Why do you get hungry?

This subpage on why you get hungry forms the first part of the theory for Biotech Academy’s material on the Body and Diet .

 

Almost all living organisms have a brain. Your brain collects everything you see, hear, feel, taste and smell. The brain has always been an interesting area, because it is precisely the reason why you breathe, feel hungry, can talk to people or play football. In short: Your brain makes you live. It is constantly changing and you are actually able to influence it yourself, e.g. in the form of physical activity, food habits and sleep rhythm.

The brain is complex and all its processes are not yet known. For example, 20 years ago, it was believed that everyone was born with the same number of nerve cells, also called neurons. Today we know that it varies from person to person.

Currently, research is being conducted into whether certain foods can have effects on the brain. This suggests that large amounts of sugar can cause lows and depression, but researchers do not yet know what effect sugar has on humans, as the experiments have only been tested on animals.

Your brain, sheltered inside the skull, contains 100 billion nerve cells. Their task is to convey information between the different parts of the brain as well as information between the body and the brain. One end of the neurons, the dendrites, receives messages that, via the axon, send the impulse out to the nerve terminals. Their job is to pass the messages on to the next neuron. This is illustrated in Figure 2. The neurons do not touch each other, but lie like beads on a string.

The contact surface between neurons is called synapses. Here come constant electrical signals that the dendrites receive. The electrical signals cause the dendrites to release neurotransmitters that bind to a receptor on the neuron next to it. This binding triggers a new signal, and the whole process is repeated until the signal has reached the “target cell”. For example, it can be a signal to the intestine to contract so that the food can be digested.

The electrical signals can be affected by food, medicine and physical activity, among other things. Some foods contain substances that affect the brain. Energy drinks can provide a sense of energy. This is due to the caffeine content. Caffeine is similar to the neurotransmitter adenosine. This substance inhibits brain cell activity and gives a feeling of fatigue. Caffeine binds to the same receptors as adenosine, but has the opposite effect and therefore increases brain cell activity.

In addition, caffeine also helps increase the secretion of another neurotransmitter called dopamine, which makes you feel good. The invigorating effect that caffeine provides also has a downside, as it can reduce your ability to concentrate for a short time.

Another important neurotransmitter is serotonin. It has an impact on mood, emotions, sleep and appetite. Energy drinks contain a large amount of sugar and, as mentioned earlier, it is believed that sugar can affect mood. This is because sugar decreases the production of serotonin.

 

Appetite regulation – what determines if you are hungry

In the brain lies an area called the hypothalamus. It regulates both your feeling of hunger and satiety. Imagine that the hypothalamus receives many different signals, both from inside the body itself and from the outside. When your stomach is empty, it produces a hormone called ghrelin. It sends signals to the hypothalamus that you are hungry. In addition to the internal processes of the body, external circumstances also affect your hunger. Here the senses play a crucial role. The way food looks, smells and tastes has a big impact on the feeling of hunger, while the temperature around you also plays a role.

Satiety is signaled with the help of the hormone leptin. It is produced in the adipose tissue, and when the content of fat is increased in the individual fat cell, more leptin will be released into the bloodstream. When leptin binds to leptin receptors in the hypothalamus, a hormone is released that signals satiety.

Appetite regulation is thus influenced by many different factors. It plays a crucial role because it ensures that the body gets enough nutrients and energy. Without nutrients, your cells don’t survive.

Your whole body is made up of cells. The body’s tissues and organs have different functions, and therefore the cells also have varying lengths. The blood cells, platelets, have a size of about 3 micrometers, while nerve cells can grow up to 1 meter. Although the cells look very different, they have many of the same characteristics. They are all circumscribed by a cell membrane made up of lipids (fats) as Figure 3 illustrates. The cell membrane acts as a protective layer for the cell’s interior, and its structure allows only small molecules such as water, oxygen and nitrogen to pass through unhindered.

Structure of cells

Larger molecules need help. In the cell membrane there are encapsulated transport proteins that allow certain larger molecules to penetrate.

The interior of the cell is liquid and is called cytosol. Just as the body contains different organs in order to function, the cell needs so-called organelles to maintain its function. The ribosome, the golgi apparatus, the endoplasmic reticulum, and the mitochondria are all located inside the cell and have different functions. In the mitochondria, the nutrients we get from food are burned, and in this way energy is formed.

 

The genetic material

In the cytosol, the cell nucleus is also found. It contains the cell’s genetic system, the genes that are located on chromosomes. The chromosomes are made up of DNA, and it is precisely DNA that codes for your genes.

