The waist–hip ratio or waist-to-hip ratio (WHR) is the dimensionless ratio of the circumference of the waist to that of the hips. This is calculated as waist measurement divided by hip measurement (W⁄H). For example, a person with a 75 cm waist and 95 cm hips (or a 30-inch waist and 38-inch hips) has WHR of about 0.79.
The WHR has been used as an indicator or measure of health, fertility, and the risk of developing serious health conditions. WHR correlates with perceptions of physical attractiveness.
According to the World Health Organization's data gathering protocol,[3] the waist circumference should be measured at the midpoint between the lower margin of the last palpable ribs and the top of the iliac crest, using a stretch-resistant tape that provides constant 100 g (3.53 oz) tension. Hip circumference should be measured around the widest portion of the buttocks, with the tape parallel to the floor.[4] Other organizations use slightly different standards. The United States National Institutes of Health and the National Health and Nutrition Examination Survey used results obtained by measuring at the top of the iliac crest. Waist measurements are usually obtained by laypersons by measurings around the waist at the navel, but research has shown that these measurements may underestimate the true waist circumference.[4]
For both measurements, the individual should stand with feet close together, arms at the side and body weight evenly distributed, and should wear little clothing. The subject should be relaxed, and the measurements should be taken at the end of a normal respiration. Each measurement should be repeated twice; if the measurements are within 1 cm of one another, the average should be calculated. If the difference between the two measurements exceeds 1 cm, the two measurements should be repeated.[4]
Practically, however, the waist is more conveniently measured simply at the smallest circumference of the natural waist, usually just above the belly button, and the hip circumference may likewise be measured at its widest part of the buttocks or hip.[5] Also, in case the waist is convex rather than concave, such as with different body types, and obesity, the waist may be measured at a horizontal level 3 cm above the navel.[6]
The WHR has been used as an indicator or measure of health, and as a risk factor for developing serious health conditions.
WHR is used as a measurement of obesity, which in turn is a possible indicator of other more serious health conditions. The WHO states that abdominal obesity is defined as a waist–hip ratio above 0.90 for males and above 0.85 for females, or a body mass index (BMI) above 30.0.[4] The National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) states that "total cholesterol levels are usually higher in persons with predominant abdominal obesity, defined as a waist-to-hip circumference ratio of ≥ 0.8 for women and ≥ 1.0 for men.[7]
WHR has been found to be a more efficient predictor of mortality in older people (>75 years of age) than waist circumference or BMI.[10] If obesity is redefined using WHR instead of BMI, the proportion of people categorized as at risk of heart attack worldwide increases threefold.[11] WHR may be less accurate in individuals with a BMI of 35 or higher, and more complex to interpret since an increased WHR may result from increased abdominal fat or decreased lean muscle mass around the hips.[12] The body fat percentage is considered to be an even more accurate measure of relative weight. Of these three measurements, only the waist–hip ratio takes account of the differences in body structure. Hence, it is possible for two people of the same sex to have different body mass indices but the same waist–hip ratio, or to have the same body mass index but different waist–hip ratios.
WHR has been shown to be a better predictor of cardiovascular disease than simple waist circumference and body-mass index.[13] The study by American Heart Association has shown that measuring waistline and comparing it to hip size might be a better way to predict heart disease risk than a widely used body mass index.[14] However, other studies have found waist circumference (particularly waist-to-height ratio[15]) to be a better indicator of cardiovascular risk factors than the waist–hip ratio,[16] body fat distribution,[17] and hypertension in type 2 diabetes.[18]
The stress hormone cortisol is regulated by the hypothalamic-pituitary-adrenal (HPA) axis and has been associated with higher levels of abdominal fat and therefore a higher WHR.[19]Abdominal fat is a marker of visceral fat (stored around important internal organs such as the liver, pancreas and intestines) and has greater blood flow and more receptors for cortisol than peripheral fat. The greater the number of cortisol receptors, the more sensitive the visceral fat tissue is to cortisol. This heightened sensitivity to cortisol stimulates fat cells to further increase in size.[20]Women who have a combination of normal BMI and high WHR experience elevated cortisol reactivity to acute stressors and failure to habituate to repeated stressors, compared to women with normal WHR.[21] This suggests that high WHR might also indicate HPA-axis dysregulation and over-exposure to cortisol.
