All animals must eat protein regularly to survive, because we cannot make protein out of fat or carbohydrate or cholesterol. Proteins form enzymes, muscles, hormones, and other vital bodily components. How much protein do we need, and does it matter where we get it from?
What is protein?
Proteins are complicated molecules of many different shapes and sizes that are essential to all forms of life. Proteins are intimately involved in virtually everything that happens inside our cells and are infinitely more diverse and complex than carbohydrates and fats.
Below are just a few examples of important bodily proteins:
- Enzymes (to run chemical reactions)
- Peptide hormones (example: insulin)
- Antibodies (immune system molecules)
- Muscle fibers
- Neurotransmitters (examples: serotonin, adrenaline, dopamine, nitrous oxide, and histamine)
- Blood carrier proteins (examples: hemoglobin, albumin)
- Hair, skin, and nails
- Melanin (skin pigment)
Below are some examples of important molecules that cannot be built without proteins:
- DNA and RNA
- Glutathione (a critical antioxidant)
- Creatine (supplies energy to muscles)
Why do we have to eat protein?
Carbohydrates, fats and cholesterol are made of carbon, hydrogen and oxygen, but proteins are unique because they also contain nitrogen. This is why the body cannot make protein out of carbohydrate, fat, or cholesterol. We can make carbohydrate (from protein), and can store some extra as glycogen. We can make cholesterol out of anything, and can recycle excess cholesterol in the bile. We can make most fats out of anything, and can store huge amounts of extra fat.
We can live a whole lifetime (after infancy) without eating any carbohydrate, and we can live for 6 months or more without eating any fat, depending on how much fat we have on our bodies to begin with. However, we have no way to store proteins and can only live for a maximum of 70 days without eating any protein.
Since we can’t make proteins from scratch, and we can’t store excess protein, protein is the only macronutrient that we absolutely must eat regularly in order to thrive. Without enough protein in the diet, the body will have no choice but to break down muscle fibers to release the protein it needs to survive.
What are amino acids?
Proteins are made up of small building blocks called amino acids. While there are hundreds of amino acids, there are only 20 amino acids used to build proteins. By combining these 20 amino acids in different sequences, cells can create thousands of unique proteins. Amino acids are like letters of the alphabet, and our cells put them together in different combinations like words in a dictionary, each one with its own meaning and purpose.
There are 9 essential (or indispensable) amino acids that we cannot make from scratch under any circumstances. We must eat all 9 of these amino acids regularly:
- Leucine (branched)
- Isoleucine (branched)
- Valine (branched)
- Methionine (contains sulfur; can be converted to cysteine)
- Phenylalanine (can be converted to tyrosine)
*Branched chain amino acids are not metabolized by the liver; they are used primarily by muscle cells.
There are 5 “conditionally essential” amino acids that we need to eat under certain circumstances (growth, stress, illness):
There are 6 “nonessential” amino acids that we can make from other amino acids:
- Aspartic acid
How much protein do I need?
This seems like such a simple question, but it isn’t . . .
According to the World Health Organization (WHO), the definition of the daily protein requirement is:
“the lowest level of dietary protein intake that will balance the losses of nitrogen from the body, and thus maintain the body protein mass, in persons at energy balance with modest levels of physical activity.”
However, the WHO also acknowledges that:
“this definition of the requirement in terms of nitrogen balance does not necessarily identify the optimal intake for health, which is less quantifiable.”
And, when it comes to exactly how much of each essential amino acid we need per day:
“At present, no method is entirely reliable for determining the dietary requirement for indispensable amino acids.”
We know that the human body goes through between 300 and 400 grams of protein every day, but that doesn’t necessarily mean that we have to eat 300 to 400 grams of protein every day; we can get most of that daily amount simply by recycling used proteins. We can’t recycle all of the proteins we eat, because some are wasted due to inefficiencies in metabolism, and some are lost through natural activities of daily living:
- sloughing of skin cells
- hair growth
- DNA/RNA breakdown
- undigested protein (eaten by bacteria in the colon)
- urine ammonia (used to regulate pH of blood)
- oxidation/gluconeogenesis (burning amino acids for energy)
Since we lose some protein every day, we have to eat protein to replace these losses. The Institute of Medicine (IOM) states that the minimum average daily protein intake for the average adult should be:
0.8 grams of protein per kg of body weight
0.36 grams of protein per pound of body weight
For example, a 150-lb person would multiply 150 lbs x 0.36 grams of protein = 54 grams of protein per day. A simple way to do a rough calculation of the requirement in your head is to divide body weight by 3. It underestimates it a bit, but it’s very close.
