3. Vitamin D

Functionally, vitamins are involved in various metabolic processes where they serve usually as coenzymes in various biochemical reactions associated with proper functioning of the whole organism [2]. They thus catalyze organic reactions by participating in the formation of hormones, cells, chemical structures of the nervous system, composition of genetic material and a host of other biological processes. They also combine with proteins to form enzymes which participate in various body reactions including in the development of body's immune system. Probably because vitamins are present in small quantities, in the past, diseases of vitamin deficiencies were treated using various vitamins supplementarily in their management; however, advancement in science has led to many biochemical and biological methods that are appropriately used in the identification, measurement

Due to the involvement of these vitamins in several metabolic processes in spite of their small quantities, their deficiencies usually manifest clinically in various forms; for example, pellagra and beriberi are clinical conditions associated with the deficiency of niacin and thiamine respectively, (sub-groups of vitamin B), scurvy is a clinical condition associated with vitamin C deficiency, while osteomalacia (in adults) and rickets (in growing children) are associated with vitamin D deficiencies. Night blindness is associated with vitamin A deficiency, deficiency of vitamin B12 or that of folic acid is associated with megaloblastic anemia while spinal bifida has been associated with the deficiency of folic acid in the mother while the baby was in-utero [2]. Because vitamins as essential nutrients are mostly derived from diets, etiology of hypovitaminosis has always been associated with either in-adequate intake from diet or abnormality of absorption whereby a large quantity of these essential nutrients remains unabsorbed even when present abundantly in diet. Generally, vitamin toxicity is associated with the fat-soluble series because of their insolubility in the aqueous medium which largely constitutes the human system; however, excessive ingestion of vitamin A has been known to result in toxic mani-

Vitamins are generally classified based on their solubility in aqueous or lipid medium. Thus, there are the fat-soluble vitamins (A, D, E and K) and the watersoluble ones (vitamins B complex and C). Although they are usually classified into these two broad groups, the classification is for convenience based on the chemical structure of vitamins; most of the vitamins have sub-groups that are no less prominent in name as the known main group. For example, while vitamin A is available either as the plant (carotenoid) or animal (retinol) types, vitamin B has several sub groups which have been distinctly classified based on structure and function. On the other hand, the name vitamin E, which is a known fat-soluble vitamin, refers to a family of eight naturally occurring homologs that are synthesized by plants from homogentisic acid. They are all derivatives of 6-chromanol and differ in the number and position of methyl groups on the ring structure. Also, vitamin D is another classical example of a lipid-soluble vitamin. The name, vitamin D, refers to about 5 different compounds generally classified as such, these are: 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), 25 hydroxyvitamin D2 (25-OH-D2). Hence, the general classification of vitamins notwithstanding, several sub-groups of vitamins exist which when considered based on structure, function and activity will make the conventional broad classification of vitamins too simplistic in terms of their overall relevance in human metabolic

and diagnosis of diseases associated with many of the known vitamins.

festations which may ultimately result in liver damage [2].

2.2 Classification of vitamins

Fads and Facts about Vitamin D

8

Vitamin D is one of the fat-soluble vitamins that has distinct biochemical functions in human metabolism. It exists in five active forms


#### 3.1 Sources and production of vitamin D

The general conception is that vitamin D is synthesized only in the body, however, evidences abound that vitamin D is available in different forms in some plants and fruits in sub-Sahara Africa. There are also reports on application of these plant sources in the treatment of some vitamin D related diseases. Vitamin D2 (ergosterol) has been identified in some plants and fungi. Vitamin D2 differs from D3 in having a double bond between C22 and C23 and a methyl group at C24 in the side chain. D2 can be considered the first vitamin D analog which is converted to D3 by ultraviolet radiation. As earlier stated, plants like perennial ryegrass contain some amounts of ergosterol which when ingested can also be readily converted to D3 in the body [3].

Vitamin D3 has many dietary sources. The parent compound (D2) is derived essentially from dietary sources like egg yolk, sea fatty fish, liver, and mushroom among others. The production of vitamin D3 (D3) in the skin is not an enzymatic process. Sea fatty fish essentially contain vitamin D2.

#### 3.2 Plant sources of vitamin D

Accidental discovery of activation of some vegetables and crops by exposure to mercury lamp led to the identification of vitamin D2 in some inert foods like cottonseed, wheat and lettuce [4]. Later, vitamin D2 was identified from solutions of ergosterol irradiated with UV light in-vitro [5]. Hence, 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. This initial concept on the presence of vitamin D in plants however changed with the discovery of a type of calcium intoxication in grazing animals similar to that caused by vitamin D toxicity that consumed certain plants [6]. This was believed to be due to vitamin D3 or a metabolite of vitamin D3 present in the plants that stimulate calcium absorption producing hypercalcemia and deposition of calcium in soft tissue including aorta, heart, kidneys, intestines, and uterus [6].

Hence, while plants like Solanum glaucophyllum Desf. (S. glaucophyllum), Cestrum diurnum L. (C. diurnum) and Trisetum flavescens Beauv. (T. flavescens) were found to cause calcium intoxication similar to that caused by vitamin D toxicity in

grazing animals, rats and even chickens in South America [7–9]; similar effects of plants ameliorating calcium intoxication were found in studies on chicken in Africa and her sub-Sahara using Moringa oleifera leaves [10]. Aside from above, provitamin D3 and vitamin D2 (7-dehydro cholesterol) have been identified in leaves of plants like the genus Solanaceae and in S. lycopersicum, S. glaucophyllum and C. annuum amongst others [11–15].

Like most plant and herbal preparations, there are lots of knowledge gap in the biochemical and physiological mechanism behind application of these plant materials as sources of nutrients and most importantly in the management of diseases. However, isolation and characterization of vitamin-like substances in some of the plant may allow for the assumption of most of the claimed empirical roles of these plants and herbal preparations (WHO, 2002) [16].

Ocimum gratissimum (Figure 1) is one of the plants commonly used (empirically) to treat open wounds in rural setting of sub-Sahara Africa. The phytochemical analysis of this plant is well documented [17]. The assumed theory is that at higher temperatures, leaves of this plant facilitate collagen and fiber formation on the open wound thus enhancing clotting and angiogenesis [18]. However, the direct effect traceable to vitamin D content and activity remain to be elucidated.