Evidence for the relationship between cortisol and central fat distribution has primarily been studied in individuals with Cushing's syndrome.[22]This is characterized by over-exposure to cortisol due to elevated activity of the HPA axis. A primary component of Cushing's syndrome is the accumulation of fat in the abdominal region, and it is hypothesized that elevated cortisol levels contribute to this accumulation. However, this hypothesis remains contested as cortisol levels only modestly explain variation in central fat distribution. It is more likely that a complex set of biological and neuroendocrine pathways related to cortisol secretion contribute to central adiposity, such as leptin, neuropeptide y, corticotropin releasing factor and the sympathetic nervous system.[22]
In general, adults with growth hormone deficiencies also have increased WHRs.[23][24] Adults with untreated congenital isolated growth hormone deficiency have increased WHRs, possibly from increased cortisone–cortisol ratios and insulin sensitivities.[24] Since these individuals have increased visceral obesity, it has been suggested that a minimal growth hormone secretion would theoretically increase insulin resistance.[24] However, because of the growth hormone deficiency, this insulin resistance point cannot be reached and these individuals are more sensitive to insulin. Increased adipose deposits are therefore more likely to form in these individuals, causing the high WHR. Growth hormone deficiencies have also been correlated with WHRs in prepubertal children; the specific baseline body statistics, such as WHRs, of pre-pubertal children with growth hormone deficiencies can predict growth response effectiveness to artificial growth hormone therapies, such as rhGH treatments.[25]
Males with congenital adrenal hyperplasia, determined by CYP21A2 mutations, have increased WHRs.[26]
Women with high WHR (0.80 or higher) have significantly lower pregnancy rates than women with lower WHRs (0.70–0.79), independent of their BMIs.[27] Men with WHRs around 0.9, similarly, have been shown to be more healthy and fertile with less prostate cancer and testicular cancer.[citation needed]
One of the factors that affects a woman's waist-hip ratio is her gynoid fat distribution, a store of energy to be expended in the nurturing of offspring, both to provide adequate energy resources during pregnancy and for the infant during the stage in which they are breastfeeding.[28] In an ancestral environment where food was scarce, a female with high levels of gynoid fat would be signalling to males that she in an optimal state for reproduction and nurturing of offspring. This can be seen in the fact that a female's waist–hip ratio is at its optimal minimum during times of peak fertility—late adolescence and early adulthood, before increasing later in life.[29]
As a female's capacity for reproduction comes to an end, the fat distribution within the female body begins a transition from the gynoid type to more of an android type distribution. This is evidenced by the percentages of android fat being far higher in post-menopausal than pre-menopausal women.[30][31]
Evidence suggests that WHR is an accurate somatic indicator of reproductive endocrinological status and long-term health risk. Among girls with identical body weights, those with lower WHRs show earlier pubertal endocrine activity, as measured by high levels of lutenizing hormone and follicle-stimulating hormone, as well as sex steroid (estradiol) activity. A Dutch prospective study on outcome in an artificial insemination program provides evidence for the role of WHR and fecundity.[32] These investigators report that a 0.1 unit increase in WHR decreases the probability of conception per cycle by 30% after adjustment for age, obesity, reasons for artificial insemination, cycle length and regularity, smoking, and parity.[33][34]
Menopause, the natural or surgical cessation of the menstrual cycle, is due to an overall decrease in ovarian production of the hormones estradiol and progesterone. These hormonal changes are also associated with an increase in WHR independent of increases in body mass.[35] Significantly, studies find that large premenopausal WHRs are associated with lower estradiol levels and variation in age of menopause onset.[36] Circulating estrogen preferentially stores lipid deposits in the gluteofemoral region, including the buttocks and thighs, and evidence suggests that menopause-associated estrogen deficiency results in an accumulation of adipose deposits around the abdomen.[37] These menopause-induced changes in body fat distribution can be counteracted with hormone replacement therapy.[38] In contrast, aging males gradually accumulate abdominal fat, and hence increased WHR, in parallel with declining androgen levels.[22]
Using data from the U.S. National Center for Health Statistics, William Lassek at the University of Pittsburgh in Pennsylvania and Steven Gaulin of the University of California, Santa Barbara found a child's performance in cognitive tests correlated to their mother's waist–hip ratio, a proxy for how much fat she stores on her hips.[39]
Children whose mothers had wide hips and a low waist–hip ratio scored highest, leading Lassek and Gaulin to suggest that fetuses benefit from hip fat, which contains long chain polyunsaturated fatty acids, critical for the development of the fetus's brain.[39] In addition, evidence suggests that children of low-WHR teens were protected from the cognitive deficits often associated with teen birth.
Studies in twins have suggested that between 22% and 61% of variability in waist-to-hip ratio may be accounted for by genetic factors.[40]
WHR is considered as one of the three determinants of female attractiveness, the other two being body mass index (BMI), and curviness.[42] The concept and significance of WHR as an indicator of attractiveness was first theorized by evolutionary psychologist Devendra Singh at the University of Texas at Austin in 1993.[43][44] Singh argued that the WHR was a more consistent estrogen marker than the bust–waist ratio (BWR) studied at King's College, London by Glenn Wilson in the 1970s.[45][46]
Some researchers have found that the waist–hip ratio is a significant measure of female attractiveness.[47] Women with a 0.7 WHR are usually rated as more attractive by men from various cultures.[48] Preferences may vary, according to some studies,[42] ranging from 0.6 in China, South America, and some of Africa[49] to 0.8 in Cameroon and among the Hadza tribe of Tanzania,