Vitamin D is one of the fat-soluble vitamins structurally derived
from cholesterol. It is essential for maintaining healthy bones and teeth and
also has several other important functions in the body such as regulating the
absorption of calcium and phosphorus and facilitating normal immune
function. It exists in five active forms: 1,25-dihydroxyvitamin
D3(1,25-(OH)2D3, 24,25-dihydroxyvitamin
D3(24R,25-(OH)2D3, 1,25-dihydroxyvitamin
D2(1,25-(OH2)D), 25-hydroxyvitamin D3(25-OH-D3) and 25-hydroxyvitamin
D2(25-OH-D2). 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 has 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 D3are the two major forms of vitamin D. Vitamin D2 differs from D3 in having double bond between C22 and C23 and a methyl group at C24 in the side chain. 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
In plants vitamin D has been discovered accidentally when some
vegetables and crops like cottonseed, wheat and lettuce got exposure to mercury
lamp and later vitamin D2 was identified from the solutions of ergosterol
irradiated with UV light in-vitro. Contamination of plants with fungi which has
a high concentration of ergosterol led to the discovery of “plants contaminated
with fungi” as veritable source of vitamin D2. Plants, specially
from Solanaceae family, have been shown to contain vitamin D3 and
its hydroxylated derivatives, including 1α,25(OH)2-vitamin D3 [1α,25(OH)D3],
a pluripotent hormone in animals. These secosteroids have also been
detected in members of the other families like Cucurbitaceae, Fabaceae and
Poaceae. Solanum glaucophyllum is the species which
accumulates 1α,25(OH)D3 to the greatest extent. Synthesis of
vitamin D takes 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.
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 fruits. 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 or other plants
could be developed as a plant-based, sustainable source of vitamin D3.
Recommended daily amount of vitamin D
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
Omotosho, I. . Plant Sources of Vitamin D and Its Medicinal
Application in Sub-Sahara Africa. In: Jr., E. T. Z. , editor. Fads and Facts
about Vitamin D [Internet]. London: IntechOpen; 2019 [cited 2022 Jun 04].
Available from: https://www.intechopen.com/chapters/64362 doi:
10.5772/intechopen.81851
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