 

DNA is made up of two long chains that resemble a ladder in appearance. The steps are made up of bases that match each other in pairs. Each DNA molecule contains four bases: A, G, C, and T, which is short for adenine, guanine, cytosine, and thymine. A complements with the base T, while C and G form pairs.

All cells contain 46 chromosomes, 23 from your mother and 23 from your father. However, germ cells contain only 23 chromosomes, but when an egg cell fuses with a sperm cell, it turns into a cell with 46 chromosomes.

The body’s organs are made up of tissues, which are made up of cells. They each have their own function, e.g. muscle cells must ensure that the muscles contract and perform a movement, while nerve cells conduct impulses to and from the brain. But in order for cells to perform their specific tasks, they need nourishment. If you want to know more, you can watch BioStriben’s videos on “Genetics”.

 

From food to energy

The human body can be compared to a complicated machine that requires fuel in order to function. Over time, it becomes worn down and needs to be repaired. At the same time as the spare parts are replaced, the body also grows and it is therefore necessary to eat proteins, carbohydrates and fats, as well as vitamins and minerals, which together are called nutrients. The nutrients we get through the food we eat. In addition to acting as spare parts and building material for the cells, food also provides energy to the body.

The body uses the energy to control all the processes that take place. For example, keeping warm, digesting food or making sure the heart beats. In short: All body functions depend on the nutrients.

 

Energy – the yield of food

Energy is a word that is used in many contexts. You probably know the feeling of being depleted of energy or having a lot of energy to play a football match. The body needs energy to function, just as a radiator also requires energy to heat a room, or a car requires energy when it has to drive from A to B. In the following section you can read what the body needs the energy for.

In the physical sense, energy is the ability to perform work. The unit of work and energy is the joule, abbreviated J. In conjunction with the body, the device kilojoule, abbreviated kJ, is more applicable. In the old days, energy associated with food was indicated in calories, cal, or kilocalories, kcal. The definitions are:

 

A joule is the work that a force of 1 newton performs when it moves an object 1 meter in the direction of the force.

A calorie is the amount of heat needed to heat 1 g of water from 14.5°C to 15.5°C.

 

ATP

Metabolism is the term for the body’s energy metabolism, i.e. its formation and consumption of energy. For the body to function, it needs energy. This energy is supplied via the substance ATP. The energy comes from the food and depending on the content of the meal and the state of the body, there are different ways to form energy. In order for the body to be in balance, an equilibrium between consumption and the formation of energy is required.

 

The body’s energy expenditure

The body uses energy to maintain physiological functions, such as converting the food you consume or being able to play a football game. The four main components of the body’s energy expenditure include:

 

Resting energy consumption

Restful energy consumption includes the energy needed for the body’s basic functions such as breathing, keeping the heart going, and regulating body temperature when the body is completely relaxed. Although the brain only makes up 2-3% of the body’s mass, it uses just over 20% of the energy supplied to the body through food.

 

Energy for physical activity

Physical activity refers to all movements that occur due to muscle contractions. This means that energy consumption increases in relation to energy consumption in a resting state.

When the body is used for everyday activities such as a football match or other physical work, it needs energy. Compared to energy expenditure in a resting state, physical activity can increase by a factor of 5-10, however, it depends on whether you run a marathon, play a football match or go for a walk. The muscle groups play a crucial role and the higher the intensity and the larger the muscle groups that are put to work, the more energy is consumed.

 

Energy for temperature control

When body temperature rises or falls, there is an increase in energy expenditure, however, it is a relatively insignificant contribution because our clothing helps to insulate the body.

 

Energy to digest and absorb the diet

After eating a meal, the food must be broken down and then absorbed into the body. This process requires energy, and about 10% of the total energy consumption per day comes from it. When the meal begins, there is an increase in energy expenditure that lasts 3-12 hours, depending on whether the meal contains many or few joules and the digestibility of it.

 

Burning the nutrients

When food is absorbed into the body, energy is generated. This is done by breaking down the various nutrients. The amount of energy supplied depends on the content of the meal. Carbohydrate and protein each provide 17 kJ per gram, while 1 gram of fat provides 38 kJ. If the body needs energy, it will always start by burning sugar, which is also called glucose. The breakdown of glucose is called glycolysis and is described in a later section. If the body’s glucose content is not sufficient in relation to the amount of energy needed, fat burning will start. The third nutrient, the proteins, is also burned by the body, but this only happens if the glucose and fat stores are exhausted. When this happens, the muscles break down and it is therefore the least useful way to generate energy.

 

 

Question:
One Snickers contains 1176 kJ. You burn 100 kJ when you jump high and fast for 2 minutes. How long do you have to jump to burn a Snickers?