Both starvation and fasting are states of abstinence from food. The main differences between the two are the purpose and length of time of that abstinence. In both cases, the physiological changes that take place help the body adapt to the nutrient deficit. While fasting is generally a safe practice, it can be harmful if prolonged.
The Body's Sources of Fuel
When you eat, your body breaks down the food into sugars, fatty acids and amino acids. These are the basic building blocks needed for maintaining bodily processes. Sugar, or glucose, is stored in the liver and muscles in a chemical form called glycogen. Fat is stored as fatty acids in adipose tissue, otherwise known as "fat cells." Amino acids make up the protein that builds muscle. When you no longer consume these important nutrients, your body must metabolize muscle protein, glycogen and stored fat to meet your nutritional needs.
Fasting Versus Starving
In the absence of food intake, hormonal changes take place that allow the body to make use of those stored nutrients. For example, insulin, epinephrine and glucagon are hormones that work together to regulate blood sugar. Insulin brings down blood sugar levels by causing cells to take up glucose, while epinephrine and glucagon elevate blood sugar by controlling the breakdown of glycogen. During starvation, insulin decreases and glucagon and epinephrine increase. This is often when you feel hungry. The interplay between these hormones allows the body to keep blood sugar levels stable during a fast. In starvation, however, glycogen has been used up and molecules called ketones are generated from stored fatty acids to be used as an alternative fuel. In addition, protein from muscle can be used to make glucose, which is when food avoidance becomes dangerous. When the body begins breaking down its own muscle for protein, distortion of cells and even death from organ failure may occur.
Reasons for Food Abstinence
Starvation and fasting have different purposes. Starvation can be the result of an eating disorder or an undesired consequence of lack of access to food. Alternatively, fasting is typically performed for a defined period of time for a particular purpose, such as a religious practice, in preparation for a medical procedure or as part of a cleanse. Fasting may include a very low-calorie diet, which has been attempted as a health-promoting practice. A small study at Washington University School of Medicine published in the "Proceedings of the National Academy of the Sciences" in 2003 suggested that caloric restriction reduced cardiovascular disease risk.
When embarking on a fast, it is important to proceed with caution to avoid starvation. Side effects may include headaches, dizziness and low blood pressure. The American Cancer Society claims that fasting for a short period of time, such as eight to 12 hours, is not likely to be harmful, provided that fluid intake is adequate.