In 1997 the World Health Organisation (WHO) Expert Consultation on Obesity recognised the importance of abdominal fat mass (referred to as abdominal, central or visceral obesity), which can vary considerably within a narrow range of total body fat and body mass index (BMI). It also highlighted the need for other indicators to complement the measurement of BMI, to identify individuals at increased risk of obesity-related diseases. Two such proxy measures of abdominal obesity, which can complement the measure of BMI are waist size (circumference) and waist-to-hip ratio (WHR).
In my post ‘What is obesity – is it merely about BMI? What is obesity? I had highlighted that over the years, starting from the early 1980s, many research studies had confirmed the notion that rather than the excess body fat per se, regional distribution of body fat at different anatomic sites was much more important in driving the risk of cardiovascular diseases and other health hazards linked to obesity. In the same post I had described ‘abdominal obesity’ (individuals with excess intra-abdominal fat) and a characteristic obesity phenotype – ‘Asian Indian Phenotype or Paradox’ (Asians, particularly people in South-East Asia and China, displaying a greater amount of intra-abdominal fat for a given waist size [circumference] or BMI).
In my post ‘Normal Weight Obesity – a myth or a reality? Normal Weight Obesity I had highlighted that even in individuals with normal BMI, increasing abdominal obesity is associated with higher risk for the development of various lifestyle diseases (chronic or noncommunicable diseases) regardless of the total body fat.
In my post ‘Body Mass Index – is it the best measure of obesity? BMI I had highlighted that even though BMI is the most widely used measure of identifying obesity, it had some serious limitations, including its failure to identify where fat is stored in the body (i.e. centrally or peripherally). Two people of the same BMI may have very distinct body shapes, depending on the distribution of body fat, and consequently, have a different risk profile. From a preventive and public health point of view, it is crucial that risk factors are identified at an early stage, in order to change and modify behaviour and lifestyle in high-risk individuals.
In view of the role played by intra-abdominal (visceral) fat in the causation of various cardiovascular diseases and type 2 diabetes and failure of BMI to identify where fat is stored in the body, it is imperative to qualify BMI with some measure of body shape, as recommended by the 1997 WHO Expert Consultation on Obesity, such as waist size (circumference) or waist-to-hip ratio to identify individuals with a high-risk pattern of body fat distribution.
Waist size (circumference) measurement
Although waist circumference and BMI are interrelated, waist circumference provides an independent prediction of risk over and above that of BMI. Research studies have shown that people with a normal BMI but a large waist size (as discussed in my post ‘Normal Weight Obesity – a myth or a reality, referred to above) have worse long-term survival than people who are overweight or obese but do not carry their weight around the middle i.e. do not have abdominal obesity (‘metabolically healthy obese subjects’ as discussed in my posts ‘What is obesity?’ and Normal weight Obesity, referred to above). Therefore, waist size measurement is particularly useful in patients who are categorised as normal or overweight on the BMI scale. At BMIs > 35, waist circumference does not add much to the information of disease risk than that provided by BMI. This is mainly because the waist circumference in patients with BMI > 35 will usually be greater than the cut-off points used to define increased relative risk. Also, in practice, it is difficult to measure waist size in very obese patients as it may be difficult to accurately palpate the bony landmarks in these patients. As waist measurements lose their predictive power at very high BMIs, it is not necessary to measure waist size in individuals with BMI > 35 kg/m2.
Methods for measuring waist and hip circumference
An important issue in using and interpreting waist size (circumference) or waist-to-hip ratio is the protocol used to obtain the measurements. Different protocols have been laid down by different organisations, the world over, for measurement of waist and hip circumference. The WHO Expert Consultation on Waist Circumference and Waist-to-Hip Ratio was held in Geneva, Switzerland, on 8-11 Dec 2008, WHO Expert Consultation Report with the aim of recommending an appropriate protocol for international use. The guidelines recommended by this expert consultation are as under:
Placement of tape
(a) Waist circumference – The WHO protocol for measuring waist circumference instructs that the measurement be made at the approximate midpoint between the lower margin of the last palpable rib and top of the iliac crest (The iliac crest is the curved superior border of the ilium, the largest of the three bones that merge to form the hip bone. It is located on the superior and lateral edge of the ilium very close to the surface of the skin in the hip region. When a person places their hands on their hips, it is the skin above the iliac crest that they rest their hands on).
The United States (US) National Institutes of Health (NIH) protocol provided in the NIH Practical Guide to Obesity (NHLBI Obesity Education Initiative, 2000) and the protocol used in the US National Health and Nutrition Examination Survey (NHANES) III indicate the waist circumference should be made at the top of the iliac crest.
(b) Hip Circumference – hip circumference measurement should be taken around the widest portion of the buttocks.
Each measurement should be repeated twice; if the measurements are within 1cm of one another, the average should be calculated. If the difference between the two measurements exceeds 1 cm, the two measurements should be repeated.
Tightness and type of tape
The accuracy of the waist and hip circumference measurements depends on the tightness of the measuring tape and its correct positioning. The measuring tape should be parallel to the floor at the level at which the measurement is made. For both waist and hip, the tape should be snug around the body, but not pulled so tight that it is constricting. The protocol also recommends the use of a stretch-resistant tape that provides a constant 100 gm tension through the use of a special indicator buckle; use of this type of tape reduces differences in tightness.
