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The significant attention gained by food-sourced vitamins has provided insights into numerous current researches; for instance, the potential reversal of epigenetic age using a diet and lifestyle intervention, the balance between food and dietary supplements in the general population, the role of diet and food intake in age-related macular degeneration, and the association of dietary supplement use, nutrient intake and mortality among adults. As relevant literature about food-sourced vitamin increases, continuous synthesis is warranted. To supplement existing information, this perspective review discussed food-sourced vitamins for consumer diet and health needs, scoping from vitamin absorption, metabolic functions, utilization, to balancing nutritional requirements. Relevant literatures were identified through a search of databases like Google Scholar, Web of Science, the Interscience Online Library, ScienceDirect, and PubMed. We demonstrated that vitamins whether from plant- and animal-based sources are prerequisites for the metabolic functions of the human body. The fat- and water-soluble classification of vitamins remains consistent with their respective absorption and dissolution potentials, underpinned by numerous physiological functions. Vitamins, largely absorbed in the small intestine, have their bioavailability dependent on the food composition, its associated interactions, as well as alignment with their metabolic functions, which involves antioxidants, coenzymes, electron acceptor/donor, and hormones. Moreover, vitamin deficiencies, in every form, pose a serious threat to human health. Vitamin toxicities remain rare, but can still occur mainly from supplementation, although it appears much less in water-soluble vitamins of which some excesses get readily removed by the human body, different from the fat-soluble ones that are stored in tissues and organs. Besides discussions of absorption, transport, and cellular uptake of vitamins, this perspective review also included approaches to meeting vitamin requirements and therapeutic strategies against micronutrient deficiency and COVID-19. We have also attempted on how to strike the balance between food-sourced vitamins and dietary supplements.
Keywords: Vitamin absorption, Vitamin transport, Micronutrient, Physiological function, Animal-based, Plant-based
Nutrients are a prerequisite to human life, and among them include vitamins that play several important physiological roles (Combs, 2008). Being low molecular weight organic compounds distinct from macronutrients such as carbohydrates, proteins, and fats, vitamins are micronutrients the human body requires for optimal cell development/growth, as well as countless metabolic functions/processes (Combs, 2008; Goncalves et al., 2015). Besides assisting in the synthesis of nervous system chemicals, blood vessel formation, hormones, and genetic materials, it is widely understood that vitamins can, when combined with proteins, serve as a catalyst to generate metabolically active enzymes necessary for life processes (Cooper, 2000; Bhagavan & Ha, 2015). Though the human body needs vitamins to work properly, most vitamins appear not synthesised within the body, at least not in sufficient amounts to meet our daily needs (SAL, 2017; Kumanyika & Oria, 2017), and therefore must be obtained from the diet. Compared to other classes of such nutrients like carbohydrate, protein, and fat, the breakdown (catabolism) of vitamins neither provides significant energy nor structural functions to the body. Instead, very small quantities (micrograms or milligrams per day) are required to perform the highly specific and unique functions of health maintenance and metabolic integrity (Bender, 2003). This, therefore, distinguishes vitamins from fatty acids and amino acids, which are nutrients required in larger amounts, and also from inorganic minerals and trace elements. Briefly, vitamins are distinguishably grouped into fat-soluble vitamins (A, D, E, and K) and water-soluble vitamins (B-complex and C) based on their absorption potential in either fat (non-polar medium) or water (polar medium), in addition to the region of their physiological activity (Zhao et al., 2019). The magnitude of their dissolution is influenced by some intrinsic and extrinsic factors which would be discussed later in this perspective review. Generally, the fat-soluble are well-known as lipophilic (oil-loving) organic compounds while the water-soluble vitamins are regarded as the hydrophilic (water-loving) organic compounds.
