Body Mass Index in Adults with Prader-Willi Syndrome

Contributed by Susanne Blichfeldt, Georgina Loughnan, Jackie Waters. IPWSO

Body Mass Index, or BMI, is an approximate measure, based on a person’s height and weight, to indicate if they are a health weight. It is calculated by dividing a person’s weight in kilograms by their height in metres squared. You don’t have to get our your calculator though – there are plenty of online calculators which will do it for you, such as the NHS Choices one at https://www.nhs.uk/Livewell/loseweight/Pages/BodyMassIndex.aspx

BMI is divided into a number of ranges

·       below 18.5 – underweight

·       18.5 – 24.9 – healthy weight

·       25 – 29.9 – overweight

·       30 – 39.9 – obese

·       40 or above – severely obese

You may also see BMI depicted as a chart like the one on the right.

How might BMI be different for people with PWS?

Remember, BMI is only an approximate measure. It does not take body composition (fat, muscle, bone) into account. People with PWS generally have a different body composition to most other people, with a greater proportion of fat to muscle mass. For instance, for an active, 150 cm tall person with PWS, weighing 80kg, who does not have difficulty moving around, lean body mass (ie muscle, bones and inner organs) will most often be around 35-40 kg. The rest is fat.

A DEXA scan (dual energy X-ray absorptiometry) is recommended to evaluate the amount of muscle and fat in the body. It can also evaluate the amount of calcium in the bones of adults, and check if the person has osteoporosis, which is quite common among adults with PWS. It is generally recommended that adults with PWS have a DEXA scan every 2-3 years.

It is also very valuable to know the amount of lean body mass, when medication is prescribed, because most medication is primarily distributed in the lean body mass. Too high doses of medication may easily create side effects if the doctor is unaware of the person’s low lean body mass.  A DEXA scan is easy to perform. A GP can refer you to the appropriate hospital service for this.

Those who have lost a lot of weight may also have excess body skin which can lie in skinfolds on their bodies.

Georgina Loughnan, who is an advisor to the International PWS Organisation, and works at the Prader-Willi Syndrome Clinic, a part of Metabolism & Obesity Services, Royal Prince Alfred Hospital, Camperdown, New South Wales, in Australia, says: 

“Regarding having all clients reach a healthy weight Range of BMI – 20-25kg/m2, I have several clients who are in this range now and yes, some, after losing much weight have excess skin on arms and legs. However, with regular effective exercise as a major part of their weight loss programme some have minimal excess skin. I have had several people lose over 50 kg (2 lost 75 kg) and had small abdominal aprons (excess abdominal skin) that just needed to be well washed and dried, but gave them no problems. The key is consistent exercise for the whole body, which is so essential for healthy weight loss.

However, reaching a BMI of 20-25kg/m2 should not be the aim for all people with PWS. A better aim is for a healthy weight that reduces comorbidities and improves physical ability and lifestyle. This is far more important and can be achieved in the overweight BMI category of 25-30kg/m2.”

Dr Susanne Blichfeldt, also an advisor to IPWSO, concurs that exercise is the key. She says, “Just sitting and losing weight means that you lose both muscle and fat tissue, so it is important if you are on a diet to move a lot. Walking is perfect, and swimming and gym are also good to build up muscles.  It is more important to have a weight that does not compromise motor activity, than to try to reach the 20-25 target”.

Does growth hormone treatment make a difference?

According to a Scandinavian study, growth hormone treatments in adults will add about 2 kg more of muscle mass  and 4 kg less of fat on average. However, worldwide there have been no large-scale longitudinal studies of the effects of growth hormone (GH) on muscle mass in adults, and only a very few adults with PWS in the UK receive it, as it is only licensed by NICE (National Institute for Clinical Excellence) up to the age of 18 years in PWS.

BMI and Children

The BMI values mentioned above are to be used for adults only.  BMI charts for children also exist for each age group and, in general, BMI values for normal weight children are lower than for adults, because of a different body composition in children. Normal BMI values are lowest in the youngest age groups.  As all children grow at different rates, you should take the advice of your consultant or dietitian as to whether your child is a healthy weight and developing along the right centile for PWS. 

Thank you

Many thanks to Dr Susanne Blichfeldt, MD, senior consultant and paediatrician, medical advisor to the International PWS Organisation and it Clinical and Scientific Advisory Board,  and to Georgina Loughnan for their contributions to this article and for checking its medical accuracy.