Hello, and welcome to today's biology lesson. Today, we are going to explore the fascinating world of nutrition — how our bodies obtain and use the substances they need to survive and thrive. By the end of this lesson, you will understand what nutrients are, why we need them, the different classes of nutrients, and how to maintain a balanced diet for good health.
Let us begin with some fundamental terms. Nutrition is defined as the supply of essential organic and inorganic chemical compounds to the body. These compounds are collectively called nutrients. Food, then, is any substance we eat or drink that contains these nutrients — think of milk, bread, eggs, fish, or apples. Your diet is the combination of food items you consume in your meals. A balanced diet is one that contains all the principal constituents of food in proper quantities for a particular person.
Now, what makes a diet balanced for you specifically? Four key factors come into play. First, your age — whether you are a child, young adult, or elderly. Second, your sex — male or female. Third, your lifestyle and nature of work — whether you are a farmer, an athlete, or someone with a sedentary job. And fourth, any special needs you might have — such as illness, pregnancy, or nursing a baby.
A meal is the food taken at one time to satisfy your appetite. Appetite is simply the desire to eat, while hunger is that uneasy sensation you feel when your body lacks food.
Why do we need nutrition at all? All living organisms require food for six main purposes. First, for growth — building new protoplasm and cells. Second, for repair — providing material to fix worn-out or injured cells. Third, for energy — powering all the life functions your body performs. Fourth, to maintain the chemical composition of cells. Fifth, to provide raw materials for manufacturing secretions like enzymes, hormones, sweat, and milk. And sixth, to protect against disease and infection.
Now let us explore the six classes of nutrients in detail. These are carbohydrates, fats, proteins, mineral salts, vitamins, and water. Each plays a unique and vital role in your body.
Carbohydrates are compounds of carbon, hydrogen, and oxygen, with hydrogen and oxygen always in a two-to-one ratio. They are your body's principal energy source. When oxidised in cells, they release the energy you need to function.
Sugars are a type of carbohydrate that dissolve in water and taste sweet. They fall into two major categories. Monosaccharides, or simple sugars, have the chemical formula C₆H₁₂O₆. These need no digestion and are absorbed directly into your body. The three main types are glucose, fructose, and galactose.
Glucose, often called grape sugar, is the most common simple sugar in organisms. When we speak of blood sugar, we mean glucose levels in your blood. One mole of glucose releases 686 kilocalories of energy. Fructose, or fruit sugar, is common in plants. Galactose is found in milk.
Disaccharides, or double sugars, have the formula C₁₂H₂₂O₁₁ and require digestion before absorption. Sucrose is your commercial sugar, obtained mainly from sugarcane or sugar-beet. It is made of glucose and fructose joined together. Maltose, or malt sugar, consists of two glucose molecules. Lactose, or milk sugar, is composed of glucose and galactose.
Starch is an insoluble carbohydrate with the formula (C₆H₁₀O₅)n, making it a polysaccharide. Plants store carbohydrates as starch. Potatoes, grains like rice and wheat, and bread are excellent sources.
Two other important polysaccharides are cellulose and glycogen. Cellulose forms plant cell walls. Humans cannot digest it, but it provides crucial roughage for gut function. Glycogen is how animals store carbohydrates, mainly in the liver and muscles.
Let us talk more about roughage, the indigestible cellulose in your diet. Since we lack enzymes to break down cellulose, it passes through undigested. Yet it serves remarkable functions. It absorbs water, keeping faecal matter soft and preventing constipation. It eases the movement of food through your intestines. It stimulates muscle contractions in the intestinal wall. And it helps remove cholesterol and toxins from your body. Good sources include salads, green vegetables, fruits like guava and banana, corn, and cabbage.
Fats are also composed of carbon, hydrogen, and oxygen, but with much less oxygen than carbohydrates. They actually produce more energy — one mole of fat releases 9.45 kilocalories. Common sources include butter, ghee, cheese, vegetable oils, and fish liver oils. Fats are made of fatty acids and glycerol.
