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Vitamins are one among the seven major nutrients required by the human body for growth and metabolism. It constitutes of a group of organic molecules called vitamers that combine together to form the functionally significant vitamin. For example – Vitamin E is a combination of four tocopherols and four tocotrienols. There are 13 known vitamins and have been classified based on solubility into -

1. Fat soluble vitamins – They are lipophilic organic compounds which are not excreted out of the body but stored in the body (liver). These include Vitamins A, D, E, and K.

2. Water soluble vitamins - They are hydrophilic organic compounds which are not stored in the body and easily excreted through urine. Therefore, these vitamins are required daily in small amounts and include Vitamin C (ascorbic acid) and Vitamin B family.

The Fat-Soluble Vitamins.

Vitamin A - It consists of retinoids like retinol, retinal and retinyl esters. The bioactive form of Vitamin A is retinal and retinoic acid which performs the essential biological functions like cell growth and differentiation playing a critical role in immune function, vision and cell to cell communication. It is mostly associated with vision as they form a component of the protein rhodopsin that help night vision. Therefore, deficiency of this vitamin is primarily implicated in night blindness. Deficiency can also lead to pregnancy complications, increased risk of infections, skin issues like hyperkeratosis, etc.

Vitamin A is an essential vitamin that can be absorbed from diet in 2 forms – the preformed Vit A that is easily absorbed and utilized and the provitamin A i.e., carotenoids that has to be broken down and converted into active Vitamin A. Preformed Vitamin A are obtained from food of the animal origin like meat, egg, fish and cod liver oil, dairy products like cheese, butter, etc. Carotenoids like beta and alpha carotene are obtained from food of plant origin like fruits especially carrot, pumpkin, sweet potatoes and vegetables like spinach, cabbage, dandelion greens, etc.

Recommended Dietary allowance for men and women are 900 μg/day and 700 μg/day respectively beyond which chances of toxicity increase. However, the upper limit of toxicity (3000 μg/day) should not be crossed. Since Vitamin A is fat soluble it is easily stored in the body and can reach high toxic levels faster leading to hypervitaminosis A.

Vitamin D – Vitamin D help regulate phosphorus and calcium homeostasis in the body. It is non-essential fat-soluble vitamin and is generally synthesized in the epidermal layers of the skin on exposure to the sunrays. Typically, the sunrays before 11am and that after 4 pm is considered to be effective. The important forms of Vitamin D are Vitamin D3 (Cholecalciferol) and Vitamin D2 (Ergocalciferol) which can also be ingested from diet and supplements. Food like fatty fish, egg, meat and fortified milk provide vitamin D. It is important for bone development in children as well as adults. The Recommended Dietary Allowance for Vitamin D is 15 μg per day and not more than that. However, the dose is much lower for infants and children. Vitamin D toxicity is rare but can lead to hypervitaminosis D having side effects like atherosclerosis, hyperparathyroidism, etc.

Vitamin D deficiency is more common in individuals nowadays because of the sedentary lifestyle with lack of sunlight exposure and in some vegan people. Deficiency of vitamin D is majorly implicated in rickets, osteomalacia, osteoporosis and other bone disorders.

Vitamin E – It is a fat-soluble essential vitamin that plays the role of antioxidant in the body. It scavenges the free radicals in the body minimizing its damages and prevent clot formation in the heart arteries. Alpha tocopherol is the active form of Vitamin E that is utilized by humans. Vitamin E is obtained from plant-based foods like oils (eg.  Sunflower, safflower & soyabean oil), fruits (eg. pumpkin, mango & avocado), nuts (eg. peanuts & almonds), vegetables (eg. Spinach & beet greens) and seeds (eg. Sunflower seeds). The Recommended Dietary Allowance is 15 mg per day for adults.

Deficiency can lead to retinopathy, peripheral neuropathy, ataxia and decreased immune function. Toxicity is rare and occurs only if supplements greater than 1000 mg is taken daily leading to bleeding. Therefore, upper limit is set at 1000 mg tocopherol supplement per day.

