By T. Jared Bunch, MD Published Apr 30, 2014

One of my colleagues and I have had an ongoing discussion regarding the “obesity paradox” – weight and how it influences our lifespan. He is over six feet tall and near 170 pounds. He has a strict whole food diet, exercises daily, and minimizes any time sitting or being inactive. His hope, in the end, is that he will live longer and better. In fact, the last time I spoke with him he was hoping to lose a few more pounds to further his longevity.

To my colleague, weight is a direct measure of health. In this regard, if all other things are equal, then the more weight a person has on their frame the less health they possess. At this point you are probably wondering why this is an ongoing discussion between my colleague and I. It is ongoing because I tell him that in most long-term health studies people that are “normal” weight, often referred to as a body mass index, BMI, between 19-25 and those considered underweight often have the highest risk of dying.

The obesity paradox refers to the inverse relationship between risk of death and body fat. This paradox is thoroughly described in a well-written book that I recently read, The Obesity Paradox by Dr. Carl J. Lavie. If this topic is of interest to you, I highly recommend this book as it covers this subject completely.

Evidence for Weight and Longevity

I observed the obesity paradox in a published study I conducted while studying at the Mayo Clinic. We looked at 226 people who experienced a heart arrest in the community and were resuscitated. What we found was that people that were slightly overweight (BMI from 25-30) had the highest 5-year survival at 78 percent. People who were underweight had a significantly lower survival at 67 percent, similar to people considered morbidly obese.

Our thoughts at that time with this study were similar to my colleague’s now. We thought that thin cardiac arrest patients died more often because they were thin for a reason other than fitness. For example, they were thin because they had cancer, or were smokers, or had some other systemic disease.

Does the possibility of underweight people having other diseases explain the obesity paradox?

In a very nice study from the Mayo Clinic of 250,152 patients from 40 different studies, the influence of weight on outcomes was further explored. The authors found that the lowest risk of all types of mortality as well as heart related mortality was in those people considered overweight (BMI from 25-30). In fact the risk of death was 12 percent lower than those people considered normal weight (BMI 20-24.9). The highest risk of mortality was in the underweight patients (BMI <20). Their risk was 37 percent higher than those people considered normal weight. Even their risk of heart disease related mortality was higher (45 percent higher) than those that were normal weight.

It was not until people had a BMI >35 that there was a clear increased risk of heart disease related morality (80 percent increased risk) compared to normal weight people. This observation is a classic “U” shaped survival curve with the highest risks seen in those that were severely underweight or overweight, with the lowest part of the curve in those considered slightly overweight (BMI from 25-30).

These high mortality rates in the underweight people were recently evaluated in a long-term study looking at people over 30 years. In a study of 31,578 people from the Swiss National Cohort, the influence of body weight on long-term death risk was further explored. The authors defined being underweight as a BMI <19. Compared to being normal weight, those that were underweight had a much higher mortality risk (37 percent increased risk) regardless of smoking status. Furthermore, the authors could not find that the higher risk was associated by cancer, pulmonary or lung disease, or chronic heart disease.

These two studies suggest that diseases that make people thin are not fully responsible for the obesity paradox.

I went to the lead author of the large study from Mayo Clinic to get his opinion. Dr. Francisco Lopez-Jimenez is a world leader in cardiology with research interests in understanding the role of weight, body fat, and heart risks. When asked why the obesity paradox exists he said “Nobody knows the cause. All that we know is that incomplete adjustment (accounting for other disease processes) does not explain it”.  He offered potential explanations.

  1. Obese people with coronary artery disease have a different disease process and often have risk factors for progression of disease that can be modified and as such respond better to treatments. For example, when we see an overweight person with high blood pressure, diabetes, and coronary artery disease we often talk about the central role of increasing activity and weight loss. Often when the weight goes down the blood pressure improves and the diabetes becomes more controllable. In this sense, the weight was related to risk factors (high blood pressure and diabetes). In underweight people with coronary artery disease and diabetes and high blood pressure the risk factors are often from genetic factors that may be less modifiable.
  2. Underweight people often have less muscle. Obese people from the mere fact that they have to carry excess weight often develop more muscle in the legs and the thighs. Muscle mass itself is also a powerful predictor of mortality. Across multiple disease states and patient populations, people with more muscle mass do better and live longer.
  3. Fat may have a protective effect. Our bodies use fat for energy storage. In the process of fighting a disease we need energy. In people that have more energy stores that may have the ability to overcome a significant body insult from an acute or chronic disease. For example, an acute disease such as pneumonia or a chronic disease such as heart failure or emphysema.

Fitness is More Important Than Being Thin

In discussing this I don’t want to promote weight gain. More importantly, I think there is more to daily health than weight and the pursuit of being underweight. In this regard, fitness is more than being thin and is more important than being thin.

Given the obesity paradox, I asked Dr. Lopez-Jimenez how he counsels patients regarding weight loss. He said “We emphasize that the paradox does not mean that an obese patient with coronary artery disease would not benefit from weight loss. Nonsmokers have a better prognosis than smokers, but smokers who quit have a better prognosis than smokers who don’t”.

I believe this is very important advice. In the United States we are facing an obesity epidemic. A lot of this is driven by lifestyles that are less active and food sources that are calorie dense. Increasing activity needs to be an essential part of everybody’s daily goals to improve their health. Healthy food choices can augment fitness and enhance muscle mass retention. Remember the mortality curves are “U” shaped, so those that were very obese (BMI >35) also had poor outcomes. Trying to maintain a healthy weight for your body frame still remains important and a fundament aspect the long-term health.

Dangers of Being Underweight

Finally, I really want to emphasize that being underweight is also significant risk factor. This may be due to lack of muscle mass or lack of energy reserve to fight acute and chronic diseases. Just as for those that are obese, if you are intentionally underweight (BMI <19), realize this likely is a poor lifestyle choice and can adversely affect your health. I would recommend focusing more on your muscle mass and general fitness and allow your body weight to normalize to your frame. This can be done with the help of your physician and a dietician.

Posted in: Heart Health