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Why do we have to get them from our diet, and if they aren't taken in our diet we will face disease? Then why we don't have the enzymes which are require for EFA synthesis?

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    $\begingroup$ It's not really like your body can decide like 'hey, lets synthesize the enzyme required to catalyze the reaction for the creation of this EFA'. Apparently, we're just fine without them due to ample EFAs in our diet. $\endgroup$ – pbond Jul 20 '14 at 18:08
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    $\begingroup$ Have a look at this topic: biology.stackexchange.com/questions/16962/… $\endgroup$ – Chris Jul 20 '14 at 18:25
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    $\begingroup$ i don't understand will u plzz elaborate it more $\endgroup$ – Haya Dar Jul 20 '14 at 19:22
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    $\begingroup$ I would speculate that since essential fatty acids are so abundant in our diet, there was no selective pressure for the maintenance in our genome of the enzymes that produce them. $\endgroup$ – canadianer Jul 20 '14 at 20:36
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    $\begingroup$ @canadianer can you add more details if you can and post your comment as answer $\endgroup$ – WYSIWYG Nov 26 '14 at 13:02
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Well, the essential fatty acids that humans fail to produce are not the same ones that other species fail to produce. For example, while cats produce their own Vitamin C and will therefore never develop scurvy, they can't produce their own taurine and will become sick if they don't consume enough of it.

To answer why a species might lose the ability to produce an essential amino acid, you have to consider what that species is eating most of the time. If its diet is rich in the amino acid anyway, individuals who lose the ability to synthesize it either feel no cost or--possibly more likely--even benefit by not wasting resources synthesizing compounds that may be abundant already in the diet and must be excreted if unused.

Turns out that this loss of ability to synthesize your own fatty acids is a particularly common consequence of evolving parasitic lifestyles, which makes sense: if you're just stealing someone else's acids, why bother to produce your own? Even outside of parasites, there's some interesting discussion on why the nine amino acids essential to all animals might have evolved from a genomics paper here, discussing which amino acids you really do have to make on your own and which you can get away with consuming depending on your species' lifestyle and dietary habits.

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