Protein molecules are composed of amino acids, which contain nitrogen and sometimes sulphur. Your body uses amino acids to produce new proteins and to replace damaged proteins.

Your body can synthesize most of the 21 amino acids that you need to make protein, with the exception of nine essential amino acids (histadine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine) that must come from your food.

Essential	Nonessential	Conditionally Essential*
Histidine	Alanine	Arginine
Isoleucine	Aspartic acid	Cysteine
Leucine	Glutamic acid	Glutamine
Lysine	Selenocysteine	Glycine
Methionine		Proline
Phenylalanine		Serine
Threonine		Tyrosine
Tryptophan		Asparagine
Valine

*Conditionally essential amino acids are ones that most humans can produce from other amino acids, and are not normally required in your diet. However, certain people have medical conditions that don’t allow them to synthesize these amino acids in adequate amounts. For example, people with the disease phenylketonuria (PKU) must keep their intake of phenylalanine extremely low to prevent serious medical complications. Because they cannot synthesize enough tyrosine from phenylalanine, tyrosine becomes essential in the diet of PKU patients.

Fortunately, all unrefined foods have varying amounts of protein with varying amino acid profiles, including leafy green vegetables, tubers, grains, legumes, and nuts. All the essential and nonessential amino acids are present in these foods in amounts that meet or exceed your needs.

But wait! Isn’t plant protein inferior to animal protein? Isn’t it incomplete? Don’t you have to carefully combine certain plant foods with certain amino acids to make “complete” protein that your body can use?

No, no, and no. So how did this myth come about?

In  1914, Thomas B. Osborne and Lafayette B. Mendel conducted studies on rats, which suggested that they grew best when fed a combination of plant foods whose amino acid patterns resembled that of animal protein. That makes sense, as all baby mammals, rats and humans included, grow best when fed the perfect food for baby mammals: their mother’s milk. The term “complete protein” was coined to describe a protein in which all eight or nine essential amino acids are present in the same proportion that they occur in animals. “Incomplete protein” described the varying amino acid patterns in plants. It’s a misleading term, because it suggest that humans (and other animals, one would assume) can’t get enough essential amino acids to make protein from plants.

Fortunately, the theory that plant proteins are somehow “incomplete” and therefore inadequate has been disproven.  All unrefined foods have varying amounts of protein with varying amino acid profiles, including leafy green vegetables, tubers, grains, legumes, and nuts. All the essential and nonessential amino acids are present in any single one of these foods in amounts that meet or exceed your needs, even if you are an endurance athlete or body builder.

The following table shows the amino acid requirements of adults as recommended by the World Health Organization calculated for a 62-kilogram (137-pound) adult, and the amino acid profile of 2530 calories of baked potatoes (9 large baked potatoes), which comprise a day’s worth of calories for a 62-kilogram adult:

Essential Amino Acid	Requirement mg/day/62 kg adult	9 large baked potatoes
Tryptophan	248	565
Threonine	930	1830
Isoleucine	1240	1830
Leucine	2418	2691
Lysine	1860	2933
Methionine+Cystine	930	1534
Phenylalanine+Tyrosine	1550	3553
Valine	1612	2682
Histidine	620	942

As you can see from the table above, you would get a sufficient amount of every essential amino acid by eating nothing but potatoes all day—and most of us don’t consider potatoes to be a particularly good source of protein.

Whenever you eat, your body stores amino acids, and then withdraws them when it needs them to make protein. It is not necessary to eat any particular food or any particular combination of foods together at one sitting, to make complete protein. Your body puts together amino acids from food to make protein throughout the day.

So am I saying that protein from plants is just as good as protein from animals? No, actually. I’m saying it’s better.

One distinction between plant protein and animal protein is that animal protein contains carnitine, which is not an essential amino acid for humans. Carnitine is another non-essential amino acid found almost exclusively in animal flesh. As with cholesterol, your body makes all the carnitine you need. The bodies of other animals also makes all the carnitine they need, so if you eat their flesh, their carnitine can end up in your gut. Within 24 hours, certain gut bacteria metabolize the carnitine to a toxic substance called trimethylamine, which then gets oxidized in your liver to trimethylamine-n-oxide (TMAO), which then circulates throughout your bloodstream. TMAO increases the buildup of cholesterol in the inflammatory cells in the atherosclerotic plaques in your arteries, and slows the removal of cholesterol from the arteries’ walls, increasing your risk of cardiac surgery, heart attack, stroke, and death.

Another distinction is that animal proteins contain very large amounts of sulfur, especially in the essential amino acid methionine and the conditionally essential amino acid cysteine. Your body makes other sulfur-containing amino acids out of these, including keto-methionine, cystine, homocysteine, cystathionine, taurine, and cysteic acid.

Even though you need a small amount of sulfur-containing amino acids, an excess of these amino acids beyond your needs causes the following problems:

1. Your body breaks down the sulfur-containing amino acids into powerful sulfuric acid. Excess acid causes your body to leach calcium from your bones to maintain pH. This can lead to osteoporosis and kidney stones.
2. Your body breaks down methionine (which is very high in animal protein) into homocysteine, which is a risk factor for heart attacks, strokes, closure of your leg arteries (peripheral vascular disease), blood clots in your legs (venous thrombosis), cognitive impairment, dementia, Alzheimer’s disease, neurotransmitter deficiency, and possibly depression. (Tryptophan, conversely, serves as a basis for creating serotonin, your brain’s primary mood-regulating neurotransmitter.)
3. Methionine feeds cancerous tumors. Normal cells can get the sulfur they need from other sulfur-containing amino acids, but cancer cells, including breast, lung, colon, kidney, melanoma, and brain cancers, need methionine. Meat and dairy products also raise the levels of a powerful growth-stimulating hormone, called insulin-like growth factor-1 (IGF-1), which magnifies the effects of methionine to promote cancer, including cancers of the breast, colon, prostate, and lung.
4. Sulfur from high-methionine (animal) protein is toxic to the tissues of your intestine, and can damage your colon, leading to a disease called ulcerative colitis.

Casein, the main protein in dairy foods, incites cancer progression.

The absence of casein and the lower amounts of carnitine, methioine, and cysteine in plant protein have several advantages:

1. Plant proteins do not incite cancer progression.
2. A diet based on plant food is low in carnitine, which keeps your arteries healthy.
3. Plant-based diets are low in both calories and methionine. Low-calorie diets slow the aging process and prolong life. Low-methionine diets also prolong life.
4. Plant protein makes you smell better. The high sulfur in animal protein causes bad breath, body odor, and flatulence that smells like rotten eggs.

Nearly all vegetables, beans, grains, nuts, and seeds contain some, and often plenty, of protein with varying amino acid profiles. Vegans who eat varied diets containing vegetables, beans, grains, nuts, and seeds rarely have any difficulty getting enough protein as long as their diet contains enough calories to maintain their weight. All the essential and nonessential amino acids are present in these foods in amounts that meet or exceed your needs, and in a ratio that is kind to your body.

A few amino acids from protein can be converted into glucose and used for fuel through a process called gluconeogenesis; the remaining amino acids are eliminated in urine. This occurs during muscle atrophy, and in quantity only during starvation.

A diet of unrefined plant foods, especially a diet that includes beans, grains, vegetables, nuts, and seeds provides all the essential amino acids you need to stay healthy.