我们都知道蛋白质对健康至关重要。究竟什么是蛋白质？我们到底需要多少？那些关于蛋白质的常识都是对的吗？

We all know protein is critical to our health.  But what is it exactly?  How much do we really need?  What are the most common protein myths?

蛋白质，碳水化合物和脂肪是对人体功能正常运转至关重要的三种宏量营养素。蛋白质是由20几个氨基酸组成的（最常见的是20个，具体数目没有定论）。你可以把这些氨基酸想象成儿童积木的单个部件。这些单件可以通过不同的组合形成不同形状和大小的各种东西。蛋白质的组合方式和这个差不多。时时刻刻，你的体内都有超过1万种的蛋白质在辛勤工作，维持着你的健康。这些蛋白质是由不同数量和不同排列的氨基酸组合成的。

Protein is one of the three macronutrients along with carbohydrates and fats that are critical for normal human functioning.  The building blocks of proteins are more than 20 amino acids (The most common ones are 20, and the exact number is not agreed upon).   Think of these amino acids as individual pieces of children’s bloc toys.  These individual pieces can form many different kinds of shapes and sizes, through different combinations.  Proteins work about the same way.  At any time there are more than 10,000 proteins at work in your body to keep you healthy.  These proteins are synthesized from different combinations and numbers of these amino acids.

在这20多种氨基酸中，有9种被称作“必需氨基酸”，因为我们人体无法自行制造这些，必须从食物中获取。

Of these over 20 amino acids, 9 of them are essential, meaning, we have to get it from food sources, as the body cannot make them on its own.

蛋白质对于构建、维持和修复我们体内几乎所有的组织都必不可少，包括骨骼、肌肉、血液、头发、指甲和器官。为许多生化反应提供动力的酶也是蛋白质构成的。在血液中携带氧气的血红蛋白也是由蛋白质组成的。此外，氨基酸还用来构建其他形式的生物分子，比如DNA和RNA。

Proteins are necessary for building, maintaining, and repairing almost all the tissues in our body, including our bones, muscles, blood, hair, nails, and organs.  It makes up the enzymes that power many chemical reactions.  Hemoglobin, which carries oxygen in your blood, is also made up of protein.  In addition, amino acids also build other biological molecules such as DNA and RNA.

当你摄入含蛋白质的食物时，你的消化系统会将其分解为氨基酸，然后你的身体会以不同的组合方式根据身体当时的需要将这些氨基酸重新组合成各种形式的蛋白质，来执行所需的各种功能。我们的身体时时刻刻都在不断分解和组成蛋白质。

When you eat protein containing foods, your digestive system breaks it down into amino acids, and then your body synthesizes these amino acids in different combinations to form new proteins to perform functions needed.  Our body is constantly breaking down and forming proteins.

在三种宏量营养素中，蛋白质通常是最受关注的。你很可能不会去想“我吃的碳水化合物够不够？”或“我吃的脂肪够不够”，但你有可能会担心自己摄入的蛋白质够不够。那么我们到底需要多少蛋白质呢？是不是越多越好？

Among the three macronutrients, protein usually gets most of the attention.  You probably don’t think much about “am I getting enough carbs?”, or “am I getting enough fats?”, but you may be concerned if you are getting enough protein.  So how much do we really need?  Is more always better?

根据中国政府公布的中国膳食参考摄入量，成年女性平均每日推荐摄入量（RDA）为 55 克，成年男性平均每日摄入量为 65 克。

According to the Chinese Dietary Reference Intakes published by Chinese government, the RDA (Recommended Daily Allowance) for an average adult woman is 55 grams a day, and 65 grams a day for an average adult man. 

美国政府公布的每日推荐摄入量略有不同：成年女性为 46 克，成年男性为 56 克。就卡路里摄入量而言，这大约相当于普通人卡路里需求的 10%。

The recommended daily intakes published by the US government are slightly different: 46 grams for adult woman, and 56 grams for adult man.  In terms of calorie intake, that’s about equivalent to 10% of an average person’s calorie need.

当然，这只是一个大致的原则，如果你体重偏高或偏低呢？

Of course this is just a general guideline, what if you weigh more or less than average? 

所以我更喜欢基于体重的估算方法：每公斤体重需要约0.8克蛋白质（或每磅体重0.36克蛋白质）。

This is why I prefer a more weight-based approach: 0.8 gram of protein per kilogram of body weight (or 0.36 gram of protein per pound of body weight).

你的蛋白质需求还取决于你的年龄和运动水平。我们的肌肉量在 30 多岁时达到顶峰。在那之后会平均每十年流失3% 到 8%。在没有任何干预的情况下，如果你的饮食和运动保持不变，那你在80多岁时会失去大约50% 的肌肉。增加蛋白质摄入量可以在一定程度上帮助你保持肌肉的强壮。

Your protein need is also dependent on your age and your activity level.  Our muscle mass peaks at late 30s.  After that, you slowly and gradually lose your muscle 3 to 8% per decade on average.  Without any intervention, you lose about 50% of your muscle mass in your 80s.   Increased protein intake can help you preserve muscle mass to a certain extent.

