Vitamins are organic compounds essential for normal physiological and metabolic functioning of the body in small quantities. Vitamins are a large group of micronutrients, mostly derived from diets with a few exceptions. These micronutrients usually exist in complexes and thus cannot be obtained from a single dietary source.

Functionally, vitamins are involved in various metabolic processes where they work most often as coenzymes in many biochemical reactions essential for proper functioning of the organism. They catalyze organic reactions by participating in composing hormones, cells, chemicals of the nervous system, genetic material and more. They also form enzymes combined with proteins to participate in various body reactions including the development of immune system. Due to the involvement of vitamins in several metabolic processes, their deficiencies manifest in various forms.

There are fat-soluble vitamins (A, D, E, K) and water-soluble ones (B complex and C).

The main focus here is vitamin D. Its main function is maintenance and regulation of calcium and phosphorus homeostasis in the body, it is critically important for the development of a healthy skeleton and maintenance of the immune system. Vitamin D insufficiency increases the risk of osteoporosis, hypertension, autoimmune diseases, diabetes and cancer.

Vitamin D synthesis and activation

Vitamin D is categorised into five classes numbered 2–6 and is one of the fat-soluble vitamins. Main forms of are ergocalciferol (D2) and cholecalciferol (D3), which differ structurally. Its five formulas:

D2: C₂₈H₄₄O

D3: C₂₇H₄₄O

D4: C₂₈H₄₆O

D5: C₂₉H₄₈O

D6: C₂₉H₄₆O

Sources and dietary intake of vitamin D

Because the body does produce D3, vitamin D does not meet the classical definition of a vitamin. The main vitamin D in food is also D3, along with its metabolites, which are part of metabolic pathways in vertebrates. Without sufficient vitamin D humans develop a deficiency disease – osteomalacia, while excessive vitamin D consumption results in toxicity. Toxic levels can’t be obtained by a usual diet, but by excessive consumption of supplements or over-fortification of food. Vitamin D itself is biologically inactive and requires activation processes. There is no set optimal standard for vitamin D. Some studies show that D2 and D3 act equally in maintaining vitamin D status, while others state that D2 is less effective than D3.

Wild mushrooms are the only known significant source of D2 and D4. Production in fungi and yeast is caused by UVB-exposure of ergosterol (provitamin D2 – biological precursor of vitamin D2). There are high concentrations of ergosterol in fungi and consequently small amounts can be found in plants contaminated with fungi. Conversion to vitamin D2 occurs by sun-exposure of the plant material during growth. Content of vitamin D2 in the plant has been shown to increase with the level of sun exposure.

Vitamin D3 is synthesized in the skin by UVB-exposure – a photochemical conversion of provitamin D3. The conversion happens by an exposure to sunlight at 290–315 nm (UVB). Provitamin D is biologically inactive and thermodynamically unstable, so it transforms to vitamin D in a temperature-dependent manner. If there is a prolonged UVB-exposure, provitamin D3 converts to inactive form to prevent vitamin D toxicity in organism. Synthesis of vitamin D in the skin depends on season and latitude. Vitamin D from the skin diffuses into the blood, then it’s transported by vitamin D binding protein (DBP) to the liver, whereas vitamin D from the diet is absorbed in the small intestine and transported to the liver via chylomicrons and DBP.

Microalgae contain D3 and provitamin D3, D3 has also been identified in several plant species. D3 and its provitamin 7-dehydrocholesterol have been identified in the leaves of several plant species mostly belonging to Solanaceae (a family including tomato, potato, eggplant and capsicum). Differences in growth conditions, such as the intensity of the light source and length of exposure have a significant impact on the vitamin D3 content. Vitamin D3 has in most studies been identified after UVB exposure, but vitamin D3 synthesis without the action of UVB has also been reported. They compared vitamin D3 in UVB- and non-UVB-exposed plants. The content of vitamin D3 in the UVB-exposed plants was 18–64 times higher than for the non-UVB-exposed plants. D₃ and provitamin D₃ have been found mainly in the Solanaceae family, but research focussed on the leaves, which are known to be poisonous. It is not yet clear whether D₃ and its metabolites are present in the edible fruits of Solanaceae.

Interesting fact – grazing animals in several parts of the world develop calcium intoxication, similar to that caused by vitamin D toxicity, from consuming particular plants. This is due to D3 or its metabolite present, which stimulates calcium absorption, producing hypercalcemia and deposition of calcium in soft tissues: aorta, heart, kidneys, intestines, and uterus. Controlled studies with various animals showed that Solanum glaucophyllum leaves or its extracts caused increased absorption of calcium and phosphorus similar to excess intake of vitamin D. Cestrum diurnum and Trisetum flavescens are also known to cause calcium intoxication. Studies with these plants led to the identification of vitamin D3 and related compounds in plant tissue.

