The most basic function of bodily fat is self-storage of food reserves.
食物储备的自我存储是脂肪最基本的功能。
In prehistoric times,natural selection favored genotypes that could endure harsh conditions by stocking the most fat.
在史前时期，以保存大量的脂肪来忍耐恶劣坏境的基因型在自然选择中胜出。
With chronic malnutrition being the norm for most of human history,genetics evolved to favor fat storage.
同时，因为慢性的营养不良普遍存在于大多数的人类历史中，遗传基因朝着脂肪存储方向进化。
So when did body fat become problematic?
那么，身体脂肪什么时候开始成为了问题?
The negative impacts of being overweight were not even noted in medical literature until as late as the 18th century.
超重的消极影响直到18世纪才在医学记载中出现。
Then,technological advances coupled with public health measures resulted in the betterment of the quantity,quality,and variety of food.
接着，科技进步与公共健康措施一起提升了食物的产量、质量和丰富性。
Sustained abundance of good food enabled a healthier population to boom economically.Output increased,and with it,leisure time and waistlines.
源源不断的良好食物使得健康人口的数目大增。随着产量的增加，人们的业余时间和腰围也随之增加。
By the mid 19th century,being excessively overweight,or obese,was recognized as a cause of ill health,and another century later,declared deadly.
到19世纪中期，极度超重或肥胖被认为是不健康的，而一个世纪以后，肥胖则被宣告是致命的。
What is the distinction between being overweight and being obese?
超重和肥胖之间的区别是什么?
A calculation called the BMI breaks it down for us.For example,if someone weighs 65 kilgorams and is 1.5 meters tall,they have a BMI of about 29.
BMI公式为我们解释了这个问题。举例来说，如果一个人体重65千克，身高1.5米，其BMI值大约是29。
Obesity is a condition of excess body fat that occurs when a person's BMI is above 30,just over the overweight range of 25 to 29.9.
肥胖意味着身体脂肪超标，即BMI超过30;BMI在25-29.9之间则是刚刚超重。
While BMI can be a helpful estimate of healthy weight,actual body fat percentage can only really be determined by also considering information like waist circumference and muscle mass.
虽然BMI对估算健康的体重有一定帮助，但身体实际的脂肪百分比只能由腰围和肌肉群等信息决定。
Athletes,for instance,have a naturally higher BMI.
就像运动员天生就拥有较高的BMI。
一个人是如何肥胖起来的?
So how does a person become obese?At its most basic,obesity is caused by energy imbalance.
那么，一个人是如何肥胖起来的?从根本上来说，肥胖是由能量的不均衡导致的。
If the energy input from calories is greater than the energy output from physical activity,the body stores the extra calories as fat.
如果来自卡路里的能量输入高于物理运动所输出的能量，那么身体存储的多余卡路里就成了脂肪。
In most cases,this imbalance comes from a combination of circumstances and choices.
大多数情况下，这种不平衡来自环境和选择的结合。
Adults should be getting at least 2.5 hours of exercise each week,and children a whole hour per day.But globally,one in four adults and eight out of ten adolescents aren't active enough.
成年人每个星期至少要拥有2.5小时的运动量，而儿童每天应有1个小时的运动时间。但是全世界有四分之一的成年人和五分之四的青少年并不积极运动。
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