Findings

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Consistent with trends in other countries,^1,2 the prevalence of obesity has been rising in Canada.  From 1978/1979 to 2004, the percentage of Canadian adults who were obese rose from 14% to 23%.³  The increasing prevalence of obesity is a major public health concern, as excess weight has been associated with type 2 diabetes, cardiovascular disease, psychosocial difficulties, osteoarthritis, and premature mortality.⁴ 

Cross-sectional data about the prevalence of obesity, however, do not provide information about rates of weight change among individuals.  Longitudinal data are needed for insight about patterns of change that are behind the increase in obesity in Canada.  For instance, a recent longitudinal study showed that almost a third of Canadians whose weight had been in the acceptable range in 1994/1995 became overweight in the following eight years, and about a quarter of those who had been overweight became obese.⁵

As an extension to that analysis, an understanding of obesity requires information about the rate at which individuals are gaining (or losing) weight.  Longitudinal studies in the United States have demonstrated that, in general, adults gain weight up to ages 55 to 60, after which they start to lose weight.^8-10  Few studies have examined rates of weight change in a representative sample of Canadians.  Analyses of data from the 1981 Canada Fitness Survey and the 1988 follow-up, the Campbell Survey of the Well-being of Canadians, demonstrated that body mass index (BMI) was relatively stable over the period between the surveys, but the researchers did not estimate the rate of change.¹¹  Another study based on the same data estimated that in families of at least two people, the weight change from 1981 to 1988 was a gain of 2.9 kilograms (kg) for fathers and a gain of 3.5 kg for mothers.¹²  However, both these studies examined only two points in time, and so could not determine whether rates of weight gain were changing or remaining stable.

The purpose of this analysis, which is based on longitudinal data from the National Population Health Survey (NPHS), is to examine two-year changes in the self-reported weight of the Canadian adult household population from 1996/1997 to 2004/2005, and to determine if the rate of change has been speeding up, slowing down, or has remained stable (see Methods, Definitions and Limitations).

Rate of gain slowing
Amount gained varies
Smaller percentage gaining
Those who gain, gain more
A dynamic process
Cumulative effects
Concluding remarks

Rate of gain slowing

Canadians continue to gain weight, but indications are that the pace has slowed down.  Over the two years from 1996/1997 to 1998/1999, the average self-reported weight of people aged 18 to 64 rose 0.96 kg for men and 0.86 kg for women (Chart 1).  During the 2000/2001-to-2002/2003 interval, average gains were higher:  1.12 kg for men, and 1.02 kg for women.  Over the next two years from 2002/2003 to 2004/2005, Canadians’ weight continued to rise, but the average amount gained was less:  0.74 kg for men and 0.57 kg for women.  Regression results (Appendix Table A) indicate that this pattern of weight gain is statistically significant. Thus, while Canadian adults were still gaining weight, they were gaining significantly less than in the earlier periods.

Amount gained varies

Changes in weight were significantly associated with sex, age group and BMI (Appendix Table A). 

Over the eight years from 1996/1997 to 2004/2005, the average self-reported weight of men and women in all age groups increased.  However, in each two-year interval, people aged 18 to 33 reported significantly greater average gains than did 34- to 49-year-olds, and people aged 50 to 64 reported significantly smaller gains (Chart 2 and Chart 3). 

The decline in the average amount gained in the 2002/2003-to-2004/2005 interval applied to men and women in most age groups.  The exception was men aged 18 to 33 who, on average, gained more weight in the last interval than in the preceding one.

An individual’s BMI was associated with how much his or her self-reported weight changed in each two-year interval (Chart 4 and Chart 5).  On average, overweight people gained 0.8 kg less, and obese individuals, 1.9 kg less, than did people whose weight was in the acceptable BMI range (Appendix Table A).  In fact, during most two-year intervals, people who were obese experienced a mean loss in self-reported weight.

The decline in the average weight gain in the last two-year interval could reflect several processes:  an increase in the number of people losing weight, an increase in the amount of weight lost, a decrease in the number of people gaining weight, a decrease in the amount of weight gained, or a combination of these factors.  Further analyses were undertaken to examine which of these processes were behind the slowdown in the amount of weight gained.

Smaller percentage gaining

During each of the first three intervals, almost half of adults reported that they gained weight, but from 2002/2003 to 2004/2005, 44% of men and 46% of women did so (Table 1).  For men, but not women, this was a significantly lower proportion than in all previous intervals. 

As well, 32% of men reported a loss in weight from 2002/2003 to 2004/2005, a significantly higher percentage than in the first two intervals.  Among women, the proportion losing weight did not differ significantly from one interval to another, ranging from 29% in the first three intervals to 32% in the last. 

Those who gain, gain more

For the men who gained weight, the average amount rose from 4.56 kg in the first interval to 4.99 kg in the last, a statistically significant increasing trend (Table 1).  The average gain among the women who gained weight varied, ranging from 4.39 kg 4.78 kg. A statistically significant trend of an increase in the amount gained among women was also observed.

Among the men who lost weight, there was no statistically significant trend in the amount lost, with the average ranging from 4.42 kg to 4.68 kg.  By contrast, among the women who lost weight, the average loss rose significantly from 4.35 kg in the first interval to 4.91 kg in the last.

Thus, the overall decline in the average weight gain in the last interval (2002/2003 to 2004/05) appears to be driven by a combination of factors; that is, a smaller proportion of men gaining weight and greater losses among the women who lost weight.

A dynamic process

An important consideration in examining trends in weight change is that the same people did not gain, lose or maintain their weight over all two-year intervals.  For example, of the women who lost weight from 1996/1997 to 1998/1999, almost 64% gained weight over the subsequent interval from 1998/1999 to 2000/2001.  Conversely, of the women who gained weight in the first interval, approximately 38% lost weight in the following interval, while approximately 39% gained weight in the subsequent interval. The pattern was similar for men with more than two-thirds of those who lost weight in one interval gaining weight in the next. 

Cumulative effects

The average weight changes among adults in each two-year interval were gains of 0.5 kg to 1 kg (1.1 to 2.2 lbs), and the overall change during the entire eight years was an average gain of 4.01 kg (8.8 lb) for men and 3.44 kg (7.6 lb) for women.  While these amounts may appear relatively small, such changes are cumulative, resulting in a further shift of the distribution of an already predominantly overweight and obese population toward unhealthy weights.³  Even a small shift in the population distribution toward excess weight can have important consequences for the incidence of weight-related diseases.¹⁷

Concluding remarks

The results of this analysis describe the pattern of weight change among Canadian adults from 1996/1997 to 2004/2005.  There has been a slowdown in the average amount of weight gained and an increase in the number of men losing weight.  At the same time, the amount gained by those who gain weight has increased.

Further research is warranted to identify the correlates and causes of these trends.  While poor nutrition and lack of physical activity have been identified as primary contributors to weight gain, alternative explanations such as environmental factors are important and should also be considered.¹⁸  Longitudinal analysis in particular is needed to inform public health strategies aimed at addressing the problem of obesity in Canada.  Analysis of subsequent cycles of the National Population Health Survey will make it possible to determine if the decrease in the rate of weight gain continues in the future.

Although Canadian adults are still, on average, gaining weight, public health messages promoting healthy eating and physical activity have proliferated, and it is possible that without these interventions, the recent rate of weight gain might have been higher.