We hear about calories all the time.
我们时常听到卡路里。
How many calories are in this cookie?
这个曲奇里有多少卡路里？
How many are burned by 100 jumping jacks, or long distance running, or fidgeting?
100个开合跳能燃烧多少卡路里？长跑呢？坐立不安呢？
But what is a calorie, really, and how many of them do we actually need?
那到底什么是卡路里呢，我们到底需要多少卡路里？
Calories are a way of keeping track of the body's energy budget.
卡路里能够跟踪身体能量收支。
A healthy balance occurs when we put in about as much energy as we lose.
当我们摄入和消耗的能量相当的时候，就达到了健康的平衡。
If we consistently put more energy into our bodies than we burn,
如果我们摄入的能量远远高于身体消耗的能量，
the excess will gradually be stored as fat in our cells, and we'll gain weight.
多出的部分会渐渐转化为脂肪储存在细胞内，然后我们变胖了。
If we burn off more energy than we replenish, we'll lose weight.
如果我们燃烧的能量比存入的多，我们就会减少体重。
So we have to be able to measure the energy we consume and use, and we do so with a unit called the calorie.
因此我们需要测量我们消耗使用的能量，于是我们用卡路里来测量。
One calorie, the kind we measure in food, also called a large calorie,
我们在食物中测量的，一卡路里，也称为一大卡，
is defined as the amount of energy it would take to raise the temperature of one kilogram of water by one degree Celsius.
它被定义为让一千克水温度升温一摄氏度所需的能量。
Everything we consume has a calorie count, a measure of how much energy the item stores in its chemical bonds.
我们吸收的所有东西通过测量它化学键里储存的能量都能用卡路里衡量。
The average pizza slice has 272 calories, there are about 78 in a piece of bread, and an apple has about 52.
每块匹萨含有272卡路里，面包含有78卡路里，苹果有大概52卡路里。
That energy is released during digestion,
能量在消化过程中释放，
and stored in other molecules that can be broken down to provide energy when the body needs it.
储存在其他的分子中它们在身体需要的时候转化为能够消耗的能量。
It's used in three ways: about 10% enables digestion, about 20% fuels physical activity,
能量有3种消耗方式：10%左右用于消化，20%左右满足身体活动消耗，
and the biggest chunk, around 70%, supports the basic functions of our organs and tissues.
而最大的一部分，占约70%，用来支持我们器官和机体组织的基本功能运作。
That third usage corresponds to your basal metabolic rate,
第三个用处对应基础代谢率，
a number of calories you would need to survive if you weren't eating or moving around.
表示你不吃不喝不活动的情况下生存所需的卡路里量。

Add in some physical activity and digestion,
考虑物理活动和消化的因素，
and you arrive at the official guidelines for how many calories the average person requires each day:
那就得出了官方指南中所说一般人每天所需的卡路里量：
2000 for women and 2500 for men.
女性需要2000卡路里，男性则需要2500。
Those estimates are based on factors like average weight, physical activity and muscle mass.
这些数据是基于诸如平均体重、活动量、肌肉量这样的因素估计得来的。
So does that mean everyone should shoot for around 2000 calories? Not necessarily.
难道每人每天都要2000卡路里吗？并不一定。
If you're doing an energy guzzling activity, like cycling the Tour de France,
如果你从事消耗量巨大的活动，像是环法自行车赛，
your body could use up to 9000 calories per day.
你的身体每天能消耗高至9000卡路里。
Pregnancy requires slightly more calories than usual, and elderly people typically have a slower metabolic rate,
孕妇所需的卡路里也比普通人多一些，老人通常有较慢的新陈代谢率，
energy is burned more gradually, so less is needed.
能量消耗更慢，所需的更少。
Here's something else you should know before you start counting calories.
然而你在开始计算卡路里前应该了解一些事。
The calorie counts on nutrition labels measure how much energy the food contains,
营养成分表上标注的卡路里含量，是指这些食物自身所含有的能量，
not how much energy you can actually get out of it.
并不是你能从中获得的能量。
Fibrous foods like celery and whole wheat take more energy to digest,
像是芹菜和全麦这样的纤维食物需要更多的能量去消化，
so you'd actually wind up with less energy from a 100 calorie serving of celery than a 100 calorie serving of potato chips.
因此你从含100卡路里的芹菜中真正能摄取的能量是少于含100卡路里的薯片的。
Not to mention the fact that some foods offer nutrients like protein and vitamins,
同时有些食物提供营养物质，像是蛋白质和维他命，
while others provide far less nutritional value.
而其他的一些食物提供很少的营养价值。
Eating too many of those foods could leave you overweight and malnourished.
吃太多的这类食物会让你体重超重并且营养不良。
And even with the exact same food, different people might not get the same number of calories.
即使是同样的食物，不同的人摄取到的卡路里也是不同的。
Variations in things like enzyme levels, gut bacteria, and even intestine length,
因为个体差异，比如酶水平，肠道细菌，甚至是肠子的长度，
means that every individual's ability to extract energy from food is a little different.
都能令每个个体从食物中提取能量的能力产生少许差异。
So a calorie is a useful energy measure,
因此，卡路里是有用的能量测量指标，
but to work out exactly how many of them each of us requires
但如果想要准确知道我们所需多少卡路里，
we need to factor in things like exercise, food type, and our body's ability to process energy.
我们要考虑活动量、食物种类和我们身体摄取能量的能力。
Good luck finding all of thaton a nutrition label.
快去看看营养成分表吧。