Food Nutrients and Diet
Senior 1 · Biology
Food Nutrients and Diet
Lesson 1 of 5
25 min
A plate of ibishyimbo (beans) with sweet potatoes looks like one meal, but to your body it is a mixture of many different substances. Some are burned for energy within minutes, some are built into new muscle, some are stored for later, and some pass straight through. Biologists sort these substances into seven groups called nutrients. This lesson is about the three that you eat in the largest amounts.
Definition
Nutrient: a substance in food that the body uses for energy, for growth and repair, or to keep its processes working. Example: protein.
Diet: all the food and drink a person usually takes in.
Macronutrients are needed in large amounts (grams a day): carbohydrates, lipids and proteins. Micronutrients are needed in small amounts (milligrams or less): vitamins and mineral salts.
| Class | Main use in the body | Examples of Rwandan sources |
|---|---|---|
| Carbohydrates | energy | cassava, sweet potatoes (ibijumba), maize, rice, sorghum, cooking bananas |
| Lipids (fats and oils) | stored energy, insulation, cell membranes | avocado, groundnuts, palm and sunflower oil, milk, butter |
| Proteins | growth and repair | beans, peas, soya, eggs, milk, meat, fish such as isambaza |
| Vitamins | keep body processes working | fruits and green vegetables (lesson 2) |
| Mineral salts | bones, teeth, blood, nerves | milk, beans, dark green leaves, iodised salt (lesson 2) |
| Roughage (dietary fibre) | keeps food moving through the gut | vegetables, fruit, whole grains, beans |
| Water | solvent and transport | drinking water, fruit, soups, milk |
Carbohydrates, lipids and proteins are all large organic molecules built mostly from carbon. Look at Figure 1 and notice two things: each is made by joining many small units together, and they contain different elements.
Figure 1: The building blocks of carbohydrates, lipids and proteins.
Carbohydrates contain only carbon, hydrogen and oxygen, with twice as many hydrogen atoms as oxygen atoms, so their general formula is .
Lipids also contain carbon, hydrogen and oxygen, but much less oxygen. A fat or oil molecule (a triglyceride) is one glycerol unit joined to three fatty acids. Fats are solid at room temperature and come mainly from animals; oils are liquid and come mainly from plants. Lipids store energy, insulate the body under the skin, cushion organs such as the kidneys, form part of every cell membrane, and carry the vitamins A, D, E and K.
Proteins contain carbon, hydrogen, oxygen and nitrogen, and often sulfur. They are chains of amino acids; about 20 kinds are joined in different orders, like letters making different words, and each chain folds into a particular shape. The body uses proteins to build and repair cells, and to make enzymes, antibodies, haemoglobin and some hormones. Nine amino acids cannot be made by the body and must come from food: these are the essential amino acids.
Nutritionists use these standard energy values, which appear on food labels in many countries:
| Nutrient | Energy released |
|---|---|
| Carbohydrate | per gram |
| Protein | per gram |
| Lipid | per gram |
A gram of fat releases more than twice as much energy as a gram of carbohydrate, which is why fat is the body's long-term energy store.
Warning
Proteins are for building, not mainly for energy. Protein can release energy, but the body uses carbohydrates and lipids first. It burns protein for energy only when it is short of the others, for example in starvation, and then it breaks down its own muscle. So "protein gives us energy" is a weak exam answer; "protein is used for growth and repair" is the strong one.
Worked example 1: the energy in a meal
A plate of rice and beans contains 60 g of carbohydrate, 15 g of protein and 10 g of fat. How much energy does it provide?
Answer: about 1645 kJ. Notice that 10 g of fat gives more than a third as much energy as 60 g of carbohydrate.
Worked example 2: which molecule is which?
Three molecules have these formulas: P is ; Q is ; R is . Identify the class of each.
Maize and most other cereals are low in one essential amino acid, lysine; beans are rich in lysine but lower in another, methionine, which cereals supply. Eaten together, as in the common Rwandan meal of maize and beans, the two foods provide all the essential amino acids. This is why families who eat little meat can still meet their protein needs.
Biomolecules (Updated 2023), Amoeba Sisters. Watch the parts on carbohydrates, lipids and proteins; the part on nucleic acids goes beyond this unit.
Tip
Memory hook: "N is for New cells." Only proteins contain nitrogen, and proteins build new cells. For energy per gram, remember "fat is double": 37 kJ against 17 kJ.
Practice: carbohydrates, lipids and proteins
Key points
Sources and credits
Figure 1, the tables, the worked examples and the practice questions are written by Ganzaa. The energy values are the standard ones used on food labels (for example, in the European Union's food information regulation of 2011).
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