Protein requirements, in grams of protein per pound of body weight:
- 0 to 6 mos: 0.69
- 7 to 12 mos: 0.54
- Age 1 to 3: 0.48
- Age 4 to 13: 0.43
- Age 14 to 18: 0.39
- Age 19 and older: 0.36
Pregnant women need additional protein to grow their babies:
- 1st trimester: + 0.5 gram per day
- 2nd trimester: + 8 gram per day
- 3rd trimester: + 25 gram per day
Breastfeeding mothers need additional protein for infant growth:
- 1st 6 mos: + 16 g/day (according to WHO)
- 2nd 6 mos: + 10 g/day (according to WHO)
[The IOM recommends adding 25 grams per day throughout lactation]
Adults with traumatic injuries (burns, major infections, head injuries, etc) need to temporarily increase protein intake for healing:
- Traumatic injury: 0.90
As for whether exercise increases dietary protein requirements:
“the question of how changes in energy flux through increased activity influence nitrogen balance may be of great practical importance. However, this important question is poorly understood.”
Common sense, though, would seem to indicate that building muscle should require additional protein.
How much protein is in food?
A thorough list of foods and their protein content can be found on the USDA Food Data Central website.
Keep in mind that grams of protein do not equal grams of meat, because meat is not pure protein. For example:
- a cooked chicken breast weighing 172 grams (6 ounces) contains 54 grams (just under 2 ounces) of protein
- a cooked hamburger weighing 172 grams (6 ounces) contains 42 grams (1.5 ounces) of protein (beef contains less protein per ounce than chicken because beef contains more fat)
Do people eating low-carbohydrate diets need extra protein?
The standard scientific references (such as the WHO and the IOM) do not address this question, because protein requirement research is conducted on people eating standard diets, which usually contain a lot of carbohydrate. People who eat a standard diet make blood sugar out of the sugars and starches they eat. However, if you eat a very low carbohydrate diet, you will need to make blood glucose out of protein, instead, because we can only make very small amounts of glucose from fat. If you don’t eat enough protein to maintain your blood sugar, your body will steal protein from your muscles to accomplish this critical task. Therefore, people eating low-carbohydrate diets probably need to increase their protein intake to maintain healthy blood sugar levels without losing muscle.
What happens if I eat too much protein?
I find no evidence that exceeding your daily minimum protein requirements is dangerous to your health, so when in doubt, err on the side of eating more, not less. According to the Institute of Medicine, there is:
“no defined intake level at which potential adverse effects of protein was identified . . . [and] there is no evidence that amino acids from usual or even high intakes of protein from foodstuffs present any risk.”
Now, some of you may have heard that eating high-protein diets can cause illness, but the high protein diets referred to in these claims were not simply high in protein—they were also either too low in fat, too low in calories, too low in nutrients, or contained high amounts of foods that can be bothersome. There is no evidence that a diet high in animal protein AND fat is harmful.
Some of you may be familiar with the concept of “rabbit starvation”, which occurred when people tried to exist on a diet of only very lean rabbit meat, and became sick. This phenomenon is often cited as a reason not to eat a diet that contains too much animal protein. However, the problem with this diet was not the presence of too much animal protein; it was the absence of adequate fat:
“An exclusive diet of any lean meat, of which rabbit is a practical example, will cause digestive upset and diarrhea. Eating more and more rabbit, as one is impelled to do because of the increasing uneasiness of hunger, will only worsen the condition . The diarrhea and the general discomfort will not be relieved unless fat is added to the diet. Death will follow, otherwise, within a few days. One would probably be better off on just water than on rabbit and water.” [Angier]
When eating an all-animal diet, which is naturally extremely low in carbohydrate, the body must use fat for energy. The body also requires dietary fat to absorb vitamins and other nutrients from foods. There have been a number of cultures (Eskimos are a good example) which have thrived on nearly 100% animal food diets for centuries without any ill effects.
You may also have heard that eating too much protein can damage the kidney, but that is also not true. [For more information about this, see my meats page]
What does the body do with excess protein?
Any extra protein your body doesn’t need is turned into one of three things:
- urea (urine)
- glucose (blood sugar)
- ammonia (urine)
When cells have extra protein they don’t need, they send it to the liver, where it is either turned into glucose or urea. Under standard conditions, 90% of excess protein is turned into urea (a non-toxic waste product) and excreted in the urine. The liver has a very high capacity for urea production and can handle up to 230 grams of protein at a time.
