Food Nutrients Selection Principles
Senior 1 · Home Science
Food Nutrients Selection Principles
Lesson 4 of 5
20 min
Let's begin
A girl in S1 feels tired all the time, is short of breath climbing the hill to school and cannot concentrate in afternoon lessons. The cause may be as simple as a lack of iron. Minerals such as iron, calcium and iodine are needed in small amounts, but a shortage of any of them shows up in blood, bones, growth and even learning.
Mineral salts (minerals) are elements that the body needs, which come from the soil and reach us through plants, animals and water. In this lesson you will learn the main minerals, how to help the body absorb iron, why iodised salt matters and why too much salt is harmful, and why water is a nutrient in its own right.
| Mineral | Main functions | Good sources | Result of too little |
|---|---|---|---|
| Calcium | Strong bones and teeth; blood clotting; muscle and nerve action | Milk, yoghurt (ikivuguto), small whole fish eaten with the bones (sambaza), dark green leaves, beans | Weak bones; rickets in children (with lack of vitamin D) |
| Phosphorus | Works with calcium in bones and teeth; energy release | Milk, fish, meat, beans, groundnuts | Rare |
| Iron | Part of haemoglobin, which carries oxygen in the blood | Meat, liver, fish; beans, amaranth, cassava leaves, groundnuts, sorghum | Iron-deficiency anaemia: tiredness, pale palms and inner eyelids, shortness of breath, poor concentration |
| Iodine | Makes thyroid hormones, which control growth, energy use and brain development | Iodised salt, fish | Goitre (swelling in the neck); in pregnancy, lasting harm to the baby's brain |
| Sodium (with chloride, as salt) | Controls the water balance of the body; nerve and muscle action | Table salt, stock cubes, salty snacks, processed foods | Rare; heavy sweating or diarrhoea can cause a shortage. Too much raises blood pressure |
| Potassium | Water balance; steady heartbeat | Bananas, beans, potatoes, avocado | Muscle weakness |
| Zinc | Growth, wound healing, fighting infection | Meat, fish, beans, groundnuts, whole grains | Poor growth; frequent infections |
| Fluoride | Hardens tooth enamel | Some drinking water, tea, fluoride toothpaste | More tooth decay |
Iron comes in two forms. Haem iron, in meat, liver and fish, is absorbed easily. Non-haem iron, in beans, leaves and grains, is absorbed much less well. Since most Rwandan iron comes from beans and leaves, it pays to help absorption:
Girls who have started menstruating, pregnant women and young children need the most iron, because they lose blood or are growing fast.
Iodised salt is table salt with a tiny amount of iodine added. Iodine deficiency affects about two billion people worldwide and is the leading preventable cause of intellectual disability, so always buy salt marked "iodised".
At the same time, most people eat too much salt. The World Health Organization recommends that adults eat less than 5 g of salt a day (about one level teaspoon), which contains 2 000 mg (2 g) of sodium. Food labels often show sodium, not salt. To convert:
salt ≈ sodium × 2.5
About 60% of an adult's body weight, and about 65% of a child's, is water. Water is not usually listed with the nutrients, but the body cannot survive more than a few days without it. It:
Water comes from drinking water and also from milk, porridge, soup, fruit and vegetables. The amount needed varies with age, activity, health and climate, and thirst is a good guide. More is needed in hot weather, during sport, and with fever or diarrhoea. Signs of dehydration are thirst, a dry mouth, dark yellow urine, headache and tiredness. A person with diarrhoea should be given oral rehydration solution (ORS) to replace both water and salts.
Task: A packet of instant noodles contains 1 400 mg of sodium. (a) How much salt is this? (b) What percentage of the WHO daily limit of 5 g of salt is it?
Solution: (a) Salt ≈ 1 400 × 2.5 = 3 500 mg = 3.5 g. (b) 3.5 ÷ 5 × 100 = 70% of the daily limit, from a single packet.
Task: Ishimwe's usual lunch is ugali and plain boiled beans, with a cup of sweet tea. Suggest two changes, at little or no cost, that increase the iron she absorbs, and explain each.
Solution: (1) Cook the beans with chopped tomato or add a few leaves of amaranth, and have an orange or passion fruit juice with the meal: vitamin C helps the body absorb non-haem iron from the beans. (2) Drink the tea an hour or more after the meal instead of with it, because substances in tea reduce iron absorption.
Tip
Memory hook: "Iron likes C, hates tea." And for labels: "sodium times two and a half makes salt".
Exam technique: for a mineral question, give the function, a Rwandan source AND the deficiency sign: e.g. "iodine: makes thyroid hormones; iodised salt; goitre".
Classify as a major or trace mineral: (a) calcium; (b) iodine; (c) iron; (d) potassium.
Solution: (a) major; (b) trace; (c) trace; (d) major.
Name the mineral whose lack causes (a) goitre; (b) anaemia.
Solution: (a) iodine; (b) iron.
Why are sambaza a better source of calcium than a fillet of tilapia?
Solution: Sambaza are small and eaten whole, with their soft bones, and bones are rich in calcium. A fillet has the bones removed.
A stock cube contains 1 000 mg of sodium. How much salt is that?
Solution: 1 000 × 2.5 = 2 500 mg = 2.5 g of salt, half of the 5 g daily limit.
List three signs of dehydration.
Solution: Any three of: thirst, dry mouth, dark yellow urine, headache, tiredness.
Summary
Sources and credits
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
