0610 · 7.1Diet

Balanced diet and nutrient needs

0:00Hundreds of years ago, on long voyages across the ocean, sailors often became very ill. Their gums bled, their teeth became loose, and small cuts would not heal. Many of them died. They had plenty of bread and salted meat, so they were not starving. The problem was something missing from their food. When ships began to carry citrus fruits, such as lemons and oranges, the illness became much rarer.

0:21That illness is called scurvy, and it shows us something important. Having enough food is not the same as having the right food. By the end of this video you will be able to describe what is meant by a balanced diet. You will know the main sources of each type of nutrient, and why your body needs it. And you will be able to state the causes of two deficiency diseases, scurvy and rickets.

0:42You met carbohydrates, fats and proteins as biological molecules in an earlier video. Now we will see where they come from in your food, and why your body needs them. Let us start with the big idea, a balanced diet. A balanced diet is a diet that contains all of the nutrients the body needs, in the correct proportions, or amounts, to meet that person's needs. Those nutrients are carbohydrates, fats and oils, proteins, vitamins and mineral ions.

1:08A balanced diet also includes fibre and water. Notice the words correct proportions. A balanced diet does not mean equal amounts of everything. You need far more carbohydrate than vitamin C, for example. And the correct amounts are different for different people. A teenager who is still growing, a pregnant woman, and a person with a very active job all have different needs.

Carbohydrates, fats and proteins

1:29First, carbohydrates. Good sources are bread, rice, pasta, potatoes and breakfast cereals. Sugary foods, like sweets and fruit juice, also contain carbohydrate. The main job of carbohydrates is to provide energy. In your cells, glucose is used in respiration to release energy. That energy is used for muscle contraction, keeping warm, growth, and all the other processes of life. Second, fats and oils. Fats are solid at room temperature, like butter.

1:56Oils are liquid, like olive oil or sunflower oil. Other good sources are cheese, nuts and oily fish. Fats are also a source of energy. In fact, a gram of fat releases about twice as much energy as a gram of carbohydrate. Fat is stored under the skin, where it acts as an energy store, and also as insulation, which helps keep the body warm. Fat stored around organs, such as the kidneys, helps to protect them.

2:18Fats are also used to build cell membranes. Third, proteins. Good sources are meat, fish, eggs, milk and cheese. Plant sources include beans, lentils, peas and nuts. Proteins are needed for growth, and for the repair of damaged tissues. That is why growing children and teenagers need plenty of protein. Proteins are broken down into amino acids, and your cells join those amino acids together to build their own proteins.

2:41These include the proteins in muscle, enzymes, antibodies, and haemoglobin in red blood cells. A common mistake is to say that the main job of protein is to give energy. The body can use protein for energy, but its main role is growth and repair. So in an exam, if a question asks why protein is needed, write growth and repair of tissues.

Vitamins, calcium and rickets

3:01Next come the vitamins and mineral ions. You need them in only very small amounts. They do not provide energy. But without them, the body cannot work properly. The syllabus asks for two vitamins, C and D, and two mineral ions, calcium and iron. Vitamin C is found in fresh fruit and vegetables, especially citrus fruits such as oranges, lemons and limes. Strawberries, peppers and broccoli are good sources too.

3:24Vitamin C is needed to make a protein called collagen. Collagen is found in connective tissue. You can think of connective tissue as the material that holds the body together. It is in your skin, your gums, and the walls of your blood vessels. Without collagen, these tissues become weak. This explains scurvy. Scurvy is caused by a lack of vitamin C in the diet. Without vitamin C, the body cannot make enough collagen, so connective tissue becomes weak.

3:48The gums bleed and swell, the teeth can become loose, small blood vessels break and cause bruising, and wounds heal slowly. Now the sailors make sense. Their diet had almost no fresh fruit or vegetables, so they got very little vitamin C. Vitamin D is found in oily fish, such as salmon and sardines, in egg yolk, and in milk and butter. Some foods, like breakfast cereals, have vitamin D added to them.

4:11There is also a second way to get it. Your skin makes vitamin D when it is exposed to sunlight. Vitamin D is needed for the absorption of calcium from the food in your intestine into the blood. Calcium is what makes bones and teeth hard and strong. That brings us to the first mineral ion, calcium. Good sources are milk, cheese and yoghurt, and green vegetables such as broccoli.

