Carbohydrates respiratory quotient

There are no convincing experimental data, based on respiratory quotients, which support the hypothesis that fatty acids are convertible to sugar. Contrasted with the marked alterations which have been observed when carbohydrate is being changed to fat, the decreases in R. Q. below the level of 0.707 for fat, which should be observed if fat were changing to carbohydrate, are very small and can be partially if not entirely explained by alterations in the R. Q. of the metabolized protein and fat, due to their incomplete, oxidation. 

Work by Voit and his associates continued so that by 1900 standard values for heats of combustion of different foods had emerged (Table 1). Respiratory quotients (RQ) were also derived, associated with the utilization of the different foods. The RQ is the molar ratio of the amount of carbon dioxide produced in the oxidation of a substance to the amount of oxygen needed for that oxidation. For carbohydrate the RQ is 1 ... 

The respiratory quotient (RQ) (of evolution of carbon dioxide to uptake of oxygen) has been found to be near 1.0 for plant tissues and for metabolized carbohydrates. Fats evolve less carbon dioxide and have an RQ... 

Amitrole had a drastic effect on the fixation of 14C02 by illuminated chloroplasts of Chlorella pyrenaidom, lowering the incorporation into sucrose by 95% at a concentration of 500 mg. per liter it did pot affect phosphorylated compounds.145 Carbohydrates hydrolyzable by acids were higher (51%) in treated, chlorotic corn leaves than in controls (32%), and this result was interpreted as due to increased metabolism of proteins and fats. The respiratory quotient of 0.8 to 0.88, compared to controls near 1.0, supported this interpretation.145... 

Studies of the respiratory quotient or respiratory exchange ratio (RER) indicate a metabolic shift in CR rats depending on food availability, while there is little change in AL rats [21,42]. Figure 7 illustrates daily variations of RER with dietary regimen. Analysis of the food composition indicates an expected average value of RER of 0.89. RER in AL rats is 0.89 0.02 over a 24-hour period and is relatively more constant than that in CR rats, indicating that AL rats metabolize a constant ratio of carbohydrate and lipid over 24 hours [21, 42]. In contrast, CR animals metabolize more carbohydrate... 

It is not yet clear which estimates of the ratio between the levels of protein and of carbohydrate metabolism during hypoxia should be regarded as reliable. It seems likely that the increase in respiratory quotient in freshwater fish to values of 2.5-2.8, as found by Mohamed and Kutty (1983a, 1986), indicates a predominance of protein expenditure over that of carbohydrate. A hypoxic environment shifts the acid-base balance of the fish towards acidosis (Kotsar, 1976), thereby inducing the redistribution of electrolytes, alteration of ion exchange and the activity of Na+-K+-Mg2+-ATPases and alkaline phosphatases. It also leads to an increased level of C02 in the blood, which enhances the bicarbonate buffer system (Kotsar, 1976). In section 2.1, we... 

Insulin is probably the most important inhibitor of lipolysis. In contrast to adults, in whom catecholamines represent the most important stimulators of lipolysis, thyrotropin (TSH) is the most important stimulator of lipolysis in the newborn. Plasma free fatty acid concentrations rise markedly in the first hours after birth in response to a marked increase in the TSH concentration and a fall in the insulin concentration. The fatty acids released from lipid stores are oxidized by some extrahepatic tissues (e.g., heart and skeletal muscle, kidney, intestine, and lung). Because the respiratory quotient (the ratio of carbon dioxide production to oxygen use) falls from a value of 1.0 (showing that carbohydrate oxidation is the primary source of energy) to a value of 0.8 to 0.9 (showing increasing oxidation of protein or fatty acids) at 2 to 12 hours of age, at a time when protein catabolism is usually insignificant, fatty acid oxidation must represent... 

Figure 2 - Utilization of carbohydrate and lipid by skeletal muscle during starvation and exercise. Respiratory quotient (RQ) is the ratio of CO2 producea/02 consumed. Data from Owen and Reichard (6), Felig and Wahren (11) and Wahren (12).

Indirect calorimetry provides the ability to measure the relative contribution of macronutrients toward energy use. The measurements of expired carbon dioxide and consumed oxygen are used to calculate respiratory quotient (RQ). An RQ of 1.0 indicates use of carbohydrate solely, an RQ of 0.7 indicates use of fat solely, whereas an RQ of 0.85 indicates mixed use of macronutrients. Data in rats demonstrates a significant increase in using fat as an energy substrate following DAG oil infusion (gastric) as observed by a decreased respiratory quotient value between 3-5 hours post-infusion (15). 

Since 1929, various workers have investigated the value of sorbitol as a substitute carbohydrate in diabetes with findings which were polemic. Typical examples of the results of various European clinicians are cited as follows. Reinwein administered sorbitol to diabetics and observed an increase in the respiratory quotient without a concomitant hyperglycemia. Contrariwise, Roche and Raybaud observed no increase in the respiratory quotient under the same experimental conditions and Donhoffer and Donhoffer observed a rise in blood sugar upon the administration of sorbitol to diabetics. Kaufmann reported that sorbitol... 

In addition to perfusion experiments, tests with tissue slices have been cited to prove the conversion of fat to carbohydrate. Here, both changes in composition of the slices and their R. Q. have been determined and certain deductions drawn from these values. When these slices are prepared from well-fed animals, the levels of R. Q. are usually from 0.75 to 0.85, which can hardly support the hypothesis that a conversion of fat to sugar is occurring. However, when the liver slices are obtained from rats in which the carbohydrate stores are depleted, either by fasting or by a previous high-fat diet, figures for the respiratory quotient below 0.70 have been reported - while Dickens and Simer found values below 0.70 only when the tests were carried out with phosphate or bi-... 

In patients fed an equal amount (on a molar basis) of carbohydrates and fats, the respiratory quotient is... 

Indirect calorimetry, a technique that measures Oj consumption and COj production, can be used when more accurate determinations are required for hospitalized patients. A portable indirect calorimeter is used to measure oxygen consumption and the respiratory quotient (RQ), which is the ratio of Oj consumed to COj produced. The RQ is 1.00 for individuals oxidizing carbohydrates, 0.83 for protein, and 0.71 for fat. From these values, the daily energy expenditure (DEE) can be determined. 

The quantitative analysis of oxidation of fuels shows that when most carbohydrates are oxidized, the ratio of carbon dioxide produced to oxygen consumed is 1. On the other hand, this ratio, which is called the respiratory quotient (RQ) or the respiratory exchange ratio, is -0.7 when fatty acids are oxidized. The measurements of O2 consumption and CO2 emission only indicate which fuels are being oxidized on average by the whole body and do not necessarily reflect which fuel is being used selectively by particular cell types or tissues. As noted in Chap. 11, the synthesis of triglycerides raises the RQ above 1. 

Animals do not normally obtain energy exclusively from either carbohydrate or fat. They oxidise a mixture of these (and protein). Consequently, in order to apply the appropriate thermal equivalent, it is necessary to know how much of the oxygen is used for oxidation of each nutrient. The proportions are calculated from what is known as the respiratory quotient (RQ).This is the ratio between the volume of carbon... 

In some situations, discussed in more detail later, heat production has to be estimated from oxygen consumption alone. If a respiratory quotient of 0.82 and a thermal equivalent of 20.0 kJ/1 are assumed, then departures from this RQ in the range of 0.7-1.0 cause a maximum bias of no more than 3.5 per cent in the estimate of heat production. A further simplification is possible in respect of protein metabolism. The thermal equivalent of oxygen used for protein oxidation is 18.8 kJ/1, not very different from the value of 20.0 assumed for carbohydrate and fat oxidation. 

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