Introduction

From boosting our immune system to maintaining healthy bones, eyes, and skin, and aiding in energy production, vitamins are indispensable to humans. Vitamins are organic compounds that help in maintaining and promoting our overall health ^{[1, 2]}. As essential micronutrients they are required in small amounts but are crucial for various functions and metabolism. Lack of vitamins in the diet can cause important metabolic deficits _{[3, 4]}. Humans either cannot synthesize vitamins or they are synthesized in inadequate amounts, so they must be obtained from various foods ^{[5, 3]}. While a balanced diet is the best way to obtain these essential nutrients or their precursors and analogs, in some cases, the required amount can be fulfilled through supplements ^[6].

Types of Vitamins: Understanding Vitamins

Vitamins are classified into two main categories: fat-soluble and water-soluble.

1. Fat-Soluble Vitamins: Vitamin A, D, E and K
2. Water-Soluble Vitamins: Vitamin C and B-Complex, which includes Thiamine (B1), Riboflavin (B2), Niacin (B3), Pantothenic acid (B5), Pyridoxine (B6), Biotin (B7), Folic acid (B9), and Cobalamin (B12).

Science and Storage of Vitamins in Humans

In the overall metabolism, Vitamins regulate and facilitate crucial chemical reactions in the body's cells, exhibiting catalytic or regulatory properties ^[7]. The absorption of both water-soluble and fat-soluble vitamins takes place in gastrointestinal tract, primarily small intestines ^{[9, 8, 10]}.

Fat-soluble vitamins (vitamin A, D, E, and K) dissolve in fat and are stored in the body’s fatty tissue, muscles, and liver for a long period of time. Liver is the most important site for vitamin storage, accounting for more than 90% of total fat-soluble vitamin storage ^{[8, 11]}. Stored vitamin A in the liver can sustain a person for 5 to 10 months, while vitamin D stores last 2 to 4 months without additional intake ^[1]. Although vitamin K is fat soluble, only a little is stored in the body, lasting only a few days ^{[12, 13]}.

However, the storage of water-soluble vitamins is relatively small, and the body tends to excrete excess amounts of these vitamins through urine ^[8]. Vitamin B12 is an exception, it can be stored in the liver for a whole year or longer ^[1]. Since they are not stored in the body for extended periods, a regular and consistent intake through a balanced diet is necessary to prevent their shortages or deficiencies in the body.

If a vitamin is low or lacking in one’s diet due to nutritional inadequacy, or its improper absorption in the human body (due to abnormalities of vitamin metabolism) a specific deficiency or disease may develop.

The Impact of Vitamins on Human Health

13 vital vitamins are integral for human metabolism. Each has a specific role to play and perform highly specialized functions in a human body. Some of the prominent functions they play are,  

Vitamin A - The Eye's Delight

Vitamin A exists in two  forms— preformed (retinoid) and provitamin (carotenoid). In animal tissues it occurs as retinol-a preformed vitamin A ^{[1, 2]}. In plant-based food it occurs as yellow and red carotenoids pigments which are called provitamins A carotenoids, with beta-carotene being the main provitamin A ^{[14, 15]}.

Vitamin A is essential for maintaining healthy vision. It is an integral part of pigment rhodopsin in our retinas and helps maintain the integrity of the cornea, ensuring that it remains clear and functional for sharp vision. Without sufficient vitamin A, we risk night blindness and, in severe cases, permanent vision impairment^{[2, 16]}.

Vitamin A prevents several other eye diseases, like dryness of eyes (xerophthalmia) ^[17], conjunctival problems ^[18], and age-related macular degeneration (AMD) ^[19].

While it’s primarily known for its role in eye health, vitamin A serves as a powerful natural antioxidant in the human body ^{[20, 21, 22]}.

It is also an anti-inflammation vitamin because of its critical role in enhancing immune function ^{[23, 24, 25]}.

It fosters the growth of fresh skin cells and repairs damaged tissues ^[26] and is a regulator cellular growth, reproduction, and development ^{[27, 26, 28]}.

B Vitamins - The Energy Boosters

Vitamin B is not a single entity but a complex of eight water-soluble vitamins, each with its unique functions and benefits ^{[29, 30]}. Together, they control different aspects of energy synthesis during the breakdown of food molecules in human energy metabolism (energy producing chemical reactions) ^{[1, 30, 31, 32]}.

Thiamine (vitamin B1) is important for the health and functioning of the central nervous system, integrity of myelin sheath of nerve fibers ^{[1, 34, 33]}, fitness and optimum functioning of heart ^[35], and gastrointestinal tract ^[36]. Continuous and severe thiamine deficiency affects these organs and their functioning, a condition commonly known as “beriberi” ^[32].

