Weight gain, which can lead to obesity, occurs when more energy is ingested from all foods than expended through normal bodily functions and physical activity. The cause of such an energy imbalance is complex and involves a number of factors including genetics, culture, physical environment, socioeconomic status, and education. All food sources of protein, carbohydrate (sugars and starches), fat, and alcohol contribute calories. All of these nutrients can be converted into body fat if eaten in greater amounts than needed by the body. Sugars, like other macronutrients, contribute calories, but do not uniquely contribute excess calories.

Overweight and Obesity Defined

Overweight and obesity are generally defined using the body mass index (BMI), which is calculated by dividing body weight in kilograms by height in metres squared. Body weight categories are defined in the Canadian Guidelines for Body Weight Classification in Adults as follows:

BMI (kg/m2) Classification Health Risk
< 18.5 Underweight Increased risk
18.5-24.9 Normal Weight Least risk
25.0-29.9 Overweight Increased risk
30 and over Obese  
30.0-34.9 Obese class I High risk
35.0-39.9 Obese Class II Very high risk
> 40.0 Obese Class III Extremely high risk

This weight classification system applies only to people age 18 years and over and does not apply to pregnant or breast-feeding women, or to very muscular people. In children, unlike adults, adiposity varies and BMI changes substantially with gender and age. Therefore, age and sex-specific cut-offs are utilized for children from 2-18 years. For additional information on assessing and monitoring growth in Canadian children, please visit the Canadian Paediatric Society.

Prevalence of Overweight and Obesity in Canada

Obesity rates among children and adults in Canada have increased substantially during the past 25 years, according to the 2004 Statistics Canada Canadian Community Health Survey, which directly measured the height and weight of a nationally representative sample of over 30,000 people.


According to the 2004 survey, 23% of Canadians aged 18 or older, an estimated 5.5 million adults, had a body mass index (BMI) of 30 or more, indicating that they were obese (see table below). Men and women were equally likely to be obese at 22.9% and 23.2%, respectively; however more men (42%) than women (30%) were overweight. Similar data on Americans show that 29.7% of Americans aged 18 or older were obese in 1999-2002, significantly above the 2004 figure for Canada (23.1%). Most of this difference was attributable to the situation among women. Whereas 23.2% of Canadian women were obese, the figure for American women was 32.6%.

Percentage distribution of Canadian adult population according to BMI, 2004

  Adults 18+ (%) Men (%) Women (%)
Underweight 2.0 1.4 2.5
Normal weight 38.9 33.6 44.1
Overweight (not obese) 36.1 42.0 30.2
Obese (BMI ≥ 30) 23.1 22.9 23.2
Overweight or obese (BMI ≥ 25) 59.1 65.0 53.4

Source: 2004 Canadian Community Health Survey: Nutrition

Children and Adolescents

In 2004, the overweight rate for 2 to 17 year-olds was 18% (an estimated 1.1 million), and 8% were obese (about 0.5 million) — for a combined rate of 26%. Compared to Americans, based on the most recent data (1999-2002), the combined overweight/obesity rate of 2 to 17 year-olds was similar in the United States and Canada, but the American obesity rate was slightly higher (10% versus 8%).

Overweight and obesity rates among Canadian children, aged 2 to 17, 2004

  Overweight (%) Obese (%) Overweight/Obese (%)
Total 18.1 8.2 26.2
Boys 17.9 9.1 27.0
Girls 18.3 7.2 25.5
Total 2 to 5 15.2 6.3 21.5
Total 6 to 11 17.9 8.0 25.8
Total 12 to 17 19.8 9.4 29.2

Source: 2004 Canadian Community Health Survey: Nutrition

Factors Contributing to Obesity

Weight gain occurs when more energy (calories) is ingested from all foods than is expended for normal bodily functions (e.g., heart beating, breathing) and physical activity. Fluctuations in energy balance (higher or lower energy intake relative to expenditure) within a meal, day or week are normal and will not necessarily lead to a persistent change in body weight. However, large increases in energy intake relative to expenditure (i.e., positive energy balance) at regular intervals or small consistent increases over a long period of time can result in weight gain, and potentially lead to obesity. All food sources of protein, carbohydrate (sugars and starches), fat, and alcohol contribute calories. All of these nutrients can be converted into body fat if eaten in greater amounts than needed by the body.

Energy In: Typical Breakdown of Caloric Intake
protein 0.15
fat 0.3
carbohydrate 0.55
Energy Out: How Calories are Used
physical activity 0.30
digesting food 0.1
basic body functions 0.6

Many factors contribute to people eating more calories than they use, including dietary and physical activity patterns, environmental and societal influences, and genetics. There is no single factor that causes weight gain.

Sugars and Body Weight

The Dietary Reference Intakes report on macronutrients (2005), published by the US Institute of Medicine in collaboration with Health Canada forms the basis of dietary advice in Canada and the United States. The DRI report reviewed available evidence and concluded that no Tolerable Upper Intake Level could be set for total or added sugars in relation to obesity. The report concluded that “there is no clear and consistent association between increased intake of added sugars and body mass index (BMI).” In fact, according to the DRI report, higher intakes of total or added sugars are associated with a lower incidence of obesity (see below Figure) as the DRI report states that “a negative correlation between total sugar intake and BMI has been consistently reported for children and adults”, and “a negative correlation between added sugar intake and BMI has been observed.” Most recently, a comprehensive expert scientific review of sugars and obesity commissioned by the World Health Organization  published in 2013 concluded that the changes in body weight were linked to changes in caloric intake (i.e. there was no unique effect of sugars as compared to other carbohydrates on body weight). Based on data from Statistics Canada, higher consumption of calories independent of the sources increased the odds of obesity. 

UK (2,197 adults) Men Women
<10 25.9 26
10-13 25.5 24.9
14-16 24.8 24.2
17-20 24.4 24.1
>20 24.1 23.8
Body mass Index (kg/m2 Added Sugars intake (% of energy)  
Scotland (11,626 adults) Men Women
Low 27.2 26.5
  26.4 25.8
Medium 26.1 25.6
  25.4 25.4
High 24.5 24.1
Body mass Index (kg/m2 Added sugars intake by quintile (% of energy)  

Association between added sugars and body mass index (BMI) from the Dietary and Nutritional Survey of British Adults (top) and the Scottish Heart Health and MONICA studies (bottom). A significant negative correlation between added sugars and BMI was observed in both studies. Adapted from US Institute of Medicine, Dietary Reference Intakes for Macronutrients (2005).

Trends in sugar consumption in Canada plotted against obesity rates support this inverse correlation (Figure 2). Other countries, including Australia, the UK, and the US are also showing a declining trend in added sugars consumption while obesity rates have plateaued or continue to rise (Barclay and Brand-Miller, 2011; Welsh et al, 2011).


The consistently observed association between higher intakes of added sugars and lower incidence of obesity may be due to the fact that higher intakes of added sugars are associated with lower intakes of fat, or that those with higher intakes of added sugars have greater energy needs (e.g., greater physical activity). It has been suggested that this association could be due to overweight individuals reducing sugars intakes after becoming overweight; however, this is unlikely because the association is observed across the entire range of BMIs, in children and adults, and in people who do or do not restrict sugars.

Obesity is a complex issue, which involves a combination of factors including but not limited to overconsumption of total Calories and physical inactivity.