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In this world I'm building, a bunch of previously inactive genes in humans have started expressing, and changed newborns' metabolic system dramatically.

As a result, a portion of population cannot digest regular food, but can eat synthetic food (let's call this population Type S, as opposed to Type R, regular humans). For clarity, regular food can be of plant or animal origin, and synthetic food can only be produced in lab-like settings.
The world's population is now divided into two groups, with their food sources not overlapping. The proportion of Type S and Type N people in population change gradually.

  • What kind of chemistry and biology can be behind this?
  • How can synthetic food differ chemically, so Type S human body can feel the difference? And how can it be so different, Type R humans are unable to digest it?
  • What are the raw materials for this synthetic food?
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  • $\begingroup$ I recall a story that featured junk food vitamins, where small evolutionary changes allowed utilization of newly available substances. A few people who shifted to a "helthier" diet got very sick, and it turns out that one lad had a slightly more efficient enzyme for something or another and it needed $\endgroup$ – JDługosz Jun 9 '15 at 12:35
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    $\begingroup$ Have you considered how monumentally difficult it will be for the parents of these newborns to figure out why their kid isn't growing as it nurses from mother? That particular detail may be important for the believability of this scenario. It's not like they have a lot of time to figure out why their kid is dying. $\endgroup$ – Cort Ammon Jun 9 '15 at 15:01
  • $\begingroup$ Yes, I have, @Cort Ammon, and there's a whole story behind it. I just need the biochemistry to be explained.. $\endgroup$ – wellagain Jun 9 '15 at 16:33
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It’s difficult to imagine how the human body could suddenly be unable to digest naturally occurring organic compounds, but its a little bit easier to imagine that humans could find them toxic. There is a rare genetic disorder in humans called Phenylketonuria (PKU). Individuals with PKU lack a specific enzyme called phenylalanine hydroxylase which is responsible for breaking down an important amino acid called phenylalanine and helping turn it into another important amino acid tyrosine. As a result, if people with PKU eat too much phenylalanine it won’t be converted into tyrosine and will build up as a toxin in the brain and cause numerous problems. An additional problem is they need to supplement their diet with tyrosine because they can’t make enough of it. Fortunately for people with PKU, all they have to do is limit their intake of phenylalanine and supplement their diet with tyrosine and they do just fine.

I think a similar sort of metabolic disorder could come close to your desired outcome. Imagine if some humans lost the ability to properly metabolize an organic compound found ubiquitously in all life. It could be a carbohydrate, or a lipid, or an amino acid, but whatever it is, anyone who can’t properly digest it will slowly die from its accumulation. However, the compound is also a precursor for another essential organic molecule (in the same way that phenylalanine is a precursor for tyrosine), and those people lacking the ability to digest it also suffer from a lack of this compound. But, this second molecule needs to be finely balanced, too much of it will also cause toxicity as it doesn’t occur at high levels naturally.

So, now we have our setup. The people who are unable to metabolize this organic compound must eat food devoid of it so it will not accumulate and be toxic, but the food must also be supplemented with the compound that the first one is normally metabolized in to. Laboratories will be capable of creating this synthetic food with the precursor removed and the product supplemented, but anyone without the disorder who eats this synthetic food will build up toxic levels of the product. So people with the disorder will find normal food toxic, and people without the disorder will find the synthetic food toxic.

It’s a little complex and it’s not exactly what you wanted. The people can still technically digest the other food, it just slowly kills them. However, I think the chemistry is reasonable. I don’t know enough about human physiology to suggest a specific enzyme, precursor, product group but there are potentially many that would work. Of course, as to how a fraction of the human population developed this change, I leave that to you.

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  • $\begingroup$ It's good, biology seems very plausible for my untrained eye. However I'd like people to exhibit fast symptoms after ingesting "the wrong" food, similar to mild food poisoning. It's also crucial for newborns not to develop properly on "the wrong" food. Will that fit? $\endgroup$ – wellagain Jun 11 '15 at 6:42
  • $\begingroup$ The speed of the symptoms would depend on how much tolerance the human body has for too much or too little of the molecules. In Phenylketonuria the body retains some ability to break down phenylalanine and will get some tryptophan from their diet, so the symptoms take years of an improper diet to manifest. For immediate symptoms the molecules would need to be present in exactly the correct concentrations with just a little too much or too little causing big problems for the human body. Neurotransmitters might be an example of a class that causes noticeable problems with tiny perturbations. $\endgroup$ – Mike Nichols Jun 11 '15 at 20:28
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Note that one way to make everything incompatible is to use a mirror image of all the sugars and proteins.

