Chapter 3: Apply knowledge of micronutrients

Micronutrients are essential nutrients required by the human body in small amounts to perform vital physiological functions. For nutrition and dietetics professionals in Kenya, understanding micronutrients is crucial to designing diets that prevent deficiencies and promote optimal health, particularly in vulnerable populations such as children, pregnant women, and the elderly. This chapter explores the classes of micronutrients, focusing on vitamins and minerals, and examines their key food sources within the Kenyan dietary context. Mastery of this knowledge enables practitioners to address micronutrient malnutrition effectively through targeted nutritional interventions.

3.1 Classes of micronutrients

Micronutrients are the vitamins and minerals the body needs in small amounts. They are sorted into classes because members of the same class behave in the same way: they are absorbed in the same way, kept in the body in the same way, and lost from food in the same way. Knowing the class of a micronutrient lets a dietitian predict how it should be cooked, how often it must be eaten, and how a supplement can go wrong. Vitamins are divided by what they dissolve in, and minerals by how much the body needs each day. The foods that supply them, their functions and the diseases caused by their lack are taught in the later topics of this chapter.

3.1.1 Vitamins

Vitamins are organic compounds that the body needs in very small amounts to work normally, and that it cannot make in enough quantity. There are a few partial exceptions: the skin makes vitamin D when exposed to sunlight, the body makes some niacin from the amino acid tryptophan, and bacteria in the large intestine make a little vitamin K and biotin. These sources do not usually meet the full need, so diet remains the main supply.

Thirteen vitamins are recognised. Nine dissolve in water (vitamin C and the eight B vitamins) and four dissolve in fat (vitamins A, D, E and K). The solvent largely decides how the vitamin is absorbed, carried, stored and lost.

3.1.1.1 Water soluble vitamins

Water soluble vitamins dissolve in water. They are absorbed in the small intestine directly into the blood and travel freely or attached to carrier proteins. The body keeps only small reserves of most of them, and any excess is passed out in the urine. This is why a regular supply from food is needed, and why a person on a poor diet can run short within weeks or months. Vitamin B12 is the exception: the liver holds a store that can last for years.

Because they dissolve in water, they are also lost from food into cooking water. A large intake from food is seldom harmful, but very large doses of some water soluble vitamins taken as supplements, for example vitamin B6, can still cause harm.

Example: a family boils kale for a long time in a lot of water and then pours the water away. Much of the vitamin C and folate has been lost into the water and by the heat, so the vegetable supplies less of them than it did raw.

The water soluble vitamins

The group has nine members:

• Vitamin C (ascorbic acid): the only water soluble vitamin outside the B group. It is easily destroyed by heat, air and light, and is lost into cooking water.
• Vitamin B1 (thiamine): sensitive to heat and to alkali, so adding bicarbonate of soda to cooking water destroys it.
• Vitamin B2 (riboflavin): stable to heat but destroyed by light, so milk kept in clear containers in sunlight loses it.
• Vitamin B3 (niacin): stable to heat and light. The body can make some niacin from the amino acid tryptophan.
• Vitamin B5 (pantothenic acid): widely found in foods, and its name comes from a Greek word meaning "from everywhere".
• Vitamin B6 (pyridoxine): a group of three related compounds, pyridoxine, pyridoxal and pyridoxamine.
• Vitamin B7 (biotin): also made by bacteria in the gut. Raw egg white contains avidin, which binds biotin and stops its absorption, and cooking destroys avidin.
• Vitamin B9 (folate): the form found naturally in foods. Folic acid is the synthetic form used in supplements and fortified foods. Folate is sensitive to heat and light.
• Vitamin B12 (cobalamin): contains cobalt and occurs naturally in animal-source foods. It needs intrinsic factor, a protein made by the stomach lining, to be absorbed.

The eight vitamins B1, B2, B3, B5, B6, B7, B9 and B12 are together called the B-complex.

Keeping water soluble vitamins in food

Careful handling protects the vitamins that dissolve in water:

• Wash vegetables before cutting them, not after, so that cut surfaces do not sit in water.
• Cook in a small amount of water, with a lid on, for the shortest time that makes the food tender.
• Use the cooking water in soups, sauces or stews instead of throwing it away.
• Do not add bicarbonate of soda to cooking water for green vegetables or beans.
• Keep milk and other riboflavin-rich foods in opaque containers, away from direct sunlight.

3.1.1.2 Fat soluble vitamins

Fat soluble vitamins dissolve in fat and not in water. They are absorbed together with dietary fat: bile from the liver breaks fat into tiny droplets, the vitamins enter these droplets, are absorbed by the cells of the small intestine, and are carried away in the lymph inside particles called chylomicrons before reaching the blood. In the blood they travel attached to carrier proteins.

The body stores them, mainly in the liver and in fatty tissue, vitamin K in smaller amounts. Stores can supply the body for weeks or months, so a short period of low intake does not cause trouble at once. The same stores explain why excess intake builds up instead of being passed out in the urine. Toxicity from food is unusual, but high-dose supplements of vitamins A and D are well known causes of it. A dietitian therefore counsels about supplements and refers the client to the clinician for dosing.

Example: a child eats a carrot cooked with a little oil. The oil helps the plant pigment beta-carotene, which the body turns into vitamin A, to be absorbed. A child fed boiled carrot with no fat at all absorbs less.

The fat soluble vitamins

The group has four members:

• Vitamin A (retinol): found as retinol in animal foods and as provitamin A carotenoids, such as beta-carotene, in plant foods.
• Vitamin D (calciferol): exists as D2 (ergocalciferol) from plants and fungi and D3 (cholecalciferol) from animals. The body makes D3 in the skin when it is exposed to sunlight.
• Vitamin E (tocopherols and tocotrienols): a group of related compounds found mainly in plant oils.
• Vitamin K: K1 (phylloquinone) comes from green leafy plants, and K2 (the menaquinones) is made by bacteria, including those of the large intestine.

