plant sources vitamin d precursers
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4981617/
Introduction
Food sources of vitamin D are scarce. Although oily fish is considered to be a good source of vitamin D3 (1, 2), its consumption and its vitamin D content is not high enough to significantly improve the vitamin D status of humans (3). Besides fish, mushrooms are often considered as another valuable source of vitamin D, in particular of vitamin D2. However, the major natural vitamin D metabolite in fungi and yeast is the vitamin D precursor ergosterol, which can be converted to vitamin D2 by UVB irradiation (4). The UVB-exposed baker’s yeast, which has been approved by the European Food Safety Authority as a reliable ingredient to enrich bakery products with vitamin D, is a prominent example for a successful application of UVB irradiation to enhance vitamin D in natural foods (5). However, less data are available on vitamin D precursors and metabolites in plants. Yellow oat grass (Trisetum flavescens) is well described for its capability to synthesize bioactive vitamin D. It contains vitamin D glycosides which can be hydrolyzed in the gut or by the gastrointestinal microflora to the biologically active 1,25-dihydroxyvitamin D (6–8). Other so-called calcinogenic plants that contain active vitamin D forms are Solanum malacoxylon, Cestrum diurnum, and Nierembergia veitchii of the Solanaceae family (6–8). These plants are presumed to cause calcinosis in grazing animals due to the hypercalcemic effect of toxic 1,25-dihydroxyvitamin D levels (9). Vitamin D metabolites were also found in Cucurbitaceae, Fabaceae, and Poaceae (10–12). Besides that, certain plants are associated with fungal endophytes (13, 14) or are capable to produce the vitamin D3precursor 7-dehydrocholesterol (7-DHC) on its own via the lanosterol pathway (15). Based on these data, we hypothesized that plant oils could also contain vitamin D precursors or metabolites. The main aims of this investigation were [1] to identify and quantify precursors and metabolites of vitamin D in plant oils that are used in human nutrition and [2] to investigate whether a short-term exposure of selected oils to UVB light could increase their vitamin D content. To elucidate possible adverse effects of UVB exposure on the quality of the oils, we analyzed oxidative biomarkers and tested the sensory quality of the UVB-exposed oils. Additional tests were conducted to assess the stability of these vitamin D metabolites subsequent to thermal treatment and storage of the UVB-exposed oil. Finally, we aimed to elucidate the efficacy of plant-derived vitamin D to improve the vitamin D status by feeding an UVB-exposed plant oil to mice......................................
ng/g; 25-hydroxyvitamin D3, 2.1 ng/g).
Discussion
The presented studies demonstrated that plant oils contain high amounts of ergosterol, but comparatively low amounts of 7-DHC. It was striking that the ergosterol concentrations in the plant oils were on average 100 times higher than the 7-DHC concentrations. It is assumed that plants are per se not capable of producing ergosterol or vitamin D2 (28), and that any of these metabolites are synthesized by endophytic fungi or by superficial fungal infections (13, 14, 29). Regarding 7-DHC, the analyses revealed 10 times higher concentration of this cholesterol precursor in the WGO than in the other oils. 7-DHC is an intermediate of the cholesterol synthesis pathway. It is well described that plants from the Solanaceae, Fabaceae, and Poacaea families are capable of producing cholesterol (30, 31), which is assumed to be used for the synthesis of glycoalkaloids and ecdysteroids (32, 33). The 7-DHC has also been proposed to function as an UV light protector (34), because the 7-DHC absorbs UVB irradiation that would otherwise damage the ribonucleic acids. The detectable amounts of 7-DHC in the linseed, rapeseed, and pumpkinseed oil suggest that cholesterol is also synthesized in plants from the Linaceae, Brassicaceae, and Cucurbitaceae families. However, in contrast to other researchers, who measured vitamin D in certain parts of the plant (12, 31, 35–37), we were not able to detect vitamin D in untreated plant oils.
The detection of vitamin D precursors in the plant oils prompted us to speculate that exposure of oils to UVB irradiation could convert ergosterol and 7-DHC into vitamin D2and vitamin D3, respectively. Among the analyzed plant oils, the highest levels of vitamin D2 and vitamin D3 in response to an UVB irradiation were found in the WGO. After an 8-min exposure of thin-layered WGO, 1 g of this oil contained 1.5 μg vitamin D2 and 0.08 μg vitamin D3. We further found that the conversion rate of vitamin D precursors to vitamin D in the WGO was reduced by 40% if the oil layer thickness was increased from 1.0 to 3.2 mm. One gram of this thick-layered WGO provided in total a vitamin D content of 885 ng. With an average consumption of 12 g oil/day (38), a total of 10.6 μg vitamin D could be supplied by intake of UVB-exposed WGO, which matches 50% of the recommended daily vitamin D intake (1).
An interesting finding of this study was that the vitamin D content in the oils increased with the time of storage and a moderate thermal treatment. It is well described that the UVB photon converts the precursors, 7-DHC and ergosterol, to pre-vitamin D which in turn isomerizes to vitamin D by a thermal reaction (34, 39). Therefore, we assume that the preformed pre-vitamin D can convert to vitamin D in conditions with absent UVB irradiation. Our data further indicate that taste and aroma, and also biomarkers that are indicative of autoxidation such as the tocopherol concentration, peroxides, and free acids were not significantly influenced by a short-term exposure of the plant oils to UVB irradiation. This makes the short-term UVB treatment of plant oils to a safe and reliable technique to produce vitamin D supplements.
To evaluate the efficiency of UVB-exposed plant oils to improve the vitamin D status in vivo, we conducted a study with mice that were fed diets with either UVB-exposed WGO, untreated WGO, or WGO with supplemented vitamin D3. Here, we found that the UVB-exposed WGO is suitable to improve the vitamin D status of the mice as the group fed the UVB-exposed oil developed higher 25(OH)D plasma levels than the group fed the untreated oil. Compared with the group fed the vitamin D3-supplemented WGO, the UVB-exposed oil was less effective in increasing the 25(OH)D plasma concentrations. However, it should be noted that the livers of mice that received the UVB-exposed WGO stored huge amounts of vitamin D2 in comparison to that of mice fed the vitamin D3 supplemented oil. The increased storage of hepatic vitamin D2 in combination with the reduced plasma concentration of 25(OH)D2 in the group fed the UVB-exposed oil suggests that vitamin D2 is less appropriate as a substrate for hepatic hydroxylation than vitamin D3. It has been a debate for many years whether both forms of vitamin D are bioequivalent. A series of studies has shown that vitamin D2 does not increase 25(OH)D serum concentrations to the same amount as vitamin D3 does (40–42). The current data confirm the different efficacy of both vitamin D isoforms. However, we cannot exclude at this stage, that photo-isomers that are produced by the UVB treatment may also impact the bioavailability of the vitamin D form in UVB-exposed oil.
To conclude, plant oils that are commonly used in human nutrition contain considerable quantities of ergosterol, but small amounts of 7-DHC. Among the different analyzed oils, WGO has the highest amounts of vitamin D precursors. A short-term UVB irradiation was successful in increasing the vitamin D content of the selected oils. The in vivo study has shown that UVB-exposed WGO can improve the vitamin D status, although less effective than vitamin D3.
Author Contributions
CB, BK, and GS conceived and designed the experiment. AB performed the experiment. AB and FH analyzed the data. AB, CB, and GS wrote the manuscript. BK and FH critically reviewed the manuscript
