Carbon dietary

Fig. 7.10 A routing model for dietary carbon. Dietary protein carbon is normally used to build body tissues including collagen. Excess protein is burned as energy. Carbon in carbohydrates and lipids in the diet are used primarily for energy except when there is insufficient protein for maintaining body tissue. Carbohydrates and lipids are burned by the body to produce energy waste products are CO, and H,O. Excess energy is stored as fat. CO is exhaled and wastes are excreted. Illustration courtesy of Tamsin O Connell...

Prostaglandins arise from unsaturated C20 carboxylic acids such as arachidonic acid (see Table 26 1) Mammals cannot biosynthesize arachidonic acid directly They obtain Imoleic acid (Table 26 1) from vegetable oils m their diet and extend the car bon chain of Imoleic acid from 18 to 20 carbons while introducing two more double bonds Lmoleic acid is said to be an essential fatty acid, forming part of the dietary requirement of mammals Animals fed on diets that are deficient m Imoleic acid grow poorly and suffer a number of other disorders some of which are reversed on feed mg them vegetable oils rich m Imoleic acid and other polyunsaturated fatty acids One function of these substances is to provide the raw materials for prostaglandin biosynthesis... 

Mobilization of Fats from Dietary Intake and Adipo.se Ti.ssne /3-Oxidation of Fatty Acids /3-Oxidation of Odd-Carbon Fatty Acids /3-Oxidation of Unsatnrated Fatty Acids Other Aspects of Fatty Acid Oxidation... 

Although /3-oxidation is universally important, there are some instances in which it cannot operate effectively. For example, branched-chain fatty acids with alkyl branches at odd-numbered carbons are not effective substrates for /3-oxidation. For such species, a-oxidation is a useful alternative. Consider phy-tol, a breakdown product of chlorophyll that occurs in the fat of ruminant animals such as sheep and cows and also in dairy products. Ruminants oxidize phytol to phytanic acid, and digestion of phytanic acid in dairy products is thus an important dietary consideration for humans. The methyl group at C-3 will block /3-oxidation, but, as shown in Figure 24.26, phytanic acid a-hydroxylase places an —OFI group at the a-carbon, and phytanic acid a-oxidase decar-boxylates it to yield pristanie add. The CoA ester of this metabolite can undergo /3-oxidation in the normal manner. The terminal product, isobutyryl-CoA, can be sent into the TCA cycle by conversion to succinyl-CoA. 

Mammals can add additional double bonds to unsaturated fatty acids in their diets. Their ability to make arachidonic acid from linoleic acid is one example (Figure 25.15). This fatty acid is the precursor for prostaglandins and other biologically active derivatives such as leukotrienes. Synthesis involves formation of a linoleoyl ester of CoA from dietary linoleic acid, followed by introduction of a double bond at the 6-position. The triply unsaturated product is then elongated (by malonyl-CoA with a decarboxylation step) to yield a 20-carbon fatty acid with double bonds at the 8-, 11-, and 14-positions. A second desaturation reaction at the 5-position followed by an acyl-CoA synthetase reaction (Chapter 24) liberates the product, a 20-carbon fatty acid with double bonds at the 5-, 8-, IT, and ITpositions. 

An alkene, sometimes caJled an olefin, is a hydrocarbon that contains a carbon-carbon double bond. Alkenes occur abundantly in nature. Ethylene, for instance, is a plant hormone that induces ripening in fruit, and o-pinene is the major component of turpentine. Life itself would be impossible without such alkenes as /3-carotene, a compound that contains 11 double bonds. An orange pigment responsible for the color of carrots, /3-carotene is a valuable dietary source of vitamin A and is thought to offer some protection against certain types of cancer. 

Terpenoids are classified according to the number of five-carbon multiples they contain. Monoterpenoids contain 10 carbons and are derived from two isopentenyl diphosphates, sesquiterpenoids contain 15 carbons and are derived from three isopentenyl diphosphates, diterpenoids contain 20 carbons and are derived from four isopentenyl diphosphates, and so on, up to triterpenoids (C30) and tetraterpenoids (C40). Monoterpenoids and sesquiterpenoids are found primarily in plants, bacteria, and fungi, but the higher terpenoids occur in both plants and animals. The triterpenoid lanosterol, for example, is the precursor from which steroid hormones are made, and the tetraterpenoid /3-carotene is a dietary source of vitamin A (Figure 27.6). 