Effect of fat intake and processing on fat soluble vitamins

These vitamins are fairly stable to cooking heat and are not lost into cooking water. They can be lost through exposure to air and light, and through repeated frying at very high temperatures. Because absorption depends on fat, a diet with very little fat, or a gut or liver condition that reduces fat absorption, can limit how much of these vitamins the body takes up even when the foods supply them.

3.1.2 Minerals

Minerals are inorganic elements that the body needs for building tissues, regulating fluids and nerve and muscle activity, and working with enzymes. They are the part of food that remains as ash when the food is burned. The body cannot make them, so they must come from food and drinking water. Minerals are divided into two classes by the amount the body needs each day: macro minerals and micro minerals.

Unlike many vitamins, minerals are not destroyed by cooking heat. They can still be lost when they dissolve into cooking water that is thrown away, or when food processing removes the part of the grain that holds them.

How minerals behave in the body

Most minerals are absorbed in the small intestine. In the blood they travel as free charged particles, such as sodium and potassium, or attached to proteins, such as iron on transferrin. Some are stored: calcium and phosphorus in bone and teeth, iron as ferritin in the liver, spleen and bone marrow, and iodine in the thyroid gland. Minerals leave the body through urine, faeces and sweat. Iron is unusual because the body has no regulated way of removing a surplus, so too much iron from supplements can accumulate.

Factors that change mineral absorption

How much of a mineral the body actually takes up, its bioavailability, is often lower than the amount in the food. Several things change it:

• Phytates: compounds in whole grains, maize and legumes that bind iron, zinc and calcium. Soaking, fermenting and germinating these foods reduces phytate.
• Oxalates: compounds in some leafy vegetables that bind calcium and make it hard to absorb.
• Tannins: compounds in tea that reduce absorption of iron from plant foods when the tea is drunk with the meal.
• Vitamin C: eaten in the same meal, it improves absorption of iron from plant foods.
• The form of the mineral: iron from animal foods (haem iron) is absorbed better than iron from plant foods (non-haem iron).

3.1.2.1 Macro minerals

Macro minerals are the minerals needed in amounts of more than 100 mg a day. The dividing line is a convention used to group the minerals, not a natural boundary. They make up the larger part of the body's mineral content, and several of them are the main electrically charged particles that control fluid balance.

There are seven macro minerals:

• Calcium: the most abundant mineral in the body. Nearly all of it is held in bones and teeth.
• Phosphorus: the second most abundant mineral. Most of it is also in bones and teeth.
• Magnesium: a large share is held in bone. The rest is in soft tissues and body fluids.
• Sodium: the main positively charged particle outside the cells. Table salt is sodium chloride and is a major dietary source.
• Potassium: the main positively charged particle inside the cells.
• Chloride: the main negatively charged particle outside the cells. It enters the diet mostly together with sodium in salt.
• Sulphur: a part of the amino acids methionine and cysteine and of the vitamins thiamine and biotin. It is obtained through protein foods, and no separate daily intake is set for it.

Example: a dietitian explaining the macro minerals at a community health talk can point out that one pinch of table salt supplies two of them, sodium and chloride, while the same plate of food also has to supply calcium, phosphorus, magnesium and potassium from other foods. A diet is judged by the whole pattern of foods, not by one mineral.

3.1.2.2 Micro minerals

Micro minerals, also called trace minerals, are needed in amounts of less than 100 mg a day, and several are needed only in micrograms. Their small quantity does not mean they matter less: the body cannot work without them. For many of them the gap between too little and too much is narrow.

The main micro minerals are:

• Iron: found in two forms in food: haem iron in animal foods, which is better absorbed, and non-haem iron in plant foods.
• Zinc: its absorption is reduced by phytates in whole grains and legumes.
• Iodine: the iodine content of plant foods depends on the iodine in the soil, so people living far from the sea need iodised salt or other reliable sources.
• Copper: needed in very small amounts, and is absorbed less well when intake of zinc is very high.
• Selenium: the amount in plant foods depends on the soil in which they grow.
• Fluoride: in small amounts it strengthens tooth enamel, while too much stains and weakens teeth, a condition called dental fluorosis.
• Manganese: needed in tiny amounts and found in whole grains, nuts and tea.
• Chromium: needed in tiny amounts and found in small quantities in many foods.
• Molybdenum: needed in tiny amounts and found in legumes, grains and nuts.

Example: a dietitian reviewing the diet of a family in a dry highland area sees that all their food is grown locally and that they use plain, non-iodised salt. The classes of micro mineral tell her that iodine intake depends on the soil and the salt, so she checks the salt before looking at anything else.

Chapter Summary

This chapter explored the various classes of micronutrients, focusing on vitamins and minerals as essential components of human nutrition. Vitamins were divided into water soluble and fat soluble types, each with distinct roles and absorption mechanisms. Minerals were classified into macro minerals and micro minerals based on the amounts required by the body. The chapter also examined common food sources rich in these micronutrients, highlighting their availability in diverse diets. The vital functions of micronutrients were discussed, emphasizing their roles in physiological processes and overall health maintenance. Attention was given to the consequences of micronutrient deficiencies, including specific conditions such as anaemia, goitre, beriberi, pellagra, scurvy, and deficiencies in vitamins A and D. Understanding these deficiencies underscored the importance of adequate micronutrient intake for preventing disease and promoting well-being.