Why do humans have such complex dietary requirements?

[Dr. Diaphanous]

Plants and many bacteria survive with only inorganic elements in very simple forms: as pure elements or simple oxides, for example carbon as carbon dioxide, nitrogen as nitrate. They combine the elements to make all the chemicals they require - proteins, sugars, poisons, enzyme cofactors etc.

Cows can survive on nothing but grass. Penguins can survive on nothing but fish. Giant tube worms are sustained only by a certain type of bacteria.

So how come humans need a "balanced diet", with vitamins and essential fatty acids and essential amino acids produced by other organisms?

If you had a community of humans living in a corn field, and you gave them as many mineral supplements as they needed, and gave them enough fats and sugars that they had plenty of calories in their food, they would die out from lack of tryptophan and lysine, because corn only has 6 of the essential amino acids. (Also vitamin deficiencies).

And not only that, humans can die of too much as well as of too little. Less than half a gram of vitamin A can cause severe problems for an average-sized adult, even though too little is also fatal.

I know humans' natural diet is varied, but why has it evolved so it MUST be varied? I don't believe our ancestors never ever lacked at least one essential nutrient.

At some point, out ancestors made all the amino acids/vitamins/etc they needed from carbon dioxide and ammonia and a few other components, or from other amino acids/vitamins. Why did natural selection not kill all the lineages where this was lost?

[Angua]

Many things like a varied diet. Penguins eat a lot of different types of fish, and cows are living off the bacteria living inside them as much as the grass they're consuming (where do you think they get all their protein from?). Humans aren't the only animals that get vitamin deficiencies - guinea pigs can't synthesise vit c and cattle can suffer from vitamin a deficiency if they don't get enough green grass. Feed another animal too much vitamin a, and they'll probably not do too well either.

It's an advantage to be able to eat a lot of different things, because you don't become too dependent on one thing. It's not that hard to get enough vitamins from things around you - the problem is that these days we have so many foods that are mainly sugar and fat that we forget to have added stuff in. You really don't need so many fruits and veg per day to survive (studies vary from 3-7 portions per day), but a higher target also helps reduce the amount of non-useful stuff that you eat (as well as meaning that if someone's shooting for 5 and only gets 3, it's not that much of a loss).

[firechicago]

Because there's a significant metabolic cost to synthesizing your own vitamins and amino acids. Even if you get enough nutrients from your environment that you never actually need to synthesize them, there's a developmental and metabolic cost just to maintaining the ability to. That's energy that you can't use to build a bigger brain, or stronger muscles, or a more robust immune system or any one of a million other things. So if you're in an ecological niche where you have access to a lot of different nutrients, it makes sense to spend that energy making yourself bigger, stronger or healthier, rather than paying a metabolic price for what you can get externally for free.

[Dr. Diaphanous]

Guinea pigs are one of the only species that can't synthesize vitamin C (along with capybaras, certain birds and fish, and most primates and bats). All other species (AFAIK) can synthesize it.

Yes, it's an advantage to be able to eat a lot of different things. But we need to eat a lot of different things. We are dependent on variety. I don't think a human would be very healthy eating only apples for a few months, then only rabbits when the apples ran out.

And as for the metabolic cost, compare the massive metabolic rate of a mammal to the tiny metabolic rates of plants and bacteria that make their own compounds. I probably have enough calories in me that I could fund the whole metabolism of a shrub. In fact, someone should make a symbiotic organism that lives in our bodies and gives us more dietary freedom. Or repair a few of our vitamin metabolism genes. A few dozen grams of fat a year for a few dozen grams of vitamins, sounds like a good trade. (Although if you are getting your nutrients from your diet, surely it can't cost much to have a few inactive genes, with maybe a few inactive gene-regulating proteins on standby to activate them.

(I know that I'm wrong in this argument - humans can't make vitamins. But I don't think I have heard a convincing argument why I am wrong.)

[Zamfir]

Our food contains the built-up reserves of months or even years of plant growth, fueled by solar power. When you eat meat (or fishes like the penguins), you're even eating the concentrated compounds in a creature that itself spend months to years collecting the compounds from plants that each took a long time to build.

[Angua]

That's still quite a number of species that can't synthesise vitamin c. Also, if we can synthesise things, then they don't count at vitamins (or we don't think of them as such - the only reason vitamin d counts is that most people live in countries where they don't get enough sun). You can live quite well off rabbit if you make sure you eat the liver btw - and I once saw an article where the journalist only ate potatoes for a month (they actually aren't too bad for vitamins). Most agricultural communities back in the day lived mainly off whatever their staple food was - varied diets are a pretty recent thing. We don't need to eat that many different things, we just happen to prefer it (probably because we're used to having to look for seasonal food, where you can't eat the same thing all the time). Personally, I think a lot of the health food craze is over rated - it's a lot more important to not eat things high in fat and sugar all the time than it is to have that varied a diet.