如果你经常做高强度的运动，你对蛋白质的需求会更高。如果你在孕期或哺乳期，你也需要更多的蛋白质。

If you have a strenuous exercise regimen, your need is higher.  And if you are pregnant or breast feeding, you also need more protein.

每日推荐摄入量（RDA） 的计算方式意味着它可以顾及到大约 98% 的人口。也就是说，比如，在研究计算RDA的总人群当中，46克蛋白质将确保群中98%的成人女性有足够的摄入量。它不是平均值，也不是中间值。所以有可能你甚至都不需要这么多。

The way RDA is calculated means it intends to cover about 98% of the population.  It means, for exmaple, that among the total population that is studied to calculate RDA,  46 grams of protein will ensure 98% of adult women in the study have adequate intake.  It is not an average value, and it is not a median value.  So it is likely you may not even need this much.

就像营养学中的很多东西一样，我们需要多少蛋白质也有着它的不确定性和争论。中国和美国政府提供的指导原则就是对这种不确定性很好的一个显示。中国人的平均身高和体重低于美国人，但中国向其公民推荐的蛋白质摄入量要比美国政府高，这是不是有点奇怪？我有看到过一个人每餐至少应该摄入50克蛋白质的说法，也读过很多关于过量蛋白质对身体极其有害的研究。

Like so many things in nutrition, there are a lot of uncertainty and debates.  The difference between China and US government guidelines serve as a reminder of that this uncertainty.  Doesn’t it strike you as a bit odd that China is recommending a higher intake of protein to its citizens than US, given the lower average height and weight of Chinese than Americans?  I have heard people argue that a person should consume at least 50 grams protein PER MEAL.  And I have read a lot about the harmful effects of excessive protein. 

目前，我的研究给予我的结论是：我们需要有足够的蛋白质，但不能过多。

At this point, based on my own research, I have come to the conclusion that we need to have enough, but not too much protein.

由瓦尔特·隆⼽（ Valter Longo） 博士（我们在以前的一篇文章中谈到过他写的“长寿饮食”一书, " 你可以信任的长寿饮食秘诀”）领导的团队曾经做过一项研究，试图理解膳食蛋白质与死亡率之间的联系。他们查看了 6800 多名50岁及以上的美国成年人的数据。这些人被分为三组：高蛋白（超过20%的卡路里来自蛋白质）、中蛋白（10%到19%）和低蛋白（低于10%）。他们的发现令人吃惊：50到65 岁高蛋白质摄入组的成年人的总体死亡率增加了 75%，并且在接下来的18年内死于癌症的可能性是低蛋白质组的4倍。与低蛋白质饮食相比，中等蛋白质饮食组的癌症死亡率增加了3倍。但是，在 65 岁以上的年龄组中，他们发现高蛋白组的所有病因死亡率的风险降低了28%，死于癌症的风险降低了60%。

A team led by Dr. Valter Longo (the scientist we talked about in our previous article that wrote the book “The Longevity Diet”， “The longevity diet you can trust")  performed a study exploring the link between dietary protein and mortality.  They look at the data of more than 6800 US adults aged 50 and over.  The people were divided into three groups: high (more than 20% of their calories are from protein), moderate (10% to 19%) and low (below 10%).   What they found is startling: Adults in the 50 to 65 group with high protein intake had a 75% increase in overall mortality and were 4 times more likely to die from cancer during the following 18 years than those in the low protein group. The moderate-protein diet group had a 3-fold increase in cancer mortality compared to the low-protein diet.  However, in the age group of 65 and older, they found the high protein group had a 28% lower risk of dying from any cause and a 60% lower risk of dying from cancer. 

而且，高蛋白饮食让所有年龄段的糖尿病死亡率增加 5 倍。

What’s more, A high-protein diet was also associated with a 5-fold increase in diabetes mortality across all ages.

澳大利亚的研究人员对小鼠进行的研究也显示了类似的结果。与喂食低蛋白质饮食的小鼠相比，喂食高蛋白质饮食的小鼠的乳腺癌和黑色素瘤的进展更快。然而，喂食低蛋白质饮食的老年小鼠不是非常的健康。

The results were echoed by the result of similar studies in mice by Australian researchers.  Mice fed a higher protein diet had increased progression of breast and melanoma tumors than those fed a lower protein diet. However, older mice don’t thrive as well on a low protein diet.

如果这些不够让人信服的话，让我们看看人类最天然的食物：母乳。既然婴儿需要最快速的生长，你会不会觉着母乳的蛋白质含量一定很高？其实不是。人类的母乳是哺乳动物中蛋白质含量差不多最低的：约 1% 的重量和 6% 的卡路里。

If all this is not convincing enough, let’s consider the most natural human food there is: breast milk.  Since infants need the most rapid growth, you would think it has high protein content?  Nope. In fact, it is the lowest protein milk among mammals: about 1% by weight and 6% by calories.  

