Food Nutrients and Diet
Senior 1 · Biology
Food Nutrients and Diet
Lesson 4 of 5
25 min
In one household in Musanze, the father carries sacks of potatoes at the market all day, the mother is expecting a baby, the grandmother is 75, and the two children are in P3 and S1. They eat from the same pots, yet each needs a different amount of food, and some need more of particular nutrients. What makes a diet "balanced" depends on who is eating it.
Definition
Balanced diet: a diet that contains all the nutrients in the right proportions, and enough energy (but not too much) to meet a person's needs.
Malnutrition: ill health caused by a diet that is not balanced. It includes undernutrition (too little energy or too little of a nutrient) and overnutrition (too much energy, leading to obesity).
Basal metabolic rate (BMR): the energy the body uses at complete rest, just to stay alive: breathing, heartbeat, keeping warm.
Figure 1: A simple plate guide to a balanced meal, with Rwandan foods.
Many health services use a plate like this as a simple guide: fill half with vegetables and fruit, a quarter with starchy foods for energy, and a quarter with protein foods. Add some fat (avocado, groundnuts or a little oil), a cup of milk for calcium, and water to drink. Eating many different foods over a week matters more than getting every single meal perfect.
The energy a person needs is their BMR plus the energy used for activity:
The activity factor is about 1.4 for a person who sits most of the day, about 1.7 for an active person, and 2.0 or more for very heavy physical work (FAO, WHO and UNU, Human Energy Requirements, 2004).
| Person | Needs more of | Why |
|---|---|---|
| Teenagers | energy, protein, calcium, iron | rapid growth of muscle and bone; girls lose iron in menstruation |
| Manual workers and athletes | energy (carbohydrates) | muscles work hard for many hours |
| Pregnant women | protein, iron, calcium, folic acid, some extra energy | building the baby's body, blood and bones |
| Breastfeeding mothers | energy, protein, calcium, water | making milk for the baby |
| Young children | protein, calcium, vitamins A and D (for their size) | fast growth; small stomachs, so food must be nutrient-rich |
| Older people | less energy, but the same vitamins and minerals; calcium and vitamin D | lower activity and BMR; bones become weaker with age |
| People recovering from illness | protein, energy, vitamins | repairing damaged tissue and fighting infection |
Worked example 1: calculating a daily energy need
Nutritionists estimate the BMR of a girl aged 10 to 18 from her mass (in kilograms) with the equation , in kJ per day (the Schofield equation used in the 2004 FAO, WHO and UNU report). Keza is 13, weighs 42 kg and is active: she walks to school and plays netball. Estimate her daily energy need.
Answer: about 8900 kJ a day, a little more than the 8400 kJ reference intake for an average adult, because she is growing and active. (For boys of the same age the equation is .)
Figure 2: The nutrition label on a packet of maize flour.
Worked example 2: using the label
Uwase makes ugali with 120 g of the maize flour in Figure 2. How much energy and protein does it give her, and what percentage of an adult's reference intake of energy is that?
Answer: about 1850 kJ, 22% of a day's energy, but only 9.6 g of protein. Adding beans to the meal would make it far better balanced.
Warning
"Balanced" does not mean "equal", and "more" does not mean "better". A balanced diet does not have equal amounts of each nutrient: you need hundreds of grams of carbohydrate a day but only milligrams of iron. And eating more energy than you use is also malnutrition: the extra is stored as fat, and obesity raises the risk of type 2 diabetes, high blood pressure and heart disease.
Rwanda's national school feeding programme gives pupils a midday meal at school, usually based on maize or rice with beans, with vegetables and, where possible, milk. A meal that combines a staple with beans provides energy and good protein at low cost, and helps pupils concentrate in afternoon lessons.
Balanced diet, FuseSchool. Listen for the food groups and the reasons different people need different amounts; compare them with the table above.
Tip
Memory hook: "Half, quarter, quarter, cup and glass": half the plate vegetables and fruit, a quarter starch, a quarter protein, a cup of milk and a glass of water.
Practice: a balanced diet
Key points
Sources and credits
Figures 1 and 2, the tables, the worked examples and the practice questions are written by Ganzaa. The BMR equations are the Schofield equations for ages 10 to 18, converted to kJ, as used in FAO, WHO and UNU, Human Energy Requirements (2004); the label in Figure 2 is an example, not a real product.
