Canadian Guidelines for Body Weight Classification in Adults

Questions and Answers for the Public

The Canadian Guidelines for Body Weight Classification in Adults is a tool intended for health professionals and researchers in Canada.

The Question and Answer (Q & A) section below provides more information about weight classification and answers to questions about weight and health.

Questions:

1. What is the Canadian body weight classification system?
2. What is the body mass index (BMI)?
3. What does a high or low BMI mean?
4. What is the waist circumference (WC)?
5. Are there limitations to the body weight classification system?
6. What factors other than weight affect my risk of developing health problems?
7. What are the health risks of a person with a normal BMI and WC who has poor health habits?
8. What are the risks for someone who is 'overweight' but who eats nutritiously and is physically active?
9. According to my BMI and WC I am at increased risk for developing health problems. Where can I get more information on body weight and health risks?
10. I am concerned that my 25-year old daughter is underweight, but according to the weight classification guidelines she is within the 'normal weight' category.
11. Can we use this weight classification system with children?

Answers:

Q1: What is the Canadian body weight classification system?

A: The Canadian body weight classification system uses the body mass index (BMI) and the waist circumference (WC) to assess the risk of developing health problems associated with overweight or underweight.

The system is for use with adults age 18 years and over with the exception of pregnant and lactating women.

Q2: What is the body mass index (BMI)?

A: The BMI is a ratio of weight-to-height.

Research studies in large groups of people have shown that the BMI can be classified into ranges associated with health risk. There are four categories of BMI ranges in the Canadian weight classification system. These are:

• underweight (BMI less than 18.5);
• normal weight (BMIs 18.5 to 24.9);
• overweight (BMIs 25 to 29.9), and
• obese (BMI 30 and over).

To determine your BMI and the associated level of health risk, refer to the Body Mass Index nomogram.

You can also calculate your BMI using this formula:

BMI = weight in kilograms
(height in metres)²

Q3: What does a high or low BMI mean?

A: Most adults with a high BMI (overweight or obese) have a high percentage of body fat. Extra body fat is associated with increased risk of health problems such as diabetes, heart disease, high blood pressure, gallbladder disease and some forms of cancer.

A low BMI (underweight) is associated with health problems such as osteoporosis, undernutrition and eating disorders.

The risk of developing weight-related health problems increases the further one's BMI falls outside the 'normal weight' category. It is important to note that sudden or considerable weight gains or weight losses may also indicate health risk, even if this occurs within the 'normal weight' BMI category.

Q4: What is the waist circumference (WC)?

A: The WC provides an indicator of abdominal fat. Excess fat around the waist and upper body (also described as an 'apple' body shape) is associated with greater health risk than fat located more in the hip and thigh area (described as a 'pear' body shape).

A WC at or above 102 cm (40 in.) for men, and 88 cm (35 in.) for women, is associated with an increased risk of developing health problems such as diabetes, heart disease and high blood pressure. The cut-off points are approximate, so a WC just below these values should also be taken seriously.

In general, the risk of developing health problems increases as WC increases above the cut-off points listed above. Even if the BMI of an individual is in the 'normal weight' range, a high WC indicates some health risk.

Q5: Are there limitations to the body weight classification system?

A: The classification system may underestimate or overestimate health risks in certain adults, such as, highly muscular adults, adults who naturally have a very lean body build, young adults who have not reached full growth, and adults over 65 years of age.

Very muscular adults, such as athletes, may have a low percentage of body fat but a large amount of muscle tissue. This can result in a BMI in the overweight range which may over estimate the risk of developing health problems.

For people who naturally have a very lean body build or for young adults who have not attained their full growth, a BMI just below the 'normal weight' range may not indicate an increased health risk.

For adults over age 65, more research is needed to determine if the cut-off points for the 'normal weight' range differ in any way from those for younger adults.

It is also important to note that BMI and WC are only one part of a health risk assessment. To further clarify risk, other factors need to be considered as well.

Q6: What factors other than weight affect my risk of developing health problems?

A: Age, inherited traits, presence or absence of other conditions such as diabetes, high blood lipids, hypertension, and high blood glucose levels also influence the development of diseases associated with overweight. Risk factors such as poor eating habits, physical inactivity, and tobacco use can play a role in the development of diseases associated with both overweight and underweight.

Consult a health care professional for a more complete assessment of your weight as it relates to health risk. It is important to discuss with your health professional what BMI and WC mean for you as an individual.

Q7: What are the health risks of a person with a normal BMI and WC who has poor health habits?

A: Maintaining a 'normal weight' is one element of good health. However, unhealthy eating habits, low levels of physical activity and tobacco use will increase the risk of health problems even for those within the 'normal weight' range.

Q8: What are the risks for someone who is 'overweight' but who eats nutritiously and is physically active?

A: Although being overweight indicates some risk to health, research suggests that regular physical activity can decrease the risk of several health problems. Equally, a nutritious diet has been shown to decrease some of the risks associated with overweight.

It is important to emphasize that a weight classification system is but one tool to assess health risks in individuals.

Q9: According to my BMI and WC I am at increased risk for developing health problems. Where can I get more information on body weight and health risks?

A: If you are concerned about your weight, consult your physician or health care provider for a more complete assessment of your individual circumstances and risk factors. You may also contact your local public health department or community health centre for more information and resources. The phone numbers of these organizations are in your telephone book.

Health Canada develops and promotes guidelines for healthy eating and physical activity. The guidelines include Canada's Food Guide and  Tips to Get Active (for children, youth, adults and older adults). These are tools to assist you in your efforts to make healthy food choices and incorporate physical activity into your daily life style and are available at: Food and Nutrition and Health Canada.

Q10: I am concerned that my 25-year old daughter is underweight, but according to the weight classification guidelines she is within the 'normal weight' category.

A: In general, Canadian adults who have a BMI within the 'normal weight' category have the least risk of developing weight-related health problems. It's important to remember that the BMI is a tool that measures body weight at one point in time. A marked weight change, either weight gain or weight loss, may place a person at risk even if they remain within the same BMI category. Using unhealthy practices such as restrictive eating habits to manage body weight can also increase a person's risk of health problems, even if they are within the 'normal weight' category.

A health care professional can provide a more complete health assessment of individual circumstances and risk factors such as weight history (i.e., patterns of weight gain and/or weight loss). You may also wish to contact your local health unit or community health centre to find out about services available in your community.

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

A: The 2003 Canadian Guidelines for Body Weight Classification in Adults applies only to Canadians 18 years of age and older. Currently, Health Canada does not have guidelines for classifying weight in Canadians under 18 years of age and there are no Canadian standards for tracking growth in children. However, Dietitians of Canada has published a collaborative statement on growth monitoring, in conjunction with the Canadian Paediatric Society, the College of Family Physicians of Canada and Community Health Nurses Association of Canada. This collaborative statement recommends the use of charts developed by the Centers for Disease Control and Prevention in the United States to monitor growth in children ( at the individual level).

For more information on this statement, consult the  Dietitians of Canada web site.

At the population level (when looking at prevalence rates), the charts developed by Cole et al are currently recommended, the reference for the article is the following:

Cole TJ, Bellizzi MC, Flegal KM & Dietz WH (2000): Establishing a standard definition for child overweight and obesity worldwide: international survey. BMJ 320, 1240-1243.

© Her Majesty the Queen in Right of Canada (2003). All rights reserved.
Cat. H49-179/2003-2E-IN
ISBN 0-662-33982-7