BMI ‘exceptions’

The BMI calculation is problematic for these groups. Why?

Athletes and sportspeople in general tend to have a higher level of muscle compared to fat. Plus muscle is denser than fat and takes up more room. So, if you are building muscle but losing fat then you will probably notice that your weight has increased but you look toned and leaner than usual.

These people often follow a prescribed diet and training regime which is based on a higher calorie intake than for normal people. This is particularly noticeable for weightlifters and bodybuilders who are trying to gain lean muscle mass.

A BMI calculation for an athlete or a bodybuilder will reflect an increase in weight by pushing them into the ‘overweight’ or ‘obese’ category – even though they are fit and healthy.

To learn more visit our section on Sports Nutrition.

The BMI calculation can be applied to children and teenagers, but with one difference: a child’s BMI must be plotted against an ‘age growth’ chart. In other words, the age and sex of the child will be used to obtain a ‘percentile ranking’ so that you can see how your child’s BMI compares against other children of a similar age and sex.

For example, if your child has a ‘BMI for age’ percentile of 55% then his/her weight is greater than 55% of children of the same age and gender.

A ‘percentile’ is a measurement used to determine a child’s size and growth patterns.

It’s a good idea to carry out this measurement on a regular basis to track your child’s growth patterns and weight.

One thing to bear in mind is that children grow at different rates so use this BMI figure as a guide only. So if you think that your child is overweight or obese, consult your GP.

With the elderly, a BMI of 25 to 27 is preferred rather than under 25 as this can help to protect against osteoporosis. Elderly people tend to be frailer and especially prone to thinning of the bones, so an extra layer of fat might help.

There is also the fact that many elderly people tend to have smaller appetites and feel full with less food than younger people. They often choose to ‘graze’ or pick at food rather than enjoying three meals a day. And if an elderly person lives alone then they may find that they can’t be bothered to cook for themselves.

The problem with this is that their immune system will be low due to a lack of essential vitamins and minerals which increases their risk of illness. If anything they require more nutrients than most other times in their lives because the body is less efficient at absorbing these nutrients.

So they need to be encouraged to eat especially if they show a BMI reading in the underweight category, good nourishing foods.

Nourishing foods are also important for pregnant women or those who are breastfeeding. A healthy, well balanced diet will ensure that both you and your baby get those essential nutrients. And what you eat can affect the development of your unborn baby and its future well being.

You can expect to put on weight during pregnancy but how much you do is governed by your calorie intake. You will eat more than usual and will also store an extra layer of fat for breastfeeding.
But, bear in mind that the more weight you gain the more there is to lose after the birth. And this extra weight can be very difficult to shift!

So how does this relate to your BMI?

Doctors and other health professionals recommend that your BMI be measured on your first antenatal appointment. He or she will need to know your height and current weight although you can have a go for yourself.

Try this (example):

  • Multiply your height (in metres) by your height in metres again. For example, if you are 5 feet 5 inches then your height in metres is 1.7 metres. Use an online ‘feet to metres’ converter to do this calculation.

    1.7 (metres) x 1.7 (metres) = 2.89

  • Then work out your weight in kilograms. If you don’t know this then again, use an online ‘pounds to kilograms’ converter.

    For example, if you weigh 120 pounds then this converts to 54 kilograms.

  • Divide your weight in kilograms by your height in metres

    54 (weight in kilograms) divided by 2.89 (height in metres) = 18.68

This measurement puts you in the healthy weight category.

If you don’t fancy doing the maths then use an online BMI calculator.

This measurement will be used by your midwife when discussing potential weight gain. He or she will advise you to stick to a daily food intake of around 2,500 calories and from healthy sources. So eat plenty of fresh fruit and vegetables, protein, carbohydrates and fats. It is okay to have a few treats but try and limit these as excess weight can be difficult to lose afterwards.

In respect of the BMI example above; if your BMI is 18.68 then try to ensure that weight gain is between 28 to 40 pounds maximum. Any more than that can put you at risk of developing high blood pressure, pre-eclampsia and gestational diabetes.

The answer is to watch what you eat and to take regular exercise. If you were exercising before your pregnancy then don’t feel that you have to stop altogether. You will find that your GP or midwife will recommend exercising but choose your activity carefully. Avoid anything which is likely to harm you and your baby.

Body Mass Index Guide Index:

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