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Plants as a Source of Vitamin D

 

Vitamin D is essential for maintaining healthy bones and teeth. It also has several other important functions in the body such as regulating the absorption of calcium and phosphorus and facilitating normal immune function. According to a study, vitamin D deficiency (serum 25-hydroxyvitamin D [25(OH)D] < 50 nmol/L or 20 ng/ml) is associated with fractures and bone loss. Severe vitamin D deficiency with a 25(OH)D concentration below <30 nmol/L (or 12 ng/ml) increases the risk of excess mortality, infections, and many other diseases. Poor vitamin D status is a global health problem. Approximately one billion people worldwide suffer from vitamin D insufficiency because most foods including plants contain little vitamin D.

Sources of vitamin D

Fish have the highest natural content of vitamin D (salmon contains 30 μg/100 g and tuna 2.9 μg/100 g). Other sources of vitamin D3 are meat (~0.6 μg/100), egg (~1.75 μg/100) and milk products (~0.1 μg/100). Vitamin D3 has also been identified in several plant species. Vitamin D2 and D3 are the two major forms of vitamin D. Microalgae contain vitamin D3 and provitamin D3. Small amounts of vitamin D can also be found in plants contaminated with fungi. In plants vitamin D2 is formed by UVB exposure of ergosterol and vitamin D3 by UVB exposure of 7-dehydrocholesterol.

Vitamin D in plants

Traditionally, only animal products have been considered a source of vitamin D3, currently it has been reported that fruits and vegetables have the potential to serve as a source of vitamin D. A recent study has shown that biofortified tomatoes can provide a new route to vitamin D sufficiency by editing genome, and modifying a duplicated section of phytosterol biosynthesis. Tomatoes synthesize 7-DHC in their leaves, when 7-DHC gets UVB exposure it produces vitamin D3. It has also been reported that, although 7-DHC is present in tomato leaves, but it does not normally accumulate in fruit. It serves as an intermediate in the formation of tomatines in green fruit and esculeosides in ripe fruit. A specific isoform of 7-dehydrocholesterol reductase (Sl7-DR2) converts 7-DHC to cholesterol for the synthesis of α-tomatine in leaves and fruit. Furthermore, the close association between cholesterol/SGA biosynthesis, 7-DHC accumulation and photosynthesis in leaves and green fruit of tomato suggests that knockouts of 7-DR2 activity in pepper, where fruit may be green when eaten, might also provide a vitamin D3-biofortified, plant-based food. It is also found that, editing of Sl7-DR2 could generate similar alterations in any elite tomato variety, meaning that tomato could be developed as a plant-based, sustainable source of vitamin D3.

Synthesis of Vitamin D in plants

Vitamin D biosynthesis is taking place along the normal sterol pathway. Vitamin D2 is formed by UVB exposure of ergosterol and vitamin D3 by UVB exposure of 7-dehydrocholesterol. Sterol biosynthesis can be divided into two parts. The first part is the mevalonic acid pathway. All isoprenoid compounds, including sterols, are formed via the mevalonic acid pathway from the common C5 isoprene building blocks isopentyl diphosphate (IPP) and its isomer dimethylallyl diphosphate (DMAPP). One molecule DMAPP and two molecules IPP is assembled to farnesyl pyrophosphate (FPP). Finally, two molecules FPP are combined to make squalene. Cyclization of squalene is via the intermediate 2,3-oxidosqualene, that forms either lanosterol or cycloartenol via a series of enzymatic cyclizations. The enzymes involved in 24-demethylsterol and 24-ethylsterol synthesis have been identified in the model plant Arabidopsis thaliana. Experiments with biosynthetic mutants and transgenic plants indicate that the enzymes regulating 24-demethylsterols and 24-ethylsterols also are involved in the regulation of 24-desmethylsterols.

The recommended daily amount of vitamin D is 400 international units (IU) for children up to age 12 months, 600 IU for people ages 1 to 70 years, and 800 IU for people over 70 years.


References

Amrein, K., Scherkl, M., Hoffmann, M. et al. Vitamin D deficiency 2.0: an update on the current status worldwide. Eur J Clin Nutr 74, 1498–1513 (2020). https://doi.org/10.1038/s41430-020-0558-y

Jäpelt RB, Jakobsen J. Vitamin D in plants: a review of occurrence, analysis, and biosynthesis. Front Plant Sci. 2013;4:136. Published 2013 May 13. doi:10.3389/fpls.2013.00136

Li, J., Scarano, A., Gonzalez, N.M. et al. Biofortified tomatoes provide a new route to vitamin D sufficiency. Nat. Plants (2022). https://doi.org/10.1038/s41477-022-01154-6


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