Food Nutrients Selection Principles
Senior 1 · Home Science
Food Nutrients Selection Principles
Lesson 1 of 5
20 min
Let's begin
Your hair, skin, muscles, blood and even the enzymes that digest your lunch are built from protein. At your age the body is growing faster than at any time since early childhood, so it needs a steady supply. In Rwanda most protein comes from plants, especially beans, and a cook who understands protein can build strong meals from cheap foods.
A nutrient is a substance in food that the body uses for energy, growth, repair or regulation. In this lesson you will learn what proteins are made of, why some protein foods are more complete than others, how to combine plant foods, and how much protein a person needs.
Proteins are long chains of smaller units called amino acids, joined together like beads on a string. About 20 different amino acids are used to build human proteins. The body can make most of them itself, but nine cannot be made by the body and must come from food. These are the essential amino acids.
During digestion, the body breaks food proteins down into amino acids, absorbs them, and then rebuilds them into the proteins it needs.
The biological value of a protein describes how well its amino acids match what the body needs.
| High biological value (HBV) | Low biological value (LBV) | |
|---|---|---|
| Meaning | Contains all nine essential amino acids in good amounts | Short of one or more essential amino acids |
| Examples | Meat, fish (tilapia, sambaza), eggs, milk, soya beans | Beans, peas, groundnuts, maize, rice, sorghum, wheat |
The essential amino acid that is in shortest supply in a food is called its limiting amino acid. Cereal grains such as maize and rice are low in lysine. Legumes such as beans and peas are low in methionine.
Protein complementation (protein combining) means eating two or more LBV foods whose limiting amino acids are different, so that each supplies what the other lacks. Together they give protein almost as good as an HBV food.
The foods do not have to be eaten in the same mouthful, or even at the same meal. Eating a variety of grains and legumes over the day is enough.
| Food | Protein (approximate) |
|---|---|
| Dry red kidney beans, uncooked | 22.5 g per 100 g |
| Kidney beans, boiled | 8.7 g per 100 g (they have absorbed water) |
| Groundnuts (peanuts), raw | 25 g per 100 g |
| Egg, one of about 50 g | 6 g per egg |
| Cow's milk | about 32 g per litre (3.2%) |
| Cassava root, raw | 1.4 g per 100 g |
Notice how little protein cassava has. A diet built mainly on cassava, bananas or sweet potatoes can fill the stomach and still leave a child short of protein.
A common guideline for healthy adults is about 0.8 g of protein for every kilogram of body weight each day. Growing children and teenagers, pregnant and breastfeeding women, and people recovering from illness need relatively more.
Task: Mrs Mukamana weighs 60 kg. (a) Using 0.8 g per kg, find her daily protein need. (b) Show that 150 g of dry beans (weighed before cooking), 1 egg and a 250 ml cup of milk come close to it.
Solution:
(a) 60 × 0.8 = 48 g of protein a day.
(b) Beans: 22.5 g per 100 g, so 150 g gives 22.5 × 1.5 = 33.75 g. Egg: 6 g. Milk: 32 g per litre, so 250 ml (0.25 litre) gives 32 × 0.25 = 8 g. Total = 33.75 + 6 + 8 = 47.75 g, about 48 g. The small amounts of protein in her maize, rice or sweet potato make up the rest, and the beans with a grain complement each other.
Tip
Memory hook: "Grain + legume = complete". Grains lack lysine, legumes lack methionine; together they cover each other.
Exam technique: when asked to distinguish HBV from LBV protein, give the definition (all essential amino acids or not) AND an example of each.
What is an essential amino acid?
Solution: An amino acid the body cannot make, so it must come from food. There are nine for humans.
Classify as HBV or LBV protein: (a) eggs; (b) peas; (c) sambaza; (d) maize flour.
Solution: (a) HBV; (b) LBV; (c) HBV; (d) LBV.
Explain why a meal of rice and beans provides better protein than rice alone.
Solution: Rice is low in lysine, and beans supply lysine. Beans are low in methionine, and rice supplies it. Together they provide all the essential amino acids (protein complementation).
A 45 kg adult needs 0.8 g of protein per kg. How much protein is that per day?
Solution: 45 × 0.8 = 36 g.
How much protein is in 250 g of boiled kidney beans?
Solution: 8.7 g per 100 g, so 8.7 × 2.5 = 21.75 g, about 22 g.
Summary
Sources and credits
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
