Body Reserves

In addition to the fat storage in adipose tissue, a small amormt of fat is present as hpid droplets in musele tissues, or in eirculation as free fatty aeids associated with albumin or as part of a hpoprotein particle. The amormt of fat both in the plasma and stored in musele varies aceording to several faetors, ineltrding energy state, fitness level, and dietary fat intake. Within the musele eells, free fatty acids are primarily oxidized for energy. 

3 Fat Metabolism during Exercise From Adipose, from Intramuscular Triglycerides (IMTG) 

Once the fatty acids are in the cytoplasm of the muscle cells, they are transported across the mitochondrial membrane by carnitine palmityol transferases I and II for P-oxidation and energy production. During exercise, total fat oxidation rates can increase more than ten-fold during the transition from rest to moderate-intensity exercise. The availability of glucose, rather than that of fatty acids, controls the rate of fatty acid oxidation because glucose can decrease the oxidation of long-chain fatty acids by inhibiting their transport into the mitochondria. 

In addition to the use of fatty acids liberated from adipocytes, or obtained from the plasma as albumin-bound long-chained fatty adds or in very low-density lipoproteins (VLDLs), research indicates that the intramuscular triglyceride (IMTG) pool can serve as a dynamic fuel source during physical activity. Because no blood transport is required, intramuscular fatty acids can be readily used for energy in exercising muscle. A detailed discussion on IMTG can be found in Section 2.4.1. 

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The most widely recognized physiological action of the insect AKH/HGHs is to mobilize lipids or carbohydrates from fat body reserves for use as energy substrates by peripheral tissues. In Leptinotarsa decemlineata, proline is the main substrate for flight muscle metabolism (16), and Lom-AKH-I promotes fat body proline synthesis... 

The state of body reserves of the vitamin may affect the extent to which it is absorbed (by affecting the synthesis of binding and transport proteins) or the extent to which it is metabolized after uptake into the intestinal mucosa [e.g., the oxidative cleavage of carotene to retinaldehyde is regulated by vitamin A status (Section 2.2.1)]. 

A priori, it would appear to be a simple matter to determine reqmrements for vitamins. In practice, a number of problems arise. The first of these is the definition of the word requirement. The U.S. usage (Institute of Medicine, 1997) is that the requirement is the lowest intake that will maintain a defined level of nutriture in an individual - i.e., the lowest amount that will meet a specified criterion of adequacy. The WHO (1996) defines both a basal requirement (the level of intake required to prevent pathologically relevant and clinically detectable signs of deficiency) and a normative requirement (the level of intake to maintain a desirable body reserve of the nutrient). 

Adequate body reserves and normal metabolic integrity. This is the (possibly untestable) goal. Both immune function and rninirnization of DNA damage offer potential methods of assessing optimum micronutrient status, but both are affected by a variety of different nutrients and other factors (Fenech, 2001). 

Serum folate below 7 nmol per L or erythrocyte folate below 320 nmol per L indicates negative folate balance and early depletion of body reserves. At this stage, the first bone marrow changes are detectable. 

Vitamin C status is generally assessed by estimating the saturation of body reserves or measuring plasma and leukocyte concentrations of the vitamin. Urinary excretion of hydroxyproline-containing peptides is reduced in people with inadequate vitamin C status, but a number of other factors that affect bone and connective tissue turnover confound interpretation of the results (Bates, 1977). The ratio of deoxypyridinolineipyridinoline compounds derived from collagen cross-links provides a more useful index, but is potentially affected by copper status (Tsuchiya and Bates, 1997). 

It is relatively easy to assess the state of body reserves of vitamin C by measuring the excretion after a test dose. A subject whose reserves are saturated will excrete the whole of a test dose of500 mg of ascorbate over 6 hours. A more precise method involves repeating the loading test daily until complete excretion is achieved, thus giving an indication of how depleted the body stores were. 

Potassium supplements are sometimes used to treat conditions of potassium deficiency, especially during or following severe loss of body fluids (e.g. chronic diarrhoea) or treatment with drugs that deplete body reserves (e.g. some diuretics). It may also be used as a substitute for natural salt (sodium chloride) in cases where sodium is restricted (e.g. in hypertension). 

