Absorption, dietary riboflavin

Akiyaroa, T,Selhub, J., and Rosenberg, 1. H. (1982). FMN phosphatase and FAD pyrophosphates in rat inte.stinal bru.sh borders Role in intestinal absorption of dietary riboflavin. /. Nutf. 112, 263-268,... 

Dietary riboflavin is present mostly as a phosphate, which is rapidly hydrolyzed before absorption in the duodenum.In humans, the rapid, saturable absorption of riboflavin following an oral dose suggests that it is transported by a carrier-mediated pathway located predominantly in duodenal enterocytes. The process may be sodium-dependent. Bile salts enhance absorption of riboflavin. Fecal riboflavin is derived from the intestinal mucosa and the intestinal flora. This is the predominant excretory route for the vitamin. 

T Akiyama, J Selhub, IH Rosenberg. FMN phosphatase and FAD pyrophosphatase in rat intestinal brush borders role in intestinal absorption of dietary riboflavin. J Nutr 112 263-268, 1982. 

In foods vitamin B2 occurs free or combined both as FAD and FMN and complexed with proteins. Riboflavin is widely distributed in foodstnffs, but there are very few rich sources. Only yeast and liver contain more than 2mg/100g. Other good sources are milk, the white of eggs, fish roe, kidney, and leafy vegetables. Since riboflavin is continuously excreted in the urine, deficiency is qnite common when dietary intake is insufficient. The symptoms of deficiency are cracked and red lips, inflammation of the lining of the month and tongue, mouth ulcers, cracks at the comer of the mouth, and sore throat. Overdose of oral intake present low toxicity, probably explained by the limited capacity of the intestinal absorption mechanism [417]. 

A minority of alcoholics develop nutrient deficiencies. In Western countries, alcoholics represcrit the largest population segment that can benefit from dietary intervention. Alcoholics are at risk for deficiencies in folate, thiamin, riboflavin, vitamin B, vitamin A, and magnesium, particularly when the intake of these substances is low. In some cases, absorption of the nutrient is impaired in others, catabolism of the nutrient is iitcreased. Thiamin deficiency is a firmly established consequence of alcoholism, as discussed in the iTiiamin section. 

The conversion of riboflavin to flavin mononucleotide (FMN) is catalyzed by flavokinase (Figure 9.73). This conversion may occur during absorption through the gut mucosa or in other organs. The subsequent conversion of FMN to flavin adenine dinucleotide (FAD) is catalyzed by FAD synthase. FAD synthase uses ATP as a source of an adenylyl group, in this conversion (McCormick et ah, 1997). Various phosphatases, including those of the gut mucosa, can catalyze the breakdown of FAD to FMN and of FMN to free riboflavin. Dietary flavins that are covalently botmd to proteins are thought to be unavailable and not to contribute to our dietary needs (Bates et ah, 1997). 

The large intestine is a part of the gastrointestinal tract with diverse microflora which synthesizes significant amounts of vitamin B2, mainly as free riboflavin. The variability of microflora in the human gut is dependent on dietary habits. It has been proven that vegetable-based diet increases the synthesis of riboflavin in the colon (linuma 1955). In vivo colonic absorption was demonstrated in rats... 

Measurements of the plasma pool of riboflavin following test doses is not a viable method of measuring absorption, because redistribution to other tissue sites plus urinary excretion takes place too rapidly for this pool to be representative of the amoimt absorbed. Although the urinary response to a test dose has been the most commonly used approach to studies of intestinal absorption in humans, it suffers from the potential disadvantage that physiological intakes, and especially low intakes of riboflavin from poor food sources, cannot be measured by this technique. Such studies of small doses are however needed, in order to determine the factors that modulate riboflavin absorption in developing countries, where dietary sources of riboflavin are minimal and clinical signs of riboflavin... 

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