Several religions place one or more of the following restrictions on food consumption: 1) the types of foods permitted for consumption in general or during particular times of the year; 2) the time of day when food consumption is permitted; and 3) food preparation . These types of restrictions can either persist year-round or be active only during special fasting periods. The text below focuses on the following fasting periods: 1) Islamic Ramadan; 2) the three principal fasting periods of Greek Orthodox Christianity (Nativity, Lent, and the Assumption); and 3) the Biblical-based Daniel Fast. The reason for the inclusion of these religious fasts in this review and the exclusion of others is that, to our collective knowledge, these are the only fasts about which scholarly research has been performed that explicitly detailed the subjects' dietary intake.
During the holy month of Ramadan, all healthy adult Muslims are forbidden from consuming any food or water from sunrise (Sahur) to sunset (Iftar). Food and fluid intake become nocturnal during Ramadan, and the common practice is to eat one large meal after sunset and one lighter meal before dawn . Ramadan is clearly the most commonly researched religious fast. The collective work highlights both the positive and negative effects of Ramadan fasting on overall health.
It was previously thought that Ramadan fasting often led to reduced energy intake and weight loss , but recent studies have found that caloric intake actually increases despite the decreased meal frequency . In relation to macronutrient composition, meals are often composed of more fat and less carbohydrate during Ramadan than during the rest of the year [126, 127].
Ramadan fasting has elicited mixed results in relation to cardiovascular health, particularly regarding lipid profiles. Hallak and Nomani  and Ziaee et al.  noted a decline in high density lipoprotein (HDL)-C levels and an increase in LDL-C levels following Ramadan fasting. However, several other studies have found an increase in HDL-C levels and a decline or no change in LDL-C levels [125, 127, 129–131]. Indeed, some studies have noted an increase in HDL-C levels by as much as 20%  and 30% . The majority of studies have found no difference in triglyceride levels [130–132], while one study noted a decrease following a period of Ramadan fasting . Salehi and Neghab  reported declining total cholesterol levels following a Ramadan fast. Conversely, Aksungar et al  did not note any changes in total cholesterol and LDL-C levels but did note decreases in the TC/HDL ratio, C-reactive protein levels, and homocysteine levels.
Changes in heart rate and blood pressure have also been observed during Ramadan. Husain et al  observed a significant lowering of resting heart rate in men, although no changes were noted regarding the resting heart rate of women. Heart rate during steady-state aerobic exercise has also been shown to decrease during the fast compared to non-fasting values [134, 135]. Regarding blood pressure, one study noted a decrease in both resting systolic and diastolic blood pressure -although this change was noted in both fasters and non-fasters, raising a question regarding the ability of Ramadan fasting to actually promote such an effect  while four studies found no difference in either variable [135, 137–139]. Ramadan (2002)  compared blood pressure changes during steady-state aerobic exercise during and after Ramadan fasting. Systolic - but not diastolic - blood pressure significantly increased during exercise when subjects were fasting. Contrastingly, neither systolic nor diastolic blood pressure changed during exercise in subjects who were not fasting.
To our knowledge, only two studies to date have examined the effects of Ramadan fasting on oxidative stress and antioxidant status, highlighting an important area for future research. Ibrahim et al  observed a reduction in malondialdehyde (MDA) erythrocyte concentrations, while no changes were noted regarding levels of either serum MDA or plasma protein-bound carbonyls. No changes were found regarding the concentration of glutathione or the activities of glutathione peroxidase and catalase in erythrocytes. Plasma levels of ß-cryptoxanthin and total carotenoids significantly decreased during Ramadan fasting, and plasma levels of vitamin C, β-carotene, lycopene, and lutein were non-significantly reduced. No changes were noted regarding plasma levels of α-tocopherol, γ-tocopherol, retinol, α-carotene, and zeaxanthin. Chaouachi and colleagues  reported that blood levels of vitamin A increased, while blood levels of vitamin E decreased during Ramadan fasting.
