It is difficult to incorporate riboflavin into liquid products because it has poor solubility in water, hence the requirement for riboflavin-5-phosphate (FMN, also called E101 when used as colorant), a more soluble form of riboflavin. The enrichment of bread and ready-to-eat breakfast cereals contributes significantly to the dietary supply of the vitamin. Free riboflavin is naturally present in animal-sourced foods along with protein-bound FMN and FAD. Cows milk contains mainly free riboflavin, but both FMN and FAD are present at low concentrations. Fortification
Some countries require or recommend fortification of grain foods. As of 2021, 56 countries, mostly in North and South America and southeast Africa, require food fortification of wheat flour or maize (corn) flour with riboflavin or riboflavin-5-phosphate sodium. The amounts stipulated range from 1.3 to 5.75 mg/kg. An additional 16 countries have a voluntary fortification program. For example, the Indian government recommends 4.0 mg/kg for "maida" (white) and "atta" (whole wheat) flour. Absorption, metabolism, excretion
More than 90% of riboflavin in the diet is in the form of protein-bound FMN and FAD. Exposure to gastric acid in the stomach releases the coenzymes, which are subsequently enzymatically hydrolyzed in the proximal small intestine to release free riboflavin.Absorption occurs via a rapid active transport system, with some additional passive diffusion occurring at high concentrations. Bile salts facilitate uptake, so absorption is improved when the vitamin is consumed with a meal. One small clinical trial in adults reported that the maximum amount of riboflavin that can be absorbed from a single dose is 27 mg.
They are also seen in patients with end-stage kidney disease on hemodialysis, Hodgkins disease, pellagra, and sickle cell anaemia.The lines remain visible as long as protein intake is inadequate or synthesis is impaired, and they should disappear upon return to normal function.In extreme conditions, Muehrckes lines may also arise from physical changes in peripheral circulation: one case study reported appearance of the lines in a healthy subject following ascent to 8,848 meters (29,029 ft) on Mount Everest. History
Muehrckes lines were described by American physician Robert C. Muehrcke (1921–2003) in 1956. In a study published in BMJ, he examined patients with known chronic hypoalbuminemia and healthy volunteers, finding that the appearance of multiple transverse white lines was a highly specific marker for low serum albumin (no subject with the sign had SA over 2.2 g/dL), was associated with severity of the underlying condition, and disappeared upon successful treatment (corticosteroids in nephrotic syndrome) or direct infusion of HSA. See also
Mees lines – a similar appearance, except the lines are in the nail and move as the nail grows
Half and half nails
Terrys nails
List of cutaneous conditions

References
Bibliography
Fawcett, Robert S.; Sean Linford; Daniel L. Stulberg (March 15, 2004). "Nail Abnormalities: Clues to Systemic Disease". American Family Physician. 69 (6): 1417–24. PMID 15053406. James, William; Berger, Timothy; Elston, Dirk (2005). Andrews Diseases of the Skin: Clinical Dermatology. (10th ed.). Saunders. ISBN 0-7216-2921-0. Muehrcke Lines of the Fingernails at eMedicine
Muehrckes Lines of the Fingernails on WebMD