Dietary protein fasting

Food can affect both the absorption and the clearance of theophylline. One study showed that the absorption of a modified-release formulation of theophylline was very slow after an overnight fast, in contrast to absorption after a test meal (86). The effect may be dose-related. More specifically, dietary protein significantly affects theophylline clearance a low-protein diet reduces theophylline clearance and a high-protein diet increases it. The implications for clinical practice have not been elaborated, but dietary extremes are contraindicated in patients taking theophylline (51). 

Boirie Y, Dangin M, Gachon P, Vasson MP, Maubois JL, Beaufrere B. Slow and fast dietary proteins differently modulate postprandial protein accretion. Proc Natl Acad Sci. 1997 94 14930-5. 

An adult has a requirement for a dietary intake of protein because there is continual oxidation of amino acids as a source of metabolic fuel and for gluconeogenesis in the fasting state. In the fed state, amino acids in excess of immediate requirements for protein synthesis are oxidized. Overall, for an adult in nitrogen balance, the total amount of amino acids being metabolized will be equal to the total intake of amino acids in dietary proteins. 

Dietary proteins are a source of amino acids which can serve as important precursors for gluconeogenesis. During a fast or starvation, a major contribution is made by alanine which is released along with other amino acids from skeletal muscle. Since labile proteins rich in alanine are not present in muscle, the released alanine appears to result from the activity of alanine aminotransferase (Section 16.1) which produces alanine from pyruvate. This is the basis of the alanine cycle which also operates between skeletal muscle and the liver. The alanine cycle functions only when peripheral tissues reoxidize glycolytic NADH through the oxidative phosphorylation pathway. In the presence of oxygen, pyruvate is not utilized in lactate production and is available for the amino transfer reaction. 

While zinc Is generally consumed with or In food, the tracer was given after an overnight fast and food was not allowed for at least five hours after administration. Administration of the tracer with food would have Introduced at least one and probably several new uncontrollable variables Into the study. Sandstrom et al.(37,38) have shown that Zn-65 absorption can be variably Increased or decreased by such dietary factors as white versus whole grain bread and levels of zinc, calcium and protein. 

Chylomicrons, large triglyceride-rich particles containing apolipoprotein B-48, B-lOO, and E, are formed from dietary fat solubilized by bile salts in intestinal mucosal cells (Fig. 21-2). Chylomicrons normally are not present in the plasma after a fast of 12 to 14 hours and are catabolized by lipoprotein lipase (LPL), which is activated by apolipoprotein C-II, in the vascular endothelium and hepatic lipase to form chylomicron remnants. The remnants that contain apolipoprotein E (see Fig. 21-2) are taken up by the remnant receptor, which may be an LDL-receptor-related protein, in the liver. Free cholesterol is liberated intracellularly after attachment to the remnant receptor. Chylomicrons also function to deliver dietary triglyceride to skeletal muscle and adipose tissue. During the catabolism of nascent chylomicrons to remnants, triglyceride is converted to free fatty acids and apolipoproteins A-I, A-II, A-IV (free in plasma), C-I, C-II, and... 

In the fasting state, 9 L of fluid enters the proximal small intestine each day. Of this fluid, 2 L are ingested through diet, while the remainder consists of internal secretions. Because of meal content, duodenal chyme is usually hypertonic. When chyme reaches the ileum, the osmolality adjusts to that of plasma, with most dietary fat, carbohydrate, and protein being absorbed. The volume of ileal chyme decreases to about 1 L/day upon entering the colon, which is further reduced by colonic absorption to 100 mL daily. If the small intestine water absorption capacity is exceeded, chyme overloads the colon, resulting in diarrhea. In humans, the colon absorptive capacity is about 5 L daily. Colonic fluid transport is critical to water and electrolyte balance. 

Liver metabolizes up to 85% of orally administered D-fructose. Fructokinase, aldolase, and triose kinase are important control enzymes for hepatic D-fructose metabolism.62 The activities of these enzymes are subject to dietary composition and hormonal control. Total fructokinase, aldolase, and triose kinase decrease to about half or less of their normal activity on fasting for 48 to 72 hours, and are restored to normal in 24 hours upon D-fructose administration. Longterm feeding of D-fructose results in the maintenance of a considerably higher level of all three enzymes, an effect also seen with rats maintained on a high-fat or high-protein diet.62... 

Sidransky, H., Sarma, D. S. R., Bongiorno, M., and Verney, E., Effect of dietary tryptophan on hepatic polyribosomes and protein synthesis in fasted mice, /. Biol. Chem., 24, 1123, 1968. 

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