Those people would also be immune from disease.

So, some awful plague caused people to take the drastic step with their children, ensuring their future by taking the DNA out of the fertilized egg, synthesizing an exact copy made from mirror biology, and inserting that into a mirror egg-surrogate.

Meanwhile, when these mirror kids were 10 years old, the plague was finally beaten.

Now there are two kinds of human that are totally incompatible biologically, who breed true naturally but can use synthesis to recode the father's contribution for in-vitro fertilization.

You can have a lot of storyline concerning these "races". The mirror kids were initially elite and humanity's best hope. Many were born to families with power (who would pay their own way, in addition to the program to produce them over a diverse population), so would not be immediately abandoned. But over generations, their disadvantage creeps up and they become oppressed.

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  • $\begingroup$ Hmm.. Different isomers of molecules? Thant might work! $\endgroup$ – wellagain Jun 9 '15 at 13:17
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    $\begingroup$ @wellagain Specifically, I believe JDługosz is referring to the chirality of amino acids. The proposition is switching humans' left-handed aminos for right-handed ones. In that case, presumably most food found on earth (derived from organic sources that use left-handed amino acids) would be non-nutritive to our new variant humans. $\endgroup$ – recognizer Jun 9 '15 at 20:51
  • $\begingroup$ @recognizer wouldn't it make mirror humans too different from regular humans then? I'm not too familiar with the subject, but, although JDługosz storyline is sound, I need those changes in human to appear naturally, not in vitro. $\endgroup$ – wellagain Jun 10 '15 at 4:54
  • $\begingroup$ @recognizer but JDługosz answer got me into thinking about isomers. I know a glucose molecule isomer can have different properties and different tolerance, and even metabolism, so wouldn't it be enough difference for an extremely sensitive body to recognise? $\endgroup$ – wellagain Jun 10 '15 at 4:56
  • $\begingroup$ Amino acids and sugars too. And every ensyme and the machinery that builds them, etc. $\endgroup$ – JDługosz Jun 10 '15 at 5:28
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Instead of thinking about how the body changes, I propose you think about how the food might change.

In your world, there could have been a cataclysmic event that removed digestive enzymes from existence (what event that might be is the subject of another question...). This means that humans can no longer digest things because the enzymes that usually do it are nonexistent.

The synthesised food, then, can be seeded with digestive enzymes in it. It would have to be eaten rather quickly to avoid the enzymes dying off, but it could be done.

Alternatively, if you're fixed on a biological difference, you can simply have a group of humans who weren't affected by the cataclysm - perhaps they were in bunkers, or out colonising Mars when the cataclysm happened. Your type S humans would be those who were affected, and have to have their food seeded with enzymes, and the type R is those who weren't and can digest normally.

Enzyme-seeded food could damage the type R humans by giving them stomach ulcers through a digestive overload - having too many digestive enzymes is a bad thing because they starts to digest your stomach lining and cause ulcers.

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  • $\begingroup$ Hmm.. I need to have the world's population divided into two groups, with their food sources not overlapping. If it's about enzymes, then you can synthesise enzymes only, and add it to regular food (grown or hunted). Unfortunately, the biological differences are a must here. $\endgroup$ – wellagain Jun 9 '15 at 12:11
  • $\begingroup$ Also, although the reasons for this new gene expression are unknown (it might have been a cataclysmic event of some kind), I need people to be born with this biological difference, and the proportion of type S and Type N people in population change gradually. $\endgroup$ – wellagain Jun 9 '15 at 12:19
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I recall a story that featured junk food vitamins, where small evolutionary changes allowed utilization of newly available substances. A few people who shifted to a "helthier" diet got very sick, and it turns out that one lad had a slightly more efficient enzyme for something or another and it needed the caramel coloring in soda drinks to work.

