In case you were looking for more evidence that losing weight is more complicated for some people than simply eating less and moving more, an interesting study published this week from a group in the US demonstrated that the degree of weight loss followed by caloric restriction is determined by an individual’s biology1. This study happened to catch my eye because, as mentioned in a previous post, I’m in the midst of taking a course on raising awareness about weight bias and stigma.
Basically what the researchers did was put 12 participants on a diet that contained 50% of the calories required for weight maintenance for 6 weeks and measured their weight loss. During the 6 weeks the participants obtained all of their calories via supplemental shakes (Ensure) and they were not allowed to engage in any physical activity. After the intervention researchers measured the relative proportion of body weight that each participant lost. As I’ve already eluded, even after accounting for various factors like baseline weight, age, gender and ethnicity, some people lost more weight than others. These results may come as no surprise for those who have struggled to lose weight while seeing others following similar regimens shed pounds more easily, but the big question is why?
The researchers did another series of measurements to try to get to the bottom of this. A few weeks before putting the participants on their calorie restricted diets, they measured their metabolism in response to a 24 hour fast. They did this using a whole body calorimeter*. For those that may not know, whole body calorimeters are contained living suites where participants “live” for a short period of time. Metabolism (ie. daily calories expended) is determined by measuring the ratio of oxygen inspired and carbon dioxide expired.
*Before moving to Egypt I managed a research laboratory at the University of Alberta that had one of only two (at least at the time) calorimetry units in Canada.
Anyways, what the researchers found was that not everyone’s metabolism responded the same way to the fast. Some people’s metabolism slowed down whereas other’s did not. What’s interesting about these results is that when correlated to their later weight loss, it was the people whose metabolism slowed down the most during the fast that lost the least amount of weight during the 6 week low calorie diet. Some people’s metabolism adjusted (ie slowed down) when challenged with less food. The researchers referred to these people as having a “thrifty” metabolism.
From an evolutionary perspective, when calories were sparse, you can understand why this would be a good thing. Our early human ancestors would’ve been able to conserve their energy reserves even when food wasn’t readily available. But in today’s environment where this is rarely the case for most people, you can see how it can become problematic.
In terms of dietary recommendations the results of this study doesn’t really change anything – in order to lose weight, people still need to create an energy deficit. However, I think it illustrates the point that for a portion of the population accomplishing this deficit is not easy. Their biology is working against them. It’s just another reason why people struggling with obesity deserve our empathy, rather than our judgement.
1. Martin Reinhardt, Marie S. Thearle, Mostafa Ibrahim, Maximilian G. Hohenadel, Clifton Bogardus, Jonathan Krakoff, and Susanne B. Votruba.A Human Thrifty Phenotype Associated With Less Weight Loss During Caloric Restriction. Diabetes, May 2015 DOI: 10.2337/db14-1881