4:33Calcium is needed for strong bones and teeth. It is also needed for blood clotting, and for muscles to contract normally. Now we can understand rickets. Rickets is caused by a lack of vitamin D, or a lack of calcium. Vitamin D can be short because of a poor diet or too little sunlight. Both lead to the same problem. Without enough calcium reaching the bones, the bones of a growing child become soft and weak.

4:56They bend under the weight of the body, so the legs bow outwards. This is a key exam point. Rickets has two possible causes, so give both. Lack of vitamin D or lack of calcium. And if you are asked to explain it, make the link. Vitamin D is needed to absorb calcium, and calcium is needed for strong bones.

Iron, fibre and water

5:12The second mineral ion is iron. Good sources are red meat, liver, beans, and dark green vegetables such as spinach. Many breakfast cereals also have iron added. Iron is needed to make haemoglobin. Haemoglobin is the red protein inside red blood cells that carries oxygen around the body. As useful context, a person who does not get enough iron cannot make enough haemoglobin, so their blood carries less oxygen, and they feel tired and weak.

5:36This condition is called anaemia. Be precise here. Iron does not carry oxygen on its own. Iron is needed to make haemoglobin, and haemoglobin carries the oxygen. Now fibre, which is also called roughage. Fibre comes from plants. It is mostly cellulose, the material in plant cell walls. Good sources are wholemeal bread, brown rice, wholegrain cereals, fruit, vegetables, beans and lentils. Here is the surprising part. Humans cannot digest fibre.

6:06It passes all the way through the gut. So why do we need it? Because fibre adds bulk to the food in your intestines. The muscles in the wall of the gut push food along by squeezing behind it, in waves. This is called peristalsis. Bulky food gives those muscles something to push against, so food moves through more easily. This helps to prevent constipation. So fibre gives no energy and is not absorbed.

6:28Its job is mechanical. It helps peristalsis and prevents constipation. Finally, water. You get water from drinks, but also from food. Fruit and vegetables, like cucumbers and melons, are mostly water. Water is needed because it is a solvent. Most of the chemical reactions in your cells happen in solution, dissolved in water. Water is also the main part of blood plasma, which transports dissolved substances, such as glucose, around the body.

6:53Water is needed to remove waste, by dissolving it in urine. And when you sweat, water evaporates from your skin, which helps to cool you down.

Rickets worked example and recap

7:01Let us try an exam-style question. A doctor sees a young child whose legs are bowed outwards. The child spends almost all day indoors, and rarely eats eggs, oily fish or dairy foods. First, name the disease the child has. Second, explain how the child's lifestyle and diet have caused it. This part is worth 4 marks. The first part is straightforward. The disease is rickets. For the second part, think about what the clues are telling you.

7:27Staying indoors means less sunlight. Few eggs, oily fish and dairy foods means a diet low in vitamin D and calcium. Now build the chain of reasoning. Here is a model answer. The child makes little vitamin D in the skin, because they get little sunlight. Their diet also contains little vitamin D. Vitamin D is needed to absorb calcium from the intestine. Their diet also contains little calcium, because they eat few dairy foods.

7:51So not enough calcium reaches the bones. Calcium is needed to make bones hard and strong, so the bones are soft and weak, and they bend under the child's weight. Each correct link in the chain can earn a mark, up to the four available. A weak answer just says the child is not eating healthily. That scores nothing, because it does not name any nutrient or explain what it does.

8:12Let us recap. A balanced diet contains all the nutrients, carbohydrates, fats and oils, proteins, vitamins and mineral ions, together with fibre and water, in the correct proportions to meet a person's needs. Carbohydrates, from foods like bread and rice, provide energy. Fats and oils, from butter, oil and nuts, provide energy, act as an energy store and insulation, and build cell membranes. Proteins, from meat, fish, eggs and beans, are needed for growth and repair.

8:40Vitamin C, from citrus fruits, is needed to make collagen for connective tissue, and a lack of it causes scurvy. Vitamin D, from oily fish and eggs, and made in the skin in sunlight, is needed to absorb calcium. Calcium, from dairy foods, makes bones and teeth strong, and a lack of vitamin D or calcium causes rickets. Iron, from red meat and spinach, is needed to make haemoglobin. Fibre, from wholemeal foods, fruit and vegetables, adds bulk to food, helps peristalsis, and prevents constipation.

9:08And water is a solvent, transports substances in the blood, removes waste, and helps to cool the body. In the next video, we will look at the digestive system, and follow the journey your food takes through your body.