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Obesity: Definition, Diagnosis and Causes

child's feet on a scale

According to the World Health Organization (WHO), overweight and obesity are characterized by “abnormal or excessive fat accumulation that may impair health.”

The WHO says that globally:

  • Obesity has nearly tripled since 1975
  • In 2016, more than 1.9 billion adults, 18 years and older, were overweight. Of these over 650 million were obese.
  • 39% of adults aged 18 years and over were overweight in 2016, and 13% were obese
  • Most of the world's population live in countries where overweight and obesity kills more people than underweight
  • 41 million children under the age of 5 were overweight or obese in 2016
  • Over 340 million children and adolescents aged 5-19 were overweight or obese in 2016

But how do you judge whether you are overweight or obese? To help you find out, you can consult Health Canada’s Canadian Guidelines for Body Weight Classification in Adults, which are based on WHO recommendations. To estimate adult overweight and obesity, the guidelines use the Body Mass Index (BMI), a simple measure of your weight relative to your height (your weight divided by the square of your height, expressed in kg/m2).

Click here to access the BMI calculator at Université Laval

Health Canada points out that the Guidelines should not be used to assess overweight and obesity for persons under the age of 18 or over the age of 65, pregnant women and breastfeeding women.

Health Canada classifies health risks based on BMI as follows:

Classification BMI category Risk of developing health problems
Insufficient weight < 18.5 Higher
Normal weight 18.5 to 24.9 Less
Excess weight 25.0 to 29.9 Higher
Obesity, Class I 30.0 to 34.9 Elevated
Obesity, Class II 35.0 to 39.9 Very high
Obesity, Class III > 40.0 Extremely high

Other factors are also taken into consideration, in addition to BMI, including the waist circumference, which makes it possible to determine excess abdominal fat (this is referred to as abdominal obesity when the waist is more than 88 cm in women and 102 cm in men) and the waist to hip circumference ratio which provides an even more precise picture of the distribution of fat in the body (the ratio is considered elevated when the result is higher than 1 in men and higher than 0.85 in women).


Obesity is a chronic, complex and evolving disease that depends on a combination of physical activity, diet, socio-economic status, ethnicity, environment and food marketing, genetic predisposition and depression.

When you burn fewer calories than you consume, obesity occurs gradually. In the past, the health sector attributed obesity to a lack of willpower and self-control that could lead to excessive eating and a lack of physical activity. Today, health professionals are now well aware that obesity is a complex medical problem that is linked to genetic, environmental, behavioural and social factors.

Recent research has also shown that certain genetic factors affect the appetite and metabolism of fats that lead to obesity and increase the risk to a person who is genetically predisposed to weight gain.

While genetic predisposition may contribute to obesity, it is not its main cause. Environmental and behavioural factors exert greater influence, including consumption of excess calories from high-fat foods and little or no daily physical activity. And various psychological factors can also contribute to obesity, such as a sense of worthlessness, guilt, emotional stress, or trauma that can trigger a defensive mechanism such as over-eating.

Some health conditions (binge eating, Cushing’s disease and polycystic ovary syndrome) can also lead to weight gain and obesity.

Risks to your Health

Obesity can lead to significant health risks including respiratory problems (e.g., sleep apnea, asthma), certain cancers (e.g., prostate and colon cancer in men, breast and cervical cancer in women), cardiovascular diseases, reproductive system disruptions, depression, diabetes, gallbladder and liver problems, gastroesophageal reflux disease (GERD), hypertension, high cholesterol and musculoskeletal problems.

In addition to physiological complications, obesity is also linked to psychosocial problems such as low self-esteem, discrimination, difficulty finding employment, and reduced quality of life.

Impacts on Your Sleep

Sleep well

Metabolic Syndrome

Obesity can lead to metabolic abnormalities known as metabolic syndrome. Metabolic syndrome is not a disease, it is a condition that, untreated, can greatly increase your risk of developing several chronic diseases. It is diagnosed when you have three of the following conditions:

  • High blood pressure (≥ 130/85 mm HG, or medication)
  • High blood glucose (≥ 5.6 mmol/L, or medication)
  • High triglycerides (≥ 1.7 mmol/L, or medication)
  • Low HDL cholesterol (< 1.0 mmol/L in men and < 1.3 mmol/L in women)
  • High waist circumference (≥ 102 cm in men, 88 cm in women: may vary depending on ethnicity)

Obstructive sleep apnea is a manifestation of metabolic syndrome. When you do not sleep much or if your sleep is disturbed, you have a tendency to eat more the following day because sleep contributes to regulating hormones such as leptin, which sends out the signal for satiety, and ghrelin, which stimulates the feeling of hunger.

Find answers with our online screening form.

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Obesity prevention must begin in childhood with good eating habits and behaviours. The following tips for parents can help establish healthy eating habits:

If you already have good eating habits and you eat well, it is easier to encourage your children to do the same, because they will follow your example. The following advice to parents can help establish good eating behaviours:

  • Eat as a family, in a calm and welcoming environment, without television, so that meals can be a pleasant activity for your children
  • It is best not to congratulate your children when they finish their plate (the important message to convey is that eating well is normal, it is not a way to please one’s parents
  • Avoid rewarding or punishing your children using food
  • Water is the ideal beverage for your child, so even consuming natural fruit juice should be limited to 1 glass per day

Why is it so important to eat well for good health? This handy guide clears up some common misconceptions.

A few recommendations from Health Canada

To achieve and maintain a healthy weight, Health Canada recommends that you moderate the amount of food you eat. Serving sizes have a direct impact on calories, so avoid eating in restaurants with large servings or buffets.

Keep in mind that alcoholic beverages and sugary beverages are high in calories, so limit those foods as well. And above all, make room for physical activity in your daily lives. When you move, you burn calories!

If you are concerned about your weight, consult a physician or other healthcare professional for a complete assessment of your weight and the risks to your health. Discuss what your BMI and waist circumference means to you.

And finally, Health Canada recommends that you avoid miracle diets. While some may result in rapid weight loss, they generally require avoiding certain types of food and the weight lost is quickly regained upon returning to a normal diet.

For more information on obesity:

World Health Organization

Public Health Agency of Canada

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