutaneous fat is health neutral, which means that you can have as much of it as you want and it does not cause disease. 

The second place fat lies in our body is in the abdominal cavity. This is called belly fat or visceral fat: This is the fat that causes heart disease. 

This MRI is of a male with a BMI of 40, otherwise known as morbid obesity. The centre of the image shows an abdomen full of fat (it’s all the beige stuff, the black stuff is stomach, bowel and bladder and above the abdomen is the heart and lungs). The abdominal cavity is a closed space and so when you fill it up with more and more fat the pressure inside increases like when we put more and more air into a balloon.

The pressure in this guy’s abdomen is 3 times atmospheric pressure, or the pressure you would find at 80 feet below sea level.

The abdominal cavity is where most of his organs are. This pressure is crushing his organs 24/7 and the pressure is pushing up and out. This pressure causes the tense beer belly. This pressure also causes the lungs to be pushed up. This MRI shows that this man’s lungs are a fraction of their normal size, which is why someone with abdominal obesity will become short of breath easily.

You really do not want to have much abdominal fat because abdominal fat cells are unique. They secrete chemicals called cytokines. These chemicals cause inflammation, and heart disease is an inflammatory disease. The inside of a vessel becomes inflamed, and then narrowed, and blocked. A blocked heart artery is what causes a heart attack. Only five things cause the inside of vessels to be inflamed, narrowed and blocked:

-       abdominal obesity

-       diabetes

-       smoking

-       high blood pressure

-       a bad diet

So if you have any of these you are at an increase risk of heart disease because of vessel inflammation.

So how do you know if you have a healthy waistline? In many ways your waist circumference is a better predictor than your weight as to whether you are at an increased risk of heart disease.

Your waist circumference should be half your height. This rule even works for children.  For example, I am 74 inches tall (6 foot 2) and so my waist circumference should be less than 37 inches. A 64 inch (5 foot 4) tall woman should have a waist less than 32 inches.

This is the reason why you always hear that weight in the centre is more dangerous. An apple shape is worse than a pear shape. If your waist circumference is more than half your height you have an elevated risk of heart disease and now you know why. An elevated waist circumference can be equivalent to smoking as far as elevated heart disease risk. You probably don’t smoke, so why carry around extra belly fat?

David A. Macklin MD CCFP