Whoever said weight loss was a straightforward process has probably never tried to go from a BMI of 40 down to 24 kg/m2. I must admit that only a few years back, in the early stages of my graduate training, I was equally naive. I thought: it’s a case of simple math – once your energy expenditure exceeds your energy intake, you lose weight.
It certainly sounds simple, and yet it rarely is that simple in practice.
Take for example a recent study published in the November issue of Obesity by Gasteyger and colleagues which investigated the effect of the size of the weight loss study, in terms of numbers of participants recruited, on the weight loss achieved among the study participants.
In the study, the authors looked retrospectively at data collected during a prior multicenter trial to see if the size of the study center (how many people were recruited to lose weight at site A versus site B, for example) had any influence on the success of the weight loss treatment.
In total, 22 different centers recruited anywhere from 4 up to 85 participants to undergo a 8 week low calorie diet. The subjects BMIs were in the range of high 30’s to low 40’s (class 2 and 3 obese).
At the end of the intervention the average weight loss among all the subjects was approximately 10 kg (or 10% of initial body weight) – not bad for 8 weeks!
However, the average weight loss for a given weight-loss center ranged from 5.8 to 11.8% – despite the exact same intervention.
Interestingly, the authors found a relationship between the number of subjects recruited at each center and the average degree of weight loss observed among those subjects.
Specifically, for every increase in 10 subjects to the center’s study population, the expected average weight loss for participants in that center increased by approximately 0.5%!
So if two obese people joined the same weight-loss study, the one who joined a bigger center, with more subjects, would be expected to lose more weight on the exact same plan.
Not so straightforward, after all.
How do we explain these findings?
Here’s the authors’ best guess:
“The main reason for the correlation between weight loss and the number of recruited subjects per center may be that principal investigators, study coordinators, and dieticians working at centers with high numbers of recruited subjects have more experience than those working at smaller centers; therefore, they are probably more efficient in treating and counseling subjects during a low calorie diet.”
Have a great weekend,
Gasteyger, C., Christensen, R., Larsen, T., Vercruysse, F., Toubro, S., & Astrup, A. (2010). Center-Size as a Predictor of Weight-Loss Outcome in Multicenter Trials Including a Low-Calorie Diet Obesity, 18 (11), 2160-2164 DOI: 10.1038/oby.2010.118