[chenille]

Guinea pigs are one of the only species that can't synthesize vitamin C (along with capybaras, certain birds and fish, and most primates and bats). All other species (AFAIK) can synthesize it.

This is a little bit of a sharpshooter fallacy, though, because it's very common for living things to have some kind of complex essential nutrients. It's true many plants and bacteria don't, but even so there are lots of things like Euglena, single-celled algae that you would expect to be self-sufficient except they turn out to need vitamin B12. So I think the question would be less how we can get by needing particular vitamins, which is not so unusual, as what allows us the particular deficiencies that we have. As it happens, people are particularly good at eating a variety of things, so I'm not surprised we ended up with a few more than average.

[Qaanol]

As is the answer to every single “why” question in evolution, the answer is simply, “Because all the other ones died, or their descendants are considered to be different species now.”

[gmalivuk]

Most agricultural communities back in the day lived mainly off whatever their staple food was - varied diets are a pretty recent thing.

You sure about that last part? My understanding was that variation *plummeted* a lot of places when agriculture started, but was fairly high before that.

[Angua]

Most agricultural communities back in the day lived mainly off whatever their staple food was - varied diets are a pretty recent thing.

You sure about that last part? My understanding was that variation *plummeted* a lot of places when agriculture started, but was fairly high before that.

My point was that they didn't all die off immediately, and weren't even that unhealthy. Though it's hard to tell what diets were like, but not all diets at that varied - the inuit diet doesn't look very varied by the standards wanted in this thread (http://en.wikipedia.org/wiki/Inuit_diet).

[Copper Bezel]

(I know that I'm wrong in this argument - humans can't make vitamins. But I don't think I have heard a convincing argument why I am wrong.)

That's a tautology. They're only "vital" because we can't synthesize them. If we depended on the same nutrient but could synthesize it, we wouldn't have to consider it a vitamin in humans.

I know little to nothing about biology, particularly nutrition, but really think the rest of your question is based on a similar assumption. We've adapted to synthesize things for which the cost of obtaining them from the environment in a particular form is greater than the cost of synthesizing them from other components. If a chemical, at whatever level of complexity, is abundant in our diet for a few million years, we stop synthesizing it, because we don't benefit from doing so. That we're not primary producers, or that we're not grazers, means we're outsourcing already, depending on the availability of certain kinds of proteins in the environment. From, like, those other organisms that put all that work into making them up.

Independence of the environment doesn't matter, because evolution works on organisms within their niches. What does confer advantage is efficiency within the environment. So it really makes more sense to work backward and say, how many of the necessary proteins are readily available in the environment? Then, which ones need to be synthesized instead?

[gmalivuk]

the inuit diet doesn't look very varied by the standards wanted in this thread (http://en.wikipedia.org/wiki/Inuit_diet).

Really? Seals, whales, fish, deer, bear, oxen, poultry, eggs, grasses, roots, berries, and seaweed isn't varied?

[Izawwlgood]

The section on nutrition on that wiki actually has a lot of interesting sources for vitamins. I would not have expected vitamin C to be in seal brains.

I think Mongolian cuisine is actually more centered around protein than the Inuit cuisine is. I remember a bizarre foods where he basically ate nothing but milk and meat products.

Oh hey, lookathat:
http://sciencelinks.jp/j-east/article/1 ... 794115.php

[gmalivuk]

So from those two examples, it looks like maybe a lot of our complex dietary requirements come from no longer typically eating every part of the animals we kill.

[Izawwlgood]

I'm curious now if most predators that rely on a singular source of food (say, polar bears on seals) also eat the entire animal, or focus on specific organs if they're particularly deprived. I remember in My Side of the Mountain (totally scientific source) a sequence where he's ravenous and feeling sick, and he catches a rabbit and for some reason the rabbits liver looks like the most delicious thing in the world to him, and he eats it without even cooking it. Later, he finds out that liver is rich in vitamin 'whatever he lacked'. D? Anyway.

[Angua]

I think it was A.

the inuit diet doesn't look very varied by the standards wanted in this thread (http://en.wikipedia.org/wiki/Inuit_diet).

Really? Seals, whales, fish, deer, bear, oxen, poultry, eggs, grasses, roots, berries, and seaweed isn't varied?