As with the reports of Ramadan's effects on markers of cardiovascular health, similarly mixed results are available regarding Ramadan fasting and the ability to improve glucoregulatory health. Two studies have noted a decrease in fasting blood glucose following Ramadan [123, 132], while a recent study noted no effect on blood glucose levels . Kassab et al  noted that serum leptin levels increased during Ramadan by 37% and 39% in obese and nonobese subjects, respectively. Additionally, the authors found a significant correlation between changes in serum leptin and serum insulin levels, which suggests that insulin may play a role in regulating leptin secretion .
Aside from the potentially beneficial effects discussed above, some studies have noted adverse effects associated with Ramadan fasting. Ramadan fasting has been shown to negatively impact nocturnal sleep by increasing sleep latency and decreasing both slow wave sleep and rapid eye movement sleep [142, 143]. Lack of sleep can cause irritability  and has been shown to lead to an increase in the intake of stimulants such as coffee and tea during the month of Ramadan . Ramadan fasting may lead to ulcer complications, as studies have reported increased gastric acidity during the day, with altered circadian patterns of levels of plasma gastrin, gastric pH, glucose and calcium . Other potential adverse health effects associated with Ramadan fasting include energy level imbalances , dehydration , decreased athletic performance , and altered circadian fluctuations in hormone levels .
Collectively, the above-listed studies have reported conflicting effects of Ramadan fasting on a number of health-related biomarkers. There are several potential confounding variables that may influence the effect of Ramadan fasting on these biomarkers, including: age, physical activity, diet, sleep cycles, and cultural habits [126, 150]. Regarding the latter, El-Ati et al.  reported that total energy intake increased among Saudi subjects and decreased among Indian subjects during Ramadan, and that this discrepancy was due to the differences in food choices between the two groups. Food choices and eating habits affect metabolism and could potentially affect the above-listed biomarkers. Future investigations should take these variables - as well as other confounding variables - into consideration.
There are three principal fasting periods for Greek Orthodox Christians. During the 40 days that precede Christmas (Nativity), dairy products, eggs, and meat are proscribed every day, while fish and olive oil are also forbidden on Wednesdays and Fridays. During the 48 days that precede Easter (Lent), dairy products, eggs, and meat are proscribed. Olive oil consumption is permitted only on weekends during this period, and fish consumption is only allowed on March 25th and Palm Sunday. During the first 15 days of August (the Assumption), dairy products, eggs, and meat are proscribed. Olive oil consumption is permitted only on weekends during this period, and fish consumption is only allowed on August 6th. Cheese, eggs, fish, meat, milk, and olive oil are also proscribed on every Wednesday and Friday that falls outside of the principal fasting periods. This latter proscription is temporarily lifted on the week following Christmas, Easter, and the Pentecost. Collectively, dietary consumption is restricted for 180-200 days each year.
Most studies have reported a decreased caloric intake during the fasting periods [151–154], which may result in lowered body mass [151, 152]. Percentagewise, carbohydrate intake appears to increase, while both protein and fat intake decrease [151–153]. Both saturated fat and trans-fatty acid consumption appear to decrease during fasting periods, while both monounsaturated and polyunsaturated fat consumption do not change [151, 153].
Both total cholesterol and LDL-C levels decrease during fasting periods [151, 152], while conflicting results exist regarding HDL-C levels [151, 152]. One study noted that the LDL/HDL ratio does not change during Greek Orthodox Christian fasting , while conflicting results have been noted regarding both the total/HDL ratio and triglyceride levels [151, 152]. Greek Orthodox Christian fasting appears to have no effect on blood glucose levels [151, 152], although fiber intake increases during fasting periods [151–155].
Both riboflavin  and calcium [151, 153, 155] intake appear to decrease during fasting periods, while magnesium intake appears to increase [151, 155]. The intake of the following vitamins and minerals do not appear to change during fasting periods: vitamin A [151, 153]; thiamin ; niacin ; vitamin B12 [151, 153]; vitamin C [151, 153, 154]; vitamin E [151, 153]; phosphorus ; potassium [151, 155]; and zinc . Mixed results have been recorded regarding the intake of both folate [151, 153, 155] and sodium [151, 155].
