5.2.4. Nutrition in Human Beings

When we walk or ride a bicycle, we are using up energy. Even when we are not doing any apparent activity, energy is needed to maintain a state of order in our body. We also need materials from outside in order to grow, develop, synthesise protein and other substances needed in the body. This source of energy and materials is the food we eat.

Nutrition in human beings refers to the process through which we obtain energy and materials necessary for our growth, development, and overall functioning. Whenever we engage in physical activities like walking or cycling, our body uses energy generated from the food we consume. Even in a resting state, our body requires energy to maintain homeostasis and fulfill essential functions such as breathing and circulation. Thus, the food we eat acts as a source of that energy and is crucial for our body's maintenance and growth.

Think of your body as a car. Just as a car requires fuel to run smoothly, our body needs food for energy and to keep all systems functioning. If a car doesn't get enough fuel, it can’t drive efficiently; similarly, if our body lacks proper nutrition, we may feel sluggish or unwell.

The general requirement for energy and materials is common in all organisms, but it is fulfilled in different ways. Some organisms use simple food material obtained from inorganic sources in the form of carbon dioxide and water. These organisms, the autotrophs, include green plants and some bacteria. Other organisms utilise complex substances. These complex substances have to be broken down into simpler ones before they can be used for the upkeep and growth of the body. To achieve this, organisms use bio-catalysts called enzymes. Thus, the heterotrophs survival depends directly or indirectly on autotrophs. Heterotrophic organisms include animals and fungi.

In the biological world, there are primarily two types of nutrition - autotrophic and heterotrophic. Autotrophs, like plants, synthesize their food using sunlight, carbon dioxide, and water through the process of photosynthesis. In contrast, heterotrophs, including humans, rely on consuming organic substances (food) created by autotrophs or other heterotrophs. Heterotrophs cannot synthesize their own food, so they depend on breaking down complex food materials into simpler forms with the help of enzymes, which are biological catalysts that speed up chemical reactions in the body.

Think about how we get our food. Just like you might buy a pizza that someone else has prepared (the heterotroph), plants create their own pizza in the kitchen of their leaves by using sunlight to mix ingredients from the soil and air (the autotroph). Therefore, without the plants, we wouldn't have our delicious pizzas!

The alimentary canal is basically a long tube extending from the mouth to the anus. Various regions are specialised to perform different functions. What happens to the food once it enters our body? We shall discuss this process here.

The human digestive system, known as the alimentary canal, is a complex system that processes food. When we consume food, it enters through the mouth, where it is mechanically broken down by chewing and mixed with saliva. This begins the digestion process as enzymes in saliva begin breaking down carbohydrates. After that, food travels down the esophagus to the stomach, where gastric juices continue breaking it down into a semi-liquid form called chyme. This chyme then enters the small intestine, where most nutrient absorption occurs. Thus, the digestion process can be thought of as a journey through a specialized tube, where food is systematically processed and broken down.

Imagine a factory assembly line. At each workstation, different parts of the food are handled and processed. The workers (enzymes) at every station (organs in the alimentary canal) have specific jobs to ensure the food is broken down properly and all the valuable nutrients are extracted efficiently, just as the factory processes different materials to create a final product.

When we eat something we like, our mouth ‘waters’. This is actually not only water, but a fluid called saliva secreted by the salivary glands. The saliva contains an enzyme called salivary amylase that breaks down starch which is a complex molecule to give simple sugar. The food is mixed thoroughly with saliva and moved around the mouth while chewing by the muscular tongue.

Saliva plays a crucial role in the digestion process beginning in the mouth. When we think of or see food, our salivary glands secrete saliva, which includes enzymes like salivary amylase. This enzyme starts to break down starch, a complex carbohydrate, into simpler sugars. This breakdown is essential, as the body can only absorb simple sugars. Saliva not only helps in digesting food but also moistens it, making it easier to swallow.

Think of saliva as a software program that prepares a huge file for processing. Just as software organizes and simplifies information into a more manageable format for your computer, saliva helps simplify food into smaller particles which can be efficiently digested and utilized by the body.

It is necessary to move the food in a regulated manner along the digestive tube so that it can be processed properly in each part. The lining of canal has muscles that contract rhythmically in order to push the food forward. These peristaltic movements occur all along the gut.

Peristalsis is a series of wave-like muscle contractions that moves food through the digestive tract. These contractions occur in the smooth muscles lining the alimentary canal, ensuring the food moves along the digestive system in a regulated manner, preventing backflow and facilitating proper processing at each organ stage.

You can think of peristalsis like the way a water slide transports a swimmer down its length. Just as the slide controls the swimmer’s speed and direction, the rhythmic muscle contractions guide food through the digestive tract, ensuring it reaches its destination in the stomach and intestines efficiently.

Digested food is taken up by the walls of the intestine. The inner lining of the small intestine has numerous finger-like projections called villi which increase the surface area for absorption. The villi are richly supplied with blood vessels which take the absorbed food to each and every cell of the body.

The small intestine is where most of the nutrients from digested food are absorbed. Its lining is covered with tiny finger-like structures called villi, which significantly increase the surface area available for absorption. Each villus is equipped with blood vessels that transport the absorbed nutrients such as glucose, amino acids, and fatty acids to the body's cells for energy, growth, and repair.

Think of the villi like little straws in a smoothie. The more straws you have, the more smoothie you can sip up quickly. Likewise, the numerous villi in the small intestine allow a large amount of nutrients to be absorbed efficiently, providing the body with what it needs.

The unabsorbed food is sent into the large intestine where its wall absorb more water from this material. The rest of the material is removed from the body via the anus.

Once the food has gone through digestion and absorption, any remaining undigested food moves into the large intestine. Here, the body absorbs additional water and minerals, which is essential for maintaining hydration and electrolyte balance. The leftover material, which is now more solid waste, is prepared to be excreted through the anus.

Consider the large intestine like a sponge soaking up excess water from a spill. Just as a sponge absorbs moisture, the large intestine extracts water and nutrients from leftover food before the waste is expelled, helping our body maintain a proper balance of fluids.

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