Fueling the Fire
Although the human body is very good at transforming materials into useable substances and storing excess energy in reserves for later use, it still relies on environmental sources for fuel and materials to maintain homeostasis.
Protein
Muscle is basically proteins organized in such a way as to facilitate work. These materials do not last for a lifetime and are in a continuous state of remodel and repair. Therefore, protein must be obtained from dietary sources to provide this critical building block for human function.
The body can transform existing materials into some amino acids (the unit of protein structure); however, some essential amino acids can only be obtained from digested food. Without a sufficient supply of dietary protein from either meats or vegetables, the human body atrophies, growth stops, and premature death can occur. A diet of proteins alone, however, does not provide a full array of essential materials for healthy body function.
Carbohydrates
Carbohydrates have been much badmouthed in the media and by weight-loss proponents. However, carbohydrates are the foundational fuel for all cellular activity in the body. Without carbohydrates, the body uses lipids and protein as survival fuel. In that case, by-products accumulate in the blood stream, resulting in a lowering of blood pH (metabolic acidosis).
That being said, not all carbohydrates are good for your health. Those carbohydrates that are broken down and flood the blood stream rapidly (high glycemic index carbohydrates) cause a spike in blood sugar, which is hormonally reduced because the excess high sugar is stored as glycogen and fat. Additionally, in a short period of time, the blood sugar rapidly drops and the individual feels weak and lethargic as the body now tries to shift metabolic gears and put stored sugar back into the blood stream.
Fats
Dietary sources of fat and cholesterol are important in the production of new cell membrane, as well as steroid hormones for the body. Excess fat consumption can lead to many health issues, including coronary artery disease, so fat intake must be carefully monitored and regulated.
Maintaining a Healthy Weight
While the composition of the foods we eat is an important aspect of nutrition and health, the amount of what is eaten when compared to level of physical activity is often overlooked. A healthy diet should include no more calories than are demanded by the body on a daily basis. Any amount of calories over this results in fat stores increasing in the body. An additional 500 calories consumed daily (that are not used for energy) results in the addition of 1 pound of fat over the course of a single week. Likewise, a daily reduction of 500 calories per day burns a pound of fat in a single week. Therefore, diets rich in protein, with carbohydrates of low glycemic index, and limited in fats along with careful control of the amount eaten compared to energy used will yield a healthy lifestyle that can be maintained for a lifetime.