Changes in the Obesity Phenotype Within Canadian Children and Adults, 1981 to 2007–2009
We investigated whether the relationships between BMI, waist circumference (WC), and sum of 5 skinfolds (S5S) have changed over time in Canadians. Anthropometric data on 7–69 year old from national representative surveys conducted in 1981 (n = 15,688) and 2007–2009 (n = 4,987) were examined. WC and S5S were regressed on BMI while controlling for age in each survey by sex and age group (child, adult). The results indicate that increases in WC and SFS for a one unit increase in BMI were higher in 2007–2009 than in 1981. For example, in 20–69 year old women in 1981 an increase in BMI of 1 kg/m2 was associated with corresponding increases of 1.98 cm in WC and 6.10 mm in SFS; these values increased to 2.22 cm and 7.60 mm, respectively, in 2007–2009. In conclusion, present day Canadians have higher WC and skinfold thickness values for a given BMI than Canadians did 30 years ago.
The BMI is the most commonly used measure of obesity in clinical and research settings. Most obesity surveillance systems rely on BMI, largely because it is practical to measure. It is widely believed that a high BMI increases morbidity and mortality risk because it is a marker of high body fat (1). Therefore, an assumption when using BMI to monitor trends in obesity over time is that the relation between BMI and body fat remains constant. Evidence is emerging that this assumption is untrue. For instance, using US adult population—based data from 1954–2004, Elobeid et al. (2) found that the increases in waist circumference (WC) over time were slightly above those expected from BMI increases. Similarly, using British child population—based data from 1977–1997, McCarthy et al. (3) found that the WC percentile scores increased more than the BMI percentile scores.
Additional studies in other population groups are needed to confirm these observations. Future studies should also consider different body composition measures in addition to WC. The purpose of this study was to investigate whether the relationship between BMI and other adiposity measures (WC, skinfolds) has changed over time in Canadian children and adults. We hypothesized that present day Canadians have higher WC and skinfold thickness values for a given BMI than they did 30 years ago.
METHODS AND PROCEDURES
Contemporary estimates of obesity were based on the 2007–2009 Canadian Health Measures Survey (CHMS), a representative sample of Canadians living in private households (4,5,6). Anthropometric measures were collected by trained technicians in a mobile exam center at 15 sites across Canada. The CHMS estimates in this article are based on 4,987 respondents aged 7–69 years. Historical estimates of obesity were based on the 1981 Canada Fitness Survey (CFS) (7,8). The CFS estimates in this article are based on 15,688 respondents aged 7–69 years. Both survey samples were representative of the off-reserve, nonmilitary, noninstitutionalized Canadian population (4,5,6,7,8).
Identical anthropometric measurement techniques were used in both surveys. Weight was measured to the nearest 0.1 kg and height to the nearest 0.1 cm. BMI was calculated as weight in kg divided by height in m2. WC was measured according to the World Health Organization protocol to the nearest 0.1 cm at the mid-point between the last rib and the iliac crest. The skinfolds were measured using Harpenden skinfold calipers (Baty International, West Sussex, UK) at five sites (triceps, biceps, subscapular, iliac crest, calf) and the sum of 5 skinfolds (S5S) was calculated (9). Skinfolds were not obtained on CHMS participants with a BMI ≥30 kg/m2, and therefore those with a BMI ≥30 kg/m2 were removed for all skinfold analyses for both surveys.
Analyses were performed separately by sex and age groups (see Tables 1 and 2). Significant differences between mean BMI, WC, and S5S estimates across the CFS and CHMS were assessed using t-tests. To determine if the relation between BMI and the other adiposity indicators changed over time, WC and S5S were regressed on BMI within each survey using linear regression. For these regression analyses, age was centered at the mid-point of the age range of each age group (e.g., 30.0 years for the 20–49 year old age group). For the adult age groups, BMI was centered at 25. For the child age groups, BMI was centered and at the sex-specific International Obesity Task Force cut-points (10) for the mid-point of the age range that were equivalent to a BMI of 25. Differences between the intercepts and slopes of the regression models for the CFS and CHMS were interpreted using the 95% confidence intervals (CI). Differences between the surveys were assumed when the 95% CI of the CFS and CHMS did not overlap.
