What Is BMI? The Science Behind Body Mass Index什么是 BMI？身体质量指数的科学原理
Body Mass Index (BMI) is a simple number used worldwide to classify body weight relative to height. It gives doctors, researchers, and individuals a quick way to screen for potential weight-related health risks without expensive equipment. Understanding how it works helps you interpret your result and use it wisely.身体质量指数（BMI）是一个在全球范围内用来按身高衡量体重水平的简单数值。它为医生、研究者和个人提供了一种无需昂贵设备即可快速筛查体重相关健康风险的方式。了解其计算与含义有助于你正确解读结果并合理使用。
The Formula and Its Origins公式与起源
BMI is calculated by dividing your weight in kilograms by the square of your height in metres: BMI = weight (kg) ÷ height (m)². For example, a person who is 1.75 m tall and weighs 70 kg has a BMI of 70 ÷ (1.75 × 1.75) ≈ 22.9. The formula was developed in the 19th century by the Belgian mathematician and statistician Adolphe Quetelet. He was studying the “average man” and how weight tended to scale with height in populations. The term “Body Mass Index” and its use as a health indicator became standard in the 1970s and 1980s, when epidemiologists linked BMI ranges to mortality and disease risk in large studies.BMI 的计算公式为：体重（公斤）除以身高（米）的平方，即 BMI = 体重(kg) ÷ 身高(m)²。例如身高 1.75 米、体重 70 公斤的人，BMI = 70 ÷ (1.75×1.75) ≈ 22.9。该公式由比利时数学家与统计学家阿道夫·凯特勒于 19 世纪提出，用于研究“平均人”及体重随身高的变化规律。“身体质量指数”作为健康指标的广泛应用则是在 20 世纪七八十年代，流行病学将 BMI 区间与死亡率和疾病风险建立联系之后。
How BMI Categories Are DefinedBMI 分类标准
Health organisations such as the World Health Organisation (WHO) use standard ranges to classify adults: under 18.5 is underweight; 18.5 to 24.9 is normal or healthy weight; 25 to 29.9 is overweight; and 30 or over is obese, sometimes split into further bands (e.g. 30–34.9, 35–39.9, 40+). These cut-offs are based on population studies that show increased risk of conditions like type 2 diabetes, heart disease, and certain cancers as BMI rises above the “normal” band, and higher risk of malnutrition or bone loss when it falls below. It is important to remember that these categories are statistical tools, not personal diagnoses. Your own risk depends on many other factors, including age, sex, ethnicity, muscle mass, and overall health.世界卫生组织（WHO）等机构采用统一区间对成人进行分类：低于 18.5 为体重过轻；18.5～24.9 为正常或健康体重；25～29.9 为超重；30 及以上为肥胖（有时再细分为 30～34.9、35～39.9、40+ 等）。这些界值基于人群研究：BMI 超过“正常”区间与 2 型糖尿病、心脏病及部分癌症风险上升相关，过低则与营养不良或骨质流失风险相关。需知这些分类是统计工具而非个人诊断，个人风险还取决于年龄、性别、种族、肌肉量和整体健康状况等因素。
Limitations of BMI: Why It’s Not a Complete PictureBMI 的局限性：为何不能代表全部
BMI is useful for population-level screening and research, but it has well-known limits when applied to individuals. Recognising these helps you avoid either ignoring a real concern or worrying unnecessarily.BMI 在人群筛查和研究中很有用，但应用到个人时存在众所周知的局限。认清这些局限有助于你既不过度焦虑，也不忽视真正需要关注的问题。
Muscle vs. Fat and Body Composition肌肉与脂肪：体成分差异
BMI does not distinguish between muscle, bone, and fat. A very muscular athlete can have a high BMI and be in excellent health, while someone with a “normal” BMI might carry a lot of visceral fat and have higher metabolic risk. Similarly, older adults often lose muscle and gain fat over time; their BMI might stay in the “normal” range even as their body composition becomes less healthy. For a fuller picture, professionals may use waist circumference, waist-to-hip ratio, or body composition tests (e.g. DEXA or bioelectrical impedance) in addition to BMI.BMI 无法区分肌肉、骨骼与脂肪。肌肉发达的运动员可能 BMI 偏高却非常健康，而“正常”BMI 的人也可能内脏脂肪较多、代谢风险更高。老年人常随年龄增长肌肉减少、脂肪增加，BMI 仍可落在“正常”区间，体成分却已变差。要更全面评估，可结合腰围、腰臀比或体成分检测（如 DEXA、生物电阻抗）等。
Age, Sex, and Ethnicity年龄、性别与种族
Standard BMI tables were largely built from data on Western adult populations. Different ethnic groups can have different relationships between BMI and health risk; for instance, some Asian guidelines use lower cut-offs for overweight and obesity (e.g. 23 and 25 instead of 25 and 30) because risk often rises at lower BMIs. Sex matters too: women tend to have more body fat than men at the same BMI. Age is another factor: the same BMI can mean something different in a young adult versus an older person, and BMI is not intended for growing children without age- and sex-specific charts (e.g. percentile curves).标准 BMI 表多基于西方成年人群数据。不同族裔中 BMI 与健康风险的关系可能不同，例如部分亚洲指南将超重和肥胖的界值定为 23 和 25（而非 25 和 30），因在较低 BMI 时风险即已上升。性别也有影响：相同 BMI 下女性体脂往往高于男性。年龄同样重要：同一 BMI 在青年与老年人身上含义不同；儿童需使用按年龄、性别划分的百分位曲线而非成人标准。