The functions of fat are fourfold. Like carbohydrates, they produce energy. They serve as an important storage form of food. They act as solvents for fat-soluble vitamins. And the fat layer under your skin insulates your body, protecting against rapid heat loss.
Proteins are large molecules containing carbon, hydrogen, oxygen, and nitrogen. Nitrogen is the most essential element in proteins. Some proteins also contain sulphur and phosphorus. The simple units of proteins are amino acids.
Proteins provide the chemical material for growth and repair of body cells and tissues. In emergencies, they can also be oxidised to release energy. Rich sources include lean meat, fish, eggs, milk, cheese, nuts, beans, and peas.
Severe protein deficiency leads to two serious diseases: kwashiorkor and marasmus. Kwashiorkor affects children aged one to five years. It occurs when breastfeeding stops early and the child receives a diet poor in proteins but rich in carbohydrates. The name comes from an African term meaning "the sickness the child gets when a new baby comes" — referring to how the older child loses mother's milk.
Symptoms include being underweight, a protruding belly, dark and scaly skin, stunted growth, loss of appetite, repeated diarrhoea, enlarged liver, anaemia, and swelling of the feet and face due to water accumulation. Treatment requires a protein-rich diet of pulses, milk, eggs, fish, and meat.
Marasmus affects infants below one year. It results from deficiency of carbohydrates, fats, and proteins — essentially a lack of energy-giving foods. It follows sudden stopping of breastfeeding with inadequate replacement feeding. Symptoms include very low body weight, wasted appearance with skin and bones, loosely folded skin, thin face and limbs, retarded physical and mental growth, and prominent ribs. Treatment requires a diet rich in both proteins and carbohydrates.
Mineral salts are needed in small quantities but are absolutely essential. Let us examine the key minerals.
Calcium and phosphorus strengthen bones and teeth. Calcium also helps blood clot. Phosphorus is vital for producing chemical energy in the form of ATP during cell respiration. Find them in milk, meat, eggs, fish, and pulses.
Iron is crucial for forming haemoglobin; its deficiency causes anaemia. Green leafy vegetables and liver are excellent sources.
Iodine is needed for proper thyroid function; deficiency leads to goitre. Sources include vegetables and mineral water.
Potassium and sodium maintain cell permeability, especially in nerve cells. Most foods and table salt provide these.
Minerals are categorised as macronutrients, needed in larger amounts, and micronutrients, needed in trace amounts. Macronutrients include calcium, sodium, potassium, phosphorus, magnesium, sulphur, and chlorine. Micronutrients include iron, cobalt, copper, zinc, iodine, fluorine, manganese, and molybdenum. Each has specific functions, and deficiency in any can lead to characteristic diseases.
Vitamins are chemical substances needed in minute amounts to maintain a healthy body. Most act as catalysts or enzymes in essential chemical changes. Some are fat-soluble — vitamins A, D, E, and K — and can be stored in your body for longer periods. Others are water-soluble — the B-complex vitamins and vitamin C — and cannot be stored long-term.
Prolonged shortage of vitamins causes deficiency diseases. However, excess vitamins can also be harmful. Water-soluble excesses are easily excreted, but fat-soluble excesses may accumulate.
Vitamin A, or retinol, promotes growth and resists skin and mucous membrane infections. It is essential for vision in dim light. Deficiency causes night blindness and xerophthalmia — dryness and ulceration of the cornea. Sources include butter, eggs, liver, milk, carrots, and leafy green vegetables.
Vitamin D, or calciferol, helps your body use calcium and phosphorus for bones and teeth. Your skin also produces it when exposed to ultraviolet rays. Deficiency causes rickets in children and osteomalacia in adults. Find it in fish liver oils, milk, and eggs.
Vitamin E, or tocopherol, prevents oxidation of vitamin A. Deficiency leads to muscle weakness, nerve damage, and immune problems. Sources include meat, milk, and whole wheat.
