Lipids herbivore diets

Figure 10.2. Schematic diagram showing how restricted conversion of fatty acids to amino acids influences the fractionation between collagen and CO3 of bone apatite LI = lipid component, PR = protein, T = total isotopic composition AP = COj component of apatite, a) Herbivorous diet (Cj plants only) b) Carnivorous diet, assuming rj = 1 (no barrier to fatty acid conversion to AAs) c) Carnivorous diet, assuming ri < 1 note that carbonate-collagen fractionation is smaller.

Herbivore diets consist only of plant materials and thus are the simplest diets to understand Herbivores eat the more digestible portions of plants so their diets consist of a preponderance of carbohydrates with only minor contributions of lipids and proteins from the plant material consumed ... 

Lipids form a minor part of the carbon content of herbivore diets As a result, despite the fact that they are used primarily for energy metabolism, their abundance is not sufficient to greatly influence the isotopic composition of blood bicarbonate For this reason, the isotopic composition of lipids will not significantly affect the isotopic ratio of carbon in bone apatite ... 

Most accounts of the larger A,p.,.o in carnivores have attributed this effect to higher proportion of lipids in the diet of carnivores. This arises because carnivores obtain all or most of their nutrition from the flesh of other animals, a significant part of which is composed of lipid. By contrast, lipids make up a much smaller fraction of the total carbon pool in the diet of herbivores, particularly mminants which get much of their energy from digestion of cellulose. Humans who selectively use seeds and grains as food sources obtain a... 

We can now appreciate that this explanation is incorrect, because the energy food for an animal is all of its diet and not just carbohydrates and lipids. Therefore we should not expect any selective offset due to the presence of lipids in the flesh of herbivores. Indeed, in general, the average 5 Cof total consumable herbivore tissues (flesh, lipids, etc.) is very close to that of the diet, and we might not expect any difference in the isotopic composition of the collagen or carbonate of a consumer of pure Cj plants as opposed to a consumer of the flesh of Cs-eating herbivores. We must seek elsewhere for the cause of the trophic level effect on A,p.co-... 

Herbivores typically eat diets with high carbohydrate and low protein and lipid. F is about 0.15, and the protein is mainly plant derived and not very different in S C from the non-protein (i.e., Dp is close to Dn). For carnivores, F is typically 0.5 or over, and carbohydrate is low. Animal protein is generally isotopically heavier, while the non-protein is much higher in lipid, so that Dp - Dn is generally quite large (>5%o). The spacing, Bcolla Bcarb. for the two diets is evaluated according to the equation above. 

Carnivores rely on a protein-rich diet and produce new biomass primarily from dietary amino acids, although the enzymes required for de novo amino acid synthesis are present (Garmes et al., 1998). Bone collagen, muscle (meat) and apatite were analyzed for a set of modern southern African herbivores and carnivores (Lee-Thorp et al., 1989). The isotopic analyses showed i C enrichment in bone collagen, apatite and muscle, and depletion in lipids. Difference in values between herbivores and carnivores indicates a trophic effect, which for carbon in bone collagen is 2.5-3%o (Fig. 2). 

The nature of the diet sets the basic pattern of metabohsm. There is a need to process the products of digestion of dietary carbohydrate, lipid, and protein. These are mainly glucose, fatty acids and glycerol, and amino acids, respectively. In ruminants (and to a lesser extent in other herbivores), dietary cellulose is fermented by symbiotic microorganisms to short-chain fatty acids (acetic, propionic, butyric), and metabohsm in these animals is adapted to use these fatty acids as major substrates. All the products of digestion are metabohzed to a common product, acetyl-CoA, which is then oxidized by the citric acid cycle (Figure 15-1). 

Vitamin A is a fat-soluble micronutrient that is required by all vertebrates to maintain vision, epithelial tissues, immvme functions, reproduction, and for life itself. It was discovered in 1913 as a minor component in eggs, butter, whole milk, and fish liver oils. It soon became apparent that vitamin A exists in two chemically distinct yet structurally related forms. The first form to be characterized was retinol, a lipid alcohol that is present only in foods of animal origin. Retinol is also known as preformed vitamin A because it can be metabolized directly into compovmds that exert the biological effects of vitamin A. A second form of vitamin A, present in deep-yellow vegetables, was characterized as /3-carotene, which is synthesized only by plants but can be converted to retinol during absorption in the small intestines. These carotenoids are sometimes referred to as provitamin A. The nutritional requirement for vitamin A can be met by preformed retinol, provitamin A carotenoids, or a mixture, and therefore it is possible to obtain a sufficient intake of vitamin A from carnivorous, herbivorous, or omnivorous diets. 

An outstanding feature of the composition of brain phospholipids is its remarkable consistency, irrespective of species or diet. The concentrations of the precursor EFA are extremely low (18 2, n-6,0.1-1.5% and 18 3, n-3, 0.1-1.0%) while arachidonic (20 4, n-6) and docosahexaenoic (22 6, n-3) acids predominate at 8-17% and 13-29% respectively in all species. This contrasts with the liver lipids where there is much greater variation between species. The precursor EFA are present in much greater concentrations than they are in brain and there are major differences in the product EFA. For example, 22 5 is the major n-3 fatty acid in the liver lipids of ruminants and other herbivores while 22 6 predominates in the carnivores and omnivores. Fatty acids of the n-6 family usually predominate in liver phosphoglycerides, even when the overwhelming dietary intake is in favour of the n-3 fatty acids. Thus, zebra and dolphin, both species that have an overwhelming excess of n-3 fatty acids in the diet, attain a preponderance of n-6 acids in the liver phosphoglycerides (Table 5.14). 

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