Commentary: optimal body mass index cut points

A standard body mass index (BMI)-based definition of overweight and obesity is essential to the monitoring and comparing obesity trends, yet the choice of BMI thresholds continues to excite considerable debate.1–3 Since the early 1990s, the World Health Organization (WHO) has recommended using BMI cut points of 25 and 30 kg/m2, respectively. However, these recommendations were challenged by later studies including those in Western3,4 and Asian populations.1,2,5–7 Some Asian populations have higher amounts of body fat for a given BMI and have health risks at lower BMI than Western populations;8,9 thus lower BMI cut points of 23 and 25 are recommended to define overweight and obesity. This is endorsed by the WHO for public health purposes,10,11 although researchers from countries in the region have also suggested using other BMI cut points.5,6 Overall, previous studies have been inconsistent regarding the need for lower BMI cut points in Asian populations. In this issue of the IJE, Pierce and colleagues examined the association between BMI and all-cause mortality in a prospective cohort study of 11 445 rural residents aged 18–75 years in Bangladesh.12 Their findings do not support the use of lower BMI cut points. Approximately 40% of their study participants were underweight (BMI < 18.5), whereas only 7% were overweight or obese if based on the WHO’s definition (BMI ≥ 25). After 6 years of follow-up, those who were severely underweight (BMI < 16.0) had the highest total mortality [hazard ratio (HR): 2.06; 95% confidence interval (CI): 1.53–2.77], whereas those in the moderate underweight category (BMI: 16.0–16.9) had higher mortality (HR: 1.39; 95% CI: 1.01–2.90) when compared with the normal weight group (BMI: 18.5–22.9).11 However, the highest BMI category (≥23.0) did not show a clear association with mortality (HR: 1.10; 95% CI: 0.77–1.53), although a marginal statistically significant association was detected when men were analysed separately (HR 1.49; 95% CI: 1.00–2.23). The number of deaths in women was small (n = 9), which may explain why no association was found for them. Strengths of this study include its high response rate (97.5%), objectively measured BMI and adjustment for potential confounders. Its limitations include the fact that this was largely a malnourished population, which therefore did not allow examination of the full range of obesity, the relatively short follow-up and relatively small sample size and small number of deaths (n = 393) when compared with other previous studies conducted in Western and East Asian countries.1–4,7 Furthermore, it is possible that factors other than low BMI itself such as infection, micronutrient deficiencies and other undiagnosed health conditions might have also affected mortality in the population. Further larger cohort studies with better study design are warranted to examine the relationships of BMI with mortality and morbidity in Bangladesh. These findings echo those from a previous cohort of 75 868 people in South India, which also found a significant elevation in mortality for the underweight group (BMI < 18.5), but no excess for those with a BMI ≥23.13 Like the current study, this was also a rural population with a low prevalence of overweight and obesity but a high rate of underweight. Existing studies have suggested a U- or J-shaped relationship between BMI and mortality in Western3,4 and East Asian populations.1,2,7 Underweight (BMI < 18.5) has been reported consistently to increase the risk of mortality in developed and developing countries.1–4,7,12,13 Based on a nationally representative sample, Flegal et al. found a U-shaped association between BMI and all-cause mortality.3 Some recent large prospective cohort studies in East Asian countries12,7 have reported a significant association of high BMI with mortality, which are consistent with many studies in Western populations3,4 and do not support the use of lower BMI cut-points in Asian populations. For example, a study based on a large nationally representative sample of 169 871 subjects in China reported a U-shaped association between BMI and mortality after a mean follow-up of 8.3 years.1 Underweight (BMI < 18.5), low normal weight (BMI: 18.5–22.9) and overweight or obesity (BMI ≥ 27) were significantly related to higher mortality than a BMI of 24.0–24.9 (the lowest mortality) in both men and women.1 In another population-based prospective study of 40 815 men and women aged 40–59 years in Japan, 1426 deaths were documented during the 10-year follow-up.7 The BMI–mortality association was again U-shaped. Compared with a BMI of 23.0–24.9, elevated risks were observed for those in the underweight and overweight categories. A large 12-year prospective cohort study of 1 213 829 Koreans aged 30–95 years reported a J-shaped association between BMI and mortality.2 The risk of death from any cause was the lowest among those with a BMI of 23.0–24.9. However, those with BMI <16.0 were excluded from the study, which may partly explain the J-shaped but not U-shaped relationship observed in this Korean population. On the other hand, some recent research also supports the use of lower BMI cut points for overweight and obesity in Asian populations.5,6,14 Body fat distribution differs across racial/ethnic groups; and some Asian populations are found having a higher amount of percentage of body fat and abdominal adipose tissue than Caucasians with the same BMI,8,9 and have higher health risks.8 A recent international study showed a racial/ethnic difference in the BMI–hypertension association. The ‘optimal’ BMI cut points that represent elevated incidence of hypertension were higher for Chinese (BMI: 23–24) than for Indonesian (BMI: 21–22.5) and Vietnamese (BMI: 20.5–21), which varied between population groups.14 The study by Pierce et al.12 has addressed an important issue regarding the associations of underweight and overweight with mortality in developing countries where malnutrition remains prevalent. Their findings, the high prevalence of underweight and its associated elevated risk for mortality, show that malnutrition is still a serious public health threat in low-income countries like Bangladesh. Great effort is still needed to address the problem both domestically and with assistance from the international community. With economic development and lifestyle shifts, many developing countries have been experiencing a rise in overweight and obesity during recent decades and some of them are facing a double burden of malnutrition and obesity.5,6 The dual burden will be a challenge for public health interventions as different tailored programmes and policies may be needed to address malnutrition and obesity problems, while often these countries have limited resources. Although the prevalence of overweight and obesity in most Asian countries remains lower than that in Western countries,5,12 it has been increasing steadily in some countries. Based on nationally representative data, we recently showed that the prevalence of overweight and obesity (Chinese standard, BMI ≥ 24) had increased from ∼20% in 1992 to 30% in 2002 in China.6 Recent data in South Korea showed that the prevalence of overweight (BMI ≥ 25) was 23.8% in men and 26.8% in women.2 In conclusion, the choice of optimal BMI cut points to define overweight and obesity in developing countries remains controversial. The use of appropriate BMI cut points is important. This not only helps to make meaningful comparisons between populations and assess time trends, but also helps to better understand the complex relationships between adiposity and health outcomes and evaluate obesity-related intervention programmes in diverse populations. We suggest that it is likely that the relationships between BMI and health risks vary between populations and by health outcomes. This may be due to between-population differences in body build, body composition and lifestyle factors, which may change rapidly over time as a consequence of social, environmental and behavioural shifts. Nevertheless, existing large cohort studies provide strong evidence for elevated health risks associated with underweight and with obesity among Asian and Western populations. In addition, the high prevalence of underweight in some low-income countries and its associated mortality risk show that malnutrition remains a serious public health issue in these countries. Considerable care is required to ensure that interventions focused on combating the global obesity epidemic do not adversely impact on the continuing threat of undernutrition and underweight in the developing world.