Clinical background
The waist-to-hip ratio serves a purpose that body mass index (BMI) cannot: it captures fat distribution, not merely fat quantity. Two people with an identical BMI may have radically different cardiometabolic risk depending on whether the fat is subcutaneous and peripheral or visceral and central. This is precisely what Ohlson and colleagues demonstrated in the prospective Gothenburg cohort of men born in 1913: the waist-to-hip ratio was associated with the incidence of diabetes independently of BMI [1]. A tool that distinguishes central obesity from total obesity therefore has independent clinical value as a complement to BMI, particularly when assessing patients in whom BMI underestimates metabolic risk.
Calculating the waist-to-hip ratio
The waist-to-hip ratio is calculated as the ratio of waist circumference to hip circumference, both in the same unit of length:
Waist circumference is measured at the midpoint between the lowest rib and the top of the iliac crest, with the patient standing, at the end of a normal expiration, with the tape in the horizontal plane and without compressing the soft tissues. Hip circumference is measured at the widest point over the buttocks, likewise horizontally.
The derivation cohort consisted of 792 men born in 1913 in Gothenburg, aged 54 years at baseline in 1967 [1]. After 13.5 years of follow-up, the baseline measurements were related to the incidence of diabetes mellitus. The waist-to-hip ratio was positively and significantly associated with the risk of diabetes even after adjustment for BMI as a measure of total fat mass.
Interpretation in practice
The WHO's established thresholds for substantially increased cardiometabolic risk are sex-specific:
| Waist-to-hip ratio | Men | Women |
|---|---|---|
| Low risk | < 0.90 | < 0.85 |
| Substantially increased risk | ≥ 0.90 | ≥ 0.85 |
A patient who crosses the threshold does not thereby have a diagnosis, but this should prompt an overall assessment of cardiometabolic risk: blood pressure, lipid profile, fasting plasma glucose and HbA1c, together with a discussion of lifestyle factors. In a patient with a normal BMI but a raised waist-to-hip ratio, the ratio is a reason not to be reassured by the BMI but to assess the actual risk. A patient below the threshold with a concurrently normal BMI has a low risk of central obesity-related metabolic disease, and additional risk markers can be given lower priority in the work-up.
Validation and performance
Association with cardiovascular disease and mortality. In the INTERHEART study, a case-control study of 27,098 participants in 52 countries, the waist-to-hip ratio showed a graded and strongly significant association with acute myocardial infarction (OR 2.52 for the highest versus the lowest quintile, adjusted for age, sex, region and smoking) [2]. The association with BMI largely disappeared after adjustment for the waist-to-hip ratio. The population attributable risk for myocardial infarction was 24.3% for the two highest quintiles of the waist-to-hip ratio compared with 7.7% for the corresponding quintiles of BMI.
A systematic review and dose–response meta-analysis of 72 prospective cohort studies with more than 2.5 million participants found an HR of 1.20 (95% CI 1.15–1.25) per 0.1 unit increase in the waist-to-hip ratio for all-cause mortality, with a positive monotonic dose–response relationship [3].
Added value over conventional risk factors. The Emerging Risk Factors Collaboration analysed 221,934 individuals in 58 prospective cohorts with 14,297 cardiovascular events over 1.87 million person-years [4]. The HR per 1 standard deviation higher waist-to-hip ratio (0.083 units) was 1.12 (1.08–1.15) after adjustment for age, sex, smoking, systolic blood pressure, history of diabetes and lipids. Adding the waist-to-hip ratio to a model with conventional risk factors did not, however, meaningfully improve discrimination (change in C-index 0.0008). The conclusion was that the waist-to-hip ratio, like BMI and waist circumference, does not contribute substantially to cardiovascular risk prediction when blood pressure, lipids and diabetes history are already known.
