Janelle McAlpine MMRes BA (Human Bioscience) BMid
Registered Midwife
BMI is a measure for indicating nutritional status in adults. Body mass index is calculated by dividing a person’s weight in kilograms by the square of their height in metres (kg/m2). For example, an adult who weighs 70 kg and whose height is 1.75 m will have a BMI of 22.9 (70 /1.75 x 1.75) = BMI 22.9 (1).
BMI is easy to calculate, and as such is the most commonly used tool to assess health risk in the broad population. Like other measurement scales it is not perfect. It accounts only for weight and height and lacks details such as sex, muscle mass and age. In some people it overestimates the body fat; in others it underestimates. Waist circumference measurements can complement BMI estimates and improve their accuracy.
BMI is recommended as a health measure in children and adolescents. However during childhood and adolescence the ratio between weight and height varies with sex and age, so BMI values are gender and age specific. If you fall in this category please speak to your doctor about what a healthy BMI is for you.
The prevalence of overweight and obesity in adults aged 18 years and over has continued to rise to 62.8% in 2011-12, from 61.2% in 2007-08 and 56.3% in 1995 (2).
BMI categories
For adults over 20 years old, body mass index falls into one of the following categories.
| BMI | Nutritional Status |
|---|---|
| <18.5 | Underweight |
| 18.5 – 24.99 | Normal range |
| 25 – 29.99 | Overweight |
| 30 – 34.99 | Obese class I |
| 35 – 39.99 | Obese class II |
| 40 or higher | Obese class III |
The ideal BMI range (classed as normal) is the BMI range which evidence suggests has the least amount of fat related risks associated with it. A BMI either below or above this range has associated health risks.
Underweight (1)
Moderate and severe thinness
A BMI < 16.0 is known to be associated with increased risk for ill health, poor physical performance, lethargy and death.
A BMI < 17.0 in adults indicates moderate to severe thinness . Low body mass index has been linked to increased illness in adults.
Underweight
About 3-5% of a healthy adult population have a BMI < 18.5. Environment, resource and government stability tends to dictate the proportion of people with a low BMI in any population. This proportion then determines whether it is a public health risk.
Women who are underweight are at risk of (3,4);
- Increased risk of osteoporosis and fractures,
- Increased risk of death from chronic obstructive pulmonary disease (COPD),
- Increased risk of death from surgery,
- Impaired fertility
Associated undernutrition can also result in;
- Anaemia,
- Poor immunity
- Hair loss
Overweight and obese (1);
At least 2.8 million people around the world die each year as a result of being overweight or obese. Women are more likely to be obese than men, with obesity among women being approximately double that among men in some countries. While ‘rich’ countries are most likely to have obesity in higher numbers, the ‘poorer’ people who are least well off in those countries have higher BMI’s than rich people.
Proportionally, more adult women living in areas of most disadvantage were overweight or obese (63.8%) compared with women living in areas of least disadvantage (47.7%) (2)
Women who are overweight or obese are at higher risk of;
- high blood pressure,
- high cholesterol and triglycerides
- insulin resistance,
- coronary heart disease,
- ischemic stroke,
- type 2 diabetes,
- infertility,
- pregnancy loss,
- breast cancer,
- colon cancer,
- uterine cancer,
- kidney cancer, and
- gall bladder cancer.
The risk of each of these increases along with your body mass index.
Adults living in outer regional and remote areas of Australia were more likely to be overweight or obese (69.5%) compared with adults living in major cities (60.2%) (5)
Some debate exists about whether BMI values are relevant across all populations. This has primarily been examined in Asian countries. Research suggests the BMI threshold for people of Asian descent is lower than standard BMI classifications for risk of type 2 diabetes and heart disease. Their risk is increased with a BMI as low as 22.
Waist circumference
Where your fat is on your body can be an important sign of your risk of developing ongoing health problems. Putting fat on around your midsection carries more risk than if you put it on your hips and thighs. Checking your waist circumference is a simple way to tell if you are carrying excess fat around your middle.
Regardless of how tall you are, an adults woman’s waist measurement of 80 cm or over is an indication of higher than healthy levels of internal fat deposits. These can coat the heart, kidneys, liver, digestive organs and pancreas. This can increase the risk of heart disease and stroke.
Measuring your waist helps to assess your risk of diseases related to body fat by estimating how much fat is carried around your middle. It is important this is done in conjunction with calculating your body mass index (6).
References:
- World Health Organisation. Global database on body mass index 2006. Last updated 2016. http://apps.who.int/bmi/index.jsp?introPage=intro_3.html
- World Health Organisation (European office). Body Mass Index. http://www.euro.who.int/en/health-topics/disease-prevention/nutrition/a-healthy-lifestyle/body-mass-index-bmi
- Kaufman, C. 2015, “Being underweight is risky business: “skinny” isn’t all it’s cracked up to be–underweight brings health risks, too. follow our tips to achieve a healthy weight”, Environmental Nutrition, vol. 38, no. 5, pp.
- Jacobsen, B.K., Knutsen, S.F., Oda, K. & Fraser, G.E. 2013, “Body Mass Index at Age 20 and Subsequent Childbearing: The Adventist Health Study-2”, Journal of women’s health (2002), vol. 22, no. 5, pp. 46-466.
- Australian Bureau of Statistics. Australian Health Survey: Updated Results, 2011-2012. Canberra: 2013.
- The Heart Foundation. http://heartfoundation.org.au/your-heart/know-your-risks/healthy-weight/waist-measurement