Unit 7 Food nutrients and diet
Key unit competence: Assess the relevance of different food nutrients to the human body.
Key ideas
- Nutrients are substances that organisms need for energy, growth, repair and good health. The six classes are carbohydrates, proteins, lipids (fats and oils), vitamins, mineral salts and water; roughage (dietary fibre) is also needed.
- Carbohydrates contain carbon, hydrogen and oxygen (C, H, O; hydrogen and oxygen in the ratio 2:1). Monosaccharides (glucose, fructose, galactose) are sweet, soluble, reducing sugars. Disaccharides (sucrose, maltose, lactose) are two sugars joined; sucrose is non-reducing. Polysaccharides (starch and cellulose in plants, glycogen in animals) are long chains of glucose: not sweet, insoluble. Sources: maize, cassava, rice, bread, potatoes, sugar, fruits. Main use: energy.
- Lipids contain C, H and O and are made of one glycerol + three fatty acids. Fats are solid at room temperature, oils are liquid. They store energy (twice as much per gram as carbohydrate), insulate the body, protect organs and carry vitamins A, D, E and K. Sources: butter, milk, groundnuts, avocado, cooking oil, fish.
- Proteins contain C, H, O and nitrogen (sometimes sulphur and phosphorus). They are long chains of amino acids (about 20 kinds). They are used for growth, repair and making enzymes and some hormones; they are denatured by high temperature. First-class proteins (meat, fish, eggs, milk) have all essential amino acids; second-class proteins (beans, peas, groundnuts, soya) lack some. Soya milk is a cheap, rich protein source.
- Vitamins are needed in small amounts: A (carrots, milk, egg yolk; night vision; lack: night blindness), B1 (whole grains, beans; lack: beriberi), C (oranges, lemons, guava, green vegetables; healthy gums, wound healing; lack: scurvy), D (fish liver oil, milk, eggs, made in skin in sunlight; calcium absorption; lack: rickets), K (green vegetables, liver; blood clotting). A, D, E, K are fat-soluble; B and C are water-soluble.
- Mineral salts: iron (liver, meat, beans, green vegetables; haemoglobin; lack: anaemia), calcium (milk, small fish; bones, teeth, clotting; lack: rickets, weak bones), iodine (iodised salt, sea fish; thyroxine; lack: goitre), sodium (nerves, water balance; lack: cramps), phosphorus (bones, energy), potassium (nerves, muscles).
- Water is the medium for chemical reactions, transports substances, cools the body by sweating, lubricates joints and removes wastes. Roughage (cellulose from vegetables, fruits, whole grains) adds bulk to food, helps peristalsis and prevents constipation.
- Food tests: starch: iodine solution turns blue-black; reducing sugar: Benedict's solution, heat in a water bath, blue → green → yellow → brick-red precipitate; protein: Biuret test (sodium hydroxide then copper sulphate), blue → purple/violet; lipid: emulsion test: dissolve in ethanol, add water, a milky-white emulsion forms; or a translucent grease spot on paper; vitamin C: DCPIP blue turns colourless.
- A balanced diet contains all nutrients in the right proportions and amounts for a person's needs (energy measured in kilojoules). Needs depend on age (children and teenagers need more protein for growth), sex (girls need more iron), activity (manual workers and athletes need more energy), pregnancy and breastfeeding (more protein, iron, calcium, folic acid, energy and water) and health.
- Malnutrition is too little or too much of nutrients. Kwashiorkor: lack of protein (swollen belly and feet, thin limbs, pale hair, children often weaned onto cassava and maize only). Marasmus: lack of almost all food, especially energy (very thin, wasted). Obesity: too much energy food (fats, sugars) stored as fat; risk of heart disease, diabetes, stroke. Constipation: lack of fibre and water.
Common misconceptions
- "Fat is bad and should be removed from the diet completely." No: lipids give energy, insulation and carry vitamins A, D, E and K; only too much is harmful.
- "Benedict's test works at room temperature." No: the mixture must be heated; without heating there is no colour change.
- "Iodine solution tests for sugar." No: iodine tests for starch (blue-black). Sugars are tested with Benedict's.
- "A person who eats a lot is always well-fed." No: a large diet of only maize and cassava lacks protein, vitamins and minerals; the person is still malnourished.
Worked examples
Example 1. A food sample turns blue-black with iodine and purple in the Biuret test, but stays blue in Benedict's test after heating. What does it contain?
Solution
Blue-black: starch present. Purple: protein present. Blue after heating with Benedict's: no reducing sugar. Example: a bean flour.