Riboflavin (vitamin B2)  plays a crucial role in maintaining healthy eyes, skin, nerves, and digestive functions ^{[1, 38]}.

Niacin (vitamin B3) is important for the health of mucous membranes of the mouth and gastrointestinal tract ^{[1, 39, 40]}, strength of muscles ^[41], DNA repair ^{[40, 42]}, and supports skin health especially related with severe sunlight sensitivity ^[1]. It also helps regulate cholesterol levels ^[43].

Pantothenic Acid (vitamin B5) is important for the metabolism of carbohydrates and fats, including cholesterol ^{[37, 44, 45]}.

Pyridoxine (vitamin B6) is involved in amino acid and protein metabolism ^[30]. It also supports immune function and brain health ^[46].

Biotin (vitamin B7) plays an essential role in metabolism of fats, glucose, and amino acids metabolism ^{[30, 47]}. It also helps in cell signaling, DNA replication, and gene regulation ^[48].

Both Folic acid (vitamin B9) and Cobalamin (Vitamin B12) are growth promoters and are crucial for red blood cell production and maturation, and DNA and RNA synthesis ^{[1, 30, 49]}. Vitamin B12 is required in neurologic function and myelin synthesis ^[50].

Vitamin C - The Immune Warrior

Vitamin C, also known as ascorbic acid, is widely recognized for its immune-boosting properties because it is pivotal in the production of infection fighting cells, including white blood cells ^{[51, 52]}.

Additionally, it supports collagen production in cells and helps in wound healing ^{[53, 54]}. It is a powerful antioxidant, protecting cells from damage ^{[55, 56]}.

It plays a crucial role in healthy bone growth and blood vessels ^{[57, 58]}. Vitamin C enhances the absorption of iron found in plant-based foods (non-heme iron) from the digestive tract ^[59]. It also promotes healthy skin ^[60].

Vitamin D - The Sunshine Vitamin

Vitamin D, also known as calciferol, is crucial in relation to overall body calcium metabolism and bone formation ^[61]. Vitamin D increases calcium absorption from the gastrointestinal tract and helps control calcium deposition in the bone ^{[1, 62]}. In this way they are critical for strong  and healthy bones ^[63], especially for older adults (protection from osteoporosis) and younger children (from rickets) ^{[61, 64]}.

It plays a role in supporting the immune system, potentially reducing the risk of infections and inflammation ^{[65, 66]}.

Furthermore, the human body relies on vitamin D for various functions, including cell growth, neuromuscular and immune function, and the regulation of glucose metabolism ^{[67, 62, 68, 61, 69]}.

Vitamin E - The Antioxidant Vitamins

Vitamin E is a powerful antioxidant that exists in eight different forms, the most biologically active and useful for human being alpha-tocopherol ^{[51, 70, 71]}. It is known for its ability to neutralize free radicals, which are unstable molecules that can damage cells and contribute to various diseases ^[55].

Vitamin E has beneficial effects in gynecological health and reproductive-related diseases in females ^{[1, 71]}.

Studies have suggested that high dietary intakes of vitamin E with other oxidants reduce the risk and progression of age related macular disease ^{[72, 73, 74, 75]}.

Studies have also proposed a role of vitamin E in reducing risks from various forms of cancer, but this has not been conclusively proven yet ^{[76, 77, 78]}.

Vitamin K - The Clotting Vitamins

Vitamin K exists in two primary forms, K1 (phylloquinone) and K2 (menaquinone). Phylloquinone is the main dietary form of vitamin K ^{[79, 80]}. Menaquinones are bacterial origin and are produced in the human gut by bacteria ^{[81, 82]}.

Vitamin K is essential for the synthesis of proteins called clotting factors, which are crucial for blood clotting to prevent excessive bleeding when there is an injury ^{[1, 79, 83]}.

Vitamin K is involved in bone metabolism and adequate vitamin K levels are associated with better bone health ^[84,79]. It helps regulate calcium within the bones and plays a role in bone mineral density, especially in people with osteoporosis ^{[85, 86]}.

Preventing Vitamin Deficiencies

Vitamin deficiency is a significant health concern that can have far-reaching consequences on our overall well-being ^{[20, 21, 87, 88]}. By understanding the importance of each vitamin, making informed dietary choices, and seeking professional advice when needed, you can take proactive steps to maintain optimal health and prevent the detrimental effects of vitamin deficiencies.

Various foods hold great significance to fulfill our daily required vitamin amount ^[89]. It is important to note that maintaining a well-balanced diet with a variety of nutrient-rich foods is the most effective way to prevent vitamin deficiencies.

