What is Body Mass Index and why is it important to my health?

The Body Mass Index (BMI) is a method of determining if an adult is over or underweight. The BMI is determined by dividing a person’s weight in kilograms by their height in metres squared.

  • An adult with a BMI of less than 18 is considered to be underweight.
  • An adult with a BMI between 18 and 25 is considered to be of a normal, healthy weight.
  • An adult with a BMI of between 25 and 30 is considered to be overweight.
  • An adult with a BMI of over 30 is considered to be obese.

An elevated BMI of over 25 is just one of many factors that can contribute to the development of chronic disease such as cancer, diabetes, osteoarthritis, high blood pressure and heart disease. BMI is only one of many factors used to assess a person’s risk for disease. Other factors that may be important to consider include waist circumference, smoking, physical activity level, diet, blood sugar levels, blood pressure, cholesterol levels and family history of disease.

A low BMI of under 18 is a risk factor that can contribute to chronic diseases such as malnourishment, osteoporosis, infertility and poor immunity. Being underweight may also be a risk factor, particularly in elderly people, for contracting respiratory disease. In some cases, being underweight or unexplained weight loss may be early signs of underlying disease.

BMI does not measure body fat and it is only one piece of a person’s health profile. Two people can have the same BMI, but a different percentage of body fat. A bodybuilder, for example, with a large muscle mass and a low percent body fat may have the same BMI as a person who has more body fat because BMI is calculated using weight and height only.

The relation between body fat and BMI differs with age and gender. Women, for example, are more likely to have a higher percent of body fat than men with the same BMI. On average, older people may have more body fat than younger adults with the same BMI.

Treatment options:

As with all conditions your Doctor should be consulted to diagnose and treat a weight problem. Your Doctor can recommend a weight management programme to suit your needs and monitor your progress at regular intervals. Your Doctor may recommend a combination of the following:

  • Long-term change in eating patterns.
  • Behaviour therapy – modification and reinforcement, nutrition education, increase in physical activity.
  • Medications.

Diet hints:

  • Eat a variety of nutritious foods including wholegrain breads and cereals, vegetables and legumes and fruits.
  • The diet should be low in fat (particularly saturated fat), sugar and salt.
  • Limit alcohol intake.

See the Weight Management – Meal Plan or the Underweight Diet topic on the Healthpoint.


A child’s level of body fat changes over the years as he/she grows. Girls and boys differ in their body fat as they mature. This is why the formula used to ascertain the BMI for children is the same as that for adults, however, the results are interpreted differently. The BMI measurement method for children, also referred to as BMI-for-age, is gender and age specific. BMI-for-age is plotted on gender specific growth charts. These charts are used for children and teenagers from 2 to 20 years of age.

Pharmacist’s advice:

Ask your Pharmacist for advice.

  • Follow the Diet Hints. A combination of moderate, regular exercise and a sensible diet is the most effective way to achieve your ideal weight.
  • Avoid ‘fad’ diets. These may cause an initial weight loss that is often the result of fluid loss from the body. Losing too much weight too quickly can be dangerous and may cause the body to gain even more weight when normal eating resumes. Eat low fat, healthy food and exercise to achieve long-term weight control.
  • Remember that even a modest weight loss of 5-10% can help to improve and control the complications of an elevated BMI.
  • If you have any queries about medications your Doctor may have prescribed as part of your weight management programme, ask your Pharmacist for advice.
  • Avoid alcohol. The average alcoholic drink contains approximately 500 kilojoules and is of little nutritional value.
  • Have regular exercise. Thirty minutes of moderate exercise, three to four times a week is recommended to maintain fitness and a healthy weight. Before undertaking any strenuous exercise, always start with a complete medical check-up.
  • If your BMI is low, consider reducing cardiovascular exercise and introduce some weight training under the guidance of a professional.
  • Drink 6 to 8 glasses of fresh, filtered water each day. The water will help to keep the bowels regular and prevent constipation. Ask your Pharmacist about the different types of water filters that are available.