Stevens C E, Hume I D
College of Veterinary Medicine, North Carolina State University, Raleigh, USA.
Physiol Rev. 1998 Apr;78(2):393-427. doi: 10.1152/physrev.1998.78.2.393.
The vertebrate gastrointestinal tract is populated by bacteria and, in some species, protozoa and fungi that can convert dietary and endogenous substrates into absorbable nutrients. Because of a neutral pH and longer digesta retention time, the largest bacterial populations are found in the hindgut or large intestine of mammals, birds, reptiles, and adult amphibians and in the foregut of a few mammals and at least one species of bird. Bacteria ferment carbohydrates into short-chain fatty acids (SCFA), convert dietary and endogenous nitrogenous compounds into ammonia and microbial protein, and synthesize B vitamins. Absorption of SCFA provides energy for the gut epithelial cells and plays an important role in the absorption of Na and water. Ammonia absorption aids in the conservation of nitrogen and water. A larger gut capacity and longer digesta retention time provide herbivores with additional SCFA for maintenance energy and foregut-fermenting and copoprophagic hindgut-fermenting species with access to microbially synthesized protein and B vitamins. Protozoa and fungi also contribute nutrients to the host. This review discusses the contributions of gut microorganisms common to all vertebrates, the numerous digestive strategies that allow herbivores to maximize these contributions, and the effects of low-fiber diets and discontinuous feeding schedules on these microbial digestive processes.
脊椎动物的胃肠道中存在细菌，在某些物种中还存在原生动物和真菌，它们可将膳食和内源性底物转化为可吸收的营养物质。由于pH值呈中性且消化物保留时间较长，在哺乳动物、鸟类、爬行动物和成年两栖动物的后肠或大肠以及少数哺乳动物的前肠和至少一种鸟类的前肠中发现了数量最多的细菌种群。细菌将碳水化合物发酵成短链脂肪酸（SCFA），将膳食和内源性含氮化合物转化为氨和微生物蛋白，并合成B族维生素。SCFA的吸收为肠道上皮细胞提供能量，并在钠和水的吸收中起重要作用。氨的吸收有助于氮和水的保存。更大的肠道容量和更长的消化物保留时间为食草动物提供了额外的SCFA用于维持能量，并为前肠发酵和粪便再食性后肠发酵物种提供了获取微生物合成蛋白和B族维生素的途径。原生动物和真菌也为宿主提供营养。本文综述了所有脊椎动物共有的肠道微生物的贡献、使食草动物能够最大限度发挥这些贡献的众多消化策略，以及低纤维饮食和不连续进食时间表对这些微生物消化过程的影响。