However, besides being a disparate group, vitamins nutritionally form a cohesive group of organic compounds that are neither chemically identical nor able to provide similar metabolic functions (Bender, 2003). Vitamins as micronutrients have assumed outstanding public health importance. As a result, there continues to be an increase in evidence regarding their physiological function and associated health implications of their deficiencies in diets. For instance, as vitamin A deficiency has been implicated to be the leading cause of blindness and high risk of death from common illnesses such as diarrhoea in children, it has been classified by the World Health Organization as a public health problem affecting children in South Asia and sub-Saharan Africa (UNICEF, 2019). Unlike macronutrient (carbohydrate and protein) deficiency, the health implications of micronutrient deficiencies are not always visible, hence the term hidden hunger. It is well recognized that malnutrition in every aspect poses a great threat to public health, as the World Health Organization estimates the number of people with micronutrient deficiency in the world to be over two billion (UNICEF, 2019). The prevalence of the most common vitamin deficiency globally is briefly shown in Table 1 . Given this, understanding the significance as well as the magnitude of (public health) the severity of this micronutrient’s malnutrition, and providing adequate control and preventive measures is very crucial (FAO/WHO, 2004). No single food source contains all the vitamins, and importantly, inadequate or suboptimal consumption of vitamins result in deficiencies. This implies that different foods are needed to meet the vitamin requirement of the human body (SAL, 2017). Notably, the occurrence of micronutrient (vitamin) malnutrition in an individual, could be an indication of vitamin deficiency, attributable not only to poor consumption of vitamin-rich diets but also due to factors influencing vitamin absorption. Besides inadequate ingestion of vitamin-rich foods, some of the factors that can cause vitamin deficiency as a result of impairment of intestinal vitamin absorption are genetic disorders in transport molecules, intestinal disease, interactions with certain drugs and some food components, and excessive alcohol consumption (Said, 2004; Iqbal & Hussain, 2009; Bowen, 2018).
| Vitamin | Prevalence of deficiency | Sufficiency | Hypovitaminosis | Deficiency | Reference(s) |
|---|---|---|---|---|---|
| B12 | >50% | >221 pmol/L | ≤148–221 pmol/L | WHO (2008); Guney, Yikilmaaz & Dilek (2016); Allen et al. (2018) | |
| A | 13% | >0.83 µmol/L | ≤0.71–0.83 µmol/L | WHO (2011); Akhtar et al. (2013) | |
| B9 | >20% | >30 nmol/L | – | WHO (2008); Bailey et al. (2015); Rogers et al. (2018) | |
| D | >20% | 50–100 nmol/L | ≤30–50 nmol/L | Roth et al. (2018); Amrein et al. (2020) | |
| C | >1.4% | 50–75 µmol/L | ≤23–28 µmol/L | Rowe & Carr (2020) | |
| E | 23–73 µmol/L | – | Institute of Medicine (2000); Charbek (2014) |
Note:
Values are the amount of blood serum vitamin.
Understanding the key mechanisms involved in vitamin absorption has been among the key challenges confronting researchers in this food-related discipline. For instance, some researchers of previous studies provided evidence suggesting that some fat-soluble vitamins were absorbed by passive diffusion while other vitamins of the same class were absorbed through carrier-dependent proteins (Goncalves et al., 2015). Interestingly, recent studies have shown that the mechanism of vitamin absorption, as well as transportation of absorbed vitamins, are somewhat compounded than earlier stated, as their mode of absorption appears to be concentration-dependent. For example, a high concentration of these fat-soluble vitamins is absorbed through passive diffusion while protein-mediated transport occurs at dietary doses (Reboul & Borel, 2011). On the other hand, particularly the water-soluble ones, it is well understood that vitamin absorption does occur via diffusion.