Doesn't that sound like mainly meat to you? Cows were classed as just eating grass, when they generally get different types of grass, as well as other plants (I don't know if people feed them extra things in their feed or not). Penguins were 'just fish' but they still eat different types of fish. Unless you count all omnivores as necessarily needing a 'varied diet'.

[mercutio_stencil]

I'm curious now if most predators that rely on a singular source of food (say, polar bears on seals) also eat the entire animal, or focus on specific organs if they're particularly deprived. I remember in My Side of the Mountain (totally scientific source) a sequence where he's ravenous and feeling sick, and he catches a rabbit and for some reason the rabbits liver looks like the most delicious thing in the world to him, and he eats it without even cooking it. Later, he finds out that liver is rich in vitamin 'whatever he lacked'. D? Anyway.

I've heard it said (although I don't feel like going through the effort to check my sources right now) that strict carnivores often eat the stomach (and the contents of it) first, possibly in an attempt to obtain a more balanced diet. This was from a zookeeper, who was explaining why the predators were fed sausages made from whole ground antelope, rather than the usual ground meat.

[Qaanol]

This was from a zookeeper, who was explaining why the predators were fed sausages made from whole ground antelope, rather than the usual ground meat.

Anyone else get a mental image of putting a whole antelope through a wood chipper?

[gmalivuk]

Doesn't that sound like mainly meat to you? Cows were classed as just eating grass, when they generally get different types of grass, as well as other plants (I don't know if people feed them extra things in their feed or not). Penguins were 'just fish' but they still eat different types of fish. Unless you count all omnivores as necessarily needing a 'varied diet'.

Still, penguins eat many kinds of fish. Inuit eat many kinds of fish, as well as many kinds of aquatic and terrestrial mammals, as well as many kinds of birds (and their eggs), as well as a variety of fruits and (marine and terrestrial) vegetables.

But still, the main issue seems to be how much of the animal is eaten. Mongols might eat almost exclusively horse-derived things, but eating *everything* you can make from horse milk along with every part of the horse is extremely different from subsisting entirely on horsemeat.

[Dr. Diaphanous]

Come to think of it, I remember reading that there is often a bit of vitamin C in fresh meat. Some form of cooking and preserving meat goes back to before fully modern humans evolved. But hunters would eat the soft organs (e.g. liver) immediately, and bring the tough meat (muscle) back home to be cooked.

That's a tautology. They're only "vital" because we can't synthesize them. If we depended on the same nutrient but could synthesize it, we wouldn't have to consider it a vitamin in humans.

Well yes, we can synthesize 12 of the 20 amino acids from each other. But that doesn't explain why we lost the ability to make the other 8.

As is the answer to every single “why” question in evolution, the answer is simply, “Because all the other ones died, or their descendants are considered to be different species now.”

Why did natural selection not kill all the lineages where [ability to synthesize all vitamins] was lost?

evolution works on organisms within their niches

But humans didn't stick to theirs. They migrated out of Africa and around the world, where they came across many different environments, all lacking the food types they were used to.

And vitamin deficiency is a thing. And not just for 18th century sailors.

Vitamin A deficiency:

[Copper Bezel]

But humans didn't stick to theirs. They migrated out of Africa and around the world, where they came across many different environments, all lacking the food types they were used to.

Yeah, but evolution can't plan ahead. And we have had some diet-based adaptations since then, like the ability to digest lactose beyond childhood in many populations,* but it's not enough time and not enough selection to make huge changes to all the dietary requirements we have.

* And that one's fairly impressive, since it's an adaptation to agriculture, which means it's had just 10k years to set in.

[idobox]

But still, the main issue seems to be how much of the animal is eaten. Mongols might eat almost exclusively horse-derived things, but eating *everything* you can make from horse milk along with every part of the horse is extremely different from subsisting entirely on horsemeat.

We're not very different from other mammals. It just makes sense that we can find everything we need in their body, even if it's not in the optimal proportions.
By the way:
http://www.smbc-comics.com/index.php?db=comics&id=1473#comic

[Xanthir]

Well yes, we can synthesize 12 of the 20 amino acids from each other. But that doesn't explain why we lost the ability to make the other 8.

Why do you think there needs to be an explanation? The whole point of evolution is that it finds "good enough" solutions to current problems only. Most change occurs through random drift that doesn't significantly affect fitness. If you're getting those 8 aminos from your diet, then randomly losing the ability to synthesize them won't hurt you, and the change that caused the loss can fixate without having to be good *or* bad.

