The human body obtains energy from food through a multi-step process that involves digestion, absorption, and metabolism.
First, when we eat, the digestive system breaks down food into smaller components. This begins in the mouth, where enzymes in saliva start to digest carbohydrates. As the food travels to the stomach, it is mixed with gastric juices, which further break it down into a semi-liquid form.
Next, in the small intestine, these smaller molecules—such as carbohydrates, fats, and proteins—are absorbed into the bloodstream. Carbohydrates are broken down into glucose, fats into fatty acids and glycerol, and proteins into amino acids. The small intestine has tiny, finger-like projections called villi that increase the surface area for absorption, allowing more nutrients to enter the blood.
Once absorbed, these nutrients are transported to various cells throughout the body. Glucose is a primary energy source; it can be used immediately or stored as glycogen in the liver and muscles for later use. Fats are used as a long-term energy reserve, while proteins can be used for energy if necessary, although their primary role is in building and repairing tissues.
In the cells, glucose and fatty acids undergo a process called cellular respiration, where they are converted into ATP (adenosine triphosphate), the energy currency of the cell. This process occurs mainly in the mitochondria and involves several biochemical pathways, including glycolysis and the citric acid cycle.
Ultimately, through these complex processes, the body transforms the food we eat into the energy needed for daily activities, growth, and overall function.