The reason you need to consume food is to take in nutrients that your body uses to maintain your cells. Some of these nutrients are used by cells to provide energy. Others become the building blocks for cellular products; still other nutrients participate in chemical reactions that are a part of cellular growth and maintenance.
Types of Nutrients
There are two major classes of nutrients -- macronutrients and micronutrients. The macronutrients include proteins, fats and carbohydrates. Cells use these to provide for immediate energy needs, and also store them in various forms for later energy use. The micronutrients include vitamins and minerals. Your body depends upon these to participate in various chemical reactions that are involved in cellular growth and maintenance, but they don't provide your cells with any energy.
Your cells engage in many processes each day, some of which burn energy. Cellular growth and maintenance of body cells, such as making chemicals for communication purposes, repairing cellular damage and forming new cells to replace old ones, are all energy-dependent processes. Explains Dr. Lauralee Sherwood in her book "Human Physiology," while your energy needs increase if you're active and moving around, your cells are tremendously active even if you're at rest, so you need a constant supply of energy molecules.
The energy-providing macronutrients -- carbohydrates, protein and fat -- are all made up of small molecules that your cells chemically burn to liberate energy. When you digest food, you break large nutrient molecules into smaller ones that your intestine absorbs, notes Dr. Gary Thibodeau in his book "Anatomy and Physiology." Cells take up the small nutrient molecules from the bloodstream and combine them with oxygen, to produce ATP, a chemical energy molecule. Your cells then use ATP to fuel their activities.
Most of the processes in the body require enzymes, which are chemicals that help reactions take place faster than they otherwise would, explain Drs. Reginald Garrett and Charles Grisham in their book "Biochemistry." Many of the micronutrients, including most vitamins, assist in enzyme function. Without vitamins, your enzymes aren't able to function, and cells can't produce energy from macronutrients. Minerals either assist in enzyme functions or help maintain fluid balance, which keeps cells alive and capable of communicating with each other.
While the body needs food substances in order to function on a cellular level, more of any nutrient than the body needs doesn't help it to function better -- in fact, too much of a nutrient can have detrimental effects. Too much energy-providing nutrition leads to weight gain and the health issues associated with obesity. Overconsumption of many vitamins -- specifically the fat-soluble vitamins like A, D, E and K -- can lead to adverse effects and toxicity reactions.