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There are many kinds of vitamins from various food groups and they participate in different body metabolism such as maintaining healthy skin and hair, building bones and releasing and utilizing energy from foods. There are 13 types of vitamins in total, including vitamin A, C, D, E, K, and 8 different kinds of vitamin B. Human body cannot synthesize most vitamins by themselves, so we have to take in those vitamins from food. Still, unlike carbohydrates, proteins and fats, our bodies only need a small quantity of vitamins. Different types of vitamins will have different solubility in lipids and water, depending on their molecular structures, which means that they’ll be absorbed in different parts of our body. 
Learn more: So many vitamins, but what’s the function of each of them?

Vitamin A
Vitamin A is also known as retinol, retinal, and “the four carotenoids,” including beta carotene.
It is well-known for its benefits in eye health and skin health, and it also plays a rule in our immune system.

Why Vitamin A gives us good eyesight at dark?
There are two major types of cells that helps us see—Cone cells and rod cells. Cone cells are present in small amount in the most sensitive part of the retina, and they are good at perceiving colors and they provide a clear vision during the day. Rod cells are relatively larger in numbers, and they are crucial in perceiving images at low light intensity—at dark. You might have heard that a lack of vitamin A leads to light blindness, so it’s very likely that vitamin A participates in the functioning of rod cells. Indeed, vitamin A participates in the production of rhodopsin, which is a well-known photosensitive protein/pigment found in the rod cells of the retina and detects light/dark contrast

Why Vitamin A is good for our skin health?
As an adolescent, I sometimes have acnes on my body, and one solution of this is to spread some vitamin A acid cream onto my skin. This is because that vitamin A may prevent the overproduction of keratin in hair follicles, which is known to cause skin disorders like acne.

The vitamin B family
All vitamin Bs are water-soluble molecules, and they are common in dairy products, fresh vegetables, fish and wholewheat grains. Taking enough vitamin B won’t be a big problem if your diet contains plenty of wholewheat grains. The list below introduces the functions of each kind of vitamin B.

Vitamin B1
Here comes the first family member of vitamin B, vitamin B1, also known as thiamine. Vitamin B1 is essential for producing various enzymes that help break down blood sugar. A lack of vitamin B1 may cause beriberi and Wernicke-Korsakoff syndrome.

Beriberi is a disease which includes wet beriberi and dry beriberi. Wet beriberi affects the heart and circulatory system and may eventually lead to heart failure, while dry beriberi damages the nerves and can lead to decreased muscle strength and eventually muscle paralysis. Beriberi can be life threatening if it isn’t treated. 

Wernicke-Korsakoff syndrome mainly leads to confusion, inability to coordinate voluntary movements, visual changes and additional eye problems. The two diseases above can both be caused by abuse in alcohol and a lack of vitamin B1. Be sure not to drink too much and regularly take wholewheat grains!

Vitamin B2
Vitamin B2, also known as riboflavin, is essential for the growth and development of body cells. If we lack Vitamin B2, we may suffer from inflammation of the lips or fissures in the mouth. Still, vitamin B2 can be obtained from asparagus, bananas, dairy products, eggs and fish. 

Vitamin B3
Vitamin B3, also known as niacin or niacinamide. Similar to vitamin B2, it can help the body cells to grow and work correctly. Lack of vitamin B3 may cause diarrhea, skin changes and intestinal upset, but you can avoid these diseases by taking in chicken, beef, eggs, dairy and fresh leafy vegetables. 
Vitamin B5
Vitamin B5 is known as pantothenic acid. It participates in the production of energy and hormones, and a lack of it may reduce in abnormal reactions of sense organs: “paresthesia”. Vitamin B5 is rich in meats, wholewheat grains, broccoli, avocados and yogurt.

Vitamin B6
Vitamin B6, known as pyridoxine, pyridoxamine or pyridoxal, is vital for the formation of red blood cells. Therefore, the symptom for lacking vitamin B6 is quite obvious—anemia. Also, it may potentially cause peripheral neuropathy. To maintain a sufficient level of vitamin B6, it’s good to take chickpeas, animal livers, bananas, squash and nuts. 
Vitamin B7
Vitamin B7 has a more common name: biotin. The word “biotin” means “materials for creatures”, and it enables the body to metabolize proteins, fats and carbohydrates. Biotin also participates in the production of keratin, which is a protein that builds up skin, hair and nails. A lack of vitamin B7 may cause dermatitis (inflammation of skin) or inflammation of the intestines. Like many other Vitamin Bs, vitamin B7 can be obtained in egg, animal liver, broccoli, spinach and cheese. 
Vitamin B9
Vitamin B9 also has a common name: folic acid. It participates in the making of DNA and RNA, thus is quite essential especially for babies. If lack of vitamin B9, the fetus’s nervous system may be affected during pregnancy. Therefore, taking enough vitamin B9 or folic acid is quite important during pregnancy. Vitamin B9 can be found in most leafy vegetables, peas and grain products. 