Vitamin K – It is fat-soluble nonessential vitamin generally produced by the normal flora of the human gut. The form of vitamin K produced inside the body is menaquinones which can also be obtained from animal and fermented food like meat, cheese and egg. The other type of vitamin K is phylloquinone which is obtained from vegetables like spinach, kale, lettuce and cabbages. Vitamin K is very important in blood clotting and bone tissue formation; for eg., Prothrombin is a vitamin K depended blood clotting factor that prevents bleeding and osteocalcin with the help of vitamin K produce new bone tissue.

Unlike other fat-soluble vitamins, vitamin K is easily broken down and excreted in urine due to which chances of toxicity is extremely rarely even with high intake. The Adequate Intake is 90 μg and 120 μg per day for women and men respectively. Deficiency is rare unless under antibiotic treatment that kills the gut microbes that synthesize vitamin K.

The Water-Soluble Vitamins.

Vitamin C – Vitamin C or Ascorbic acid is an essential water-soluble vitamin. Since it dissolves in water, it is not in any form stored inside the body and therefore daily intake of this vitamin via food or supplements is recommended. Vitamin C helps wound healing, control infections, perform antioxidant functions and collagen synthesis. It is also used as cofactor is several enzymatic reactions. Fruits and vegetables are rich source of this vitamin. Citrus fruits like orange, lemon, kiwi, grapefruit, strawberries and vegetables like tomatoes, broccoli, cabbage, cauliflower, brussels sprouts and white potatoes.

Deficiency of vitamin C primarily cause Scurvy, Malaise and Iron deficiency anemia due to malabsorption of non heme iron. The Recommended Dietary Allowance is 90 mg and 75 mg per day for men and women respectively. However, the Upper Intake Limit is 2000 mg per day beyond which it leads to gastrointestinal distress and diarrhea.

Vitamin B – It is a family of 8 types of Vitamin B – B1(Thiamine), B2 (Riboflavin), B3 (Niacin), B5 (Pantothenic Acid), B6 (Pyridoxine), B7 (Biotin), B9 (Folic Acid), B12 (Cobalamin). All these are essential vitamins which is to be obtained from food or supplements. The entire family helps release energy from carbohydrates and fats, breakdown amino acids, and transport of energy and nutrients throughout the body.

B1(Thiamine) - It helps metabolism, digestion, appetite and nerve function. Food like meat, fish and whole grains are a rich source. The Recommended Dietary Allowance for it is 1.1 mg and 1.2 mg per day for women and men respectively. Deficiency is observed in cases of Beriberi, Wernicke-Korsakoff syndrome where peripheral neuropathy and loss of muscle coordination is common. There is no toxicity observed for this vitamin as it is not stored in the body.

B2 (Riboflavin) - It is a key component of coenzymes used in the metabolism of fats and energy production. Food like meat, fish, vegetables like spinach, Nuts (Almonds) and fortified food (Cereal and bread) are a rich source. The Recommended Dietary Allowance for it is 1.1 mg and 1.3 mg per day for women and men respectively. Deficiency can lead to Anemia, Cataracts, swelling of mouth and throat, skin rash, etc.

B3 (Niacin) – The two forms of Vitamin in food are nicotinamide and nicotinic acid. It is a precursor of the coenzymes NAD and NADP that help catabolism of fat, carbohydrate, protein and synthesis of fatty acid and cholesterol respectively. Food that provides vitamin B3 are meats, poultry, seafood, milk, eggs, legumes, fortified breads and cereals. The Recommended Dietary Allowance for it is 14 mg and 16 mg per day for women and men respectively. Deficiency can lead to Pellagra, Depression, Hallucinations, Memory Loss, etc. The Upper Intake Limit is 35 mg per day for adults; toxicity arise from long term use of niacin supplements leading to Dizziness, low blood pressure, headache, impaired glucose tolerance, etc.