One of the most striking features of the common fatty adds is that they have an even number of carbon atoms (Table 27.1, p. 1062). This even number results because all fatty acids are derived biosynthelically from acetyl CoA by sequential addition of two-carbon units to a growing chain. The acetyl CoA, in turn, arises primarily from the metabolic breakdown of carbohydrates in the glycolysis pathway that weTl see in Section 29.5. Thus, dietary carbohydrates consumed in excess of immediate energy needs are turned into fats for storage. 

It is immediately clear that Acanthomyops need not rely on dietary sources of terpenes but can synthesize citronellal and citral from either acetate or mevalonate. The higher total activity of the citronellal as compared with the citral probably reflects the natural preponderance of citronellal (ca. 90%) in the ant secretion. As the specific activities show, these results are consistent with a common biogenetic origin of both terpenes. In the mevalonic acid pathway as described from other organisms (13), the radioactive carbon of l-C14-mevalonate is lost upon formation of isopentenyl pyrophosphate. 

Hair and bone collagen reflect similar dietary information but there is a difference in the diet-hair and diet-collagen spacing for both carbon and... 

Ambrose, S.H. and Norr, L. 1993 Experimental evidence for the relationship of the carbon isotope ratios ofwhole diet and dietary protein to those ofbone collagen and carbonate. In Lambert, J.B. and Grupe, G., eds.. Prehistoric Human Bone Archaeology at the Molecular Level. Berlin, Springer-Verlag 1-37. 

In carbon, an uncertainty exists which arises because we have neither sufficient empirical data nor an accepted theoretical model to explain the quantitative relationships between dietary carbon isotope signals and those... 

Walker, PL. and DeNiro, M.J. 1986 Stable nitrogen and carbon isotope ratios in bone collagen as indices of prebistoric dietary dependence on marine and terrestrial resources in Soutbem California. American Journal of Physical Anthropology 71 51-61. 

The properties described above have important consequences for the way in which these skeletal tissues are subsequently preserved, and hence their usefulness or otherwise as recorders of dietary signals. Several points from the discussion above are relevant here. It is useful to ask what are the most important mechanisms or routes for change in buried bones and teeth One could divide these processes into those with simple addition of new non-apatitic material (various minerals such as pyrites, silicates and simple carbonates) in pores and spaces (Hassan and Ortner 1977), and those related to change within the apatite crystals, usually in the form of recrystallization and crystal growth. The first kind of process has severe implications for alteration of bone and dentine, partly because they are porous materials with high surface area initially and because the approximately 20-30% by volume occupied by collagen is subsequently lost by hydrolysis and/or consumption by bacteria and the void filled by new minerals. Enamel is much denser and contains no pores or Haversian canals and there is very, little organic material to lose and replace with extraneous material. Cracks are the only interstices available for deposition of material. 

One further difference between the tissues should be noted briefly—that of turnover—which holds implications for the nature of the isotopic signal recorded and its interpretation. Bone is constantly resorbed and reformed during life, i.e., it turns over , whereas enamel and dentine do not, although secondary dentine can be later accreted. Enamel and dentine form during a discrete period in the individual s life. This means that carbon isotope dietary signals in bone, for both collagen and apatite, reflect diet integrated over years, whereas those in enamel and dentine increments reflect diet at time of formation. 

In the million year range, however, there is clearer evidence for isotopic alteration which also apparently increases with age. Unfortunately, at present there are few reliable browser and grazer data beyond the Late Pleistocene, i.e., in the Middle Pleistocene, which would allow determination of the periods during which bone apatite and enamel begin to deviate. The indications are that the timing should be different. Material from the site of Florisbad falls within the late Middle Pleistocene ( 125-200 Ka), but at present carbon isotope data are only available for three species of uncertain and/or opportunistic diets (Brink and Lee-Thorp 1992), so that extent or direction of isotopic alteration is impossible to separate from dietary vagaries. 

Wang, Y., Cerling, T.E. and MacFadden, B.J. 1994 Fossil horses and carbon isotopes new evidence for Cenozoic dietary, habitat, and ecosystem changes in North America. Palaeogeography, Palaeoclimatology, Palaeoecology 107 269-279. 

Larsen, C.S., Schoeninger, M.J., van der Merwe, N.J., Moore, K.M. and Lee-Thorp, J. 1992 Carbon and nitrogen isotopic signatures of human dietary change in the Georgia Bight. American Journal of Physical Anthropology 89 197-214. 

---