How are animals that eat only one kind of food not horribly malnourished? Do they need a narrower set of nutrients than humans?


How are animals that eat only one kind of food not horribly malnourished? Do they need a narrower set of nutrients than humans?


It’s nature. If you eat the thing no one else wants then you can eat as much as you want. For example pandas and koalas.

They eat food nothing else eats, but it isn’t very nutritious, so they have to always be eating. Which is fairly common for plant eaters.

And they do have health problems, it makes it hard to have babies for pandas. And unhealthy things in koalas relating to similar organs.

Different animals are able to digest different things and have different nutritional requirements. Example is dogs and cats. You can’t give a cat just dog food because it lacks an amino acid that dogs produce on their own but cats need through their diet. But you can’t give a dog just cat food because it’s too high protein.

I think most people don’t realize just how high our standards are now. We, by and large, expect a safety level that is far beyond what is simply needed to stay mostly healthy.

Humans can do a lot of things considered unsafe (like eating raw meat, or consuming certain chemicals, etc) and life a long, healthy life. However, with the law of large numbers, we can consider it very unsafe.

For example, take something based on eating a meal. It could have a very low chance of actually getting you sick. Lets say only one in a million meals eaten produces a ugly, fatal reaction.

If you eat 3 meals a day, 365/year, and do it for 100 years, that is only 110,000 meals over your life. You can do that activity every day and still pretty likely never hit the reaction.

The US population is 330 Million. If everyone did that then you’d be looking at 1,000 preventable deaths a day.

There have been humans who have only eaten one type of food and been seemingly healthy. Hell, there was the one guy who lost a ton of weight by not eating for over a year. Doesn’t mean it is good advice for the population at large, or that you’d be advised to take whatever risks are involved.

Most people would consider a 1 in 10 chance of getting severe nutritional deficiencies unacceptable, even though that means 9 in 10 people don’t have problems.

tl;dr: We live in an era of high standards.

Animals (including humans) are able to synthesize some of what their bodies need, and the things the body can’t produce on its own need to be directly ingested.

An example is vitamin c. Humans don’t produce it, so we need to eat it to avoid nasty things like scurvy. Cats and dogs need it, but their bodies can self-produce the needed amount, so they don’t need to consume any from food.

To be fair, I don’t know much beyond that, but I’d guess that animals that subsist on very narrow diets probably have a combination of relatively minimal nutritional needs (in terms of mineral/vitamin diversity) and the ability to synthesize whatever different things they do need.

edit: adding some additional info, inspired by several of the other comments below and elsewhere to the post. Also added a couple “just ok” links, which do provide some jump-off points to better sources.

“Narrow diets” might not be as narrow as they seem to us. For example, whereas humans tend to eat mainly muscle tissue (mostly protein), carnivorous animals eat muscle + organs, which provides a lot of nutrition. Also, surviving != thriving. [link](

Similar to the synthesis concept I mentioned, is a more symbiotic approach, such as that in cows. Cows eat a lot of grass, yet are able to meet their protein and nutritional needs. Their four chamber stomachs break down the grass more completely than ours, and hosts bacteria that feed on the grass as this happens, and the cow in turn ends up deriving nutritional value from the bacteria. [link](

In summary, it seems like the main reasons animals get by how they do is because: they’re still here. They’re either specialized to process their diets, and/or we’re ignorant about what they’re actually getting in their diets.

In many cases, animals that eat almost exclusively one kind of food will have rare diet supplementation and they actually do get all the necessary vitamins. Wild cats eat grass, even though they are carnivores. There’s lots of videos of horses/deer eating mice and birds that get too close, even though they are herbivores. Or goats and other animals traveling far distances to get salt intake. Housepets will eat flies or bugs that they encounter around the house (just for a relatable example).

They may not do it very often and it is a negligible part of their calorie intake, and have adapted to conserve and more aggressively absorb the rarer nutrients in their diets. Common nutrients less so – like Vitamin C, which some animals produce themselves, but our diet expects it and we get scurvy without it.

The most basic life forms only consume one thing and their bodies produce the rest of what they need. As diet diversifies, we get more of what we need. If your diet contains a lot of Vitamin C, for example, you won’t be affected by genetic defects that stop your body producing Vitamin C.

So, over the millennia, organisms lose the ability to produce vital nutrients and if their diets compensate, then all is well and the genes are passed on. If not, they die just like every other unsuccessful genetic variant.

Humans are actually kind of rare among mammals in how few nutrients we can self synthesize . Vitamin C is the one I’m most aware of.

Equine/bovine animals (like cows and horses) can survive pretty much exclusively on grass – which I assume is one of the good examples for your question.

All living things consume energy. Energy is what allows the replication of cells to preserve life. Many plants survive by absorbing sunlight and converting the energy in that warm sunlight into “food” through photosynthesis. They also, however, absorb basic nutrients, like nitrogen, from solid particles in the soil, which supplements the energy they get from photosynthesis. They absorb these nutrients through their roots with the help of water which helps to absorb the minerals. You might think of plants as taking sunlight to heat their oven and the minerals as the ingredients in the meal they “cook” to “eat.”

Animals which primarily depend on grasses and grains as food eat those plants, which have already converted minerals and sunlight into a new, more complex form of energy. The animals “steal” this energy from the plants, having already done a lot of legwork.

These animals have specialized digestive systems and organs which produce the more complex proteins they need to develop muscle and fat, which aren’t obviously available in plants (but the building blocks are there).

Carnivores take this a step further and “steal” the nutrients from animals which already converted the energy from plants into valuable and readily available proteins (among other nutrients). This same concept occurs throughout the chain of life, with fruits and vegetables and grains all creating different concentratioms of sugars, fiber, carbohydrates (a sugar) and proteins and other basic vitamins and minerals which animals eat to quickly and easily obtain the energy they specifically need to live and reproduce.

Humans generally need a wide variety of foods because we have some of the most energy-intensive processes. Our basic living functions could be sustained with some basic sugars and carbs and a little bit of protein, but we have evolved to be able to grab more of that protein through carnivorous diets, and we have successfully been able to reach a wider variety of fruits and vegetables to obtain these nutrients for such a long time. Our brains are also extremely energy-intensive and require fats, oils, and specialized proteins to fuel the specialized higher-thinking regions of the brain. Dolphins are another highly intelligent species and depend on meat for their sustenance, and because advanced predators can obtain these easily through eating meat, our bodies developed to harness that advantage and has allowed our brains to develop further than pretty much all other organisms.

When an animal produces waste, there are insects and bacteria which take *that* specific form of energy and break it down even further into the more basic nutrients found in fertile soil, allowing plants to then reabsorb it in a new cycle. The abundance of specific organisms, and the concentration of certain animals producing that waste, affects the stability of that ecosystem. Grass and plants can only absorb so much of certain nutrients in a given timeframe. Same with the bacteria and insects which break down the waste of animals.

All of this is limited by the availability of basic minerals and nutrients in the earth’s topsoil (where small organisms can break them down and plants can absorb them theough their root systems) and how quickly plants can absorb sunlight for useful energy.

So again, a recap: animals with simple diets obtain simple nutrients, and are very limited in their evolutionary advancement with respect to brain development and advanced processes. However, all nutrients are basically formed by basic building blocks, like nitrogen, hydrogen, oxygen, sodium and even metals, combined with a fuel for combining and using those minerals for “life” processes, which is harnessed from the sun via plants and photosynthesis in relatively large quantities.

Edit: Equine is horses and bovine is cows