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Its called the obesity paradox. Basically, its the observation that in many chronic disease states, people who are heavier seem to live longer than those who are lighter.
This is a finding that is ripe for jumping to conclusions, and headlines often imply that years of your physician telling you to lose a little weight can now be safely ignored.
A large study in the Annals of Internal Medicine examines the relationship between BMI and overall mortality in a population of type 2 diabetics with the title The Obesity Paradox in Type 2 Diabetes Mellitus.
This terminology, in my opinion, is a bit of a disservice to some of the great paradoxes like dividing by 0 and this sentence is a lie. Heres the deal:
This was a large prospective, English cohort of around 10,000 patients. The authors assigned the participants into the classic BMI categories at their first visit (underweight, normal weight, overweight, and obese) and then followed them for about 10 years. It turned out that survival was lowest in the underweight group, and kept right on improving as weight increased.
Now, after adjustment for the things youd want to adjust for (age, sex, smoking, chronic diseases, duration of diabetes, etc), it turned out that, compared to those of normal weight, the overweight people lived longer, the underweight people lived shorter. The obese people turned out to have similar adjusted rates as the normal weight people.
There are two things going on here that may explain these results, and neither is a paradox.
The first has to do with the one-time measurement of weight. Put simply, its very likely that change in weight is more important than what the weight actually is. Imagine a train hurtling down the tracks towards a ravine we might care just as much about the speed of travel as its position.
Has the individual been gaining weight? Losing weight? At what rate? These factors may tell us much more about the health of the individual than what the weight actually is.
The second issue is that type 2 diabetes is a disease that is caused by overweight and obesity. An otherwise healthy diabetic will be an overweight diabetic. Thats simply who gets diabetes. In the general population, its relatively clear that overweight and obesity are associated with higher mortality. In diabetics, wed expect that curve to shift to the right. In other words, a survival benefit of being overweight is exactly what wed expect to see in a disease that is caused by being overweight. No paradox, and importantly, NO suggestion that gaining weight would benefit the type 2 diabetic with a normal BMI. But of course, if you want to avoid paradoxes, youll be fine if you dont go crossing your own timestream.