You know that plants take in energy from the sun and convert it to glucose. But how does that energy get to humans and animals? In this activity, you will learn about getting energy from food.

First of all, when many glucose molecules are formed through photosynthesis, they combine to form a larger molecule called starch. The sugars and starches in food are sometimes referred to as carbohydrates. When you eat the plant or when an animal eats the plant and you eat the animal, that energy is taken into your digestive system. It is then transported throughout your body to your cells by the circulatory system.

What do you do with this energy? The energy you take in through food powers your body and enables it to carry out all its complex functions. It keeps your heart beating, your lungs breathing, your brain thinking, and so on. Without it, you would not be alive! If more glucose is taken in than is immediately needed, this extra glucose is converted to glycogen, a starch-like substance. Glycogen is stored in the liver and muscles. When we need energy, glycogen is converted back into glucose.

Carbohydrates

Experts recommend that we take in 55–65% of our daily calories in carbohydrates.

Sometimes, people, especially athletes, will take in more. For example, many athletes get their energy from what is referred to as “carbo loading.” This means eating a large amount of carbohydrates a day or two before an athletic competition. The carbohydrates are stored in the liver as glycogen and can be drawn on quickly when energy is needed for endurance events.

The image above shows foods rich in carbohydrates. When you need energy, your body uses carbohydrates first.

Fats are sometimes thought of in a negative way. Too much fat can be a problem, but bodies need fat for good health.  

Fats

Body fat is used for insulation, to conserve energy, and as a source of energy. When carbohydrate stores of energy run low, the body will turn to fat to power its activities. Fat delivers almost twice as much energy as carbohydrates do.

The fats we eat are classified in two groups: saturated and unsaturated.

• Saturated fats come from animal fats and from some oils. These types of fats have been associated with increasing the risk of heart disease.
• Unsaturated fats include most vegetable oils, such as olive oil and canola oil. These fats have been associated with reducing the risk of heart disease.
• It is recommend that fat intake should be less than 30% of the daily calorie intake. It is also recommended that people should replace saturated fats with unsaturated alternatives where possible.

Proteins

Did you know that one gram of protein contains the same energy as one gram of carbohydrate? The difference between proteins and carbohydrates are that it is harder for the body to break down proteins to release the energy, so it takes longer. That is why the body will use proteins as energy sources only when there are little carbohydrates or fats left.

Proteins also provide about half the energy provided by fats. Although protein is a poor source of energy, it is important for other body functions. Proteins are used for growth and to help build and maintain body tissues. That is why children and women who are pregnant or breast-feeding need larger amounts of protein.

It is important to eat proper quantities of carbohydrates and fats because when proteins are used as an energy source, they are unable to perform their growth and repair functions effectively. 

The image above shows foods rich in protein. Experts recommend that about 10% of our daily diet should be in the form of proteins.