Fasting glucose Fatty acid

In pregnancy, there is a sharp increase in the plasma fatty acid level after about 12 hours of fasting, much sooner than in the non-pregnant woman. This may be important in maintaining the plasma glucose level not only for the mother but also the foetus. This maintenance is achieved via the glucose fatty acid cycle (Chapter 16). 

Table 7.5 Arteriovenous differences of oxygen, glucose, fatty acids, acetoacetate and hydroxybutyrate across the brain of obese females during prolonged fasting...

The term "caloric homeostasis" was coined by Fredickson and Gordon (1958) to express the joint and complementary roles which free fatty acids and glucose play in the blood in supplying the respiration fuel. The interconversion of carbohydrate and fat has been described by Randle et al. (1963) as the glucose-fatty acid cycle (see Figure 2) Reactions 1 and 2 occurs when there is a surplus of carbohydrate (or of ingested fat). Reaction 3 occurs on fasting. The plasma non-esterified fatty acid... 

Muscle Fast twitch Slow twitch Rapid movement Sustained movement Glycolysis Aerobic pathways, eg, p-oxidation and citric acid cycle Glucose Ketone bodies, triacylglycerol in VLDL and chylomicrons, free fatty acids Lactate Lipoprotein lipase. Respiratory chain well developed. 

As we have seen, normally pyruvate would be the substrate for pyruvate dehydrogenase complex to form acetyl-CoA, but during fasting in the absence of glucose, acetyl -CoA for the TCA cycle is derived from fatty acid (3-oxidation (see Section 7.5.2) so pyruvate is diverted into oxaloacetate by the enzyme pyruvate carboxylase. Thus any amino acids whose carbon skeletons can be converted into pyruvate, OAA or another substrate of the TCA cycle, can be used for glucose synthesis. 

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