We’ve all been told that weight loss should lower the risk of heart disease, diabetes, cancer, and other degenerative diseases. Presumably, weight loss should then also extend life. The problem is that this cannot be assumed by judging the changes in biomarkers of health that changed with weight loss, such as changes in blood pressure, blood sugar regulation, or inflammation; the increased longevity must be observed. The closest thing we have to that are the longitudinal studies which track groups of people for a certain period of time, which for the most part agree that weight loss is associated with increased mortality, while weight gain is associated with decreased mortality.
Weight loss doesn’t mean fat loss of course; it could mean loss of muscle or wasting away of the body due to disease. This reverse causation hypothesis, that weight loss could have been associated with preexisting disease, has no evidence behind it. Anyway, subjects with preexisting illnesses have been excluded from these longitudinal studies, yet those who lost the most weight seemed to carry the highest incidence of cardiovascular disease, diabetes, and in some studies, cancer.
This relationship held true in a study on middle-aged Israeli men. All-cause mortality decreased with increasing weight gain over a follow-up of 18 years! They made sure to ask the participants before the follow-up period if they intended to lose weight, and still found increased mortality among those lost weight intentionally. Similarly the 1946 British Cohort Study, published earlier this year, found that those who lost more than 5kg between the ages of 43 and 53 saw a four-fold increase in mortality from all causes before the age of 65 compared to the moderate weight gain group (between 2 to 5 kg), a finding which isn’t new to this study.
In the Melbourne Collaborate Cohort Study, which was also published this year, researchers measured more than just weight, they looked at waist circumference too in order to track abdominal fat loss. They found that despite 35% of the participants losing weight, only 16% lost size on their waist. However, among those that did, obesity-related cancer deaths and cardiovascular disease deaths were the highest. Those who gained between 8.5 and 13 centimeters on their waist had the lowest rate of obesity-related cancer deaths, and those who gained between 5 and 13cm had the lowest cardiovascular disease deaths.
A recent meta-analysis found a more precise explanation: those who were healthy and obese saw an increase in mortality with weight loss, but those who were unhealthy, saw a decrease. There is more to the story but what most researchers conclude is that recommending weight loss to obese patients is premature, especially if they are healthy. In another study on overweight diabetic subjects, those merely intending to lose weight, whether or not they actually did lose weight, saw a 23% decreased mortality rate. Unintentional weight loss, as in other studies, was found to increase mortality by 58%.
Despite these findings, a plethora of studies, like this NEJM one, have associated abdominal adiposity with increased mortality. Although these longitudinal studies found increased mortality among those who lost weight, there was no information on the metabolic health of the subjects. We know that being obese can be unhealthy metabolically, but that it’s also possible to be lean and have the metabolic syndrome, and a Korean study showed that normal weight metabolically unhealthy people have higher mortality rates than metabolically healthy obese subjects. Metabolically healthy obese subjects however still seem to have higher mortality than metabolically healthy normal weight individuals in some studies. And of course, to contradict myself one more time, in another study, weight loss by personal choice decreased mortality, whereas for health reasons, it increased mortality.
So there is no conclusion here, except that we need much more than waist circumference to tease out the details, and continue to search beyond what can see with our eyes. If weight loss causes stress to the body in those who intend to lose weight, or muscle loss as researchers speculate, it should be measured more precisely to understand why the mortality is higher. Perhaps there are studies like these but so far, it doesn’t seem like there is a precise answer, except that it’s about what’s inside that counts, and fat-shaming is still based on intuition rather than reason.