By Thiyagarajan Sivapriya, M.Sc, M. Phil
Many religions incorporate fasting for both spiritual and physical benefits 
Fasting is defined as a partial or total abstention from all foods, or a selected abstention from prohibited foods. As a potential non-pharmacological intervention for improving health and increasing longevity, fasting has been the subject of numerous scientific investigations. The three most common fasts are caloric restriction, alternate-day fasting, and dietary restriction.
Calorie Restriction is the reduction of kilocalorie (kcal) intake by a certain percentage. Calorie restriction has been demonstrated to improve health and increase longevity  Additionally, Calorie restriction appears to delay the onset of autoimmune diseases, atherosclerosis, cardiomyopathies, cancer, diabetes, renal diseases, neurodegenerative diseases, and respiratory diseases .
Dietary restriction is a reduction of one or more components of dietary intake (typically macronutrients) with minimal to no reduction in total kcal intake. Alternate-day fasting consists of two interchanging days; one day, subjects may consume food ad libitum and on the other day, food is reduced or withheld altogether .
Evidences from different religions
A recent research of outpatients found that patients who were religious adherents were more likely to hold beliefs that could affect health-care choices than those with no adherence to any religion 
During the holy month of Ramadan, which varies according to the lunar calendar, Muslims abstain from eating or drinking from sunrise (Sahur) to sunset (Iftar) for a period of 28 – 30 days. The common dietary practice of Ramadan fasting is to eat one large meal after sunset and one lighter meal before dawn.
Greek Orthodox Christians fast for a total of 180 – 200 days annually including the following main fasting periods: the Nativity Fast (40 days prior to Christmas), Lent (48 days prior to Easter), and the Assumption (15 days in August).
The Biblical-based Daniel Fast typically incorporates a 21-day fasting period in which individuals refrain from consuming animal products, refined carbohydrates, food additives, preservatives, sweeteners, flavourings, caffeine, and alcohol
In a group of Jewish adolescents, religious observance was associated with favorable levels of plasma lipids and lipoproteins. Orthodox Jews, who did not consume dairy and meat products together, had certain constraints on food-buying habits and on eating out, had a better lipid profile and lower mean body mass index than secular Jews 
A group of Hindus and Buddhists eat no meat, fish, seafood, eggs and dairy products throughout life time, with the exception of milk. The underlying principle behind fasting is to be found in Ayurveda . This ancient Indian medical system sees the basic cause of many diseases as the accumulation of toxic materials in the digestive system. Regular cleansing of toxic materials keeps one healthy. By fasting, the digestive organs get rest and all body mechanisms are cleansed and corrected. A complete fast is good for heath, and the occasional intake of warm lemon juice during the period of fasting prevents the flatulence 
Zen Buddhists were known for their strict discipline and ascetic exercise since they used to be strict vegans and abstain from drinking and smoking, and other social behaviours 
People from the Japanese island of Okinawa have a high life expectancy among the highest in the world and one of the highest rate of centenarians in the world. It is intriguing that they consume lesser calorie and higher longevity than the main land Japanese. It is found that the elderly Okinawans have negative energy balance in earlier life with resultant low body mass during their latent life, decreased age related diseases and extended lifespan when compared with main land Japanese and Americans.
These religious dietary fasting has shown to decrease oxidative stress, a condition thought to contribute to aging and disease, increase longevity. In addition, enhanced verbal memory performance has been reported in elderly individuals on fasting regimen  Fasting reduces blood lipids, reduces the ratios of total cholesterol (TC): HDL cholesterol and LDL: HDL cholesterol, while it increases significantly HDL cholesterol and apo A-1, the main protein component of HDL cholesterol. Temizhan et al. (2000) found significant reductions in body weight after Ramadan fasting, with the changes being greater in men than in women of middle socioeconomic status 
However, fasting is not only a part of worship, but a great instrument for self-discipline too. It is a training of the mind and the body to endure and harden up against all hardships, to persevere under difficulties and not give up. According to Hindu philosophy, food means gratification of the senses and to starve the senses is to elevate them to contemplation. Luqman, the wise once said, “When the stomach is full, the intellect begins to sleep. Wisdom becomes mute and the parts of the body restrain from acts of righteousness.”
Evidence that calorie restriction could prevent age related diseases in humans are emerging. Further research is needed to identify the nutrient sensing pathways implicated in antiaging effect as well as their downstream effects. Although calorie restriction is associated with positive health benefits and longevity, the quality of life on a restricted diet as well as the ability to maintain the diet for long time are concerns that must be considered in humans 
- Sarri KO, Higgins S, Kafatos AG: Are religions “healthy”? A review of religious recommendations on diet and lifestyle. Ecology, Culture, Nutrition, Health and Disease 2006, 7-20.
- Spindler SR: Caloric restriction: From soup to nuts. Ageing Res Rev 2009, 9(3):324-353.
- Imai S, caloric restriction: an insight into possible trade-offs between robustness and frailty. Curr Opin Clin Nutr Metab Care 2009, 12:350-6
- Ayala V, Naudi A, Sanz A, Caro P, Portero-Otin M, Barja G, Pamplona R: Dietary protein restriction decreases oxidative protein damage, peroxidizability index, and mitochondrial complex I content in rat liver. J Gerontol A Biol Sci Med Sci 2007, 62:352-360.
- Ehman, J.W., Ott, B.B., Short, T.H., Ciampa, R.C. and HansenFlaschen, J.: Do patients want physicians to inquire about their spiritual or religious beliefs if they become gravely ill? Arch. Intern. Med., 159: 1803-1806 (1999).
- Friedlander, Y., Kark, J.D. and Stein, Y.: Religious observance and plasma lipids and lipoproteins among 17-year-old Jewish residents of Jerusalem. Prev. Med., 16: 70-79 (1987).
- Ogata, M., Ikeda, M. and Kuratsune, M.: Mortality among Japanese Zen priests. J. Epidemiol. Comm. Health, 38: 161-166 (1984).
- Subhamoy das, Why fast, http://hinduism.about.com/od/basics/a/fasting.htm
- 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.
- Temizhan, A., Tandogan, I., Donderici, O. and Demirbas, B.: The effects of Ramadan fasting on blood lipid levels. Am. J. Med., 109: 341-342 (2000).
- Ruth and Polotsky, Can we live longer by eating less, Maturitas, 2011