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Measuring fat

Measuring fat

Part 1 of the 3-part series on measuring your health.

We hear a lot these days about how our health can be measured. It’s not just the number on the scales we have to worry about any more; it’s BMI, waist-to-height ratio, waist measurement, cholesterol levels, V02 max and more. It can be a bit confusing, especially when it seems like the health experts can’t even agree on which ones are the most important. So how can we keep a good track of our health, and know how we really measure up?

In this 3-part series we cover measuring body fat, heart health and fitness.

New Zealand, along with the rest of the developed world, is getting fatter. All the talk about the ‘obesity epidemic’ is not just hype: there’s been a huge change in our collective weight that has happened really fast in evolutionary terms, which is a big worry for health experts.

Excess body weight is one of the most important risk factors for diseases that are killing us that can actually be changed. Overweight or obesity often results in high blood pressure, high cholesterol and abnormal blood glucose responses, which in turn are key in the development of heart disease, stroke, several common cancers, as well as type 2 diabetes.

And while they might not kill you, being overweight or obese also makes it more likely you’ll suffer from osteoarthritis, gout, gallstones, sleep apnoea and decreased fertility.

This is not an exhaustive list, but you get the general idea: being overweight or obese is not a healthy way to be. So how do you know if you’re overweight or obese? There are a number of different measures that are used.

To measure your actual body fatness – in other words, how much fat there is in your body – you would need to have an MRI or CT scan. This is not an option for most of us, so other measures are used as a reasonable estimate of body fatness.

The most common measure that’s been used for many years is body mass index or BMI.

  • BMI is calculated as weight (in kilograms) divided by height squared (in metres). For example, Mary is 1.6m tall and weighs 60kg:
  • Her BMI equals 60 divided by (1.6 x 1.6) = 60 divided by 2.56 = 23. This BMI puts her into the ‘healthy’ weight range.
  • If Mary was 70kg her BMI would be 70 divided by 2.56 = 27. This BMI would put her into the ‘overweight’ weight range with an increased risk of health problems.
  • If Mary was 85kg her BMI would be 85 divided by 2.56 = 33. This BMI would put her into the ‘obese’ weight range with a greatly increased risk of health problems. (To find out your BMI, see the BMI calculator.)

Higher cut-offs for overweight and obesity are used for Maori and Pacific peoples because it’s been shown that at any given BMI they have a lower level of body fat than Europeans.

Lower BMI cut-offs have been proposed for Asian ethnic groups.

Healthy weight18.5-24.918.5-25.9Lower risk
Obese30+32+Greatly increased

The cut-offs are, of course, arbitrary guides only. Your health risk doesn’t suddenly increase dramatically when you go from a BMI of 29 to a BMI of 30 for example. It actually works on a continuum, so that as your BMI increases past the healthy range (or decreases below it), your risk of health problems increases too.

Work our your BMI with our BMI calculator

The BMI does have some limitations: for example your average All Black would fall into the obese category. Because it’s only using height and weight, BMI can’t distinguish between fat and muscle mass, but given most of us are not built like your average All Black it’s not that much of a problem.

Ask yourself this question: “Am I a heavily-muscled elite athlete or body builder?” If the answer is “no”, the BMI will be a pretty good guide for you. Basically it’s designed to be relevant to adults aged 18 and over, excluding anyone who is heavily muscled.

While the BMI is used extensively, researchers have been investigating whether there are other simple measures that can be used as indicators of body fat and health.

If you carry your excess weight around your hips, you may or may not like your pear shape, but it’s a lot healthier than being an apple shape with excess weight around your middle. Excess fat in the abdominal area has been shown to be much more closely related to developing type 2 diabetes and heart disease than body fat in other areas.

Our bodies contain a small amount of essential fat, needed by the organs for normal physiological functions, and the remainder is storage fat., which is the main energy reserve of our bodies. This fat is found under the skin and around internal organs and helps to protect these organs from damage or injury. But not all storage fat is equal when it comes to health.

An excess of fat stored around our organs (called ‘visceral fat’) is thought to be much more harmful than an excess around our hips. Fatty materials from visceral fat are more mobile than from other fat storage areas, that is the visceral fat releases more fat into our bloodstream more readily than other storage fats.

One of the liver’s activities is to control levels of fat in the blood, but because the visceral fat has a direct link to our liver it seems, when it’s in excess, it can interfere with this function. Abdominal obesity is linked to insulin  resistance, type 2 diabetes and high cholesterol, which are in turn linked to cardiovascular disease.

The waist-to-hip ratio has been used as a measure of body fat distribution.

But with more and more research on obesity, it’s been found that a simple measurement of waist circumference is a better indicator of abdominal obesity and its accompanying health risk. The waist is halfway between the bottom of the ribs and the top of the hips (usually around the belly button). For bigger people, measure the waist at the widest point.

Again, there’s some debate about what the actual cut-off points should be, but to a certain degree this is just semantics; what you need to understand is your health risk increases as your girth increases.

Increased risk80cm or more94cm or more
Substantial risk88cm or more102cm or more

Another quite simple measure is the waist-to-height ratio. This one’s nice because it’s easy to remember the guideline: your waist circumference should not exceed half of your height.

So for example, if you are 170cm tall, your waist circumference should not be more than 85 cm. This measure applies to both men and women.


With the advent of the obesity epidemic, there has been an extraordinary amount of research done around these and other simple measures that can be used to help determine whether or not people are at risk of developing cardiovascular and other chronic diseases.

While there is still a lot of debate about which measure most accurately predicts risk, it has been shown that each of these three measures – BMI, waist circumference, and waist-to-height ratio – are all useful guides.

Part 2: Measuring heart health
Part 3: Measuring physical fitness

First published: Aug 2007

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