Riboflavin, absorption above

Probably as a result of its limited solubility and limited gastric absorption, no adverse effects have been associated with ingestion of riboflavin appreciably above RDA levels. One study reported no short-term side effects in 49 patients treated with 400mg/day of riboflavin with meals for at least 3 months. Because of a lack of data for risk assessment, no tolerable upper intake level has been proposed for riboflavin. ... 

The increased absorption above 500 m observed for riboflavin in the presence of a number of transition metals has been confirmed and extended by Radda and Calvin who also noted a similar effect with sodium and magnesium ions. This is likely the result of a perturbation of molecular-orbital energies by the electrostatic effects of a coordinated metal ion, rather than a charge-transfer phenomenon. The shift of the long-wavelength absorption maximum of riboflavin from 447 mjx to 430 mp in the presence of ferrous ion in a pyrophosphate, but not a phosphate or maleate buffer , is unexplained. 

The amormt of absorbed riboflavin that can remain within the body and the circulation (in blood plasma) is strictly regulated by glomerular and tubular filtration and tubular reabsorption in the kidneys. The latter is an active, saturable, sodium-dependent transport process, with characteristics similar to those of active transport in the gastrointestinal tract. It is responsible for the very sharp and characteristic transition between minimal urinary excretion of riboflavin at low intakes, and a much higher level of excretion, proportional to intake, at higher intakes. This transition point has been extensively used to define and to measure riboflavin status and requirements (see below), and to permit studies of intestinal absorption in vivo (see above). Excretion of riboflavin is affected by some chemicals (such as boric acid, which complexes with it), and by certain diseases and hormone imbalances. 

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