Although often closely associated with religion and spirituality, it is becoming increasingly common for people to embark on a period of fasting, irrespective of religious affiliation. This is because there are many health benefits associated with fasting. Just as there are many different reasons why people fast, there are also many different ways to approach fasting.
Different types of fasting
Fasting can broadly be categorised into three different types:
A Complete Fast: A complete restriction of all forms of food for a set period - For example, some people will completely fast for a week or even a fortnight; i.e. they will not have any food for the entire period, however they will usually have water. This type of fasting is usually performed for health reasons (e.g. some doctors use it to induce remission in rheumatoid arthritis patients ) and is always conducted under the watchful and continuous care of a certified medical doctor who is specialised in fasting.
Intermittent Fasting: A restriction of food and drink to specific hours of the day - Intermittent fasting is common to both religious and nonreligious fasting groups. Although commonly known religious fasting periods consist of one block of consecutive days each year, some Hindus fast on certain days of the month, week or for multiple blocks of days during the year.
Similarly, some diet regimes (e.g. the Warrior diet) recommend this type of fasting on a regular basis – sometimes on a daily basis, sometimes every two to three days.
Partial Fasting: A restriction of certain types of food for a set period - This type of fasting is commonly performed during lent, but also includes other forms of elimination fasting including juice fasts and detox fasts. Another common kind of fasting undertaken by Hindus in southern and north western India involves excluding all forms of food except for fruits and milk every Tuesday. 
Health Benefits to Fasting
There is a lot of literature examining the effects of fasting on a persons' health – both religious fasting and nonreligious fasting. The drawback to research studying the health effects of fasting in a population undergoing religious fasting is that there are often many confounding factors to take into account.
For example, both the Muslim and Baha'i Fasts discourage smoking during the fasting period. This means that a subpopulation of any study will naturally have health-related benefits during the fasting period simply by virtue of smoking less. Additionally, it is not uncommon for people to binge on highly refined foods after a day of fasting – this practice is highly likely to negatively impact on health. Lastly, religious fasting is often a period of intense prayer and meditation; this in itself has positive effects on health, independent to fasting. Hence, many of the health benefits reported below are based on findings from non-religious fasting studies, or those religious studies that include criteria or parameters to reduce the effect of confounding effects.
The Health Effects of Fasting on your Weight
People who undergo complete fasts will always lose weight, however intermittent fasting is a little more complicated. During an intermittent fast some people lose weight, others remain the same, and others will actually gain weight during the fasting period. This is because some people naturally reduce exercise and physical movement through the fasting period, while others do not. Additionally some people change their eating habits during the fasting period to include more refined foods in larger quantities.
If binging is removed from the equation, intermittent fasting can lead to weight loss in overweight individuals: A study of 28 overweight young male volunteers (aged 20 to 26) placed on 'a balanced diet of 2000 calories' during Ramadan reported significant reductions in both weight and BMI.
The Health Effects of Fasting on Cancer
Studies in animals have found that calorie restriction can reduce the risk of cancer.  Intermittent fasting then may reduce your long-term cancer risk (particularly if coupled with these healthy eating habits), however The American Cancer Society state that the available evidence does not support fasting as a cancer treatment (often referred to as metabolic therapy) in humans.[5, 6]
Interestingly, early reports from the National Institute on Aging have found that intermittent fasting may enhance normal cells' ability to cope with external stresses (like chemotherapy), without affecting cancer cells. Similarly, there is evidence in animal studies that short-term fasting may make cancer cells more sensitive to chemotherapy treatments. Together these findings may mean more effective chemotherapy treatments with less side effects; but it is still early days, so ‘watch this space’.
The Health Effects of Fasting on your Heart
There is some evidence that people who fast periodically have better cardiovascular (heart) health, possibly due to the way the body metabolises cholesterol and sugar during fasting. [8, 9] Under normal conditions your body produces enough cholesterol to sustain life. A high fat diet (particularly high in animal fats) can lead to an abnormal elevation in fat and cholesterol in the body. A group of enzymes called sirtuins typically promote fat storage and cholesterol synthesis during these times of abundance. 
When the body undergoes a short period of fasting, sirtuins suppress fat storage and cholesterol synthesis, leading to reductions in 'bad' cholesterol. Similarly, it is thought that fasting may improve the way your body metabolises sugar.  Both of these health effects (in addition to any weight lost while fasting) can contribute to improved cardiovascular health.
The Health Effects of Fasting on your Brain
Anecdotally people often report increased mental acuity during fasting periods; this has been echoed in the literature in caloric restriction studies. A study of 19 normal to over-weight people (average age of 60 years) found that calorie restriction (by 30%, minimal intake of 1,200 kcal per day) resulted in significantly improved verbal memory scores, by as much as 30% compared to pre-study scores.  This improvement occurred over a period of 3 months, which is longer than your typical religious fast but may be shorter than other periods of intermittent fasting.
There is also some evidence that calorie restriction and/or regular intermittent fasting may protect the brain against degenerative diseases like Alzheimer’s, Parkinson’s, Huntinton’s and other ailments. [11, 12] Again, 'watch this space'.
Who should not fast?
Despite the many health benefits attributed to fasting, fasting is not appropriate for everyone. Even if you are healthy, it is important that you see your doctor before commencing any type of fasting period, religious or not. In general, fasting is not advisable for the very old and young, for pregnant or breastfeeding women, or for anyone who is unwell. In some religious settings women who are menstruating are also exempt from fasting.
Fasting in an attempt to induce remission of an autoimmune disease is not recommended in people taking immunosuppressive drugs like methotrexate and azathioprine (Imuran®). Furthermore, there are many other medications that do not mix well with fasting – which again is another reason why you should see your doctor before you think about embarking on any period of fasting.
 Fuhrman J, Sarter, B, Calabro DJ. Case Studies of Medically Supervised Water-only Fasting Resulting in Remission of Autoimmune Disease. Alternative Therapies 2001;8(4):1-3.
 Wikipedia, Fasting
 Trepanowski JF, Bloomer RJ. The impact of religious fasting on human health. Nutrition Journal 2010, 9:57
 M. Salehi, M. Neghab, 2007. Effects of Fasting and a Medium Calorie Balanced Diet During the Holy Month Ramadan on Weight, BMI and Some Blood Parameters of Overweight Males. Pakistan Journal of Biological Sciences, 10: 968-971.
 Elias SG, Peeters PHM, Grobbee DE, van Noord PAH. Transient caloric restriction and cancer risk (The Netherlands). Cancer Causes Control. 2007 February; 18(1): 1–5.
 American Cancer Society, Fasting
 National Institute on Aging, Study finds fasting may help reduce negative side effects of chemotherapy
 HarvardScience: With Fasting, Enzyme turns off Body’s production of Fats and Cholesterol
 Mayo Clinic, Fasting Diet, Can it improve my heart health?
 Witte AV, Fobker M, Gellner R, Knecht S, Floel A: Caloric restriction improves memory in elderly humans. Proc Natl Acad Sci USA 2009, 106:1255-1260.
 Nature Medicine, Profile: Mark Mattson
 National Institute on Aging, Fasting forestalls Huntington’s disease in mice
 Lee C, Raffaghello L, Brandhorst S et al. Fasting Cycles Retard Grwoth of Tumors and Sensitize a Range of Cancer Cell Types to Chemotherapy. Science Translational Medicine, Published online 8th Feb 2012