Tuesday, January 12, 2010

The Mathematics of Healthy Weight

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Okay Big Boys (and Girls): Who's been spending too much time on the Computer ?

There is a very easy way to tell.

We simply do it mathematically by calculating how fat we are.

This involves calculating our "Body Mass Index" or "BMI".

My son and I both put ourselves to the BMI test, and found like most nerds, that we are definitely overweight.
(From doing too much computing!)

But all is not lost. There is another mathematical equation that can tell us just how fast we need to get our heart beating; in order to burn away those big body bits we need to get rid of. This is called our "Target Heart Rate" and is discussed later on in this Blog post.

Body Mass Index (BMI)

Body mass index (BMI) is used to estimate our total amount of body fat. It is calculated by dividing our weight in kilograms by our height in metres squared (m2). Or we can use Pounds and Feet if we are in the USA.

Differences in BMI between people of the same age and sex are usually due to body fat.

BMI and Children Under 21

The healthy weight range for adults of a BMI of 20 to 25 is not a suitable measure for children.

For adults who have stopped growing, an increase in BMI is usually caused by an increase in body fat. But as children grow, their amount of body fat changes and so will their BMI.

For this reason a BMI calculation for a child or an adolescent must be compared against age and gender percentile charts.

There is a special BMI Calculator for persons aged 2 to 20 years old at this web page:


BMI Calculation

We can use either of the charts below to work out where our BMI is.
We find our height on one axis, and then trace across until it matches up with our weight.

For Big Passy that is 185cm matching up with 97kg, which puts him as being definitely Overweight.

A couple of years ago at the peak of his Pizza eating and Computing addictions, Big Passy was actually 185cm lining up with a whopping 125kg = Very Obese.

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Source: http://www.vertex42.com

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Source: http://www.dost-dongnai.gov.vn

There are also Online Calculators for BMI at these websites:


This calculator also tells us the percentiles of Americans:

This calculator is for Children and Adolescents under 21 years of age:

Interpreting BMI Values

Once you have measured your BMI, you can determine your healthy weight range.

If you have a BMI of:

  • Under 18 – you are very underweight and possibly malnourished.
  • Under 20 – you are underweight and could afford to gain a little weight.
  • 20 to 25 – you have a healthy weight range for young and middle-aged adults.
  • 26 to 30 – you are overweight.
  • Over 30 – you are obese.

Risks of Being Overweight

(by playing too much computer games and being physically inactive):

If you are overweight (BMI over 25) and physically inactive, you may develop:
  • Cardiovascular (heart and blood circulation) disease
  • Gall bladder disease
  • High blood pressure (hypertension)
  • Diabetes
  • Osteoarthritis
  • Certain types of cancer, such as colon and breast cancer.

Risks of being Underweight

If you are underweight (BMI less than 20), you may be malnourished and develop:

  • Compromised immune function
  • Respiratory disease
  • Digestive disease
  • Cancer
  • Osteoporosis
  • Increased risk of falls and fractures.

Body Fat Distribution and Health Risks

A person’s waist size is thought to be a better predictor of health risk than BMI.
Having fat around the abdomen or a ‘pot belly’, regardless of your body size, means you are more likely to develop certain obesity-related health conditions.

  • Least risk – slim (no pot belly)
  • Moderate risk – overweight with no pot belly
  • Moderate to high risk – slim with pot belly
  • High risk – overweight with pot belly.

Waist Size and Health Risks

Waist measurement can be used to indicate health risk.

For men:
  • 94cm or more – increased risk
  • 102cm or more – substantially increased risk.

For women:
  • 80cm or more – increased risk
  • 88cm or more – substantially increased risk.

Genetic factors:
The tendency to deposit fat around the middle is influenced by a person’s genes. However, you can take this genetic tendency into account and do something about it.

Being physically active, avoiding smoking, reducing computer gaming, and eating unsaturated fat instead of saturated fat have been shown to decrease the risk of developing obesity.

BMI Information Source for all of the above: http://www.betterhealth.vic.gov.au

Training Heart Rate

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Source: http://img.brothersoft.com

Yes obviously we are going to have to get off the computer, quit the junk food, and do some serious exercise.

We need to start off slow though, and then work up to productive exercise where our lungs are burning, and our heart is beating out of our skin.

Anyone over the age of 35, as well as younger people who are overweight should check with their doctor or personal trainer before beginning any type of exercise programme.

There is actually a certain speed we are supposed to get our heart up to, for exercise to be beneficial.

This heart rate is often called our "Training Heart Rate", or "Target Heart Rate".

Our "Resting Heart Rate" is our normal heart beat when we first awake, or while we
are sitting down relaxing before doing our daily exercise.

To determine our Heart Rate in beats per minute, we can:

  • Take our pulse for 15 seconds, and multiply this answer by 4 or
  • Take our pulse for 30 seconds, and multiply this answer by 2

The heart rate we need to aim for during exercise, depends on our age, and is shown in the following graph:

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Source: http://www.healthgoods.com

There is also a great target heart rate calculator here:


The basic mathematics behind this is that your Maximum Heart Rate (MHR) is 220 minus your age. You then need to workout below this rate at a "Target Heart Rate" of about 80% of your MHR.

There is a great web page that explains all of the maths and percentage rates about ideal Heart Rate here:


Remember to always use "The Talk Test"

You should be able to carry on a conversation while exercising. This doesn't mean you shouldn't breathe hard, but if you can't speak and can't catch your breath, that's too much exertion, so slow down!

Well that's it folks. Walk away from the Computer right now, and start your New Year's health kick!

But just before you do:

Big (Overweight) Passy Wasabi

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