To our knowledge, only one study has examined the effects of Greek Orthodox Christian fasting on other hematological variables . The authors reported that fasters experienced a relative increase in serum ferritin levels, a relative decrease in MCHC levels, and no relative change in levels of hemoglobin, serum iron, and transferrin when compared with non-fasters during the Christmas fasting period. The authors also reported that the non-fasters' hematocrit declined significantly (42.7 ± 5.1 vs. 41.2 ± 5.1%) when compared to the changes in fasters' hematocrit (41.2 ± 3.9 vs. 40.0 ± 5.3%). However, this change is of little to no clinical relevance, as all values were within normal range.
There are conflicting findings on the effects of Greek Orthodox Christian fasting on blood pressure. One study found that systolic blood pressure increased during fasting periods , while another study found no change in systolic blood pressure when fasters were compared with non-fasters . One study reported that non-fasters' diastolic blood pressure decreased significantly during fasting periods when compared to the changes in fasters' diastolic blood pressure , while another study reported that fasters' diastolic blood pressure did not change during fasting periods .
In summary, Greek Orthodox Christian fasting appears to lower caloric intake and body mass, and both total and LDL-C decrease during fasting periods. The intake of most vitamins and minerals do not appear to change during these periods, although riboflavin and calcium intake each appear to decrease, and magnesium intake appears to increase. More research remains to be performed on hematological variables and blood pressure during fasting periods due to both the lack of previous research and the inconclusive findings. Also, future studies should examine each of the three principal fasting periods both separately and aggregately, because each fasting period has unique food proscriptions and durations.
A popular fast practiced by many Christians and Jews is the Daniel Fast, based on the Biblical story of Daniel (1:8-14), in which Daniel resolved not to defile himself with the royal food and wine, and he asked the chief official for permission to provide to him and his three friends nothing but vegetables to eat and water to drink for 10 days. Later in the same book (Daniel 10:2-3), Daniel again partook in a 21 day period of "clean" eating, during which time he ate no choice food (meat or wine). Based on these two stories, a modern day Daniel Fast involves ad libitum intake of certain foods, but the food choices are restricted to essentially fruits, vegetables, whole grains, nuts, seeds, and oil. In essence, this plan is a form of DR and resembles a vegan diet, which has been reported to yield health-enhancing properties [156, 157]. However, a Daniel Fast is much more stringent, because preservatives, additives, sweeteners, flavorings, caffeine, and alcohol are each forbidden. However, because individuals traditionally follow this fast for religious purposes in an attempt to become "closer to God" during a time of extended prayer, the anecdotal reports have indicated excellent compliance.
To test the health benefits of the Daniel Fast within a laboratory-based protocol, we recently enrolled 43 subjects (13 men; 30 women; 35 ± 1 yrs; range: 20-62 yrs) to complete a 21-day Daniel Fast. Pre and post intervention, subjects underwent a variety of tests including measures of body weight and composition (measured via dual energy x-ray absorptiometry), resting blood pressure and heart rate, fasting blood measures of oxidative stress, inflammation, blood lipids, insulin, and glucose. Subjects' self reported compliance, mood, and satiety in relation to the fast were also recorded.
We noted excellent compliance to the fast (98.7 ± 0.2%; mean ± SEM), as well as excellent mood and satiety (7.9 ± 0.2 using a 10 point scale). The following variables related to cardiovascular disease risk were significantly (p < 0.05) lower following the fast as compared to before the fast: total cholesterol, LDL-C, SBP, and DBP. Insulin, HOMA-IR, and C-reactive protein, although lowered to a clinically meaningful extent, were not of statistical significance. Unfortunately, due to the drastic decrease in total cholesterol, HDL-C was lower after the fast as compared to before the fast (55.65 ± 2.50 vs. 47.58 ± 2.19 mg·dL-1). Although body weight and body fat were reduced slightly, no significant difference was noted . In reference to measures of oxidative stress, we noted a significant increase (p < 0.05) in Trolox Equivalent Antioxidant Capacity, and a significant decrease (p < 0.05) in malondialdehyde and hydrogen peroxide . Similar results as presented above have been noted in a recent follow-up investigation of the Daniel Fast (unpublished findings).
The above data demonstrate that a Daniel Fast can significantly improve several biomarkers of overall health, particularly those related to cardiovascular and metabolic disease. Larger scale, randomized studies will be needed to extend these preliminary findings. Future studies should consider extending the duration of the fast, as well as modifying food choices in an effort to maintain HDL-C levels.