Vitamin B12
Here comes the last member of vitamin Bs, Vitamin B12. It is also known as cyanocobalamin, hydroxocobalamin and methylcobalamin. Vitamin B12 is very important for a healthy nervous system, so lack of it may results in neurological problems or potentially anemia. Vitamin B12 can be obtained from fish, shellfish, meat, eggs and many dairy products. 

Vitamin C
Vitamin C, or ascorbic acid, is probably the most well-known type of vitamins. It’s water soluble, and it contributes to collagen production, wound healing and bone formation. Furthermore, vitamin C is good to blood vessels, immune system and the absorption of iron. You might have heard of the disease “scurvy”, which is caused by a lack of vitamin C. Scurvy will causes bleeding gums, loss of teeth and poor tissue growth and wound healing. Still, many kinds of fruit and vegetables contain plenty of vitamin C. Make sure not to put vitamin C under high temperature as it will change the molecular structure of it.
You might have heard that vitamin C is an antioxidant, but do you know why? 
Click here to learn some interesting chemistry stuff about vitamin C and the mechanism of oxidation. 

Why is vitamin C an antioxidant: some organic chemistry stuff
First, let me introduce the process of free radical oxidation, which might happen without an antioxidant.
The oxidation reaction happening inside our body is partially free-radical oxidation, which is considered as one of the main causes of aging. Free radicals are organic molecules/atoms which have single electrons, instead of bond electrons or electron pairs, so they tend to form bond with other molecules, making them very reactive. Free radical oxidation is the process of using a free radical (often obtained from sporadic homolytic bond breaking of oxygen molecules) to attack a normal molecule to make it into another free radical. This process in organic chemistry is called “propagation”, meaning to generate new free radicals. This newly formed free radical can then bind with oxygen atom, in this case oxidized. 

So why vitamin C acts as an antioxidant?

Actually, instead of inhibiting the free radicals to from, vitamin C itself is very reactive and will react with the free radicals. Therefore, the less reactive molecules (for example our body tissues) can be protected and remain unoxidized. The reason why vitamin C is reactive is as follows.
As shown on the picture, vitamin C contains an allyl hydrogen atom, which is highly reactive and can react with potential free radicals (in this case an oxygen free radical). This is because that when the bond between the allyl hydrogen and the allyl carbon breaks to form free radicals, the free radical formed can be easily stabilized via the resonance effect—the electrons on the bottom double bond can “slide over” to the allyl position and stabilize it. Therefore, it’s reasonable to say that since the product (free radical) is easier to form, the reaction is easier to happen, thus the vitamin C molecule is more reactive.

Vitamin D
Here comes vitamin D, or ergocalciferol, cholecalciferol. (These two are vitamin D2 and vitamin D3 respectively, having slightly different molecular structures). Vitamin D is a fat-soluble molecule, and it’s necessary for the healthy mineralization of bone. Therefore, a lack of vitamin D results in problems of bones, including rickets and osteomalacia (softened bones). It’s worth mentioning that human bodies cannot produce vitamin Ds ourselves under dark conditions. Thus, exposure to sunlight or other light source is crucial in synthesizing vitamin D. You can also take in eggs, fish and livers to gain extra supplement of vitamin D. 

Vitamin E
Vitamin E is chemically known as tocopherol and tocotrienol (also two different structures with slightly different molecular structures), and it’s also a fat-soluble molecule. Similar to vitamin C, vitamin E contains allyl position hydrogen (explained in the Why is vitamin C an antioxidant: some organic chemistry stuff section) and is reactive. Therefore, vitamin E also acts as an antioxidant, which makes it able to prevent oxidative stress. Note that oxidative stress will increase the risk of widespread inflammation and various diseases, since your body is getting “oxidized” and damaged. A lack of vitamin E won’t have very common deficiencies. Anyway, it’s still advisable to take in vitamin E as it’s helpful, and you can achieve this by takin gin wheat germ, kiwis, nuts, eggs and various types of vegetables. 

Vitamin K
Here comes the last member of the vitamin family: vitamin K! It is a fat-soluble molecule and is also known as phylloquinone or menaquinone, depending on their slightly different molecular structures. Vitamin K is necessary for blood clotting. Although it doesn’t directly cause an increase in number of platelets, it still helps in production of blood-clotting proteins. (Blood clotting is a complicated process and involves many different molecules) Therefore, a low level of vitamin K may cause an unusual susceptibility to bleeding. You can obtain plenty of vitamin K from natto, leafy greens, pumpkins, figs and parsley. 
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