Summary
Highlights
The human body is constantly acquiring, processing, and discarding materials. Even the components the body holds onto are temporary, requiring continuous replacement and reconstruction. This dynamic process of breaking down and rebuilding is fundamental to survival and requires a constant supply of raw materials and energy from food. These processes, occurring at the cellular level, are collectively known as metabolism, and they link together various bodily systems like digestive, endocrine, circulatory, and respiratory systems.
Metabolism isn't a single process but a reconciliation of two conflicting chemical processes: anabolism and catabolism. Anabolism involves building complex molecules from simpler ones, consuming energy in the process (e.g., forming carbs, fats, and proteins from monomer building blocks). Catabolism, conversely, breaks down larger molecules into smaller ones, releasing energy that the body uses for various functions like staying warm, movement, and fueling anabolic reactions. The body continuously cycles through these processes, breaking down and rebuilding nutrients.
The body requires six major groups of nutrients: water, vitamins, minerals, carbohydrates, lipids (fats), and proteins. Water is the most vital, making up the majority of our bodies. Vitamins and minerals do not provide fuel or building blocks but are crucial in facilitating the body's use of other nutrients. Examples include Vitamin C for iron absorption, Vitamin K for blood clotting, and calcium for bone hardening.
Carbohydrates, primarily from plants, are essential for energy. Glucose, a monosaccharide, is the main molecular fuel for cells to produce ATP, the energy currency of the body. While ATP is unstable for storage, energy is stored as glucose, glycogen, or converted to fats. Neurons and red blood cells rely exclusively on glucose for energy. Excess glucose can be stored as glycogen in the liver and muscles or converted into triglycerides.
Lipids (fats) are crucial for energy storage, cushioning organs, and storing fat-soluble vitamins. They also form myelin for neuron insulation, provide oil for skin, and are a vital calorie source in breast milk. Important lipids like cholesterol are precursors for hormones, and phospholipids form cell membranes. The body breaks down triglycerides into glycerol and fatty acids for ATP production or to create other fatty acids and new triglycerides. Essential fatty acids, like omega-3 and omega-6, cannot be synthesized by the body and must be ingested for functions like brain synapse formation and inflammation signaling.
Proteins are fundamental for almost every bodily function. They form muscle and connective tissue, ion channels, pumps in neurons, and enzymes that drive chemical reactions. Nutritionally, proteins come from meats, dairy, eggs, legumes, nuts, and cereals. All proteins are composed of 20 amino acids. When ingested, proteins are broken down into their component amino acids, which are then reassembled into new, unique proteins dictated by an individual's DNA. The body cannot synthesize nine essential amino acids, which must be obtained through diet. Combining foods, like beans and rice, can provide all necessary amino acids.