Saturday, 30th October 2021


‘A bear, however hard he tries, grows tubby without exercise.’ A. A. Milne

 How is weight assessed?

The standard way of measuring weight compared with height is body mass index (BMI), which is weight in kilograms divided by height in metres squared. So a 70kg (about 11 stone) person who is 1.75m (about 5 foot 9 inches) tall has a BMI of 70 divided by 1.75 squared, which is 70 divided by 3.0625, which comes to about 22.9. You do not need to work out your own BMI so laboriously. Just Google ‘BMI’ and there are plenty of websites that will work it out for you using either pounds and feet or kilograms and centimetres.

The ideal BMI is generally accepted as between 18.5 and 25. Between 25 and 30 is ‘overweight’, between 30 and 40 is ‘obese’ and over 40 is ‘morbidly obese’. For the purposes of longevity, a BMI between 20 and 25 is optimal. Weights higher than this carry an increasing risk of coronary disease and type 2 diabetes, of which much more later.

How good is BMI as a measure?

There is more to weight than BMI. It also matters where you store your excess fat. Obesity can be predominantly either central or peripheral. Those with central obesity store fat in the abdominal cavity and have paunches or beer bellies – the apple shape. Those with peripheral obesity store it on their hips, bums and thighs – the pear shape. Being an apple is much more dangerous than being a pear. So when it comes to assessing risk from overweight and obesity, mainly risk for cardiovascular diseases and premature death, waist measurement seems to give a better picture than BMI. The upper limit of recommended waist size for men is 102cm (40 inches) and for women 88cm (35 inches). With higher measurements, the risk for cardiovascular disease (CVD), diabetes and high blood pressure all rise steeply. An alternative parameter is the waist/hip ratio (waist circumference divided by hip circumference). For men this should be below 1.0 and for women below 0.8.

Many argue that BMI is too crude a measure to be useful. For instance, some body shapes give a misleadingly high BMI for people with normal body fat content – ie bodybuilders. However for the average citizen, BMI does give a reasonable guide to body fat content which is its main purpose, and it is better than all the other contenders.

Obesity is not a new condition. One of the oldest surviving human carvings, the Venus of Willendorf, created some 27,000 years ago, depicts a strikingly obese woman.


Hippocrates, in about 500 BC, reported that obese people were at increased risk of sudden death. More recently, in 1727, a British physician named Thomas Short wrote, ‘No age has seen more instances of corpulency than our own’. To which the proper response might be, ‘Man, you ain’t seen nothin’ yet!’ Obesity is a huge and growing problem in most Western countries and the figures published about its extent are as gross as the problem itself. The weight of the average person in the UK has risen by more than 1.5kg (3 lb) every decade since 1970. The horrifying statistics go on and on. The UK is the most overweight nation in Western Europe and our levels of obesity are growing even faster than those of the US.

Starting in childhood

The seeds of obesity are sown in childhood. In Reception year (aged 4–5) 9.5 per cent of children are obese and by Year 6 (aged 10–11) this has increased to 19 per cent and is still rising.  The rate of severe obesity has grown from 3.2 per cent to 4 per cent in the last decade and the inexorable increase in childhood obesity is behind the very serious problem of rising levels of childhood type 2 diabetes. Government initiatives to reduce childhood obesity have failed miserably and interventions in schools have also mostly been unsuccessful. This may not be unrelated to the selling off of school playing fields. Maternal lifestyle is also a very prominent contributor to the risk of obesity in the child, so targeting overweight and obese parents may be a better option.

There has been a marked increase in the proportion of adults in the UK who are obese, from 13.2 per cent in 1993 to 26.0 per cent in 2013 for men, and from 16.4 per cent to 23.8 per cent for women over the same period. The proportions that were overweight or obese increased from 57.6 per cent to 67.1 per cent in men and from 48.6 per cent to 57.2 per cent in women. Globally, more than one billion adults are overweight, of whom some 300 million are obese.

The effect of Covid-19

Unfortunately the pandemic has seen a rise in average BMI particularly in children. Among a cohort of 432,302 persons aged 2–19 years, the rate of  BMI increase approximately doubled during the pandemic compared to a pre-pandemic period. Persons with pre-pandemic overweight or obesity and younger school-aged children experienced the largest increases. Among people with diabetes and pre-diabetes the tendency has been for weight gain in the region of several kg over the past year or so. For non-diabetic adults there was an initial rise in BMI but a later fall to nearly pre-pandemic levels but with wide variation in individual changes. Working age people have had greater rises in weight while the retired have, on average, experienced a small fall.

Diet versus exercise

The simple view of body weight is that changes in BMI are due to the balance between energy intake and energy output. If you take in more calories in the form of food than you expend in the form of exercise you will gain weight — and vice versa. This fact is an extension of the First Law of Thermodynamics: “Energy can neither be created nor destroyed”. True, but knowing it is not a lot of help to those who are trying to lose weight.

Calories-in: Diets are notoriously difficult to follow and equally notoriously ineffective. And there is more to energy intake than just the apparent content of the food consumed. For some foods the calorie content is less bio-available than for others. Some foods increase appetite. Some affect the gut microbiome which may affect how much is absorbed. Prolonged dieting leads to slowing of metabolic rate which reduces the effect of lessening intake. Humans find it difficult to assess their food intake – we eat about 50% or more than we acknowledge in questionnaire surveys.

Calories-out: These too are notoriously difficult to quantify – we talk of metabolic rate but this is mainly a factor of movement. Measurement of exercise levels has improved hugely with the introduction  of  accelerometers but cannot adequately assess continuous activity such as fidgeting which, though small in terms of calories per minute, soon mount up when so much of our time is spent at rest or low level activity. Exercise can stimulate appetite so that we can easily reverse the effects of exercise by just eating more.  As with calories-in, our ability to assess how much exercise we take is highly inaccurate – we exercise about two thirds of the amount we declare in questionnaires.

The rate of change

Changes in weight are gradual over months and years. Weight gain and weight loss are caused by small differences in calories in and calories out over long periods of time. Most attempts to reach or maintain a normal weight start in adult life which makes it so important to have a good start – to tackle the problem in childhood.

Next week I will look at the role of exercise in weight control.


  1. Jill Brian says:

    Will you cover whether there is a seasonal difference?
    Children are able to play more outside in the summer than in the winter and with climate change, in the south we see less snow, my now middle-aged children have forgotten the snowman they built during a long cold spell and it was still standing until well into spring, in April.
    Gardeners as well, cannot dig if the soil is covered in snow or ice, so must wait until spring .
    Fewer families have space at home although some are lucky to have places in childcare where there is encouragement to be active.
    Fewer children can walk to school , partly because parents are too busy and it is easier for them to drop them off by car, partly because of fears about traffic or ” stranger danger”.
    {We know about “anti-vaccers” accosting children at the school gates.}

    • Hugh Bethell says:

      Thanks Jill. Seasonal changes in exercise habit must be very marked for some people. It is not a topic which I have explored – I should do. I suspect too that fitness levels are higher in warm countries where outdoor exercise and games can take place throughout the year – without getting too cold.

  2. Suzanne Lott says:

    Thank you, , Hugh
    Very informative, as always.
    This is a tough one but as you said in your last blog, the key to converting the inactive is to find an activity they enjoy…there are so many fun ones!!

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