Vitamin K, or phylloquinone, is essential for blood clotting. Deficiency causes haemorrhage — delayed clotting time. Leafy greens like cabbage and spinach provide it, and your intestines also synthesise it.
Among water-soluble vitamins, B-one or thiamine increases growth and appetite, aids digestion, and supports nervous system function. Deficiency causes beriberi, with nerve inflammation and muscle weakness. Whole grains, yeast, liver, and lean meat are rich sources.
B-two, or riboflavin, regulates food oxidation. Deficiency causes eye and skin irritation, intestinal disorders, and tongue inflammation. Eggs, liver, milk, and green vegetables provide it.
B-three, or niacin, promotes skin and nervous system health. Deficiency causes pellagra, with dermatitis, memory loss, diarrhoea, and skin lesions. Lean meat, liver, milk, eggs, and groundnuts are good sources.
B-five, or pantothenic acid, synthesises and metabolises fats and proteins, promoting healthy skin, hair, eyes, and liver. Mushrooms, avocados, sweet potatoes, and lentils contain it.
B-six, or pyridoxine, helps interconvert amino acids. Meat, fish, eggs, and cereal bran provide it.
B-nine, or folic acid, synthesises haemoglobin. Deficiency causes anaemia. Liver and leafy vegetables are rich sources.
B-twelve, or cobalamin, ensures normal functioning of red blood cells. Deficiency causes pernicious anaemia, with severe weakness, gastrointestinal disturbance, and spinal cord degeneration. Liver, meat, milk, cereals, and pulses contain it.
Vitamin C, or ascorbic acid, promotes capillary wall function. Deficiency causes scurvy, with spongy bleeding gums and loose teeth. Fresh citrus fruits, tomatoes, and germinating seeds provide it — but remember, heat readily destroys it.
Water is indispensable for life. About two-thirds of your body weight is water. It serves multiple critical functions.
It acts as a solvent for thousands of substances in your body. It produces digestive juices. It transports digested foods and oxygen throughout your body. It helps excrete soluble wastes. And it maintains your body temperature.
You lose water constantly through sweat, urine, and breath. Replace it by drinking liquids and eating water-rich foods like fruits and vegetables. Aim for about seven to eight glasses of water daily.
Let us now bring everything together with the concept of a balanced diet. A balanced diet contains all principal constituents of food in proper quantity for a particular person. It must provide sufficient calories — remember, a kilocalorie is the amount of heat needed to raise one gram of water by one degree Celsius.
Calorie needs vary: a two-to-three-year-old child needs 1000 to 1400 kilocalories daily, while a teenager needs 1800 to 2400. An adult woman requires 2000 to 2400, and an adult man 2600 to 3000. Pregnant women need 2200 to 2600, and labourers 2800 to 3200.
Malnutrition is the condition of suffering due to an unbalanced diet — this includes both deficiency and excess of nutrients. Undernutrition specifically refers to poor nutrition from not having enough essential nutrients. Both conditions harm health and development.
Let us recap the key takeaways from today's lesson.
First, nutrition supplies essential chemical compounds called nutrients to your body, obtained through food.
Second, the six classes of nutrients are carbohydrates and fats for energy, proteins for growth and repair, mineral salts as body constituents, vitamins for special functions, and water as a universal solvent and transport medium.
Third, prolonged deficiency of vitamins and minerals causes specific deficiency diseases — from night blindness and rickets to anaemia and goitre.
Fourth, severe protein deficiency causes kwashiorkor in children aged one to five, while overall energy deficiency causes marasmus in infants under one year.
Fifth, roughage or indigestible cellulose is essential for proper gut function, preventing constipation and aiding waste removal.
And sixth, a balanced diet provides all nutrients in proper quantities tailored to your age, sex, lifestyle, and special needs.
Understanding nutrition empowers you to make informed choices about what you eat. Every meal is an opportunity to nourish your body and support your health. Apply this knowledge to build healthy eating habits that will serve you throughout your life.
Thank you for joining today's lesson on nutrition. Stay curious, stay healthy, and I look forward to our next biology exploration together.