When BMI Can Be MisleadingBMI 何时会误导
BMI can be misleading in people who are very short or very tall, because the simple squared-height formula does not scale perfectly across all heights. It also says nothing about where fat is stored: abdominal fat is more strongly linked to cardiovascular and metabolic risk than fat in the hips and thighs. Pregnant women, people with oedema, or those with certain muscle-wasting conditions may find that BMI does not reflect their true health status. In all these cases, BMI is best used as one of several tools, not the only measure of health.身高极矮或极高时，仅用“身高平方”的简单公式可能不够准确。BMI 也不反映脂肪分布：腹部脂肪与心血管和代谢风险的关系强于臀腿脂肪。孕妇、水肿者或某些肌肉萎缩患者可能发现 BMI 无法反映真实健康状况。在这些情况下，BMI 应作为多项指标之一，而非唯一健康标准。
Using BMI Wisely: Health Suggestions and Next Steps如何正确使用 BMI：健康建议与下一步
If you have used our calculator and want to put your result in context, the following practical steps can help you stay balanced and avoid both over-interpreting and under-interpreting the number.若你已使用本计算器并希望将结果放在整体健康背景下理解，以下建议有助于你理性看待数值，既不过度解读也不忽视风险。
When to Take Action何时需要行动
If your BMI falls in the underweight range, consider whether your diet and lifestyle support healthy weight gain (e.g. enough calories, strength training). If you are in the overweight or obese range, small sustainable changes—such as more movement, better sleep, and more vegetables and protein—often work better than drastic diets. In either case, a single number does not tell the whole story. If you have symptoms (e.g. fatigue, joint pain, shortness of breath) or a family history of weight-related disease, or if you are unsure how to proceed, a doctor or registered dietitian can help you set safe, realistic goals.若 BMI 处于偏轻范围，可评估饮食与生活方式是否支持健康增重（如充足热量、力量训练）。若处于超重或肥胖范围，可持续的小幅改变（如增加活动、改善睡眠、多吃蔬菜与蛋白质）往往比极端节食更有效。无论哪种情况，单一数字都不能说明全部。若有症状（如疲劳、关节痛、气短）或体重相关疾病家族史，或不知如何下手，可咨询医生或注册营养师，制定安全、现实的目标。
Combining BMI With Other Health Metrics结合其他健康指标
Use BMI alongside other indicators: waist size, how you feel day to day, energy levels, sleep quality, and any blood tests your doctor recommends (e.g. blood sugar, cholesterol). Together, these give a better picture than BMI alone. If you are active and eat a varied diet, a slightly high or low BMI may still be acceptable for you; the aim is long-term health and well-being, not hitting a specific digit on the scale.可将 BMI 与腰围、日常体感、精力、睡眠质量以及医生建议的化验（如血糖、血脂）等结合使用，综合判断优于仅看 BMI。若你坚持运动、饮食多样，略高或略低的 BMI 仍可能处于可接受范围；目标是长期健康与生活质量，而非追求某个具体数字。
Practical Tips for a Balanced Lifestyle平衡生活的实用建议
Focus on habits that support a healthy weight and overall wellness: regular physical activity (including strength training to preserve muscle), plenty of vegetables and fibre, adequate protein, and enough sleep. Reduce highly processed foods and sugary drinks where possible. Manage stress and stay hydrated. These steps benefit everyone, regardless of BMI. Remember that weight is only one aspect of health; mental well-being, relationships, and quality of life matter just as much. Use this BMI calculator as a starting point for reflection, not as a final verdict on your health.把重点放在支持健康体重与整体健康的习惯上：规律运动（含力量训练以维持肌肉）、足量蔬菜与膳食纤维、适量蛋白质与充足睡眠。尽量少选深加工食品和含糖饮料，管理压力、足量饮水。这些对所有人都有益，与 BMI 高低无关。体重只是健康的一方面，心理健康、人际关系与生活质量同样重要。请将本 BMI 计算器作为反思的起点，而非对健康的“终审判决”。
Summary小结
BMI is a useful screening tool when you understand its strengths and limits. It is based on a simple formula (weight divided by height squared) and standard categories from major health bodies. It does not account for body composition, age, sex, or ethnicity, so it should be combined with other measures and professional advice when needed. Taking small, sustainable steps in diet and activity, and viewing your result in context, will help you use your BMI result in a balanced and constructive way.在了解其优点与局限的前提下，BMI 是一种有用的筛查工具。它基于简单公式（体重除以身高平方）和权威机构的分类标准，但不考虑体成分、年龄、性别与种族，因此应与其它指标及必要时与专业建议结合使用。在饮食与活动上采取可持续的小步改进，并结合自身情况解读结果，有助于你更理性、建设性地使用 BMI。