Posture of the subject during the measurement
The posture of the subject at the time measurement is taken influences the accuracy of the measurement. Thus, the WHO protocol recommends that the subject stands with arms at the sides, feet positioned close together, and weight evenly distributed across the feet.
Phase of respiration at the exact point of measurement
The phase of respiration determines the extent of fullness of the lungs and the position of the diaphragm at the time of measurement; thus, it also influences the accuracy of the waist circumference. The WHO protocol suggests that the waist circumference should be measured at the end of a normal expiration (the exhalation of breath from the lungs) when the lungs are at their functional residual capacity.
Abdominal tension at the point of measurement
The tension of the abdominal wall influences the accuracy of the waist circumference measurement. Lowering the tension of the abdominal wall increases waist circumference, whereas increasing the tension (by sucking in) reduces waist circumference. Many individuals unconsciously react to waist measurements by sucking in the abdominal wall; hence, a relaxed posture is best for taking waist measurements. The WHO protocol recommends advising the subject to relax and take a few deep, natural breaths before the actual measurement is made, to minimize the inward pull of the abdominal contents during the waist measurement.
Influence of stomach contents at the time of measurement
The amount of water, food or gas in the gastrointestinal tract will affect the accuracy of the waist measurement. Gibson (1990) suggests that a waist measurement be made after the subject has fasted overnight or is in a fasted state, to reduce this effect. However, none of the protocols evaluated address this issue, perhaps because it would entail the subject being notified in advance of the measurement, and being present the morning after an overnight fast.
Population-specific cut-off points for waist circumference
In my post ‘What is obesity – is it merely about BMI?’ referred to above, I had highlighted that both, the total body fat and the regional distribution of body fat are influenced by gender, age and ethnicity. Accordingly, there is substantial evidence of these factors influencing waist circumference (and waist-to-hip ratio). Commonly, used cut-off points for waist circumference and waist-to-hip ratio are based on studies undertaken predominantly in populations of European origin. However, compared to Europeans, Asian populations have greater intra-abdominal (visceral) fat and African populations, and possibly Pacific Islanders have less intra-abdominal fat or percentage of body fat at any given waist circumference. Given the ethnic and population-specific differences in disease risk for any particular anthropometric measure, like BMI, universal cut-off points for waist size (circumference) and waist-to-hip ratio are not appropriate for use worldwide.
As higher levels of abdominal fat for a given waist size and at relatively lower BMI in Asian populations are associated with increased risk of type 2 diabetes and cardiovascular diseases, lower thresholds for these indicators might be needed for these populations than for the European reference populations. Data for Africans and Pacific Islanders possibly indicates a need for higher cut-offs than those used for European reference populations. However, due to the lack of adequate data in African and Pacific Islanders, at present, cut-off values for European populations are recommended to be used.
In my post ‘What is Obesity – is it merely about BMI?’ referred to above, I had highlighted that starting from the late middle age until the 80s or later, the volume of subcutaneous fat declines and there is a redistribution of fat from subcutaneous to visceral depots. Consequently, waist circumference assumes a greater value for estimating risk for obesity-related diseases at older ages. As discussed in my above post, fat patterning is also influenced by gender. Women in early adulthood have a more peripheral distribution of fat. However, following a childbirth, there is a redistribution of body fat resulting in greater central (abdominal) body fat (and consequently increases in waist size); this increases with the number of births. Menopause is also associated with an increase in body fat and a redistribution of fat to the abdominal area.
Significance of waist size measurement in disease risk prediction
To highlight the role of abdominal obesity in the causation of various metabolic abnormalities and obesity-related diseases, the World Heart Federation chose the theme of ‘Healthy Weight, Healthy Shape’ for the ‘World Heart Day 2005’. To coincide with the World Day on 25th Sep 2005, a survey called ‘Shape of the Nations’ was conducted in 27 countries, during July 2005, amongst the general public, people identified as at risk for heart disease, as well as primary care physicians to assess knowledge and understanding of the increased risk of heart disease posed by excess abdominal fat. The results of the survey showed that in spite of the impact that body fat distribution has on health risks associated with obesity, the general population is still focussed on absolute body weight, rather than focussing on where they carry their weight.
Significance of abdominal obesity in predicting the future development of type 2 diabetes and cardiovascular diseases can be further gauged from the fact that abdominal obesity is a defining parameter for metabolic syndrome, a cluster of the most dangerous heart attack risk factors, which will be discussed later in this post.
In my post ‘Body Mass Index – is it the best measure of obesity?’ I had highlighted that WHO had convened an expert consultation on BMI in Asian populations, which met in Singapore from 8-11 Jul 2002. One of the recommendations of this expert consultation was that “where possible, in populations with a predisposition to central obesity and related increased risk of developing the metabolic syndrome, waist circumference should also be used to refine action levels on the basis of BMI. Its prognostic (predicting the likely course of a disease or ailment) capacity is improved when used alongside BMI.
