Food Nutrients Selection Principles
Senior 1 · Home Science
Food Nutrients Selection Principles
Lesson 2 of 5
22 min
Let's begin
Walking to school, playing football and even thinking all use energy, and almost all of it comes from two nutrients: carbohydrates and fats. Rwandan meals are rich in carbohydrate foods such as sweet potatoes, cassava, matoke, maize and rice. The question for a good cook is not whether to eat them, but which kinds to choose: whole grains or refined flour, water or a sugary drink, vegetable oil or hardened fat.
In this lesson you will learn the types of carbohydrate and fat, what each does in the body, simple tests to detect them, and the guidelines for sugar and fat.
Carbohydrates are nutrients made of carbon, hydrogen and oxygen. They are grouped by the size of their molecules:
| Group | Meaning | Examples and food sources |
|---|---|---|
| Monosaccharides (simple sugars) | One sugar unit | Glucose; fructose (fruit, honey); galactose (from milk sugar) |
| Disaccharides (double sugars) | Two sugar units joined | Sucrose (table sugar, sugar cane); lactose (milk); maltose (sprouted or malted grain, such as sorghum malt) |
| Polysaccharides | Many sugar units joined in long chains | Starch (cassava, potatoes, sweet potatoes, matoke, maize, rice, bread); glycogen (the store of carbohydrate in the liver and muscles); cellulose, the main part of dietary fibre (vegetables, fruit skins, beans, whole-grain flour) |
Functions:
Too much: extra carbohydrate is stored first as glycogen and then as body fat, which can lead to overweight. Sugars also feed the mouth bacteria that cause tooth decay. The World Health Organization's 2015 guideline is that free sugars (sugars added to food and drinks, plus those in honey and fruit juice) should give less than 10% of a person's daily energy. For a typical adult diet, that is about 50 g of sugar a day.
Fats are solid at room temperature; oils are liquid. Both are made of units called fatty acids joined to glycerol. Fats and oils are grouped by their fatty acids:
| Type | At room temperature | Main sources | Effect on health |
|---|---|---|---|
| Saturated | Usually solid | Butter, ghee, beef and goat fat, full-fat milk, coconut and palm kernel oil | Large amounts raise the risk of heart disease |
| Unsaturated | Usually liquid | Sunflower, soya and groundnut oils; avocado; groundnuts; fish | Healthier choice; replacing saturated fat with these lowers heart disease risk |
| Trans | Solid | Formed when oils are partly hardened in factories: some margarines and shortenings, many fried snacks and biscuits | The most harmful type; keep as low as possible |
Two fatty acids, one omega-3 and one omega-6, are essential fatty acids: the body cannot make them, so they must come from food such as vegetable oils, soya, groundnuts and fish.
Functions of fat:
Visible fats can be seen and measured, such as cooking oil and butter. Invisible fats are hidden inside foods, such as the fat in groundnuts, avocado, egg yolk, fried mandazi and chips. Much of the fat people eat is invisible.
Task: The label on a 500 ml bottle of soda reads "Sugars: 11 g per 100 ml". (a) How much sugar is in the whole bottle? (b) Compare it with the WHO limit of about 50 g of free sugar a day.
Solution:
(a) 500 ml is 5 × 100 ml, so sugar = 11 × 5 = 55 g.
(b) 55 ÷ 50 × 100 = 110%. One bottle is more than a whole day's limit of free sugar, and it provides energy with almost no other nutrients. Water, milk or a piece of fruit is a better choice.
Tip
Memory hook: "Mono, di, poly = one, two, many" sugar units. For fats: "Saturated sets solid; unsaturated stays runny."
Exam technique: in label questions, first convert the label's "per 100 g" or "per 100 ml" to the whole packet, then compare with the guideline. Show both steps.
Name the group of carbohydrate each belongs to: (a) starch; (b) sucrose; (c) glucose; (d) lactose.
Solution: (a) polysaccharide; (b) disaccharide; (c) monosaccharide; (d) disaccharide.
Why is whole-grain maize flour a better choice than refined white maize flour?
Solution: Whole-grain flour keeps the outer layers of the grain, so it has more fibre (and more vitamins and minerals). The fibre prevents constipation.
Give two functions of fat other than supplying energy.
Solution: Any two of: carries vitamins A, D, E and K; insulates the body; cushions organs; forms part of cell membranes; supplies essential fatty acids.
A drop of iodine on a slice of Irish potato turns blue-black. What does this show?
Solution: The potato contains starch.
A biscuit packet of 200 g says "Sugars: 30 g per 100 g". How much sugar is in the whole packet?
Solution: 200 g is 2 × 100 g, so 30 × 2 = 60 g of sugar.
Summary
Sources and credits
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
