What are micronutrients (vitamins and minerals)?

In one line

Micronutrients are nutrients the body needs only in very small amounts but cannot do without; the word covers vitamins and minerals.

What are micronutrients (vitamins and minerals)?
The names lined up on supplement bottles
Supplement bottles and food labels list names like vitamin C, vitamin D, calcium and iron. Together these are called micronutrients. What are they, and why 'micro'?
In a word
Nutrients needed in tiny but essential amounts
Micronutrients are nutrients the body needs only in very small amounts; the word covers vitamins and minerals. The amounts are small, but running short of even one causes health problems. Carbohydrate, protein and fat, which are needed in large amounts, are called macronutrients.
How small?
Milligrams, not grams
Carbohydrate, protein and fat are eaten in grams (g) each day. Micronutrients are needed in milligrams (mg), a thousandth of a gram, or micrograms, a thousandth of that. The daily vitamin C recommendation for adults is 75–90 mg, far less than 1 g.
Vitamins
13 substances the body needs to work
Vitamins are carbon-based (organic) substances found in food. People need 13 of them, named with letters such as A, C, D, E, K and several B vitamins. The body cannot make enough of most of them on its own, so they come from food.
Two groups of vitamins
Fat-soluble and water-soluble
Vitamins A, D, E and K dissolve in fat and are stored in the liver and body fat. Vitamin C and the 8 B vitamins dissolve in water, and most of what is left over leaves the body in urine. So water-soluble vitamins need to be topped up often.
Minerals
Inorganic nutrients the body cannot make
Minerals are nutrients without a carbon backbone (inorganic), such as calcium, iron and zinc. The body cannot make them, so they all come from food. Minerals make up about 4% of body weight, and the most plentiful are the calcium and phosphorus in bone.
Two groups of minerals
Needed in small amounts, or in tiny amounts
Some minerals, such as calcium, phosphorus, magnesium, sodium and potassium, are needed in larger amounts. Others, such as iron, zinc, copper, iodine and selenium, are needed only in tiny amounts. The second group is called trace minerals.
What they do
No energy, but they keep the body working
The calories in food come from carbohydrate, protein and fat. Micronutrients have no calories, but they help the body use those three. Calcium and phosphorus also make bones hard, and iron is used by the blood to carry oxygen.
When you lack them
Too little iron causes anemia
Not having enough of a micronutrient is called a deficiency. The most common deficiencies in the world are iron, vitamin A and iodine. With too little iron, the blood cannot carry enough oxygen around the body; this is called anemia.
When you take too much
Fat-soluble vitamins build up
Because fat-soluble vitamins are stored, taking large amounts as supplements causes harm. Many harmful effects have been reported in people who took large amounts of vitamin A supplements for several months or more. So agencies set a daily upper intake level for each nutrient, above which the risk of harm goes up.
Common myth / Is extra water-soluble vitamin always flushed out?
Vitamin B6 damaged nerves
No. Vitamin B6 dissolves in water, but people who took large daily amounts as supplements developed nerve damage: numb hands and feet and unsteady walking. All of them got better after they stopped. The US upper intake level for B6 is 100 mg a day, but cases have also been reported at much lower amounts.
Summary
Small amounts, none missing
Micronutrients are the vitamins and minerals the body needs only in very small amounts but cannot do without. Vitamin B6 showed that even a water-soluble vitamin causes harm when taken in large amounts. Related terms: macronutrients, upper intake level, fat-soluble and water-soluble vitamins.