Sunday, April 21, 2013

Endocrinological Hypothesis and Obesity: Cause or Mechanism?

I wanted to comment on a well-written essay recently published in the British Medical Journal by Gary Taubes, entitled:


The science of obesity: what do we really know about what makes us fat?


From the essay: The history of obesity research is a history of two competing hypotheses. Gary Taubes argues that the wrong hypothesis won out and that it is this hypothesis, along with substandard science, that has exacerbated the obesity crisis and the related chronic diseases. If we are to make any progress, he says, we have to look again at what really makes us fat


This essay, in my view, has two components - the first is the comment on the competing hypotheses for the cause of obesity, while the second concerns the 'sub-standard' research that has been undertaken throughout the past half-century in obesity research, regardless of the hypothesis.

1) I feel as though Taubes presents a straw-man of the energy balance hypothesis of weight gain. While it may be true that die-hard supporters will cite the 1st Law of Thermodynamics until they are blue in the face, I expect that most of the researchers, clinicians and academics who spend their lives trying to better understand obesity also understand that this Law its limits with respect to weight gain. Clearly, an individual who lives with a chronic, mild negative energy balance will not waste away until they disappear. There are metabolic compensation mechanisms to prevent this from happening (reduction in basal metabolism). Finally, the circular logic presented by Taubes that weight gain is caused by overeating, and that we can tell if an individual is overeating if they have weight gain, does not refute this hypothesis - it only supports it.

2) Taubes refers to the endocrinological hypothesis of obesity - although this hypothesis is never explicitly stated. It goes like this: due to intrinsic physiological abnormalities (say, altered insulin response) a positive energy balance is the result, rather than a cause of the condition of obesity. Sure, so weight gain is the result of aberrant physiological processes. This seems to substitute one mystery for another - what causes these aberrations in physiology? I think that Taubes is confusing this endocrinological hypothesis for a cause, when it is actually a mechanism of the cause. The cause of obesity is now believed to be an obesogenic environment, one that promotes abnormalities in a number of physiological processes associated with weight gain. But these processes are not the cause of weight gain, they are the mechanism through which weight gain results.

3) Taubes also compares smoking and lung cancer with eating and obesity. As previously mentioned on my blog (Obesity Prevention: applying lessons from anti-tobacco campaigns), eating is compulsory, often beneficial, and complex, while tobacco is optional, not beneficial, and simple. A poor comparison, and one that obesity researchers, clinicians and academics alike should discontinue using, especially in published literature.

4) Kudos to Taubes for pointing out that the majority of the research used to inform policy and public health messages is not exactly top-notch. He is correct to suggest that we absolutely need better quality research - that we need to go beyond associations to assess causation. However, there are many ethical considerations that cannot be avoided in this type of research. Also, I would personally agree with Taubes that the money we will save in health care expenditures and personal health (and productivity, quality of life, etc) will outweigh the amount of money that we put in to research obesity, should we conduct the required studies. Convincing the powers that be of this long-term financial benefit is another matter.

Overall, an interesting piece. I don't expect people to abandon the energy balance hypothesis just yet. We do need better quality research - more longitudinal studies and intervention studies - and I think the field is headed in the right direction here, albeit slowly. Do we have to "look again at what really makes us fat", as Taubes suggests? I don't believe so...but have a look and decide for yourself :)

Comment!





2 comments:

  1. But if we gain weight simply because we eat too much and NOT because there are problems inherent in our metabolism, then wouldn't it be at least partly our own fault and not merely extrinsic causes? Imagine!

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  2. You're right Michelle. On an individual level, there is absolutely some personal responsibility for the development of obesity in the vast majority of cases (save for a few, legitimate, cases where there are factors outside of the individual's control). In the case of children, especially before the age of awareness, there is certainly parental responsibility as well. On an individual level it is certainly about energy in and energy out.

    However, there are many people who will simply never be lean. Their genetics dictates that they will have an endomorphic body profile for life. Of course, this doesn't mean that they can't be healthy - this is premise of the relatively new concept that individuals can be metabolically healthy, but obese. This doesn't mean that all individuals who are obese and currently metabolically healthy will be healthy for the rest of their lives...surely some will develop co-morbidities as a result of their obesity; however, there are others who will not develop these co-morbidities.

    On a population level, there are some strong environmental correlates of obesity, such as access to junk food or television watching. When we begin to view obesity on a population level, as opposed to an individual level, it becomes something more. For example, with food intake or exercise patterns the variation within-individuals is much greater than the variation between-individuals. This is because people eat and exercise in patterns that are more similar to one another than they are day-to-day in their own lives- they eat at the same time as one another, and exercise as similar times as well. Take holidays for example, such as Christmas, where everyone over-eats and drinks too much...the variability between people will be quite low (they're all over-eating and drinking too much), but it doesn't follow that they will over-eat and drink too much tomorrow, or the next day, or next Wednesday for that matter. Applying individual-based causes of obesity to a population approach doesn't make much sense...when viewed in the context of a population of people obesity becomes something more than energy in = energy out.

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