How does weight-loss fasting differ from fasting for religious purposes in terms of personal experience and the attitudes of society?

What a great question! Religious fasting and weight-loss fasting differ primarily in their motivations, but also to some degree in the lived, personal experience of the fast and the opinions others may have of a fasting person. Fasting is a spiritual practice in many religions, and in the Abrahamic traditions it may even be demanded of those who are physically capable during certain holy days, weeks, or months. Depending on the tradition and personal preference, fasting may involve only eating at certain times of day for an extended period of time, eating very plain food, and/or limiting the liquids one consumes. Religious fasting is typically performed as a means of re-orienting or "touching base" with one's faith and as a demonstration of devotion. For example, the Muslim holy month of Ramadan is a time for fasting during the hours of daylight and an increased attentiveness to prayer and charity. 

Weight-loss fasting, on the other hand, is performed with the goal of shedding body weight for health or aesthetic purposes. People who engage in weight-loss fasting may also restrict their eating to certain times of day, very small quantities, and/ or very plain foods. Some people will "fast" by consuming only liquids for an extended period of time. The idea behind the practice of fasting for weight loss is that the body will go into "starvation mode" and begin burning excess fat through a process called ketosis. Some people feel that fasting and entering a state of ketosis gives their digestive tract a chance to rest and heal itself.

So, how do the personal experiences of religious fasting and weight-loss fasting differ? For one, the lifestyle activities which may surround the act of fasting are very different. People who fast for religious reasons spend a lot of time in prayer or meditation. People who fast for the purpose of weight loss do not typically engage in more prayer than they might otherwise, but may be interested in other activities which they feel benefit their process, such as gentle exercise. (Some people do engage in strenuous exercise to burn calories, but this is not necessary and can be hazardous to health.) 

As far as the physical, mental, and emotional experience of fasting, both people who fast for religious and weight-loss reasons report initial feelings of fatigue, followed by a sense of determination, focus, and purpose. Fasting stimulates the growth of neurons in the brain, and in the state of ketosis, ketones in the blood stream are like a superfood for brain activity! Some people feel they experience a rather dreamy sense of awareness for the duration of their fast. The particularities of brain and body chemistry and what this means in terms of experience has more to do with the amount and frequency of any food being taken in rather than the act of (or motivation behind) fasting. That is, someone who fasts on only water for a week would experience a more significantly altered mental state than someone who only abstains from food during the daylight hours.

As far as the attitudes of society, especially people close to the person engaging in fasting, most people have equal parts respect and worry for those who fast. Religious fasting is offered a little more honor in most cases because it is perceived as being for unselfish and spiritually beneficial reasons. With weight-loss fasting, however, some people are judgmental and believe it will cause damage to a person's health or is a sign of an unwillingness to participate in other means of weight loss. Some people who fast for weight loss have been criticized as "lazy" because others think they are not willing to put in the effort to exercise or make longer-term changes in their diet. Recent studies showing some of the health benefits of fasting have helped to shift this attitude a little bit more toward the positive end of the spectrum, but the thought of weight-loss fasting can still worry loved ones. That's not to say that people don't worry about their loved ones who fast for religious reasons—these people are just as at risk for health troubles as someone who fasts for weight loss. Where attitudes toward people fasting differ, it really seems to be more of a judgment of the motivation for fasting rather than the act of abstaining from food in itself.