Sex and Gender-based Analysis of this topic
Body Mass Index (BMI) is calculated by dividing a person’s weight by their height (kg/m2). BMI is the most popular measure used to understand body size and shape. Health Canada uses the World Health Organization’s BMI classification system, which includes the following categories: Underweight (<18.50; further classified as: Severe thinness <16.00; Moderate thinness 16.00-16.99; Mild thinness 17.00-18.49), Normal range (18.50-24.99), Overweight (more than 25.00), Pre-obese (25.00-29.99), Obese (more than 30.00; further classified as Obese class I 30.00-34.99; Obese class II 35.00-39.99; Obese class III more than 40.00). According to data from the 2007/2008 Canadian Community Health Survey (CCHS), approximately 52% of women in Canada have BMIs that correspond to a “normal” or “healthy” body weight, while 27% of women in Canada are deemed overweight, 16% obese, and 4% underweight . Both overweight and underweight categories are associated with various health issues such as diabetes, hypertension, and depression .
Men are more likely than women to be overweight  but women are more likely to be severely obese (BMI more than 40), which is associated with a high risk of developing various health problems. Women are also more likely to occupy both the underweight and overweight extremes of the weight continuum compared to men . Clinical research has demonstrated that women more readily gain and retain body fat than men, independent of physical activity level . Women’s weight also fluctuates more widely during the life course. Because of altered physiology and metabolism, women are most predisposed to weight gain during adolescence, pregnancy, and menopause.
Canadian popular culture presents images of very thin women as socially desirable, which has important implications for women’s psychological and physical health. Women’s perception of having a socially desirable weight and body type has health consequences regardless of their actual body weight. Research suggests that women are more likely than men to equate self-worth with what they think they look like and what they believe other people think they look like . Excluding underweight, women in all weight categories experience more dissatisfaction with their body weight and shape than men do and more frequently engage in weight loss efforts, which may lead to eating disorders . Chronic dieting is very common among women and may be responsible for some health consequences attributed to obesity or overweight.
Higher rates of obesity are found among women in middle and upper-middle income groups. This differs from the pattern demonstrated for men, among whom obesity is most common in high income groups . Women with higher incomes typically have resources available to achieve a socially desirable weight, whereas women in low and middle income groups experience more challenges, including difficulty paying for community recreation and nutritious foods, a lack of leisure time, or lack of public transit services and safe pedestrian paths . Environmental, social and lifestyle factors, which commonly vary with ethnicity, also contribute to population differences in body weight . First Nations (on reserve) and individuals of Aboriginal descent (off-reserve) are more likely than other Canadians to be overweight. Furthermore, women in these populations have higher rates of obesity than their male counterparts (e.g., 41.1% for First Nations women versus 31.8% for men), as well as men and women in the general population [9,10].
There are important limitations to BMI data that are relevant to analyses of women’s body weights. As the BMI only accounts for weight and height, it cannot distinguish between very muscular individuals and those with excess fat. As men, on average, are more muscular than women, the BMI tends to underestimate overweight and obesity among women and overestimate these conditions among men . The BMI may also introduce bias into comparisons of body weight for different racial or ancestral sub-populations (e.g., Caucasian and Black Canadians) whose fat-free body mass, fat patterning, and body dimensions or proportions differ significantly . Recent Health Canada guidelines recommend combining the BMI with a waistline measurement, as centrally distributed body weight has independently been associated with poor health outcomes .
Recent revisions to the BMI guidelines have prompted some concerns for women’s health. A reduction in the underweight cutoff from 20 to 18.5 kg/m2 may delay identification of individuals, primarily young women, in the early stages of anorexia nervosa and of malnutrition among the elderly. Secondly, the decrease in the overweight cutoff from 27 to 25 kg/m2 may not be warranted by evidence of health risks and may lead to undue concern, lower self-esteem and unhealthy weight loss among women who are categorized as overweight according to the new cutoff . Reliance on cross-sectional data and the BMI as an indicator of healthy body weight fails to address the health impacts of dieting, weight fluctuation or sudden weight loss, and psychological health issues that are independent of actual body mass, all of which disproportionately affect women.