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Nutritive influences on hair colour in dogs - A survey of literature



Pigmentation depends on the presence and relation of reddish phaeomelanin and black eumelanin, and the density and distribution of the melanins in the hair cortex and/or the hair medulla as well as along the hair shaft. However, discoloration of the coat seems to be a problem of practical importance in canines. This includes general fading of hair colour but also yellowish or reddish hairs in white or black individuals. The article gives an overview on important nutritional influences on hair colour, that may also affect hair structure. In this connection, specific amino acids (above all tyrosine, phenylalanine), trace elements (above all copper, zinc), and vitamins (above all B complex, B2, H) are of particular significance for the appearance of the coat.
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The influence of sunlight on cosmetically pretreated human hair was systematically investigated. Thus, bleached, permed, and dyed hairs were irradiated for four weeks with simulated sunlight, which was split into the specific ranges UV-B, UV-A, visible light (VIS), and IR. Subsequent color measurements, strength tests, and cholesterol determinations of untreated and irradiated samples showed characteristic degradation patterns in the melanin, protein, and lipid components of human hair. Comparison of the results with irradiated, untreated brown hair point to a synergistic effect between cosmetic treatments and sunlight. In particular, chemical bleaching increases a photochemically initiated brightening (VIS), loss in strength (UV-A), and degradation of cholesterol (VIS).
In the course of studies of pantothenic acid deficiency in dogs, it was found that pups survived depletion approximately 1 month, whereas adults could survive 6 months or longer1 (Seeler and Silber, unpub lished). Schaefer, McKibbin, and Elvehjem ('42) reported that grow ing pups have a greater requirement for pantothenic acid than adult dogs, and Unna and Richards ('42) observed a decrease in requirement with increase in age in rats. In the above dog studies it was impossible to ascertain the degree of pantothenic acid deficiency prior to the stage of collapse. Further studies therefore seemed indicated, with attempts to correlate excretion and tissue levels of pantothenic acid with other signs which might permit a more accurate determination of the onset of the deficiency.
Synopsis This study uses field emission scanning electron microscopy (FESEM) to monitor the effects of UV irra-diation on the physical nature of hair fibers. Long-term UV irradiation/humidification cycling causes thinning and fusion of the surface cuticle cell, as well as fusion of the cuticular sheath into a solid, rigid, and brittle unit. While intercellular cohesion within the cuticular sheath is high, possibly due to crosslink-ing of the proteins in the intra-and intercellular domains, the cells themselves are brittle. A newly observed fracture pattern of long-term UV-exposed fibers suggests fusion of the regions attacked most severely by UV light into one rigid and brittle mass, incapable of extension due to loss of all original elastic properties. Unlike chemical oxidation, which results in partial dissolution (1 h H202) and then complete solubilization (4 h H202) of the melanin granules, photochemical oxidation produces entirely different results. Even after long-term UV irradiation/humidification (95% RH) cycling, the melanin granules appear physically intact. Loss of color does not occur as long as the melanin granules are intact. The severity of photodegradation during UV irradiation/humidification cycling becomes apparent upon brief (seconds) contact of these fibers with alkaline hydrogen peroxide. Such contact results in instantaneous disintegration of the components within the cuticle cells. Formation of sac-like structures (AllwiSrden sacs) occurs due to osmotic pressure within seconds of exposure to alkaline hydrogen peroxide caused by pho-tochemically degraded proteins within the surface cuticle cells. The cells swell until they burst and their contents drain, leaving behind cuticular membranes, which may detach or fuse to the fiber surface. UV irradiation has also severely photodegraded the melanin granules and preconditioned them for accelerated solubilization upon contact of the fibers with alkaline hydrogen peroxide. The effects of both relative humidity and spectral energy distribution on the photochemical oxidation of the hair fiber are studied. Results obtained at various relative humidities in two different fading units, namely, the QUV Accelerated Weathering Tester and the Atlas Weather-Ometer © ("AW") are compared. Scale thinning and fusion observed during UV/humidification cycling are greatly reduced with exposure at low humidities without humidification cycles. Upon post-treatment with water, fibers irradiated at a constant 10% RH in the QUV show scale thinning and fusion similar to that of fibers exposed to UV/humidification cycling. This indicates that photodegradation occurs at low humidity as well. Fibers exposed at constant 20%, 50%, and 70% humidity in the "AW" show only moderate scale thinning, even after post-treatment with water. The total solar spectrum used in the "AW" apparently causes less severe photodegradation of the proteins than the UV light of the QUV.