Canadian Guidelines for Body Weight Classification in Adults

Questions and Answers for Professionals

These questions concern the classification system described in the report Canadian Guidelines for Body Weight Classification in Adults.

A summary of the classification system is also available in a "Quick Reference Tool" which has been developed to accompany the report.

Questions:

What is the classification system?
If it is body fat that is associated with weight-related health problems, are the BMI and WC accurate measures of this?
Why was this document produced?
How does the new system differ from the 1988 guidelines?
Why is a comprehensive health assessment recommended when using the weight classification system with individuals?
What are the special considerations for adults over age 65?
I work with a multi-ethnic population, what special considerations should I be aware of when using the system?
How does physical activity affect health risk?
Is an overweight or obese individual who is fit at increased risk of developing health problems?
What other groups need special consideration when using the weight classification system?
Can we use this weight classification system with children?

Answers:

Description of the body weight classification system

Q1: What is the classification system?

A: The system is a tool used to identify health risks associated with body weight in individuals and in populations. The system uses two measures: the body mass index (BMI) and the waist circumference (WC). Both measures are associated with the long term health of individuals and populations. BMI is an index of weight to height (kg/m²). Because BMI is strongly related to the diseases associated with underweight and overweight, it can be used as an indicator of health risk. WC is positively correlated with abdominal fat. It is used as an independent indicator of health risk associated with abdominal obesity.

Q2: If it is body fat that is associated with weight-related health problems, are the BMI and WC accurate measures of this?

A: Excess body fat and particularly excess abdominal fat is linked to health problems. However, methods to directly measure total body fat or abdominal fat are not practical for widespread use. BMI and WC are not direct measures of body fat but they are significantly correlated with total body fat and abdominal fat respectively. However, individuals with body builds or proportions that vary considerably from the average, for example highly muscular individuals, may not be at the level of risk indicated by their BMI or WC.

Q3: Why was this document produced?

A: Research has added new knowledge about the relationship between weight and health since the Canadian Guidelines for Healthy Weights were released in 1988. In addition, a weight classification system was released by the World Health Organization (WHO) in 2000 and it was recommended by WHO that this system should be adopted internationally.

Comparison to the 1988 Canadian Guidelines for Healthy Weights

Q4: How does the new system differ from the 1988 guidelines?

A: Age limits
The 1988 guidelines were intended for use with people aged 20 to 64 years. The updated system can be used with people age 18 and over with no upper age limit.

Labels for the BMI Categories
The updated system provides a label for each BMI category and uses the terms 'underweight', 'normal weight', 'overweight', and 'obese'.

Lower cut-off point for the 'normal' BMI category
The lower cut-off point of the 'normal weight' range has been lowered from 20.0 to 18.5.

Range for the 'overweight' category
The BMI range for the 'overweight' category is 25.0 to 29.9.

Subcategories for the 'obese' category
The 'obese' category has been divided into three subcategories: Obese Class I (30.0 - 34.9), Obese Class II (35.0 - 39.9) and Obese Class III (40.0 and over).

Use of the waist circumference
The updated system uses WC in place of the waist to hip ratio (recommended in the 1988 guidelines).

Comprehensive health assessment

Q5: Why is a comprehensive health assessment recommended when using the weight classification system with individuals?

A: The weight classification system is based on data collected from studies in large populations. Due to the wide variation in individuals within a population, additional factors must be considered to help clarify an individual's health risk associated with weight and distribution of fat. Risk of developing a particular health problem is also dependent on the presence of other risk factors (for example, hypertension, dyslipidemia, genetic predisposition, individual weight history, and age). Other influencing factors include health behaviours such as tobacco use, eating habits and physical activity patterns. It should be noted that with weight management, the cut-off points included in the weight classification system are not meant to represent intervention targets for individuals. These targets should also be based on individual characteristics.

Adults over age 65

Q6: What are the special considerations for adults over age 65?

A: The weight classification system is recommended for use with adults over age 65, however, the following points should be considered when using the system with this group:

'Normal weight' category
Research suggests that the cut-off points for the 'normal weight' category for adults over age 65 may be slightly different than for younger adults. The lower cut-off point for 'normal weight' may begin somewhat above 18.5 and the upper cut-off point for 'normal weight' may extend into the overweight range.
Waist circumference
Although there is conflicting evidence about the use of waist circumference with seniors, at the present time the WC cut-off points provided in the updated system can be used as one indicator of health risk in seniors.

Ethnic and racial groups

Q7: I work with a multi-ethnic population, what special considerations should I be aware of when using the system?

A: The updated weight classification system is recommended for use with all Canadian adults (except for pregnant and lactating women). However, data used to support the classification system were derived predominantly from studies in Caucasian populations. There is evidence that certain ethnic or racial groups may differ from Caucasians in their levels of total body fat at a given BMI, in their fat distribution patterns, and in their degree of health risk. Differences may be influenced in part, by differences in body build or body proportions. For some groups such as Chinese and people from South Asia (India, Pakistan, Bangladesh and Sri Lanka), the health risks may be greater than for Caucasians at the same BMI. For others, such as Black populations, health risks may be lower. For First Nations, Inuit and other Canadian Aboriginal populations, research is required to determine if the association between weight and health risk differs from that in Caucasians. At the present time, for all groups within the Canadian population, the BMI and WC values provided in the updated weight classification system can be used as one indicator of health risk.

Physical activity / Physical fitness

Q8: How does physical activity affect health risk?

A: Research has shown that regular physical activity can decrease the risk of several health problems and improve health and longevity. Canada's Physical Activity Guide recommends that Canadian adults accummulate 30-60 minutes of moderate physical activity most days, preferably daily.

Q9: Is an overweight or obese individual who is fit at increased risk of developing health problems?

A: Some research suggests that cardiorespiratory fitness can modify some of the negative effects of excess body weight and body fat. However, broadly based long-term studies in a variety of population groups are needed to substantiate this finding and to determine to what extent physical fitness can modify health risks.

Special considerations for other specific groups

Q10: What other groups need special consideration when using the weight classification system?

A: The BMI may overestimate risk in the following groups: young adults who have not reached full growth, adults who naturally have a very lean body build and highly muscular adults.

Children

Q11: Can we use this weight classification system with children?

A: The updated system applies only to Canadians age 18 years and over. However, a weight classification system for children and youth is needed. Factors that make the development of a classification system for this age group a challenge include the difficulty in deriving numbers based on weight-related health outcomes plus the wide variation in growth rates among children and youth. Work has been carried out in the USA to develop a system for children and youth and there is currently an international committee working on the development of a system for this age group. This work would inform future work to develop a weight classification system for Canadian children and youth.

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