If a person stops eating or participates in a fast, their metabolism slows down, they may compromise their immune system if they have an underlying condition and they enter a state of ketosis, WebMD states. In addition, people who fast may experience headaches, dehydration, constipation and increased stress levels due to the lack of nutrients. Such effects occur with prolonged periods of fasting.
Although it is not dangerous to fast for 24 to 48 hours, prolonged periods of fasting have a negative effect on the body. While some may experience weight loss initially, this is primarily fluid loss, and the body eventually clings to fat reserves when it becomes aware that food intake is low. As a result, the faster's metabolic rate slows.
In addition, individuals suffering from a chronic condition that affects their immune system, such as diabetes, may suffer further detrimental immunological effects. This makes them more susceptible to pathogens. When the body does run out of carbohydrates and proteins to burn, it consumes lipids. This forces it into a state of ketosis, which some believe may encourage the body to lose toxins.
Other side effects that arise when a person stops eating include headaches and dehydration. As many people do not drink enough while fasting, they become dehydrated. Headaches arise as a result of hunger. In addition, when a person does not eat enough fiber, they become constipated. Finally, not eating enough causes the body to enter a state of stress.