Vitamin D biosynthesis

Sterols are precursors of vitamin D and essential for all eukaryotes. They are components of membranes and have a function in regulation of membrane fluidity and permeability. Sterols are also precursors of many steroid hormones including vitamin D and a wide range of secondary metabolites. Plant tissues contain an average quantity of 1–3 mg sterols per gram dry weight. The sterol composition of plant species is genetically determined and varies considerably. Cholesterol is the major sterol in animals, but is also present in plants. Usually, cholesterol accounts for 1–2 % of total plant sterols, but higher levels are present in especially Solanaceae.

Vitamin D biosynthesis runs along the normal sterol pathway, i.e., vitamin D2 is formed by UVB exposure of ergosterol and vitamin D3 by UVB exposure of 7-dehydrocholesterol.

Both D₃ (deriving from sun exposure and dietary sources) and D₂ (deriving only from dietary sources) are metabolised:

1. in the liver through a hydroxylation pathway to the intermediate compound,
2. second hydroxylation pathway, mainly in the kidney, also in other tissues, producing the active form of vitamin D.

• Sterol biosynthesis to vitamin D2—fungi: the major sterol end product in fungi is ergosterol synthesized via lanosterol. Sterols from fungi differ from animal sterols by the presence of a methyl group at C24. Plants are not known to produce ergosterol, so any vitamin D2 is derived from endophytic fungi or a fungal infection.
• Sterol biosynthesis leading to vitamin D3—plants : sterols in plants may be synthesized by two biosynthetic routes, via cycloartenol and/or via lanosterol. As a result cholesterol and 7-dehydrocholesterol may be formed in plants through lanosterol as is known from animals.
• Sterol biosynthesis to vitamin D3—animals: the major end product of the animal sterol pathway is cholesterol synthesized via lanosterol. The conversion of lanosterol to cholesterol requires enzymes, removal of three methyl groups, reduction of double bonds and migration of a double bond in lanosterol to a new position in cholesterol.

Vitamin D in algae

Microalgae as the basis of the food chain is the origin of the high content of vitamin D3. However, data for vitamin D in algae are limited and not consistent. Amounts found: D2 1.9–4.3 μg/100 g, D3 5.0–15 μg/100 g and their provitamins 260–1450 μg/100 g. High concentrations of vitamin D2, D3 and their metabolites were reported in freshwater microalgae. Both D2 and D3 are available for fish in their diet, but vitamin D2 is almost absent in fish. This suggests that the bioavailability of vitamin D2 is lower than for vitamin D3. Microalgae usually live at the surface of the water and vitamin D is synthesized by sun exposure of provitamins D. Microalgae caught in August were higher in vitamin D than in October and December. Microalgae are an extremely diverse group. It is, therefore, difficult to make any conclusions about algae’s production of D2 and D3. Species differences and geographic differences are to be expected.

Analytical methods to study the vitamin D forms in plants

Research into vitamin D in plants is limited, presumably due to limitations in selectivity and sensitivity of the analytical methods available. Determination of vitamin D in food has always been a challenge due to low amounts of vitamin D combined with the existence of multiple vitamin D active compounds. Plants are a complex matrix, which makes the analysis of vitamin D even more challenging. Selective and sensitive methods are necessary.

The function of vitamin D in plants

There may be some similarities between plants and animals in the way in which calcium and vitamin D are associated in regulatory processes. Vitamin D has a critical role in calcium and phosphate homeostasis in animals. When blood calcium concentrations fall, it results in increased intestinal absorption of calcium, along with decreased renal excretion, to restore normal blood calcium concentrations. When this mechanism is insufficient, bone metabolism is upregulated to release calcium from skeletal stores. Plants have similar calcium channels and pumps to those found in animals and calcium ions are a core regulator of plant cell physiology. Calcium is required for stimulation of growth, root initiation and promotion of germination in plants.

Conclusion

Vitamin D enables intestinal absorption of calcium and stimulates absorption of phosphate and magnesium ions. In the absence of vitamin D, efficiency of dietary calcium absorption is low. Absorption of calcium is enhanced by vitamin D by stimulating the expression of a number of proteins involved in transporting calcium from the lumen of the intestine across the epithelial cells into the blood. Vitamin D receptors are present in most cells in the body. Experiments using cultured cells have shown that vitamin D has potent effects on the growth and differentiation of many types of cells. These findings suggest that vitamin D has physiologic effects much broader than a role in mineral homeostasis and bone function. As one example, many immune cells not only express vitamin D receptors, but are capable of synthesizing active vitamin D and deficiency in vitamin D has been associated with increased incidence of autoimmune disease and susceptibility to diseases.

D3 and its metabolites are formed in certain plants. Fruits and vegetables have the potential to be a source of vitamin D in diets. Especially the Solanaceae family and also planktonic marine microalgae. An important thing to consider before putting sources into producing plants or algae with a high amount of vitamin D is the bioavailability, there are different levels of bioavailability from specific food.

Sources:

• Rie B. Jäpelt* and Jette Jakobsen. Vitamin D in plants: a review of occurrence, analysis, and biosynthesis. 2013
• Ishiaq Omotosho. Plant Sources of Vitamin D and Its Medicinal Application in Sub-Sahara Africa. 2019
•  https://pubchem.ncbi.nlm.nih.gov