Here is a list of some common sources of essential vitamins:

• Vitamin A: The  retinol forms of vitamin A are supplied primarily by foods of animal origin such as liver, fish, egg, and dairy products. Beta-carotene comes from fruits and vegetables, especially carrots, sweet potato, pumpkin, dark green leafy vegetables, broccoli, and tomatoes.
• Vitamin B-Complex: Vitamin B1 is found naturally in whole grains, cereals, lentils and meat, especially pork and fish.

Vitamin B2 exists in eggs, milk, lean beef and pork meats, liver, spinach, soybeans, almonds, avocado, and dates.

Nuts, legumes, grains, poultry, beef, and fish are rich sources of Vitamin B3.

Rich sources of vitamin B5, B6, and B7 are beef, chicken, and organ meats, mushrooms, broccoli, potato, salmon, and egg.

Vitamin B9’s richest sources are dark green leafy vegetables, brussels sprout, asparagus, beans, seafood, and liver.

Vitamin B12 can be found in beef liver, fish and shellfish, poultry, and eggs.

• Vitamin C: Fruits and vegetables are rich sources of Vitamin C, especially, red and green pepper, citrus family fruits, strawberries, kiwi, broccoli, and brussels sprouts.
• Vitamin D: Apart from sunlight that helps build Vitamin D in skin, fishes like trout, tuna and mackerel including fish oil are richest sources of it.
• Vitamin E: Vegetable oil, seeds, and nuts are rich sources of vitamin E. Significant amount are available in almonds, sunflower seeds, and green leafy vegetables.
• Vitamin K: soybeans, collard, turnip leaves, kale, spinach, broccoli, edamame, pumpkin, and pomegranate are excellent sources.

Recommended Doses

The Food and Nutrition Board (FNB) at the National Academy of Medicine develops nutrient reference values called the Dietary Reference Intakes (DRIs) for vitamins and other nutrients that are required for optimal human growth ^{[2, 30, 51, 62]}.  For 19 years and older, the recommended dietary amount of various vitamins are as follows,

Vitamin	Recommended dietary amount (RDA)	Upper Limit (UL) per day
Vitamin A	M: 900 mcg, W: 700 mcg	3,000 mcg
Vitamin B1	M: 1.2 mg, W: 1.1 mg	Not known
Vitamin B2	M: 1.3 mg, W: 1.1 mg	Not known
Vitamin B3	M: 16 mg, W: 14 mg	35 mg
Vitamin B5	M: 5 mg, W: 5 mg	Not known
Vitamin B6	M: 1.3-1.7 mg, W: 1.3-1.5 mg	100 mg
Vitamin B12	M: 2.4 mcg, W: 2.4 mcg	Not known
Vitamin B7	M: 30 mcg, W: 30 mcg	Not known
Vitamin C	M: 90 mg, W: 75 mg	2,000 mg
Vitamin D	31–70: 15 mcg; 71+: 20 mcg	50 mcg
Vitamin E	M: 15 mg, W: 15 mg	1,000 mg
Vitamin B9	M: 400 mcg, W: 400 mcg	1,000 mcg
Vitamin K	M: 120 mcg, W: 90 mcg	Not known

M: Male; W: female

Recommendations and Tips

Here are some tips to ensure you're getting enough vitamins:

1. Eat a Variety of Foods:Incorporate a wide range of fruits, vegetables, whole grains, lean proteins and dairy products into your diet to cover all the essential vitamins.
2. Consider Supplements:If you have a specific vitamin deficiency or follow a restrictive diet, consult with a healthcare professional to determine if supplements are necessary.
3. Get Sun Exposure:For vitamin D, spending time in the sun can help your body produce this essential nutrient. Just be cautious about sunburn and skin damage.
4. Avoid Overcooking:Overcooking or boiling foods for extended periods can destroy some vitamins. Choose cooking methods that preserve the nutritional value of your food.
5. Dietary supplement:Sometimes, despite our best efforts, we might struggle to obtain all the essential vitamins from our diet alone. In such cases, dietary supplements can fill in the gaps.

Conclusion

Vitamins are integral to human health and well-being, and understanding their role is crucial for maintaining a healthy lifestyle. While it's essential to get these nutrients through a balanced diet, supplements can be helpful when needed. Remember, a holistic approach to nutrition and overall well-being, including regular exercise and a well-balanced diet, is key to unlocking the full potential of these tiny but powerful compounds in promoting human health.

Consulting a healthcare professional is essential before adding supplements to your routine. Health care professionals can provide expert and personalized guidance based on your specific needs.

 

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