This week's Ask a ScienceBlogger question is:
A reader asks: Is severely regulating your diet for a month each year, as Muslims do during Ramadan, good for you?
There is no way I can get out of this one! As far as I know, I am the only one here who actually did research on fasting! Mind you, it's been about 5 years since I last delved deep into the literature on the effects of fasting and feeding on various body functions, mainly body temperature and circadian rhythms, but I can try to pull something out of my heels now.
I'll try to somehow systematize this, by breaking the problem down into two components first: the effects of fasting vs. effects of feasting, as the Ramadan rules encompass both. Then I'll try to put the two together in the end.
The term 'fasting' is used in the literature in two ways. One is 'calorie restriction' in which animals/subjects are fed at normal times but small quantities of low-calorie food. The other is complete food deprivation (water only) lasting at least 24 hours and often several days (larger the animal, longer the experiment can last before the food needs to be reintroduced).
Effects of fasting on body temperature
You should really go and dig through this post which explains the whole field in great detail. In short:
In a normally fed diurnal animal (e.g., human), body temperature is the lowest just before dawn. It slowly rises during the day reaching its daily peak in the late afternoon. It then starts dropping during the night until it reaches its nightly trough value just before dawn. The amplitude (difference between the daily high and the nightly low) is species-specific: about 0.5 degrees Celsius in humans, more than 1 degree in small mammals and birds.
In an animal completely deprived of food, the temperature appears normal during the day and reaches the same value of the peak in the afternoon. During the night, the temperature drops faster than normal and the pre-dawn trough reaches a lower value than in the fed animals. If this is repeated over two or more successive days, the peak still remains normal, but the nightly low goes gradually lower and lower on each night. Several factors (e.g., light, presence of predators, photoperiod, etc.) can modify this response (see the link just above). Presumably, during the day, the animal needs to be as alert about enemies as possible as well as capable of foraging (though in the end it is to no avail due to experimenters). During the night, in the safety of the burrow or nest, the animal lowers its temperature in order to save energy, i.e., to compensate for the energy loss of the previous day.
During caloric restriction, it appears, at least in some species, that both the daily peak and the nightly trough gradually go down.
Does the human body interpret Ramadan schedule as food deprivation that is interrupted every night, or as caloric restriction? Either way, the expectation is that, due to daytime fasting, body temperature would drop during the night below the normal values, but would this be corrected by nighttime feasting?
Exposure to light during the day is the cause of the daily peak remaining normal. In other words, light perceived by the eyes tends to preserve the peak. This is at odds with the activity of the circadian clock which tries to preserve the amplitude of the rhythm. During Ramadan, people may expose themselves to artifical light during the night (raising the nigthly temperature) while hiding from light to take naps during the day (thus lowering the daytime body temperature). As a result, the body temperature would remain pretty flat throughout the 24-hour period. That is exactly what they found in this paper:
During Ramadan oral temperature decreased at 09.00, 11.00, 13.00, 16.00 and 20.00 h and increased at 23.00 and 00.00 h.
There is a large body of research on the effects of scheduled feeding on circadian rhythms. While the light-dark cycle is the most powerful and the best studied environmental cue that entrains circadian clocks, many other non-photic cues also are capable of phase-shifting and entraining circadian rhythms.
Research on scheduled feeding suggests that a food-entrainable clock exists in many vertebrates, from fish to mammals, including humans. This clock is more or less independent from the main circadian clock, depending on the species, and may involve intergeniculate leaflet either as site of the clock or as a relay-station to the clock.
In normal life, appearance of light in the morning, the noise coming from the street and the radio, the morning dog-walk, and the breakfast, all conspire to entrain the circadian clock to the normal diurnal schedule. During Ramadan, physical activity is fragmented (i.e., occurs in shorter periods throughout the 24-hours), sleep is fragmented, exposure to light is fragmented, and the food is scheduled at unusal times (1-3 meals during the night depending on the local custom). Each of those factors is going to have a different effect on the circadian clock. All those factors are pulling each their own way, often managing to pull away a subset of the circadian system - either a subset of the SCN neurons or peripheral clocks - away from the other subsets. The result is internal desynchronization of the circadian system, something we usually like to call "jet-lag".
The effect of temperature on the clock
As we are warm-blooded animals, our clock is shielded from the environmental fluctutations in temperature. However, the SCN clock neurons are, in a dish, sensitive to temperature and entrainable by temperature cycles (the way lizards, for instance, can be entrained by temperature cycles of very low amplitude). Thus, fluctuations in body temperature driven by the clock also feed back on the clock. During daytime fastin/night-time feasting schedule, the flattenning of the body temperature rhythm removes this reinforcing effect of temperature on the clock and allows it to split into many sub-components.
Return of food after fasting
As far as I know, our paper is the only one noting that return of food after fasting (equivalent to the evening feast after daytime fasting of Ramadan) elicits large phase-shifts of the circadian clock.
There has been some research published on the effects of Ramadan on circadian rhythms and temperature. The fast/feast schedule was shown to alters various circadian parameters including hormones, sleep quality, patterns of drug intake not conducive to proper healing and a variety of other parameters.
Many of these effects persist for weeks after the Ramadan is over. Thus, about 1/6th of one's year is spent in the state of jet-lag, something that has been shown in other studies to lead to increased incidences of some heart diseases, peptic ulcers and some cancers. The effects are more prominent in years in which Ramadan falls during summer when the day (fasting) is long and the night (feasting) is short. It is difficult to eat at an "out-of-whack" time of day so the total intake of energy may not match the total loss of energy due to the absence of the nocturnal drop in body temperature. Losing weight, though not a bad idea for some people, is not universally a good thing.
The literature warns of the dangers of Ramadan for diabetics (who are excused from fasting by the Q'uran but often choose to fast/feast anyway), but the disintegration of the unity of circadian rhythms suggests that this schedule should not be used by patients suffering from Seasonal Affective Disorder and Bipolar Disorder as well. While it is not clear if healthy adults suffer any long-term damage, no data exist about effects of Ramadan schedules on psychiatric patients.