Table 1. Mean BMI, waist circumference, and sum of sum of 5 skinfolds values according to sex and age, 1981 Canada Fitness Survey and 2007–2009 Canadian Health Measures Survey
Table 2. Modeling of waist circumference and sum of 5 skinfolds from BMI according to sex and age, 1981 Canada Fitness Survey and 2007–2009 Canadian Health Measures Survey
Descriptive information on the obesity measures in the 1981 and 2007–2009 samples are shown in Table 1. Regardless of age and sex, the BMI, WC, and S5S estimates were significantly higher in the 2007–2009 CHMS than in the 1981 CFS.
Table 2 shows the coefficients from the multivariate model of WC and S5S centered at a BMI of 25 kg/m2 for adults (or equivalent age-appropriate BMI for children) and the mid-point of the age ranges. The slopes represent the linearly increasing relationship between BMI with WC and S5S. As shown by the slope coefficients and their 95% CI, 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 S5S of 6.10 mm; this increased to 7.60 mm in 2007–2009.
The intercepts in Table 2 represent the predicted WC and S5S for an individual with a BMI of 25 kg/m2 at the mid-point of the age range. At a BMI of 25 kg/m2 (or age appropriate BMI for children), Canadians in 2007–2009 had higher WC values than Canadians in 1981. These differences were 1.1 cm for men, 4.9 cm for women, 1.6 cm for boys, and 4.1 cm for girls. Similarly, at a BMI of 25 kg/m2 (or age appropriate BMI for children), Canadians in 2007–2009 had higher S5S values than Canadians in 1981. These differences were 5.0 mm (9.0%) for men, 9.8 mm (11.5%) for women, 7.8 mm (13.7%) for boys, and 11.1 mm (14.3%) for girls.
Consistent with our a priori hypothesis, the findings of this study indicate that present day Canadians have higher WC and skinfold thickness values for a given BMI than they did 30 years ago. Because WC and S5S are strong correlates of abdominal (both subcutaneous and visceral) and total fat (11,12), these findings suggest that body composition has changed more adversely than body weight since 1981.
A secular change in the obesity phenotype to one with more abdominal fat, as assessed by WC, is troubling as WC is a stronger predictor of obesity-related morbidity and mortality risks than BMI (13,14,15). The 1–5 cm higher WC values for Canadians with a BMI of 25 kg/m2 in 2007–2009 compared to Canadians with the same BMI in 1981 are not trivial. In fact, previous findings indicate that each 1 cm increase in WC in adults is associated with a 4–5% increased odds of hypertension, a 2–7% increased odds of dyslipidemia, and a 6–7% increased odds for having a clustering of at least three cardio-metabolic risk factors (13). Similarly, the 9–15% higher skinfold thickness values for Canadians with a BMI of 25 kg/m2 in 2007–2009 than Canadians with the same BMI in 1981 could be expected to contribute to higher risks for many obesity-related comorbidities.
In addition to changes in diet and physical activity, recent reviews have highlighted a number of other potential contributors to the recent global obesity epidemic (16). Some of these factors, such as sleep deprivation, endocrine disruptors, and some pharmaceutical agents can also impact body fat and fat distribution. As we did not study these factors directly, we can only speculate about their role in our findings. Future studies need to explore the causes of the shift in the obesity phenotype.
Although BMI is a useful surveillance tool in monitoring the prevalence of obesity, a key message of this study is that caution needs to be applied when relying on BMI alone, particularly when examining changes in obesity over an extended period of time (e.g., a few decades). For instance, the prevalence of obesity as assessed by BMI in Canadian adolescents increased approximately threefold (3%–8%) between 1981 and 2007–2009 (17,18). During the same time frame there was a sevenfold increase (2%–13%) in the percentage of Canadian adolescents with abdominal obesity as assessed by WC (19). These findings suggest that even in the absence of a change in population obesity prevalence as determined by BMI, the population health consequences of obesity seem likely to increase more than anticipated.
The authors declared no conflict of interest.