Think about how that might happen: the mutation spreads and eventually someone has copies only of the new form.

Look at history: humans can't make vitimin C, and it's readily available in our diet.

If done intentionally through genetic engineering, new metabolic mechanisms can be invented that are highly desirable. And to save on engineering more pathways to synthesize various parts it is decided to supply that ready made. Maybe some stuff can't be readily made with normal metabolisms, but is synthesized at high temperatures or with toxic feedstocks.

Now lets make that new required thing be toxic to unmodified humans and most animals.

As for whole scale nutrition, note that food is made of the same stuff as our bodies. For normal proteins, sugar, etc. to not work, the body would have to be very different.

Perhaps transhumans add more features to themselves over time. Something better than collegen that doesn't wear out and is stronger, used in skin and other places; bones are totally reingeneered chemically. They will need more materials innthe diet, perhaps including titanium instead of calcium.

Now it seems odd that old-style biology food would be not tolerated. Even if some constituents are useless, what's wrong with it? There needs to be some reason why something common in normal food woukd be dangerous to them. How about: the digestive system is changed to handle the wierd feedstocks needed by the upgrades, which are things that are non-biological and normally toxic. Something common in normal food interferes with the new pathways. It's hard to make them coexist since in digeation everything is mixed together: how do you keep A moecules away from B processing and vice-versa?

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  • $\begingroup$ You are right, food is made of the same stuff as our bodies. And this is true to this world, Type S humans are made of the same stuff as Type N, and have the same features. But i need those nutrients not to come from plants or animals, but from other sources (perhaps inorganic components). And this is where i'm stuck, cause i can't figure out the biology and chemistry behind it. $\endgroup$ – wellagain Jun 9 '15 at 13:16
  • $\begingroup$ Yea, you boxed yourself in. Digestion works on the "same stuff" to rebuild it. Look at different digestive stategies in nature: can a cow and a vulture share a meal? $\endgroup$ – JDługosz Jun 9 '15 at 23:23
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I have a friend who is hyper allergenic to almost everything. She cannot eat eggs or diary products, thus letting us having a delicious Bearnaise sauce that contains both eggs and milk (butter) was a difficult process. But she told us that she could eat the canned version since there was a substitute for eggs and the butter was replaced with oil and taste products.

I think that story could relate to your dilemma. Where in synthetic food some of the products that causes allergies would be replaced with cheaper, more non allergen products.

Also and this is total speculation, people who has been eating synthetic food had their stomachs gotten used to it, have a had time digesting "ordinary food", this is based on my wife being vegetarian and most of our food is vegetarian but when i go to enjoy meat, my stomach is not used to the meat bacteria and sometimes reacts badly.

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    $\begingroup$ Meet and meat are rather different things... $\endgroup$ – ArtOfCode Jun 9 '15 at 12:10
  • $\begingroup$ Yeah i know, brain slip. Thanks for fixing it $\endgroup$ – Magic-Mouse Jun 9 '15 at 12:10
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As to my understanding (please correct me if i'm wrong), different isomers of the same chemical formula have different properties, and metabolised in the body through different metabolic pathways. In this way it fits the requirement of body to feel the difference. But it's not that simple as "there's two mirror versions and we can only use one".

Some isomers (like Isoleucine, an isomer of Leucine) are an essential nutrient (are not synthesised in the body, and must be ingested), and are important for human health (e.g. muscle tissue). Others, like D-glutamic acid, are present in tissues, but when ingested, largely escape most deamination reactions (unlike L-glutamic acid). Yet another isomers, like L-isomer of Glucose, cannot be metabolised by human body at all, and only a bacterium, Burkholderia caryophylli, contains the enzyme which is capable of oxidising L-glucose.

But what if some genetic changes made S Type people capable of metabolising isomers which previously were not metabolised at all, or make them have other physiological consequences, so the body can process it as normal and recover important nutrients? Moreover, what if these changes made their metabolic system isomer-dependent? Now only opposite isomers can be used to fulfil their diet.

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  • $\begingroup$ As people has pointed out elsewhere, there's no real problem with metabolising isomers in nature. So looks like it's a dead-end $\endgroup$ – wellagain Jun 19 '15 at 8:05

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