Assessment of risk status
The ultimate aim of these anthropometric measurements is to identify individuals at increased risk for type 2 diabetes and cardiovascular diseases. However, like BMI, waist circumference is not a diagnostic tool for disease risks; it is more of a screening tool. Therefore, rather than using these measurements independently, incorporating both BMI and waist circumference in the classification of overweight and obesity provides a better indication of disease risk.
It is important to note that these categories denote relative risk, not absolute risk. They indicate the need to institute weight loss therapy and do not directly define the required intensity of risk factor modification. The later is determined by estimation of absolute risk based on the presence of associated disease or risk factors.
Waist-to-Hip Ratio (WHR)
Waist circumference is not the only measure of abdominal obesity. Waist-to-hip ratio is also commonly used for the assessment of abdominal obesity. The ratio is obtained by dividing waist circumference with hip circumference. It was introduced, mainly as a result of Swedish research, on the assumption that it would predict the fat distribution better than waist circumference alone. Subsequent research, however, showed that it did not. One important limitation of waist-to-hip ratio is that it requires two measurements – waist and hip circumference, which may contribute to summative (additive) measurement errors. Also, a high waist-to-hip ratio could be due to either a relatively large waist or a small hip girth due to lower muscle mass. In contrast, an individual may have a dangerous level of abdominal fat, but the waist-to-hip ratio may be low if the hip circumference is especially large. For e.g. women with ‘pear-shaped’ bodies, who carry weight mainly on their hips and thighs, may still have a large at-risk waist circumference, but their waist-to-hip ratio may not correctly identify central obesity due to the large hip circumference. Results of recent studies have supported the view that waist circumference is a better indicator of the accumulation of intra-abdominal (visceral) fat than the waist-to-hip ratio.
Waist-to-hip ratio cut-off points
The cut-off points for waist-to-hip ratio are also population specific. Males with a waist-to-hip ratio > 1.0 and females with a waist-to-hip ratio > 0.85 are considered to be at increased risk. However, for Asian populations, these figures are > 0.9 for males and > 0.8 for females.
The metabolic syndrome is a cluster of the most dangerous heart attack risk factors: diabetes and raised fasting plasma glucose, abdominal obesity, high cholesterol and high blood pressure. It is also known as Metabolic syndrome X, Syndrome X, Insulin Resistance syndrome, and Raven’s syndrome. It is estimated that around 20-25 per cent of the world’s adult population have the metabolic syndrome and they are twice as likely to die from and three times as likely to have a heart attack or stroke as compared with people without the syndrome. In addition, people with metabolic syndrome have a fivefold greater risk of developing type 2 diabetes.
While the underlying cause of the metabolic syndrome and each of its components is complex and not well understood, insulin resistance (one of the causative factors for the development of type 2 diabetes) and central obesity are considered as important causative factors. Early identification of individuals with metabolic syndrome is imperative so that lifestyle intervention and treatment may prevent the development of diabetes and/or cardiovascular diseases.
A number of expert groups have developed clinical criteria for the metabolic syndrome. All groups agreed on the core components of the metabolic syndrome: obesity, insulin resistance, dyslipidemia and hypertension. However, as a multitude of definitions has caused confusion and difficulty in clinical practice, the International Diabetes Federation (IDF) has come up with ‘The IDF consensus worldwide definition of the Metabolic Syndrome. Central (abdominal) obesity, easily assessed using waist circumference and independently associated with each of the other metabolic syndrome components, including insulin resistance, is a prerequisite risk factor for the diagnosis of the syndrome in the new definition. Insulin resistance, which is difficult to measure in day-to-day clinical practice, is not an essential requirement.
Limitations of waist size (circumference) measurement
The major limitation of waist size (circumference) measurement are ethnic, age, and gender-related differences in body fat distribution that modify the predictive validity of waist size measurement (and waist-to-hip ratio) as a surrogate marker for abdominal fat. As had been highlighted in my post ‘What is obesity – is it merely about BMI’ referred to above, body fat may be located at various sites in the body – mainly beneath the skin (subcutaneous fat) and within and around internal organs in the abdominal cavity (intra-abdominal or visceral fat). Fat in the abdomen can be stored either subcutaneously, i.e. directly under the skin or it can be stored inside the abdominal cavity, wrapped around the various organs, what is called as visceral adipose tissue. As has been highlighted before, not all fat is the same; it is the intra-abdominal fat which is responsible for the various metabolic abnormalities and increased risk of type 2 diabetes and cardiovascular diseases. Though our interest lies in measuring the intra-abdominal fat, waist circumference measurement fails to distinguish visceral from subcutaneous fat; an enlarged waist size could be due to increased abdominal subcutaneous or visceral fat depots or both.
The rationale for using waist circumference (or waist-to-hip ratio) is simple – the greater the waistline for a given BMI, higher the likelihood for relatively more amount of intra-abdominal fat. As a result, waist size measurement is a pretty good surrogate marker of abdominal obesity. For adult patients with a BMI of 25 to 34.9 kg/m2, sex-specific waist circumference cut-offs should be used in conjunction with BMI to identify increased disease risk.