Obesity is associated with increased risk of certain health outcomes – diabetes, cardiovascular disease, etc. However, as Peter has discussed previously (here and here), a relatively large minority of individuals with obesity are metabolically healthy. This is due in large part to their body fat distribution, which is typically characterized by large amounts of subcutaneous fat in the lower body, and relatively little abdominal fat. Fat cells in these two depots differ in important ways, which means that accumulating fat in the abdomen is generally much worse for your health than accumulating fat in your legs and butt (for the full details, visit this post examining how fat cells can actually protect you from chronic disease).
Why am I bringing this up?
Because new research suggests that for a given BMI, Canadians now carry more abdominal fat (and more fat in general) than they did in 1981.
This new analysis was performed by Dr Ian Janssen and colleagues, and compares directly measured waist circumference and skinfold data from the Canada Fitness Survey (1981) and the Canadian Health Measures Survey (2007-2009). Combined, the two surveys included 20,000 children and adults, and are representative of the Canadian population.
As you might expect, BMI increased in every age group between 1981 and 2007-2009. No surprise there. However, they also found that for a given BMI, current Canadians have higher waist circumferences and skinfold thicknesses than in 1981. These differences were fairly substantial – waist circumference for a given BMI was increased by 1.1 cm for men, 4.9 cm for women, 1.6 cm for boys, and 4.1 cm for girls.
So if you were to find a Canadian who was the same age, sex, and BMI as me back in 1981, it is likely that they had less body fat and a smaller pant size than I do (on average). One might reasonably expect that they would have had more muscle than me as well.
In addition, the impact of a 1-unit increase in BMI has also increased since 1981. From the paper (emphasis mine):
…the increase in WC and SFS for a 1 unit increase in BMI was higher in 2007–2009 than in 1981 for most age and sex subgroups. For example, in 1981 an increase in BMI of 1 kg/m2 in 20–69 year old women was associated with a corresponding increase in WC of 1.98 cm; this increased to 2.22 cm in 2007–2009. In 1981, an increase in BMI of 1 kg/m2 in 20–69 year women with a BMI <30 kg/m2 was associated with an increase in [sum of 6 skinfolds] of 6.10 mm; this increased to 7.60 mm in 2007–2009.
What does all this mean? Your guess is as good as mine, although the implications are almost certainly bad. As the authors point out, waist circumference is a better predictor of obesity-related death and disease than BMI, so the fact that Canadians are now carrying disproportionately more abdominal fat than in the past bodes poorly for us.
Our friend Yoni Freedhoff has recently suggested that their may be less metabolically healthy obese individuals than in the past. If that’s true, this shift in body fat distribution could be one explanation (population-level reductions in physical activity and/or increases in sedentary behaviour would also be likely contributors). I haven’t seen any quantitative analyses to support Yoni’s assertion (he was just eye-balling a figure in a recent paper), but it would be a pretty straightforward analysis with the right dataset. I wouldn’t be surprised to see something on this topic soon, if it isn’t out there already.
Janssen, I., Shields, M., Craig, C., & Tremblay, M. (2011). Changes in the Obesity Phenotype Within Canadian Children and Adults, 1981 to 2007–2009 Obesity DOI: 10.1038/oby.2011.122