The fate of excess protein is determined by the hormonal state of the body. For example, if blood sugar is falling, glucagon and other hormones will turn on gluconeogenesis in the liver, which turns amino acids into glucose to maintain healthy blood sugar levels. All amino acids except for leucine and lysine can be turned into glucose [leucine and lysine can be turned into ketone bodies, instead, which most of the body’s cells can burn for energy]. When insulin levels are high, gluconeogenesis turns off, and the liver is asked to turn glucose into fat, so it is possible to turn excess amino acids into fat under those conditions.
It is possible for excess protein to cause an increase in blood sugar levels, however this potential varies from one person to the next, and the blood sugar elevations that do occur in some people are no match for the big spikes that can be caused by excess carbohydrates (especially refined and high glycemic index carbohydrates—see carbohydrates page).
Ammonia is another way for the body to get rid of protein, but it’s the least important way. Everyone has a small amount of ammonia in the blood, and most of this comes from bacteria in the colon. Bacteria break down undigested proteins in our food, excreting toxic ammonia as a by-product, which gets absorbed by our bloodstream. The liver quickly pulls this ammonia out of circulation so that the level stays very low. The kidneys are capable of generating ammonia, too, but they excrete it into the urine (as a way to regulate the acidity of the blood).
What is the difference between plant proteins and animal proteins?
- Most plant foods contain less protein per ounce than animal foods.
- Most plant foods are missing at least one of the nine essential amino acids (exceptions include soy and quinoa).
- Plant foods tend to be lower in lysine, threonine, and the sulfur-containing amino acids cysteine and methionine. Wheat protein is particularly low in lysine. Corn is especially low in tryptophan. Legumes (including soybeans) are especially low in sulfur-containing amino acids.
- Plant proteins are more likely to cause food allergy and sensitivity than proteins from most animal meats. Five of the top nine food allergens are plant proteins: gluten (a protein found in wheat and related grains), soy, corn, nuts, and peanuts [the other four being milk protein, egg protein, shellfish and fish].
- Some plant proteins are less digestible (less bioavailable) than animal proteins. Protein from corn and beans are the least digestible, at about 70%, compared to meat, which is about 94% digestible. Glutens contain stretches of repetitive amino acid sequences (rich in proline and glutamine) that are particularly difficult for our enzymes to digest, so we cannot completely break this protein down into its individual amino acids. [Gutiérrez 2017]
- Grains, beans, nuts, and seeds (the primary protein sources for plant-based diets) contain anti-nutrients including protease inhibitors which interfere with the body’s ability to digest proteins, and fiber which interferes with the absorption of protein. These plant protein sources also include phytic acid which interferes with mineral absorption. [See my grains, beans, nuts, and seeds page for more information about these anti-nutrients. See also my "Foods that Cause Hypothyroidism" article to read about the risk of goiter from eating soy.]
What happens if I don’t eat enough protein?
Protein deficiency is very uncommon in the developed world. Studies of vegans and vegetarians in the developed world find that they tend to get adequate amounts of protein in their diets. We used to think that vegans had to eat special combinations of plant foods at every meal to make sure they were getting all nine essential amino acids their bodies needed. However, we now know that the body can hold on to amino acids for several hours, so, as long as vegans are getting all nine essential amino acids in their diet at some point during the day, they don’t have to worry about eating them simultaneously at every meal. However, vegans must be careful to eat a variety of plant protein sources in order to obtain all necessary amino acids. If rice, corn, or wheat is the SOLE source of dietary protein, essential amino acid requirements will not be satisfied.
Unfortunately, protein deficiency is a very common cause of malnutrition in underdeveloped countries. Protein malnutrition stunts growth, reduces immune function and increases susceptibility to infection.
- Third-world diets are often too low in calories of all kinds, not just protein calories. Inadequate calorie intake can lead to burning of proteins for energy instead of using them to make important body molecules.
- Third-world diets are often poor in essential vitamins which are needed to properly build, utilize, and recycle proteins.
- Third-world diets are often very high in plant foods, because they are less expensive than animal foods. Many plant foods contain “protease inhibitors” which interfere with the body’s ability to digest proteins, and fiber itself interferes with the absorption of protein.
Bottom line about protein
- We must eat high quality sources of proteins regularly because we cannot make them from scratch and we cannot store them.
- Animal sources of protein are ideal, because they contain all 20 of the amino acids our cells use to build proteins, and because they are easier to digest and absorb. However, a carefully planned vegan diet can provide adequate amounts of protein.
- Growing children/teens, pregnant/breastfeeding women, people with traumatic injuries or severe infections, and people eating a low-carbohydrate diet have higher protein requirements than the average person.
- There is no evidence that eating protein in excess of estimated daily requirements is harmful to health.
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