简单说下这背后的科学解释，为什么过多的蛋白质会让人老得更快并可能对身体造成伤害。人类生长激素的通道，包括雷帕霉素靶点 (TOR) 和胰岛素样生长因子 (IGF-1)，都是由蛋白质控制的。人在婴儿时期两者的水平都很高，因为婴儿需要快速的生长，但在成年后水平会大幅下降。然而，过量的蛋白质会上调这些通路，从而加速衰老并导致不必要的组织生长，甚至可能促成癌症的发生。

Here is some science explanations as why excessive protein ages you and may potentially cause harm.  Growth hormone pathways, including TOR （Target Of Rapamycin） and IGF-1 （Insulin-like Growth Factor), are controlled by proteins.  Humans have high levels of both as infants when rapid growth is needed, but the levels drop significantly when in adulthood.  However, excessive proteins up-regulate these pathways, thus accelerating aging and cause unnecessary tissue growth and could even promote cancer.

除了老化和不必要的组织生长之外，过量的蛋白质还可能造成其他的潜在危害。其中之一是肾脏问题。因为身体需要分解过量氨基酸中的氮将之通过尿液排出，所以持续地过量地摄入蛋白质会对肾脏造成过大的压力。对于肾功能本身就不很健全的人来说，这尤其会是一个隐患。

Besides aging and unnecessary tissue growth, there are other potential harms that excess protein could cause.  One of them is kidney problems.  Because the body needs to break down the nitrogen part of exessive amino acids  and excrete it through urine, constant and excessive consumption of protein could put too much stress on kidney.  This especially could be a problem for people already with kidney dysfunction. 

过量的蛋白质，就像过量的碳水化合物和脂肪一样，如果身体摄入的卡路里超过其所需，最终都会被转化为脂肪储存起来。尤其是如果你不经常运动的话。

Excessive protein, just like excess carbs and fat, could be eventually converted to fat in the body if the calories taken in exceed what is needed by the body.  This is especially true if you have a sedentary lifestyle.

所以，很多人经常担心自己摄入的蛋白质是否足够，而实际上我们可能更需要担心的是我们是否摄入过多。在现实生活中，蛋白质缺乏是非常罕见的。如果你吃的食物种类多样，卡路里摄入量正常，那一般就不用担心蛋白质足量的问题。

So when we are walking around worried about if we getting enough protein, in reality what we probably need to worry about is whether we’re getting too much.  In real life, it is very rare to have protein deficiency.  If you eat a variety of food and have a normal calorie intake, you should be getting enough.

总结一下，我们应该摄入足够的蛋白质以确保正常的身体机能，但不要摄入过多，给身体带来不必要的负担。如果你是65岁以下，也没有经常进行剧烈的体育锻炼，那么你的目标可以是每公斤体重0.7至0.8克的蛋白质（即每磅 0.31 至 0.36 克）。65以上，可以做适度的增加。10%到15%的卡路里来自蛋白质就足够了。如果超过20%，那恐怕就太多了。

To put all this together, we should get enough to ensure proper bodily functions, but not too much to give your body too much load.   If you are under 65 without regular strenuous physical training, aim for 0.7 to 0.8 gram of protein per kg of body weight (that is 0.31 to 0.36 gram per pound).  If you are above 65, increase it modestly.  Having 10% to 15% of your calories come from protein is adequate.  More than 20% would be too much.

下一篇文章，我们看看这些量的蛋白质翻译成餐桌上的食物应该是什么样。

In the next article, we’ll look at how this much protein translates into real food on your plate. 

References:

• Food Revolution Network.  Can You Get Your Protein From Plants?

• Harvard T.H Chan Public School of Health: Protein

• Medicine LibreTexts.  Protein Digestion, Absorption and Metabolism

• National Institute of Health. Dietary Reference Intakes (DRIs): Recommended Dietary Allowances and Adequate Intakes, Total Water and Macronutrients

• 中国居民膳食营养素参考摄入量：宏量营养素

• Harvard Health Publishing. How Much Protein Do You Need Everyday?

• Recommended Dietary Allowances: 10th Edition. Protein and Amino Acids

• Mayo Clinic Health System.  Are You Getting Too Much Protein?

• American Journal of Clinical Nutrition. Protein and Healthy Aging

• Nutritionfacts.org.  The Great Protein Fiasco

• Danny Lennon.  Researchers Point to the Optimal Protein Dose, Timing & Distribution to Maximize Muscle

• NMorgan E. Levine,a,1 Jorge A. Suarez,a,b,1 Sebastian Brandhorst,a,b Priya Balasubramanian,a,b Chia-Wei Cheng,a,b Federica Madia,a,h Luigi Fontana,c,d,e Mario G. Mirisola,a,b,i Jaime Guevara-Aguirre,j Junxiang Wan,a,b Giuseppe Passarino,f Brian K. Kennedy,g Pinchas Cohen,a,b Eileen M. Crimmins,a and Valter D. Longoa,b,2.  Low Protein Intake is Associated with a Major Reduction in IGF-1, Cancer, and Overall Mortality in the 65 and Younger but Not Older Population

• MyHealthSciences. Optimal Protein Intake for Health and Performance

• Lynn Goldstein.  Digestion of Proteins

• Oregon State University.  Protein Metabolism