Onions are believed to originate in central Asia, in modern-day Iran and Pakistan, and they have been among the earliest cultivated crops.

They were grown in Chinese gardens as early as 5,000 years ago, and are referred to in the oldest Vedic writings from India. As early as the sixth century B.C., a medical treatise, the Charaka Sanhita, celebrates this popular vegetable as a potent natural medicine, a diuretic, and remedy for the eyes, joints, and digestion.

In Egypt, onions were planted as far back as 3500 B.C. and they were worshiped as symbols of eternity due to their circle-within-a-circle structure. There were numerous paintings of onions on the inner walls of pyramids and other tombs.

Onions were buried with mummies, as people believed their strong scent had magical powers that would prompt the dead to breathe again.

Onions are also mentioned in the Bible, and the Greeks used them to fortify athletes for the Olympic games. The Romans consumed these vegetables regularly as well and used their countless medicinal properties.  Romans used onions to treat mouth sores, dog bites, poor vision, sleep issues, toothaches, dysentery, and lumbago.

By the Middle Ages, they were among the three main vegetables of European cuisine, together with beans, and cabbage, and were used to treat headaches, snakebites and hair loss.

Onions were brought by the Pilgrims on the Mayflower, but they found that Native Americans were already using wild onions in various ways.

Onion is an incredibly healthy vegetable and a rich source of antioxidants, vitamins, minerals, and amino acids. Onions belong to the Allium genus, together with garlic, scallions (green onions), leeks, and chives.

This vegetable can be of many varieties, and all of them are highly beneficial. The onion bulb can be red, white, or yellow, and the taste can range from mild and sweet to sharp and pungent.

The yellow variety is the most common in cooking, and white onions have a milder and crispier flavor. Red onions are also referred to as purple onions, and they aren’t as sharp and astringent as the yellow ones but are the highest in antioxidants.

The three varieties also differ in their nutritional value, but they are all rich in vitamins, minerals, and other beneficial micronutrients. They all have 40 calories per 100 grams, and a medium-sized onion contains nearly 2 grams of fiber. 100 grams of onions contain 9.3 grams of carbs and 4 grams of sugar. They all contain trace amounts of vitamins C and B-group vitamins, as well as minerals such as calcium, iron, manganese, and magnesium.

Onions contain decent amounts of several vitamins and minerals, and these are the main ones:

Vitamin C- which strengthens the immune function and improves skin and hair health 

Vitamin B6- It is essential for the formation of red blood cells

Potassium- This essential mineral lowers blood pressure and improves heart health 

Folate (B9)-- This water-soluble B-vitamin is vital for cell growth and metabolism, and is especially important for pregnant women

Onions are abundant in amino acids, such as lysine, threonine, leucine, and phenylalanine which are important for the metabolic processes in the body.

Scallions are sometimes referred to as green onions, and a large one provides 52 mcg of vitamin K (65% RDI), 4.7 mg vitamin C (8% RDI), and all the essential amino acids other onions contain.

Onions are high in unique antioxidants and dietary flavonoids. A kilogram of fresh yellow onions contains between 270 and 1187 mg flavonoids, and red onions contain even more, between 415 and 1917 mg per kilo. Red onions are also rich in anthocyanins which prevent numerous diseases.

Onions also contain quercetin, which has powerful antimicrobial properties and lowers blood pressure. The sulfur-containing compounds in onions also have potent anticancer properties. Anthocyanins are powerful antioxidants and pigments that give onions their reddish color, while thiosulfinates are sulfur-containing compounds that may inhibit the growth of harmful microorganisms and prevent the formation of blood clots.

Onions are incredibly versatile, so take a look at their numerous health benefits:

• The daily consumption of onions reduces inflammatory responses in the body and prevents chronic diseases like heart disease
• Onions regulate blood glucose spikes after meals and reduce fasting blood glucose in people with type 1 diabetes and type 2 diabetes
• Onions lower the risk of cardiovascular diseases, reduce blood pressure, and support heart health
• The consumption of onion skin tea has been found to support weight loss and to prevent obesity
• Onions have potent antimicrobial properties that destroy various bacteria and germs, such as Methicillin-sensitive Staphylococcus aureus (MSSA)
• Onions boost blood circulation and prevent blood clotting
• Onions lower blood cholesterol levels
• Onions improve digestion and prevent Clostridium difficile-related diarrhea (a bacteria that causes colon inflammation). They have a type of dietary fiber called oligofructose, which has probiotic effects and treats diarrhea associated with C. diff infections.
• Onions are rich in flavonoids and antioxidants that boost bone health and density, lower the risk of fractures, osteoporosis, and brittle bones
• Red onions prevent prostate problems and lower the risk of prostate cancer
• Raw onions boost the immune system and prevent infections, and increase white blood cell count which is an essential factor in the immunity 
• The high amount of beneficial antioxidants lower the risk of cancer, especially stomach cancer, colorectal cancer, esophageal cancer, and prostate cancer
• Onion destroys intestinal parasites, especially tapeworm and threadworms
• Onion juice boosts hair growth and prevents scalp infections that can lead to hair loss

You should store onions in a cool, dry, and well-ventilated place, in a mesh bag or a loosely covered paper. You can keep properly stored onions in the refrigerator for up to 2-3 months.

When it comes to the recommended daily dosage of onions, average daily doses of 50 g of fresh onion, 50 g of fresh onion juice, or 20 g of dried onion have been suggested.

When it comes to their adverse effects, onions can lead to stomach pain, bad breath if consumed in high amounts, odorous gas, and contact dermatitis if the juice irritates the skin.

Additionally, people have used onions as folk natural remedies in the following ways for centuries:

• Onions make you cry while you cut them, which can be helpful in the case of debris or other irritant caught in the eye
• Add onion juice to peppermint tea to treat vomiting
• Place a fresh slice of onion on mosquito bites to soothe the itching
• Sleeping with onion slices in the socks has been used as a natural way to detoxify the body and treat colds and the flu
• Apply onion flesh on warts, secure with a bandage, and leave it to act overnight to get rid of these skin growths
• Chew onion with the wisdom tooth to soothe the pain
• To eliminate excess earwax and soothe an earache, place an onion core into the ear, leave it overnight, and remove it in the morning
• Onion syrups have been used to treat common colds and all their symptoms

Sources:
www.healthyandnaturalworld.com
www.healthline.com
www.livescience.com