Tuesday, 9th July 2019


Earlier this year BMJ Open published a fascinating article on the association between exercise and obesity based on results from the UK Biobank study1.

To quote the study literature

“UK Biobank aims to improve the prevention, diagnosis and treatment of a wide range of serious and life-threatening illnesses – including cancer, heart diseases, stroke, diabetes, arthritis, osteoporosis, eye disorders, depression and forms of dementia. It is following the health and well-being of 500,000 volunteer participants and provides health information, which does not identify them, to approved researchers in the UK and overseas, from academia and industry”.

This is a massive undertaking and will, over the years, vastly increase our understanding of many of the non-communicable diseases which ail us and which lead to chronic ill health, particularly in old age.

The outcome of the study

The nub of this particular investigation is that the more you exercise the less likely you are to be fat – so far so obvious!

I will come back to some of the finer points of the association between body weight and exercise in future blogs. The really eye-catching aspect of this study is the fact that the investigators actually measured the amount of exercise taken by the subjects under observation – they did not just rely on what the subjects said they did.

80,000 UK Biobank participants, all adults aged 40-70 at recruitment, provided data.

During the baseline assessment centre visit, participants completed a touch screen questionnaire which included questions on sociodemographics, lifestyle, health and medical history which included a physical activity questionnaire.

They subsequently had their physical activity assessed by wearing a wrist accelerometer. (An accelerometer is a sort of super-charged pedometer which measures all physical activity). Each participant wore the device continuously for seven days and then sent it to the study centre for analysis. 

As expected those who took more exercise had lower body mass indices (BMI, a measurement of weight compared to height – see glossary) and lower body fat than those who took less exercise.  

For me the really important finding was that the correlation between physical activity and weight was twice as good for measured activity compared to self-assessed activity. The fatter the subject, the more marked was this disparity – in other words the fatter you are, the more you overestimate your exercise level.

An inflated idea of general physical activity

This study highlights the inaccuracy of questionnaires for assessing physical activity, particularly in overweight and obese subjects. Most studies of the amount of exercise which people take rely on questionnaires and so give an inflated picture of general physical activity. This also makes it very difficult to measure the effects of exercise in a variety of conditions.

Measurement of physical fitness is probably a better way to assess exercise than relying on self-reporting, but that is another story. I will be banging on about that in future blogs!

  1. Doi:10.1136/bmjopen-2018-024206

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