Our food is primarily made of carbohydrates, proteins, fats, minerals, vitamins and water. We need to regularly ingest food to sustain ourselves.
Living things do not follow the pattern of decay seen in the rest of the natural world. Unlike nonliving things (such as rocks), living things
new cells, tissues and organs in the process of development. Once mature, organisms can
existing biological structures by cellular division. This process, also called mitosis, allows cells to use molecules such as water, amino acids (building blocks of proteins), carbohydrates and lipids to create new cells. This process requires the transfer of energy among chemical bonds.
Our cells are regularly being replaced and recycled and this requires a regular influx of energy and matter. We obtain energy in the form of food calories; new matter in the form of the molecules contained in our food.
Not all molecules are digestible by all animals. These unneeded molecules may be eaten along with more desirable molecules. This is part of the reason why animals must have a caudal opening (anus) in their digestive system.
A balanced diet for humans includes several servings of breads, cereals, fruits and vegetables every day. Also essential are smaller servings of milk, dairy, meat and eggs. Fats, oils and sweets should make up a very small portion of a balanced diet.
Human beings eat several hundred pounds of food a year. From this food we obtain all the molecules we need so our bodies can operate, grow and replace the cells lost to wear, old age, or damage.
Animals, such as humans, have evolved intricate strategies of intercellular communication. Feedback, feed–forward, and inhibition allow us to maintain steady–state, or homeostasis.
We get hungry because we want to eat. This seemingly simple cause and effect is the result of the empty stomach secreting the peptide hormone ghrelin into the bloodstream. When this hormone is received by hypothalamus, it stimulates the release of growth hormones, which makes us want to eat. After eating to satiation, the release of the peptide hormone leptin causes the hypothalamus to inhibit hunger sensations. Leptin is released by a variety of tissues including adipose (fat cells), stomach, bone, pituitary, and the liver