So, as a biochemist, I am truly interested. What ARE the biochemical differences between carnivores, herbivores, and omnivores? And what nutrients can not be obtained from consuming an entire prey animal, brain, entrails, bone and all? Please educate…
ah, this is an interesting area of study. I admit to being but an amateur student of the topic.
In general, animals designed to eat plants have heavy adaptations to digest the plants- plants are DIFFICULT to digest. They have hard cell walls that have to be broken open, and much of their content cannot be digested by mammals, only by bacteria. So the animal designed to eat lots of plants has teeth designed to grind open cell walls, and usually has a very impressive digestive tract with lots of places for the food to hang out and be broken down by bacteria. The cow is a wonderful example of an animal designed to eat plants.
The cat is a fine example of a carnivore. The cat does not have grinding teeth and can’t chew up plant material. The cat also has a very simple digestive tract, and can’t digest plant material- your cat can eat enormous quantities of plant material and still starve to death.
Some animals are “in transition”- look at humans. Some millions of years ago, our ancestors were adapted to eating primarily plants- flat grinding teeth, strong jaw muscles, big fiber-fermenting guts. We’ve been evolving towards a meat-eating form since then. We can compare human biochemistry/anatomy to our closest living relative, the chimp, and see how much simpler our gut has gotten, and how our jaw/teeth has reduced in size, indicating we are less able to digest plants and more able to digest meat.
The most interesting part of studying ecological biochemistry is looking at key dietary vitamins and nutrients- all mammals started out with the same biochemistry, yet some have lost the ability to synthesize certain chemicals. If an animal loses the ability to synthesize a key chemical, it will probably just die. If the entire species manages to survive despite not being able to synthesize the chemical, it’s often because that chemical is abundant in the diet- take vitamin C. Most mammals can make it; a few species cannot, and all of these species commonly consume diets full of vitamin C. A fruit-eating monkey can easily survive after losing the ability to synthesize vitamin C; a cat, probably not so easily, thus cats are still able to synthesize vitamin C (evolutionary pressure). Or vitamin B12- most animals adapted to eat plants get their vitamin B12 from their colonies of gut bacteria. Meat eaters get their vitamin B12 by eating the flesh of these animals. Humans, having adapted towards meat-eating, have lost the ability to create and absorb sufficient B12 from our gut bacteria, so now we have to eat meat to obtain our B12 just like a lowly cat. Or taurine- plant-eaters are able to synthesize taurine, but cats have to obtain it from their diet (the natural cat diet is rich in taurine), and there is some evidence to indicate that dogs, or at least some dogs, also cannot synthesize sufficient taurine and need to eat a diet rich in the substance- evolutionary pressure keeps non-taurine-eating animals capable of synthesizing it, but animals who naturally eat a lot of meat don’t have that pressure.
If you look at a “whole prey animal”, http://www.nal.usda.gov/awic/zoo/WholePreyFinal02May29.pdf, you’ll note the most obvious nutrients that are not-present in any quantity are fiber and carbohydrates. Dogs and cats and other biochemical carnivores have no dietary requirement for these nutrients- their guts don’t require fiber to work, and their bodies don’t need carbohydrates to run. Humans, being derived from a plant-eater, do have requirements for dietary fiber and possibly for carbohydrates- if you don’t eat fiber, your gut malfunctions, and if you eat absolutely no carbohydrates at all apparently your brain can stop functioning (some people dispute this). Plus there’s that pesky inability to synthesize vitamin C humans have inherited- not much vit. C in meat.
The real problem with feeding non-species-appropriate diets to animals though, is not one of obvious “severely constipated” or “you’ve got scurvy!”, it’s more a matter of developing disorders due to exposure to too much or too little of this or that macronutrient. Look at horses- horses aren’t adapted to eating grains, and some respond to the unnatural diet by developing diseases like IR and EPSM. Or cats, who aren’t adapted to eating carbohydrates, and if we feed them a grain-based diet, their teeth decay away and some of them go on to develop diseases like diabetes and renal disease.