Most dietary retinoids are stored in the liver as esters of long-chain fatty acids (e.g. palmitate, stearate, oleate). Fish also store retinyl esters in their kidneys71. Measurement of hepatic retinyl ester concentrations provides an indication of total body reserves. However, with both retinoid and didehydroretinoid forms mediating physiological functions17,44, all varieties require consideration when assessing the retinoid status of fish. A study with brook trout (Salvelinus fontinalis) characterized... 

The RDA for ascorbate (Appendix IV) is the amount needed to cure or prevent scurvy while allowing adequate body reserves. However, rates of ascorbate synthesis in animals and the amounts needed to maintain serum... 

The stoichiometry of conversion of /3-carotene to retinol is still an unsettled issue. Central cleavage of /3-carotene theoretically yields 2 mol retinol per mole /3-carotene (Goodman and Huang, 1%5 Olson and Hayaishi, 1965 Olson, 1989) and eccentric cleavage of /3-carotene yields 1 mol retinol per mole /3-carotene (Olson, 1989, Krinsky et al., 1994). Brubacher and Weiser (1985) determined the retinol equivalent of /3-carotene in vivo using rats and chicks and found that 1 mol of absorbed /3-carotene yielded 1 mol retinol. Because the body reserves of retinol in these animals were low, the yield of retinol from /3-carotene was probably maximal. Based on these in vivo results, a ratio of 1 mol retinol per mole /3-carotene (after absorption) was used in constructing the present compartmental model of the dynamics of /3-carotene metabolism. 

Cherel, Y., Charrassin, J.-B. Handrich, Y. (1993). Comparison of body reserve buildup in prefasting chicks and adults of king penguins (Aptenodytes patagonicus). Physiol Zoo/., 66, 750-70. 

V.B12 occurs predominantly in animal tissues and animal products. It is synthesized principally by bacteria. Green plants contain little or no V.B,2. Deficiency symptoms are sometimes observed in strict vegetarians, most often in breast-fed infants whose mothers consume no animal products. The body reserves of cobalamin are usually so large that an adult can survive for many years on them in the absence of a dietary intake. 

Iron-deficiency anaemia is not common in lambs and calves because in practice it is unusual to restrict them to a milk diet without supplementary feeding. It does, however, sometimes occur in laying hens, since egg production represents a considerable drain on the body reserves. 

When ruminants are confined to cobalt-deficient pastures it may be several months before any manifestations of pine occur because of body reserves of vitamin... 

This replacement in the mixture of carbohydrate by fat is accompanied by a decline in the non-protein respiratory quotient, and when the theoretical value for fat (0.7) is reached it can be assumed that energy is being obtained only from body reserves. In ruminants, an additional indication that the post-absorptive state has been reached is a decline in methane production (and therefore digestive activity) to a very low level. 

For mammals, net requirements for the growth of the foetus increase exponentially during gestation and are quantitatively significant in the last one-third of this period. Nutrients are also required for the growth of the foetal membranes, uterus, mammary glands and (in some cases) the body reserves of the mother. 

Despite the negative balances that occurred over considerable periods early in lactation, there was a net positive balance over the lactation and dry period as a whole. It has therefore become normal practice to consider the complete lactation in assessing calcium and phosphorus requirements early negative balances are regarded as normal, since no ill effects are evident as long as subsequent replenishment of body reserves takes place, and daily requirements are formulated on the basis of total production over the lactation. However, although the lactation approach is... 

Ewes that have been severely undernourished in pregnancy show a more rapid decline in milk production during the subsequent lactation than adequately nourished animals. This accords with independent observations of reduced metabolic capacity in ewes severely undernourished during pregnancy. Where restriction is less severe and ewes come to parturition in lean condition, they have been shown to milk as well when adequately nourished as do ewes with better condition scores at lambing. Ewes will not maintain high milk production at the expense of body reserves, and even relatively small restrictions of intake depress milk production. Severe restriction of nutrient intake (to provide maintenance only) may reduce milk production by as much as 50 per cent in 2-3 days. If restriction is continued beyond the time when peak yield is normally achieved, then recovery of yield may not be accomplished even if subsequent intake is raised. 

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