View Single Post
Old 08-05-2005, 07:35 PM
Blake Blake is offline
Join Date: Mar 2001
Posts: 11,049
CC has pretty much nailed it. Nothing is easier to use than simple sugars, and there are precious few of those in meat, whereas fruits are loaded with them. Simple sugars can pass straight thorugh the gut and into the bloodstream ready for use. In contrast meat is mostly meat and trigylceride, which need processing. So it's not true to say that plant matter is harder to digest. Herbivores have longer digestive tracts for two reasons.

One is that most of the energy in non-fruit plant material is in the form of cellulose. Animals can't digest cellulose, period. Any animal living on plant material that isn't fruit is wasting almost all the avaolbale energy in it. To get aorund that problem herbivores have developed a system where they populate the gut with microbes that can digest cellulose, and the animal then digests the microbes. It takes quite a while for microbes to digest (read: rot) plant material in an animal's gut, so the guts of many herbivores have gotten longer or developed pouches and pockets to allow the food to sit for longer and rot some more.

The other reason herbivores, even frugivores, tend to have longer guts is because of the nutrient density of thier foods. Fruit is usually fairly energy dense, often more energy dense than meat, but it is invariably unbalanced, and as a result the nutrient density is low. Or to put it another way, an animal has to go through a load of plant matter, even fruit, to get the nutrients it needs to stay alive. Plant matter is typically deficient in protien and fats for example. An animal might need to eat 100X its daily energy requirement in apples to get its daily protein and fat requirement, whereas if a carnivore gets enough energy it auotmatically gets more than enough of all essential nutrients. To overcome that problem animals adopt one of three tactics. They either supplement theiir fruit diet with meat as most birds do, or they go through massive amounts of food as horses do, taking out the good bits and producing ciopious amounts of solid waste, or they develop an enlarged gut as a microbial nutrient production vat.

Microbes are clever little critters and produce most anyhting they want if they have acess to air and an energy soucre. Animals exploit that by feeding the microbes in their gut plant matter as an energy souce and then letting them synthesise proteins and fats to overcome the nutrient deficiencies. Cattle and other ruminants take this to the extreme and feed their bodies' urea waste back into thir own stomachs rather than excreting it as urine. The microbes in their stomach then use the urea ot make more protein, which the cow then uses to make more cow. But as with the use of micorbes to digest plant matter this requires fairly long gut to give the microbes time to work.

So it's not quite as simple as saying that meat is easier to break down than plant matter. Some meat is easier to break down than some plant matter, but the opposite is also true. The difference is in the quality of the food, not the ease of use or the energy available in it. Sugar is an easily digested, high energy plant food, but it's low quality, which is why refer to it as 'junk food' and 'empty calories'. It's junk food to animals just as much as it is to humans.