Performance in patients with type 2 diabetes. In the ADVANCE trial, in which 11,140 patients with type 2 diabetes were followed for a mean of 4.8 years, the waist-to-hip ratio was the strongest predictor of cardiovascular events and death among three anthropometric measures [5]. The HR per 1 SD (0.08) was 1.12 (1.05–1.19) for cardiovascular events and 1.19 (1.09–1.31) for cardiovascular death. BMI was not significantly associated with any outcome. The AUC did not differ significantly between the three measures, but the integrated discrimination improvement was greater for the waist-to-hip ratio than for BMI.
Sex and age differences. In the German KORA cohort (13,307 participants, aged 25–74 years, 15.4 years of follow-up), the waist-to-hip ratio was significantly associated with all-cause and cardiovascular mortality in both sexes, but the association was particularly marked in women [6]. Women in the third and fourth quartiles had substantially increased all-cause mortality compared with the reference quartile, while the pattern in men was more complex, with contributions from several anthropometric indices.
Reproducibility. In a study of self-measured and technician-measured waist and hip circumferences, the intraclass correlation between self-measurement and technician was 0.97 for waist and 0.96 for hip, with small systematic differences [7]. The Emerging Risk Factors Collaboration found, however, lower reproducibility for the waist-to-hip ratio (regression dilution ratio 0.63) than for BMI (0.95) and waist circumference (0.86), reflecting the fact that a ratio of two measurements accumulates measurement error from both [4].
Limitations
The waist-to-hip ratio does not add to cardiovascular risk prediction beyond established risk factors such as blood pressure, lipids and diabetes history [4]. Its principal clinical value lies in identifying central obesity in patients in whom BMI does not reflect the metabolic risk, not in replacing formal risk calculation.
Reproducibility is lower than for BMI, and measurement technique is decisive. The landmarks (the midpoint between the lowest rib and the top of the iliac crest, and the widest point over the buttocks) must be identified consistently. Different measurement protocols can give systematically different results and hamper comparison over time and between care providers.
The thresholds of ≥0.90 for men and ≥0.85 for women are derived at population level and may need adjustment for certain ethnic groups in whom body composition and fat distribution differ. The waist-to-hip ratio does not apply to pregnant women, children or patients with conditions that alter abdominal or hip configuration (ascites, large abdominal tumours, a stoma, lumbopelvic surgery). In very elderly patients, altered body composition and muscle mass may affect both waist and hip circumference and hence the interpretability of the ratio.
References
- Ohlson LO, Larsson B, Svärdsudd K et al. The influence of body fat distribution on the incidence of diabetes mellitus. 13.5 years of follow-up of the participants in the study of men born in 1913. Diabetes 1985;34(10):1055–8. PMID: 4043554
- Yusuf S, Hawken S, Ounpuu S et al. Obesity and the risk of myocardial infarction in 27,000 participants from 52 countries: a case-control study. Lancet 2005;366(9497):1640–9. PMID: 16271645
- Jayedi A, Soltani S, Zargar MS et al. Central fatness and risk of all cause mortality: systematic review and dose-response meta-analysis of 72 prospective cohort studies. BMJ 2020;370:m3324. PMID: 32967840
- Emerging Risk Factors Collaboration, Wormser D, Kaptoge S et al. Separate and combined associations of body-mass index and abdominal adiposity with cardiovascular disease: collaborative analysis of 58 prospective studies. Lancet 2011;377(9771):1085–95. PMID: 21397319
- Czernichow S, Kengne AP, Huxley RR et al. Comparison of waist-to-hip ratio and other obesity indices as predictors of cardiovascular disease risk in people with type-2 diabetes: a prospective cohort study from ADVANCE. Eur J Cardiovasc Prev Rehabil 2011;18(2):312–9. PMID: 20628304
- Rost S, Freuer D, Peters A et al. New indexes of body fat distribution and sex-specific risk of total and cause-specific mortality: a prospective cohort study. BMC Public Health 2018;18(1):427. PMID: 29609587
- Barrios P, Martin-Biggers J, Quick V et al. Reliability and criterion validity of self-measured waist, hip, and neck circumferences. BMC Med Res Methodol 2016;16:49. PMID: 27145829
