Dynamics of sterols and fatty acids during UV-B treatment of oyster mushroom

Highlights

• UV-B treated oyster mushrooms accumulated >100 μg of vitamin D₂ g⁻¹ dry matter.

• Photoproducts lumisterol₂, tachysterol₂ and previtamin D₂ increased concurrently.

• Likewise, vitamin D₄ accumulated instantly and reached saturation after 1 h.

• Illumination did not significantly alter the concentrations of fatty acids.

• UV-B pretreated oyster mushrooms are a rich source of vitamins D.

Abstract

Fruiting bodies of the oyster mushroom Pleurotus ostreatus were illuminated with UV-B with a light intensity maximum at 310–320 nm and 11.5 W/m² for 60 min at 20 °C. Changes of the sterol and fatty acid spectrum were quantified. The onset of ergocalciferol (vitamin D₂) formation was immediate in fruiting bodies illuminated from the lamella side, in sliced fruiting bodies, and in the stipes. Saturation concentrations above 100 μg/g of dry matter were reached after 1 h. At the same time, the concentrations of the photo-isomers lumisterol₂, tachysterol₂ and previtamin D₂ increased in this order. 22-Dihydroergocalciferol (vitamin D₄), showed the same course of increase and reached a maximum concentration of around 20 μg/g dry matter. With the exception of linoleic acid in cut fruiting bodies, fatty acid concentrations remained almost constant. One serving of UV-B pretreated sliced oyster mushroom covered the weekly demand of vitamin D of an adult.

Introduction

Calcitriol (1,25-dihydroxycholecalciferol), the bioactive form of vitamin D₃ formed upon UV-B illumination from 7-dehydrocholesterol, is the major player in Ca²⁺ human homeostasis. Apart from preventing osteomalacia and rickets, the diseases of a severe vitamin D deficiency, the interaction of calcitriol with specific receptors results in an involvement in autoimmune diseases and in slow hormone actions. More recently, a number of studies have demonstrated anti-cancer activity (Chiang & Chen, 2013). Other unexpected links to physiological disorders have been reported (Gominak & Stumpf, 2012). Because of its hormone-like efficiency, the fortification of food by calcitriol is strictly regulated in most countries to avoid hypercalcaemia. There are conflicting opinions on the sufficient daily intake of vitamin D₃. While in Europe and the US doses of 5–20 μg (200–800 IU) per day are commonly recommended, a daily dose of 100 μg was regarded safe and defined as tolerable upper intake level per day by Health Canada (http://www.hc-sc.gc.ca/fn-an/nutrition/vitamin/vita-d-eng.php). For the treatment of rickets a single dose of 7500–12,500 μg is used.

There is an increasing body of evidence that populations residing in countries with a lower exposure to sunlight, especially during the cold season, are latently vitamin D deficient, even if a balanced Western diet is consumed. Elderly or otherwise immobilized persons bear an increased risk, as do individuals with higher skin pigmentation, the obese, and individuals with an excessive use of sunscreens. The situation is worsened for vegetarians and vegans, because the occurrence of the natural metabolic precursor of vitamin D₃, 7-dehydrocholesterol, is limited to animals (and lichens).

Edible mushrooms are the only non-animal source of vitamin D. The concentrations are usually low (Simon, Borzelleca, De Luca, & Weaver, 2013), but may be enhanced by illumination with UV-B (Fig. 1). In full analogy to the ring opening reaction of 7-dehydrocholesterol, ergosterol (ergosta-5,7,22-trien-3-ol), a constituent of yeast and fungal cell membranes, is transformed to ergocalciferol (vitamin D₂). 22-Dihydroergosterol opens to 22-dihydroergocalciferol (vitamin D₄) (Fig. 1).

Early observations of the reaction (Kiribuchi, 1992, Mau et al., 1998) resulted in patenting (Takamura, Hoshino, Toyomasu, Tanaka, & Nakasawa, 2000). Stimulated by the revision of the physiological roles of vitamins D, a number of studies appeared more recently, extending the concept to various mushroom species and illumination regimes (Beyer et al., 2008, Jasinghe and Perera, 2006, Jasinghe et al., 2006, Kidder et al., 2008, Ko et al., 2008, Kristensen et al., 2012, Teichmann et al., 2007, Williams, 2009). The initial 4–5 mg ergosterol per g dry matter of a common white button mushroom were converted, depending on conditions used, from less than 2 μg to more than 20 μg vitamin D₂ per g of dry matter. Feeding studies confirmed the bioavailability of vitamin D₂ in animals (Calvo et al., 2013, Jasinghe et al., 2006), and it was shown to be as effective as vitamin D₃ in humans (Ferreira et al., 2008). A recent study concluded that “Vitamin D₂ from UVB-exposed mushrooms is bioavailable, safe, and functional in supporting bone growth and mineralisation in a growing rat model without evidence of toxicity” (Calvo et al., 2013). Recent research focused on button mushroom (Agaricus bisporus), the commercially most important species, but intensity, duration and type of light varied over a broad range, as did the other experimental parameters. Chemical changes were measured at seemingly arbitrarily chosen points of time.

The present study followed the formation of vitamin D₂ and D₄ in oyster mushrooms induced by effective doses of UV-B light over a period of 1 h. Concurrent formation of non-vitamin sterols as side-products of the photoreaction and the change of concentrations of saturated and unsaturated fatty acids were assessed to determine the side-effects of UV-B light.