ELI5: How are animals that eat only one kind of food not horribly malnourished? Do they need a narrower set of nutrients than humans?

Read the Story

Show Top Comments

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.


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.


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.


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.


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.