Example 2. A two-year-old child in a family that eats mainly cassava has a swollen belly, thin arms and pale hair. Name the disorder, its cause and how to prevent it.
Solution
Kwashiorkor, caused by lack of protein. Prevent it with protein foods: milk, eggs, beans, soya milk, small fish, and continued breastfeeding with a balanced diet.
Practice questions
Name the six classes of food nutrients and give one source of each.
Show answer
Carbohydrates (maize), proteins (beans), lipids (cooking oil), vitamins (oranges), mineral salts (milk for calcium), water (drinking water).
Common mistake: Leaving out water.
Describe how to test a food sample for reducing sugar and give a positive result.
Show answer
Add Benedict's solution to a solution of the food, heat in a boiling water bath for a few minutes; a change from blue through green and yellow to a brick-red precipitate shows reducing sugar.
Common mistake: Forgetting to heat.
Describe the emulsion test for lipids.
Show answer
Shake the food with ethanol, pour the liquid into a test tube of water; a milky-white emulsion shows lipid.
Common mistake: Adding water first.
Give three functions of proteins in the body.
Show answer
Growth (making new cells); repair of damaged tissues; making enzymes and some hormones; also antibodies.
Common mistake: Saying the main function of protein is energy.
Name the deficiency disease caused by lack of each: vitamin C, vitamin D, iron, iodine.
Show answer
Scurvy, rickets, anaemia, goitre.
Common mistake: Mixing scurvy and rickets.
Explain three factors that affect the diet a person needs.
Show answer
Age: children need more protein for growth; activity: manual workers need more energy; pregnancy: more protein, iron, calcium and folic acid; also sex and health.
Common mistake: Listing factors without explaining how they change the diet.
Distinguish between kwashiorkor and marasmus.
Show answer
Kwashiorkor: lack of protein; swollen belly and feet, thin limbs, pale hair. Marasmus: lack of most foods, especially energy; very thin, wasted body without swelling.
Common mistake: Saying both are caused by lack of vitamins.
Plan a balanced day's meals for a Senior 1 learner using local foods.
Show answer
Breakfast: sorghum porridge with milk and a banana. Lunch: rice, beans, cooked green vegetables (dodo) and avocado. Supper: sweet potatoes, small fish (isambaza) and cabbage; fruit; plenty of clean water.
Common mistake: Choosing meals of only starchy foods.
Matching and drag-and-drop
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. Lack of vitamin C causes: a. Glycogen 2. The main function of roughage is to: b. help food move along and prevent constipation 3. Which food is the richest source of protein? c. scurvy 4. Which is the storage carbohydrate in animals? d. Beans Show answer
1 → c (scurvy) · 2 → b (help food move along and prevent constipation) · 3 → d (Beans) · 4 → a (Glycogen)
- Scurvy: bleeding gums and slow wound healing.
- Fibre adds bulk and stimulates peristalsis.
- Beans, soya and groundnuts are good plant proteins.
- Glycogen is stored in the liver and muscles.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
balanced diet · rickets · she loses blood during menstruation · special diet- A diet with all nutrients in the right proportions is a .
- Lack of vitamin D or calcium in children causes .
- A teenage girl needs more iron than a teenage boy of the same age mainly because .
Show answer
1: balanced diet · 2: rickets · 3: she loses blood during menstruation
- It meets all the body's needs.
- Soft bones bend under the body's weight.
- Iron replaces haemoglobin lost in menstrual blood.
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. Lack of iron in the diet causes: a. goitre 2. The elements found in all carbohydrates are: b. amino acids 3. The building units of proteins are: c. carbon, hydrogen and oxygen 4. Iodised salt prevents: d. anaemia Show answer
1 → d (anaemia) · 2 → c (carbon, hydrogen and oxygen) · 3 → b (amino acids) · 4 → a (goitre)
- Iron is needed to make haemoglobin.
- Carbohydrates contain only C, H and O.
- About 20 kinds of amino acids join in chains.
- Iodine is needed for the hormone thyroxine.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
lipid and protein · B1 · A · one glycerol and three fatty acids- Night blindness is caused by lack of vitamin .
- A food gives a translucent spot on paper and a purple Biuret test. It contains .
- A lipid molecule is made of .
Show answer
1: A · 2: lipid and protein · 3: one glycerol and three fatty acids
- Vitamin A is needed for a pigment in the retina.
- Grease spot: lipid; purple: protein (for example groundnuts).
- This is a triglyceride.