B5 (Pantothenic Acid) – It is used in the synthesis of Coenzyme A which build and breakdown fatty acids, Acyl carrier protein and aids energy metabolism. Almost all food contains this vitamin, best sources being organ meats, egg. Yogurt, Nuts, vegetables like avocados, mushrooms, potatoes, etc. The Recommended Dietary Allowance is 5 mg per day for adults. Deficiency results in Irritability, restlessness, muscle cramps, disturbed sleep and so on.

B6 (Pyridoxine) – Pyridoxal 5’ Phosphate (PLP) coenzyme is used in multiple metabolic reactions within the body like breakdown of carbohydrates, fats and proteins. It maintains the optimum level of homocysteine in blood to avoid heart problems; it helps nerve functions and hemoglobin synthesis. This vitamin is found in both animal and plants food like tuna, beef liver, poultry, bananas, papaya, oranges, green leafy veggies, etc. The Recommended Dietary Allowance is 1.2 mg and 1.3 mg per day for women and men respectively. Deficiency can cause peripheral neuropathy, anemia, depression and so on. Toxicity, although rare could emerge from supplements of 1000 mg per day leading to Ataxia, Neuropathy of hands and feet and Nausea.

B7 (Biotin) – Like most of the other B vitamins, biotin also share similar functions like the metabolism, regulation of signals and gene expression. Biotin is essential for the biotinylation of histone proteins for chromatin stability. Beef liver, Avocados, Nuts, Seeds, Salmon, Pork, Sweet potato are rich source of Biotin. There is no established Recommended Dietary Allowance instead there is Adequate Intake level for biotin which is 30 μg/day for both men and women. Deficiency results in thinning of hair, brittle nails, scaly skin and rashes. There is no toxicity observed because the excess is excreted in the urine.

B9 (Folic Acid) – Folate is the natural form of Vitamin B9 found in food while in supplements it exists as folic acid which has a better absorbance value. Folate in essential in the synthesis of DNA, RNA, RBC’s and protein metabolism. It is especially important during growth period and pregnancy. Dark green leafy veggies (eg., broccoli, spinach), Fresh fruits, Seafood, Whole Grains, Eggs, Fortified food are good source of Vitamin B9. Recommended Dietary Allowance for adults is 400 μg/day. Deficiency include anaemia, shortness of breath, irregular heartbeat, etc. Although toxicity is rare, the upper limit is set to 1000 μg/day because it tends to mask Vitamin B12 deficiency.

B12 (Cobalamin) – It is naturally found in animal food and is very critical for DNA and RBC synthesis. It is also very important for nerve and brain cell development. Sources of cobalamin include fish, meat, eggs, dairy products, fortified nutritional yeasts, cereals, etc. The Recommended Dietary Allowance for both men and women are 2.4 μg/day. Deficiency leads to Anaemia, Dementia, Nerve damage, seizures, etc.

Reference:

https://www.hsph.harvard.edu/nutritionsource/vitamin-a/

https://flexbooks.ck12.org/cbook/ck-12-college-human-biology-flexbook-2.0/section/23.2/primary/lesson/nutrients-chumbio/

https://ods.od.nih.gov/factsheets/VitaminA-HealthProfessional/

https://www.healthline.com/nutrition/vitamin-a#what-it-is

https://www.webmd.com/vitamins/ai/ingredientmono-929/vitamin-d

https://www.hsph.harvard.edu/nutritionsource/vitamin-e/

https://www.hsph.harvard.edu/nutritionsource/vitamin-k/

https://www.hsph.harvard.edu/nutritionsource/vitamin-c/

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2646215/

https://www.hsph.harvard.edu/nutritionsource/vitamins/vitamin-b/

https://www.readersdigest.ca/health/healthy-living/13-essential-vitamins-your-body-needs-stay-healthy/