[iChef]

It makes perfect sense that we need a large and varied diet. Look at the complex biological machine we are trying to keep running. Different organs made of different kinds of cells doing hundreds of different functions.

As to getting your vitamins in ancient times, what do you think a lot of traditional herbal medicine was based on? Some of it is superstition and most of it today is pure woo, but there are things like making tea from pine needles which gives enough vitamin C to get by. In a time when many diseases could be caused by poor diet eating roots and greens were good ways of getting nutrients staple foods don't give. As far as eating every part of the animal modern nations only stopped doing that about 75 years ago or so and mostly in the West there are lots of places where liver, sweet breads (thymus glands) and tripe are still on the menu. Look at France, the French will eat anything that isn't fast enough to get away. Try some tete-de-vaeu, a cow's tongue with sweet breads wrapped up in a calf's face cooked with a few root vegetables and served with gribiache sauce (kinda like tartar sauce). Now that's all the nutrients you need right there and it's actually pretty tasty if made right once you get past the texture. Don't even get into the Chinese diet they eat things even the French won't touch. Whole birds, snakes, bugs you got it. Although much of this is either in poor areas or it is more intended as medicine than food. Once thing the Chinese have a knack for is edible items that will "make you strong" which basically translates into giving you a huge strutting erection. Both during my trip to Beijing and with my Cantonese speaking friends here in the states I've been offered strange sea creatures, bitter teas and strange parts of every land beast available that will "make you strong".

[Xanthir]

Once thing the Chinese have a knack for is edible items that will "make you strong" which basically translates into giving you a huge strutting erection.

If by "have a knack for" you mean "make up ridiculous woo ideas about with no connection to reality, and end up killing endangered animals to make 'medicines' that have no actual effect".

...I suppose it would be weird if you meant that, since it doesn't grammatically fit into the sentence.

[chenille]

At some point, out ancestors made all the amino acids/vitamins/etc they needed from carbon dioxide and ammonia and a few other components, or from other amino acids/vitamins. Why did natural selection not kill all the lineages where this was lost?

Or here's another way to look at it: it didn't at first because the nutrients were available in the environment. Creatures that could not make vitamin C were able to survive fine, and by chance people developed from them. Later on, once people moved to other environments or reached populations where they were not so plentiful, it did start killing some off, but not enough to wipe out the line because of other advantages it picked up. So to this day, people may be afflicted by scurvy, but it has not stopped our species from spreading. It is much too late to recover our ability to synthesize vitamin C, but that handicap is not fatal to the population as a whole.

[Soralin]

At some point, out ancestors made all the amino acids/vitamins/etc they needed from carbon dioxide and ammonia and a few other components, or from other amino acids/vitamins. Why did natural selection not kill all the lineages where this was lost?

Or here's another way to look at it: it didn't at first because the nutrients were available in the environment. Creatures that could not make vitamin C were able to survive fine, and by chance people developed from them. Later on, once people moved to other environments or reached populations where they were not so plentiful, it did start killing some off, but not enough to wipe out the line because of other advantages it picked up. So to this day, people may be afflicted by scurvy, but it has not stopped our species from spreading. It is much too late to recover our ability to synthesize vitamin C, but that handicap is not fatal to the population as a whole.

Although "people" here should be primates, the mutation that broke our vitamin C production, along with that of other modern primates, is estimated to have happened in our ancestors approximately 58-63 million years ago, before there were any humans.

And perhaps not too late to recover it, we still have most of what's required to make it:
http://en.wikipedia.org/wiki/Vitamin_C

In 1957, the American J.J. Burns showed that the reason some mammals are susceptible to scurvy is the inability of their liver to produce the active enzyme L-gulonolactone oxidase, which is the last of the chain of four enzymes that synthesize vitamin C.[185][186]

All we'd have to do is fix that last step. And we do already have the gene to make that, it's just been rendered non-functional. Maybe not something that evolution would easily stumble across, but something that seems like it would be relatively simple to engineer, given that we already know what we want and the sequence to make it.

And this is also interesting:

In 2008, researchers at the University of Montpellier discovered that, in humans and other primates, the red blood cells have evolved a mechanism to more efficiently utilize the vitamin C present in the body by recycling oxidized L-dehydroascorbic acid (DHA) back into ascorbic acid, which can be reused by the body. The mechanism was not found to be present in mammals that synthesize their own vitamin C.[14]

[Dr. Diaphanous]

I found this article about vitamin C and why humans can't make it. It has some ideas that haven't been brought up in this thread (particularly the last section)