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Why can't humans make the 8 amino acids which we need toget from the diet? Is it a loss of a past ability, or are we maybe gaining the ability? What amino acids do you acquire from beans and rice, versus milk, and versus tofu?
Question Date: 2000-06-28
Answer 1:

I think the amino acid thing is a loss, strictly speaking. Although plants can synthesize the full compliment of amino acids (20) needed to make proteins de novo, most animals cannot, so humans are not alone in that respect. The amino acids that an animal cannot make are called essential because they need to come from the diet, but they're no more structurally essential than the ones that are synthesized.
According to my Purves biology textbook ("Life: The Science of Biology"), each animal "species" (shouldn't this be "taxa"?) has its own group of essential amino acids, and herbivores have less essential amino acids than do carnivores. To me, the greater inability to synthesize protein building blocks in animals that eat mostly protein implies that this is an evolutionary loss ... a movement towards simplicity in the biosynthesis pathways. Milk, eggs, meat and tofu all have all eight amino acids required for humans. (Milk would have to be a perfect food, since baby mammals don't eat anything else.) Except soy beans, no one plant food has all eight, but you CAN get all eight by combining them in one meal (beans with grains/seeds/nuts). Both grains and legumes have valine, threonine, phenylalanine and leucine. In addition, grains have tryptophan and methionine while legumes have isoleucine and lysine.

What I didn't know is that plants have unique amino acids, over 250 (protein and non-protein). I would have thought that plants, being primary producers, could synthesize all of their required amino acids, but it is a puzzle to me why, if each plant has all 20 amino acids for protein synthesis, this doesn't carry over into an animal's diet. Apparently plants have proportionally more glutamic acid and glutamine, which animals can synthesize. Perhaps a better way to put it is that beans versus grains simply have *more* of a specific type rather than none of other types. It's still odd that different plants have such different amino acid composition, versus animals which all require the same 20 at pretty much the same proportions. I guess the amount and type of proteins are more variable in plants.

The list of vitamins required also varies from species to species.
For example, I didn't know that all mammals except primates can make vitamin C, which is why only humans get scurvy in polar climates, and why eating fresh meat (e.g. fresh frozen seal meat like in Shackleton's expeditions) can correct the deficiency. The whole vitamin thing is very wierd, actually. I can maybe understand why humans lost the ability to synthesize amino acids and vitamins since at some point we were foragers, and it makes sense to me that the health of a population would depend on quality and variety of food in the area, but why would plants, which cannot move and are at the
mercy of soil, lose the ability to synthesize some vitamins?

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