Briefly, the absorption of vitamins begins with chewing of ingested food, where enzymes start to play their role from the mouth, together with continued churning and mixing with other chemical constituents in the stomach. The churned food makes contact with the small intestine, which is the nutrient absorption center in the body. Further, the water-soluble vitamins diffuse through the intestinal walls into the bloodstream while the fat-soluble vitamins are emulsified and packaged in lipid-rich mixed micelles containing fatty acids, bile salt, and phospholipids. Subsequently, fat-soluble vitamins would be moved through the brush-like projections (villi) and absorbed into the lymphatic circulation where they are transported to tissues, target cells, or organs (liver) (El-Kattan & Varma, 2012; Khammissa et al., 2018). As relevant literature regarding food-sourced vitamins increases by the day, there is a need for continuous synthesis to update existing information. To supplement existing information, this perspective review discussed food-sourced vitamins for consumer diet and health needs, scoping from vitamin absorption, metabolic functions, utilization, to balancing nutritional requirements. For emphasis, this perspective review has been constructed as follows: (a) classification of vitamins; (b) sources of vitamins; (c) metabolic functions of vitamins; (d) absorption of vitamins; (e) vitamin transport; (f) characteristic interactions associated with food-sourced vitamins (g) vitamin deficiencies and toxicities; (h) approaches to meeting vitamin requirements; (i) vitamins as a therapeutic tool against micronutrient deficiency and COVID-19; as well as (j) establishing a balance between food-sourced vitamins and dietary supplements. This work intends to disseminate a succinct summary of existing information about key vitamin research studies and at the same time, revisit the state-of-the-art. In considering its exceptional public health importance, the physiological functions, and associated health implications of respective vitamin deficiencies, the need for the continuous literature synthesis on the severity (of the diverse vitamin deficiency) around the globe, therefore, makes this review very fitting. Following this, the current perspective review is essential, not only for the general public, but also for nutritionists, dietitians, food scientists, public health, and health-related specialists, together with their respective organizations, to understand better the mechanisms of vitamin transport and utilization, including the rationale for the impairment of vitamin absorption.
The formulating of research questions was the very step in actualising this perspective review. We made effort to ensure the research questions were directly relevant to the intended objective, and at the same time, considered the intended/target audience. After this, a search strategy was developed by consulting and collating published papers and their references from selected scientific databases, including Google Scholar, ISI Web of Science, ResearchGate, Interscience Online Library, ScienceDirect, Semantic Scholar, and PubMed/Medlin. Each database was searched using keywords and search terms such as vitamin absorption, mechanism of vitamin absorption, utilization of vitamin, transportation of vitamin, and cellular uptake of the vitamin. Further, the inclusion and exclusion criteria were established, which through the help of search strategy helped to determine the research articles that are appropriate, relevant, and specific to the objective of this perspective review.
Primary research studies reporting the fundamental aspects of vitamin in nutrition and health were included. Specifically, studies were selected if they had evaluated vitamin and their associated body functions, vitamin and food component interaction, as well as vitamin requirement in human nutrition. Articles reporting food-sourced vitamins especially from the plant and animal origin contexts were equally included because we considered them relevant for this review. Excluded studies were those that evaluated and reported other sources of vitamins such as vitamins exclusively produced from microorganisms, vitamins as nutritional supplements, vitamin-like compounds, and vitamin mimic. It is important to reiterate here that all the publications in languages other than English were also excluded. Additionally, we applied the discretion to strike a balance between the year of publication and the relevance of the information to the objective of this review. Next, the information gathered from the selected literature/studies were analysed. Essentially, all the information deemed relevant were incorporated, which helped to expatiate the discourse of this conducted perspective review.
In human nutrition, the traditional and widely accepted means of classifying the well-known thirteen vitamins have been based on their solubility, which includes: fat-soluble (hydrophobic) and water-soluble (hydrophilic) vitamins. The flow diagram showing the classifications of vitamins is shown in Fig. 1 . This classification of vitamins is basically dependent on their absorption potential either in fat or water (Bhagavan & Ha, 2015), together with where they act in the body (Delaney & Barke, 2017). The degree of vitamin dissolution in fat or water varies and is influenced by intrinsic and extrinsic factors. The fat-soluble vitamins are oily hydrophobic (lipophilic) organic compounds, usually not excreted out of the body, but rather stored in the body (liver), which include vitamins A, D, E, and K. In contrast, the water-soluble vitamins are hydrophilic (lipophobic) organic compounds not stored in the body and easily excreted through urine. Water-soluble vitamins are, therefore, required daily in small amounts, which include vitamin C (ascorbic acid) and the members of the vitamin B group, namely thiamine (vitamin B1), riboflavin (vitamin B2), niacin (vitamin B3), pantothenic acid (vitamin B5), pyridoxine (vitamin B6), biotin (vitamin B7), folate (vitamin B9), and cobalamin (vitamin B12) (Ball, 2004). Further, it is well-known that almost all vitamins are considered essential for their role in physiological processes and must be obtained from food, but some vitamins that can be synthesized endogenously in the body are not technically considered “essential” and they include vitamins D, B7, and K. The chemical structure of all the vitamins are shown in Fig. 2 .