These notes were written for the old IB syllabus (2009). The new IB syllabus for first examinations 2016 can be accessed by clicking the link below.

IB syllabus for first examinations 2016

Balanced diet

A balanced diet must contain (in the correct proportions):


Carbohydrates are the most important source of energy. They contain the elements Carbon, Hydrogen and Oxygen. The first part of the name "carbo-" means that they contain Carbon. The second part of the name "-hydr-" means that they contain Hydrogen. The third part of the name "-ate-" means that they contain Oxygen. In all carbohydrates the ratio of Hydrogen atoms to Oxygen atoms is 2:1 just like water.

Both sucrose and glucose are sugars, but sucrose molecules are too big to get into the blood, so the digestive system turns it into glucose. When we use glucose in tissue respiration we need Oxygen. This process produces Carbon Dioxide and water and releases energy for other processes.

We obtain most of our carbohydrate in the form of starch. This is found in potato, rice, spaghetti, yams, bread and cereals. Our digestive system turns all this starch into another carbohydrate called glucose. Glucose is carried around the body in the blood and is used by our tissues as a source of energy. Any glucose in our food is absorbed without the need for digestion. We also get some of our carbohydrate in the form of sucrose; this is the sugar which we put in our tea and coffee.

In general, sugar is not a healthy food as the body does not need to process the sugar and obtains the energy rather too easily. This can cause problems of excess, leading to people that are overweight and diabetes. This is a considerable problem in the developed world as large percentages of populations become obese with the corresponding decrease in general health, causing diabetes, heart disease and musculoskeletal problems.


Proteins are required for growth and repair. Proteins contain Carbon, Hydrogen, Oxygen, Nitrogen and sometimes Sulphur. Proteins are very large molecules, so they cannot get directly into our blood; they must be turned into amino-acids by the digestive system. There are over 20 different amino-acids. Our bodies can turn the amino-acids back into protein. When our cells do this they have to put the amino-acids together in the correct order. There are many millions of possible combinations or sequences of amino-acids; it is our DNA which contains the information about how to make proteins. Our cells get their amino-acids from the blood.

Proteins can also be used as a source of energy. When excess amino-acids are removed from the body the Nitrogen is excreted as a chemical called urea. The liver makes urea and the kidney puts the urea into our urine.


Like carbohydrates, fats contain the elements Carbon, Hydrogen and Oxygen. Fats are used as a source of energy: they are also stored beneath the skin helping to insulate us against the cold. If you eat too much carbohydrate and protein, you will convert some of it into fat, so you will put on weight. You must balance the amount of energy containing foods with the amount of energy that you use when you take exercise.

You must have some fat in your diet because it contains fat soluble vitamins.


Vitamins are only required in very small quantities. There is no chemical similarity between these chemicals; the similarity between them is entirely biological.

Vitamin A: good for your eyes.

Vitamin B: about 12 different chemicals.

Vitamin C: needed for your body to repair itself.

Vitamin D: can be made in your skin, needed for absorption of Calcium.

Vitamin E: is implicated in many of the body's processes

Mineral Salts

These are also needed in small quantities, but we need more of these than we need of vitamins. The body is a vast chemical reaction system with millions of different reactions all going on producing the substances that we need to survive. If we are lacking any of the vitamins and mineral s then these reactions cannot continue and we become ill.

Iron: required to make haemoglobin.

Calcium: required for healthy teeth, bones and muscles.

Sodium: all cells need this, especially nerve cells.

Iodine: used to make a hormone called thyroxin.

There are several other trace elements that are essential such as copper and selenium These are often implicated in enzyme catalysis for very important reactions.


We do not/can not digest cellulose. This is a carbohydrate used by plants to make their cell walls. It is also called roughage. If you do not eat foods materials which contain fibre you could develop problems of the colon and rectum. The muscles of you digestive system mix food with the digestive juices and push food along the intestines by peristalsis (rhythmic muscular movement); if there is no fibre in your diet these movements cannot work properly. These muscles are like any other - if you don't use them you lose them!

A Balanced Diet

You must have carbohydrate, protein, fat, vitamins, minerals salts and fibre in the correct proportions. If there is not enough protein, you will not be able to grow properly and you will not be able to repair yourself i.e. wounds will not heal properly.

If you do not have enough energy containing foods you will feel very tired, you will not have enough energy. If you have too much energy containing foods you will become overweight.

Finally beware of 'fad' diets that suggest you eat excess of one particular food group - these can be very dangerous.

If you are putting on weight - eat slightly less and excercise more. DO NOT try to lose weight fast by following extreme diets. If you are losing weight eat more BUT maintain your excercise regime. Don't forget there is natural seasonal variation in both hunger and weight.