Study Suggests Lower Mortality Risk For the Overweight - Rebuttal
Posted Jan 03 2013 9:15pm
New research published yesterday titled "Association of All-Cause Mortality With Overweight and Obesity Using Standard Body Mass Index Categories" in the Journal of American Medical Association (JAMA) reviewed 97 studies containing a sample size of more than 2.88 million people. This large meta-analysis concluded overweight but not obese individuals seem to have a lower mortality rate than their normal weight counterparts. Unfortunately many media headlines neglected to state the correlative nature of this paper. Instead, they definitively scream "Those Slightly Overweight Live Longer" or "Fat People Like Me Live Longer!". Obviously there is more to the story. Is this study conclusive? No, far from it and here is why:
1. All of its data is based on a flawed measure called the body mass index (BMI). The formula solely measures height and weight. This metric does not take into account gender, race, age, muscle mass, fat distribution, body frame and fitness level. These factors are all known to be strong health risk phenotypes. The BMI is an imperfect measure to say the least. Micheal Jordon, Arnold Schwarzenegger and Me in our prime had BMI's listing us as obese! "The BMI doesn’t give a precise readout. It can be horrible as an individual gauge." - Cleveland Clinic, Dr. Eric Topol.
2. This study is an epidemiological study which looks at patterns in a given population. It does not show cause and effect. It shows a correlation which is based on observed data. The author of this study admits "because the errors in self-reported data tend to differ by sex, there may be an offsetting effect when analyses combine men and women". Self reported data is commonly flawed. Many people lie about their age and weight or just guess or get it wrong.
3. The study looks at the correlation between BMI and death from all causes including car accidents and other accidental deaths. Furthermore, many people who die of chronic disease tend to be much thinner when they pass away. Once terminally ill persons are removed from the data, the "overweight people live longer" conclusion may no longer be statistically significant. Being slightly overweight also is thought to confer some benefit during physical injury (like a car accident or fall in the house) due to the extra cushioning. So if you'd like to increase your chances of survival from a bathroom fall and such, gain a few pounds, but too much extra fat will raise your inflammation levels and put you at a higher risk of death from all other causes.
4. Our population is living longer than ever but the number of years spent in poor health has increased as well. This study does not take into account overweight people suffer disease at higher rates and may have prolonged lives due to advancements in medicine. And these individuals are consequently visiting the doctor more often and receiving preventative care. The authors speculate that overweight people may show signs of disease earlier or be more likely to get screenings for weight-related diseases. It is likely these older overweight people are living with chronic diseases such as diabetes, heart disease, arthritis and cancers. Research has shown a direct causal relationship between being overweight and having increased chance of developing a chronic disease/illness. So I ask you, what is their quality of life?
5. The author chunks the study participants into 3 categories: Obese, Overweight and Normal Weight (which is actually just everyone else). Thus she fails to consider those at the lowest end of the BMI are often not healthy; anorexics, cancer patients etc.
6. This study looked at mortality, and is thus skewed towards those who died in old age. Being overweight as a young adult is different from being overweight as a senior. The effects on health are more severe for the young because it drastically increases predisposition to a slew of major diseases, with problems accumulating over time, increasing mortality over time. Such people will die earlier, and by old age only those who are overweight would be the healthiest amongst the survivors.
The BMI is like the bathroom scale, it gives us only one small metric of health. There are many other factors to consider when looking at one's health and longevity. Supposing the study is definitive (and it is not!) people with a "normal" BMI should not try to gain weight on purpose in order to live longer. Instead focus on your fitness level. Good fitness is a result of strategic exercise and a proper diet. In fact most research on the subject shows fit "normal" weight individuals (with good genetics) have the best protection from disease and death. Fitness and picking genetically superior parents is the key to health and longevity.