What’s the Difference Between Plant and Animal Protein?

They’re different biochemical languages that profoundly affect health

Brent R. Stockwell, Ph.D.
Aha! Science

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Image of one hand holding a burger and the other hand holding oatmeal with berries
Image: Adobe Stock

Plants and animals look different, but they both contain a lot of protein. And since protein is one of the most crucial nutrients in the foods you eat, you’re wise to understand what it is, how it works, and which kind is best. Bodybuilders think they need it to build muscle. Vegans worry they don’t get enough of it. Many of us are just baffled by it.

What is protein, and is there any difference in where it comes from? Is eating plant protein and animal protein equivalent?

What we call protein in food is a mixture of thousands of different flavors of protein. These many large molecules are built from repeating units called amino acids — there are 20 common types of such amino acids. Each type of protein has a unique sequence of amino acids strung together, like letters that form words and sentences.

Let’s make a simple analogy: Sentences in English can make very different use of the letters of the alphabet. The sentence “The quick brown fox jumps over the lazy dog” uses every letter once, and efficiently too. “An assassin sins” is a phrase that uses only four different letters. If you ate one, each could have very different effects on your body.

But since you don’t typically eat your words (or at least hope not to) here’s some relevant advice: “Eat food. Not too much. Mostly plants,” from writer Michael Pollan. So, then, what’s the real difference in plant and animal protein when we eat them?

Protein letters have different effects on health

When you eat foods with protein, your stomach digests it into individual amino acids, breaking the protein words down into single letters. The amino acid letters are reused by different parts of the body to build new protein words and sentences.

Some of the amino acids from the protein you eat get converted into energy, or into other molecules your body can use. The specific amino acids in the protein you eat have a dramatic effect on what happens to your body after you eat — how the whole story of your health turns out. So why don’t we eat the protein sources that are best for our health?

“Life expectancy would grow by leaps and bounds if green vegetables smelled as good as bacon,” suggested Doug Larson, newspaper columnist.

Plant and animal proteins use letters differently

The alphabet that is used to write both animal and plant proteins is the same set of 20 amino acids. The amino acids are the different letters. A particular protein can be like the word banana that uses more of the letter A, while another protein might be like the word beekeeper that has a lot Es, and an exotic protein might be built from an unusually large number of Ms, such as mammogram.

Plants and animals build their proteins differently, just as words are built differently in Spanish and English. The proteins from plants and animals contain different amounts of each amino acid letter. Plant proteins tend to have more of the amino acids that benefit health, whereas animal proteins often have more of the amino acids harmful to health. The protein in foods we eat thus have different amino acid compositions, which in turn affect health in positive and negative ways.

But different plants themselves use different proteins. Soy, made from soybeans, uses most of the 20 amino acid letters, whereas corn and rice only use a subset of the amino acid letters in their proteins, even though they function fine as plants. This is impressive — like writing an entire book without using seven letters of the alphabet.

The deadliest letter

So which amino acids are good and which are bad? The most dangerous amino acid seems to be methionine. Mice on a diet high in methionine develop anxiety, and have increased risk of heart disease. Eating foods with less methionine slows aging and promotes health. And yes, plants have less methionine than animals. Pork, chicken, and beef are high in methionine. Fruits, mushrooms, and broccoli are low in methionine. One slice of cured ham has the same methionine content as 1,100 apples.

The bottom line: Plant and animal proteins are built from amino acids, but in different proportions. The way we digest and metabolize these different amino acids affects health. Proteins vary tremendously in size and composition. The littlest are under fifty letters, like this sentence. The longest are more than 34,000 letters, about seven times as many as in this entire story.

There is still a vast amount of science to be unraveled about how different amino acids affect the many aspects of health. Understanding that dietary proteins are not interchangeable is the first step to decoding the precise impact of the protein we eat on our well-being.

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Brent R. Stockwell, Ph.D.
Aha! Science

Chair and Professor of Biological Sciences at Columbia University. Top Medium writer in Science, Creativity, Health, and Ideas