Body mass index (BMI) is a measure of human physical fitness that is designed to provide a standard metric for evaluating individuals’ weight, relative to their height. Specifically, BMI is calculated as weight divided by height (squared), as shown below. As such, BMI can be thought of as the ratio of weight to height, per unit of height.
BMI =kg/m2or, BMI =lbs/in2* 703
Based on BMI, an individual’s weight status can be classified as underweight, normal, overweight, or obese. Some professionals use an additional weight category of “extreme obesity” (or “morbidly obese”) when classifying weight status.
Weight Classifications Among Adults
Within adult populations, healthy body mass index values range from 18.5 to 24.9. Adults with a BMI of less than 18.5 are considered “underweight.” BMI values of between 25.0 and 29.0 classify individuals as “overweight,” and BMI values of equal to or greater than 30.0 classify individuals as “obese.” These weight classifications are based upon research findings associating higher BMI scores with increased incidence ofhealthcomplications such as heart disease, diabetes, and asthma.
儿童的体重分类
Although the same formula for calculating BMI is used with children and adults, the process for identifying normal versus unhealthy growth patterns differs between populations. Because body mass index changes substantially with age, rather than using specific cutoff values, weight classifications for children between ages 2 and 20 are determined by plotting body mass values along growth charts for age and sex. Children whose BMI values fall between the 15th and 85th percentiles on BMI-for-age-and-sex growth charts are considered to be of healthy weight. Children whose BMIs fall above the 85th or 95th percentiles for their age and sex are classified as “overweight” or “obese,” respectively. Similarly, individuals with BMI values below the 15th percentile are considered to be “underweight.”
Alternative Methods For Classifying Weight
Alternative methods exist for determining weight status including waist circumference, skin fold measurement, underwater weighing (hydrostatic weighing), and bioelectrical impedance. Waist circumference is a general method for determining unhealthy weight status and simply involves measuring a person’s waist circumference. For women, a waist circumference greater than 35 inches is considered unhealthy and 40 inches is unhealthy for men. Skin fold measurement involves using a caliper device to measure the thickness of a fold of skin and its underlying layer of fatty tissue. Specific locations that are to be representative of overall body fat include back of arm, front of thigh, and lower abdomen. Hydrostatic weighing is a method where an individual’s “dry” weight is obtained while wearing minimal clothing. Next, the subject is lowered into a water filled tank where they are weighed underwater. These two weights are then compared to determine the person’s body fat percentage. Bioelectrical impedance uses medical technology to determine body composition including percentage of fat, muscle, and water.
Why Use BMI?
Compared to procedures such as skin fold measurement, waist circumference measurement, underwater weighing, and bioelectrical impedance, body mass index proves to be a relatively uncomplicated, inexpensive, accurate, and reliable tool for classifying weight status. Whereas techniques such as underwater weighing and bioelectrical impedance require both extensive training and sophisticated equipment, BMI calculations require only a scale, height rod, and basic mathematical skills. Furthermore, unlike skin fold and waist circumference measurement, body mass index can be consistently calculated over time and by different people.
但是,体重指数计算也与许多局限性有关。尽管与重量状态的其他评估技术高度相关,但BMI计算不能直接测量体内脂肪百分比。BMI可能高估了高于预期肌肉质量(即运动员)的个体的体内脂肪,并且可能低估肌肉质量很少(即老年人)的个体的体内脂肪百分比。同样,BMI也不考虑个体内部脂肪的分布,这与某些类型的健康风险高度相关。因此,两个具有相同BMI得分的个体可能具有截然不同的身体组成。
与任何分类系统一样,将健康与不健康权重分开的特定值和百分位数在某种程度上是任意的。也就是说,各自的BMIS的前瞻性健康状况(指定为“超重”)和30个(指定的“肥胖”)可能比不同标签所建议的更相似。尽管BMI与使用分类系统的其他测量系统共享了这个基本问题,但BMI衡量的重量状态已反复被证明是研究文献中健康问题的强大预测指标。当前,BMI是用于分类体重状态的最广泛使用,准确和具有成本效益的方法之一。
References:
- Cole, T. , Bellizzi, M. C., Flegal, K. M., & Dietz, W. H. (2000). Establishing a standard definition for child overweight and obesity worldwide: International survey.British Medical Journal,320(7244), 1240–1243.
- Centers for Disease Control and Prevention (CDC). (2000).
- Growth charts.从...获得http://www.cdc.gov/Growtharts/
- Centers for Disease Control and Prevention (CDC). National Center for Chronic Disease Prevention and Health (n.d.).Chronic disease prevention。从...获得http://www.cdc.gov/nccdphp/
- 卫生与公共服务部。民族心脏,肺和血液http://www.nhlbi.nih.gov/
- Kraemer,C.,Berkowitz,R。I.,&Hammer,L。D.(1990)。肥胖研究中的方法论困难:测量问题。行为医学年鉴,12,112–118。