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Various creatures have different dietary requirements and different tolerances for various substances. For example, felines need taurine and react poorly to citrus, while humans need citrus (some source of vitamin C, anyway). Onions, grapes, chocolate and alcohol are bad for various animals. Birds aren't affected by capsaicin.

This makes me sad.

Let's say I am a brilliant geneticist / wizard / supreme being. Without resorting to permanent changes to the laws of physics / ongoing magic, and without making changes to their normal diets, would it be possible to change all mammals so that they can safely eat any food that humans can eat? Does biology somehow necessitate that an animal eating a certain diet, or just certain animals, must be susceptible to certain substances that are (mostly) non-toxic to humans?

Notes:

  • If it helps, you can ignore bats and anything that lives mainly in the water.
  • I'm not talking about eating a human diet in entirety; e.g. cats would still be obligate carnivores and I don't mind if they still need taurine, horses would still mainly eat gress. I just don't want my cats/horses to die if they consume a modest amount of chocolate.
  • They don't need to eat human quantities of food; that would be silly. Similar average lethal doses (mg/kG) is fine.
  • This isn't about chewing or digesting human food. Yes, carnivores may not want to eat much grain because their gut won't be able to break it down well enough, but they should be able to enjoy an occasional chocolate martini.
  • They shouldn't be subject to cumulative poisoning from any substance that is not a cumulative poison to humans.

Edit (thanks to John for a fascinating discussion!): I can live with not accounting for non-"linear" scaling effects being a problem for really small animals. I'm also meddling in sizes, such that my "mice" are closer to maybe 20 kg and my "elephants" are closer to 500 kg. Don't think too hard about the additional effects from this, it's just to say that I can mostly ignore the toxicity problems to the extent that raw size is the main cause.

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    $\begingroup$ chocolate and alcohol are bad for humans too it is just humans have a ridiculously high tolerance for certain toxins, particularly maillard compounds. It is believed to be due to humans adapting to eating cooked food for so long. even among humans digestion varies, milk and peanuts come to mind. $\endgroup$
    – John
    Jan 8, 2021 at 17:32
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    $\begingroup$ All food ever is bad for all animals, because food is a living thing that has evolved for million of years to not be eaten... Various animals have various tollerances to various foods... But they are still all bad, some more than others. Your mother's milk is probably the only non toxic thing you ever ate. $\endgroup$
    – user81643
    Jan 8, 2021 at 17:35
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    $\begingroup$ @user81643 source? because there are plenty of human populations that don't eat fish. $\endgroup$
    – John
    Jan 8, 2021 at 18:00
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    $\begingroup$ @user81643 Omega−3 fatty acids are widely distributed in both plant and animal life, algae have the highest concentration but that is very different than the only significant source. there are thousands of sources including the common chicken egg and most seeds. $\endgroup$
    – John
    Jan 8, 2021 at 18:46
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    $\begingroup$ @user81643: Those would be omega minus three and omega minus six. Explanation: in a fatty acid, the carbon atoms are counted starting with 1 at the COOH head. Since various fatty acids have a different number of carbon atoms, the convention is to call the last one "omega". Omega minus three fatty acids are polyunsaturated fatty acids, with a double (= unsaturated) bond between the 3rd and the 4th carbon atom from the end. There are several omega minus three fatty acids; the only essential one in healthy people is the alpha-linoleic acid, which is common in all vegetable oils. $\endgroup$
    – AlexP
    Jan 8, 2021 at 20:00

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It's all about toxins

First, they already kind of can eat anything in very small doses. Almost all animals have the ability to metabolize small amounts of things that we would say they "can't eat" but the devil is in the dose. So to solve this you could either

a. Upgrade the livers and kidneys of all the animals so that they can more easily handle the toxins.

b. Make the plants and other food sources produce less toxins. But this might make the caffeinated beverage people mad because caffeine is one of these toxins.

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  • $\begingroup$ With sufficient waste management it’s possible to metabolise nearly anything. $\endgroup$
    – Joe Bloggs
    Jan 8, 2021 at 17:51
  • $\begingroup$ @John, again, this is not about animals eating a human diet. This is just about them not dying because they ate a modest quantity of something that would be harmless to humans in similar (mg/Kg) quantity. For example, a dog eating an occasional chocolate bar or a couple onion rings, or a cat eating a grape or a slice of an orange. A cat eating a whole orange should be about as dangerous as eating a similar quantity of, say, blueberries... which might still be unhealthy for various other reasons. $\endgroup$
    – Matthew
    Jan 8, 2021 at 18:36
  • $\begingroup$ @Matthew I can see you edited your question to be more clear, please don't be upset at posts that came out before your edit. $\endgroup$
    – John
    Jan 8, 2021 at 18:42
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    $\begingroup$ changing liver enzyme production is not too hard, their livers may end up a little larger. humans have very acidic stomachs which combines with our wide range of enzyme production make it very plausible. You will still run into the size problem, humans get around many toxins just because we are big. Small animals will always run into more toxin sensitivity. $\endgroup$
    – John
    Jan 8, 2021 at 18:52
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    $\begingroup$ @Matthew yes it changes relative dose. to use simple number for clarity 1 kg might be safe for a 100lg human but even 1/1000 might be lethal to a 1kg mouse. biochemical scaling is not a straight volumetric function. Now child sized is a lot closer to human sized so dosage is much closer to normal mass scaling. expect the dangerous dosage for a child sized being (25kg) to be between a quarter and a tenth of the dose of an adult human. there are several reasons for this including the square cube law and non linear metabolic scaling. $\endgroup$
    – John
    Jan 8, 2021 at 20:01

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