Followers of fad diets and fitness enthusiasts adhere to a concept that one can never be too slim, however, this can lead to unhealthy and unrealistic body shape objectives.  Indeed, extreme deviations from an ideal body weight can lead to significant health risks. Overweight and obese people have much higher chance of having diabetes, high blood pressure, cardio-vascular diseases, cancers, joints and metabolic problems. On the other end of the spectrum, significantly underweight people will have risks of endocrine pathologies, osteoporosis, bone fractures and eating disorders.

A healthy individual will have a proportional body for their age and sex, with a balanced composition of fat, muscle and bone mass.  There is a range of body shapes that can be accepted as healthy, and this can be evaluated with objective methods.  The approach takes into account an individual’s physique, anatomical structures and overall constitution. The measurements are then correlated to one’s sex, age and level of physical fitness.  Healthy ranges for body fat percentages are considered to be:

  • Adult men 10% to 22%
  • Adult women 20% to 32%

Body Mass Index

Body mass index (BMI) is the most popular way to estimate healthy body fat percentage ranges.  It is based on the relationship between an individual’s height and weight. The formula used for BMI in adults is calculated as weight/height2, where weight is in kilograms and height in metres.  For example, someone weighing 63 Kg and measuring 1.73 metres tall would have a BMI of roughly 21.  For adults, a BMI under 18.5 is considered dangerously thin, 18.5-25 is a healthy range, 25-30 is overweight, and 30+ is obese.

BMI is an estimate that is not accurate for all individuals.  For example, BMI is not so useful for people with highly developed muscle bulk and low body fat percentages (for example, athletes). Indeed, muscle is denser than other tissues, and fat is less dense.  This individual composition will bias the BMI estimate upwards, incorrectly suggesting obesity.

Another case where BMI may not provide accurate estimates is for older people who tend to lose muscle bulk and bone density, and accumulate more fat. Their BMI might be in the normal range, but they may have an unhealthy percentage of body fat.

Direct Body Fat Measurements

More accurate measurements of body fat can be obtained using direct measurements. There are easy, popular ways to test this that are reasonably accurate, and when required, there are also highly accurate techniques, however these are more involved, and more expensive.

Simple tests of body fat percentage include:

Caliper measurements of skinfold thickness

In this approach, the thickness of fat in a fold of skin is recorded at one or more places on the body an averaged to produce an estimate.  The value is then found in a table, which indicates the individual’s body fat percentage.  It requires some experience to use this approach accurately.

 

Bioelectrical impedance 

For this, a weak electrical current is passed through the body measuring the body's electrical impedance. The recorded value is used along with the height and weight to calculate body fat percentage. Results can vary based on how much water is in the body and how the electrodes are aligned.

 

More accurate tests include:

Water tank weighing 
Also called hydrodensitometry, this method measures the volume of water displaced when completely submerging an individual in a water tank, and divides this by the person’s weight to obtain their corporal density. 

DXA 
DXA is dual-energy X-ray absorptiometry. It is a medical imaging technique that can tell how much fat tissue one has, as well as providing detailed information about how the fat is distributed.

Effects of Fat Accumulation

Statistically, women accumulate fat more on their hip areas (thus appearing pear-shaped), and men at the torso and abdomen (apple-shapes). However both sexes can store excess fat around the midriff.  These observations mean that a chest-to-waist-to-hip ratio can be used to evaluate the aesthetics of body proportions.

The ideal aesthetic female body shape is considered to be an hourglass silhouette. For males, it is an inverted triangle.  This give rise to some ad-hoc ways of evaluating an individual’s silhouette.

Waist to hip ratio test
Comparing the waist at the level of the belly button with the hips measured at their widest level provides a ratio that can be used to evaluate the body aesthetic. For men the ratio should be from 0.9 to 1.0, and for women it should be 0.8 to 0.9.