Hot off the presses from the Journal of the American Medical Association is a new large analysis of the risk of dying based on body mass index (JAMA 2013;309(1): 71-82). It has found its way into the media because of its surprising results. It has long been my assertion based on the data that existed at the time that as your BMI increases above 30 it is clear that your risk of dying increases. At a BMI between 25 and 30 it is not clear but your risk of dying probably increases.
This new study turned that assertion on its head. They analyzed over 141 different studies included 2.88 million individuals. They found that if your BMI was over 35 your had a clear increased risk of dying, if it was 30-35 there was no clear increased risk, and if your BMI was 25-30 you had a small decrease in your risk of dying. We are all looking for something to let us off the hook, so I’m sure this study will spread like wildfire through the media. They always find a way to tell us what we want to hear.
So why the different findings? Let’s analyze some of the important issues.
1. BMI has flaws in general. It does not account for the impact of location of body fat (abdominal fatis more harmful than hip or gluteal fat). It does not account for the impact of other conditions that may be present such as diabetes or high blood pressure. It doesn’t account for the impact of cardio-respiratory fitness.
2. BMI is an imperfect predictor of metabolic risk. There are some people with normal BMI who have all the obesity-associated illness (i.e. diabetes, high blood pressure, high cholesterol). There are some people with high BMI who have none of these risk factors.
3. The comprehensive review quoted above includes more recent studies than previous reviews and meta-analyses. Previous large reviews that showed increased risk of mortality for BMI 25-35 were done in a time where the treatments for obesity-associated conditions were not as effective. Since we are better at treating these conditions, people may live longer than they used to. In other words, there is an increased risk of death with a higher BMI but it is masked by the treatments that are provided for obesity-related conditions.
4. This study did not include morbidity. Based on what I wrote above in #3, even if the people with BMIs 25-35 don’t die, they may suffer from a greater number of chronic medical conditions. They’re not dying from these conditions but the weight is contributing to their overall misery.
5. People who are overweight may get earlier and better treatment. The slightly lower mortality in the overweight group may be because of more aggressive intervention by the medical community in treating chronic illnesses in overweight individuals. In other words, if an overweight person comes into a doctor’s office, the doctor is more likely to screen and aggressively treat things like diabetes or high cholesterol in overweight patients.
6. The improved mortality is only from the contribution of weight alone. In other words, if you are overweight and have diabetes, high cholesterol, or other risk factors, the fact that you’re overweight doesn’t help you any.
The Bottom Line:
My personal practice philosophy is that the more severe the problem, the more intense the intervention. In other words, the more obese someone is or the more problems the weight is causing, the more strict they should be in attacking the problem. This data doesn’t change that general philosophy, it just allows people to be less strict if their BMI is under 35 than they would have been previously. Here are my takeaways from the study:
1. If your BMI is over 35, nothing changes. You really need to find a way to lose weight to decrease your risk of dying before your time.
2. If your BMI is 30-35 and you have diabetes, high blood pressure, high cholesterol, heart disease, or any other condition associated with obesity, you should do whatever you can to lose weight.
3. If your BMI is >25 and you have primarily abdominal obesity you should ignore the above data and do what you can to lose weight.
4. If your BMI is <30 and you have no other medical problems, you can relax a bit.
5. If your BMI is <30 and you want to lose weight to look better, all the information in this blog and the book can still be applied to you (hopeless self-promotion)
6. If your young and your BMI is 30-35, remember that your metabolism slows as you get older and you’re likely to end up in the danger zone. It is easier to maintain your weight as you age if you work on healthy lifestyle habits starting at a younger age.
7. Whatever your BMI, I strongly believe a commitment to regular exercise is one of the most important things you can do to promote a long and healthy life.
8. Beware of the media oversimplifying the data and making it sound like your weight doesn’t matter. Just ignore them. In fact, ignore everything they have to say about health or medicine. It’s a good general rule to live by.
Happy New Year!