Fasting can be defined as the voluntary abstinence from or reduction of some or all food, drink, or both (absolute) for a period of time lasting typically between 12 h and 3 weeks i.e., in a short term, long term/prolonged or an intermittent pattern [1
]. Fasting is a common practice in different religious disciplines, including Islam, Christianity, Judaism and Hinduism. In Islam, the practice entails abstinence from eating and drinking between dawn and sunset [2
]. Fasting is distinct from starvation, which is a chronic and severe deficiency in caloric energy intake below the level needed to maintain life.
Health benefits of intermittent fasting have been demonstrated in both randomized controlled trials and observational studies [3
]. Caloric restriction (CR) has also been shown to prevent several chronic degenerative and inflammatory diseases [5
] and to prolong life in more primitive species including Escherichia coli
and yeast [6
]. In humans, the evidence on the positive effects of CR on longevity is indirect; for example the increased life expectancy in the Okinawan population, from the Kyushu Island in Japan, has been attributed at least in part to low calorie intake [7
]. Mechanistically, the effect of CR on longevity has been attributed to fasting-induced modulation of neuroendocrine systems, hormetic stress responses, increased systemic production of neurotrophic factors, reduced mitochondrial oxidative stress, decreased pro-inflammatory cytokine production and insulin resistance, as well as decreased aging-associated signals and autophagy promotion [5
Prolonged fasting has also been associated with positive effects on mood due to the alteration in physiology at a cellular level via increases in availability of central endogenous neurotransmitters, endogenous opioids and endocannabinoids [10
]. Cancer studies demonstrated that fasting and fasting-mimicking diets (FMDs) positively promote differential effects in both normal and malignant cells via reduction in insulin-like growth factor (IGF-1), insulin and glucose with paralleled increases in ketone bodies [11
]. In contrast, negative effects of fasting have been reported for instance on non-communicable diseases [8
], on changes to sleep patterns, cognitive function, [13
] and have also been associated with fluctuations in mood, weight and a plethora of other changes [15
Fasting is a state of negative energy balance, and as such different fasting regimens have been used to achieve weight loss, as well as other health benefits. In the context of Muslim Ramadan-type fasting, changes in energy intake depend on social, cultural and individual factors and can range from a reduction to an increase in weight [17
]. Whether this is accompanied by changes in energy expenditure is not well-known and merits further exploration for its possible implications in weight loss management strategies in general [20
This review will be examining current knowledge about different aspects of energy balance in the context of the Ramadan fast as a commonly practiced model of intermittent fasting. In the broader context, potential positive implications include the use of for such strategies to help with weight maintenance, is not weight loss, and thus a multitude of other consequential positive health benefits. Relevant literature (Table 1
and Table 2
) directly and indirectly related to the Ramadan fast, including short- and long-term fasting and also prolonged and intermittent type fasting will be explored. In the context of Ramadan, changes in energy dynamics (intake versus expenditure) have been extrapolated based on our previous quantitative studies, knowledge of physiology and alterations in energy utilization during feeding and non-feeding periods. The aim of this review is firstly, to discuss the various aspects influencing energy modulations during Ramadan fasting; secondly, to shed light on key knowledge gaps in our understanding of energy balance in relation to changes in both body composition and physiological adaptation in various models of fasting to include key periods such as the Ramadan fasting period and; lastly, to contribute to the focused directionality of future studies in key aspects that warrant further detailed investigations.
2. Energy Expenditure (EE)
When body weight is in a relatively stable state, there is equilibrium between energy intake (EI) and energy expenditure (EE). High EI levels in combination with low EE results in a positive energy balance and storage of energy, primarily as body fat. Total (daily) Energy Expenditure (TEE) consists of Resting Metabolic Rate (RMR), Thermic Effects of Food (TEF) and Activity Energy Expenditure (AEE) [20
]. Different components of EE have been reviewed elsewhere [21
] and will only be discussed briefly here.
Resting Metabolic Rate (RMR) is the quantity of energy at rest needed to maintain body temperature, repair internal organs, support cardiac function, maintain ionic gradients across cells, and support respiration. In most people, this constitutes approximately two-thirds of total energy expenditure [25
]. RMR is influenced by age, sex, body weight, pregnancy, and hormonal status. The highest rates of energy expenditure per unit of body weight occur during infancy and decline through childhood. In adult life, the decline continues at approximately 2% per decade because of a decline in lean body mass. Females have a lower energy expenditure per unit of weight than do males, probably because of the higher proportion of body fat and less lean body mass in women [26
]. Thermic effect of food is the rise in energy expenditure that occurs with food intake [26
]. This rise is in part due to the ‘obligatory’ energy cost of ingestion, digestion, and metabolic processing of nutrients, and in part due to a ‘facultative’ component arising from the sensory aspects of food and meal stimulation of the sympathetic nervous system. Different macronutrients have different thermic effects; protein causes a greater rise in EE than fat or carbohydrates. Although TEF is normally a small component of TEE (~10%) it is nonetheless an important component in energy imbalance states as it is influenced by meal size and composition, the nature of the previous diet, insulin resistance, physical activity, and ageing influence TEF [27
2.1. Short-Term Fasting
Metabolically, fasting can be divided into three distinct key stages: Stage 1: a post-absorptive phase ~6–24 h after beginning fasting where the central nervous system (CNS) and many other issues preferentially use glucose produced from glycogen breakdown. Lipolysis and ketogenesis and gluconeogenesis increase, but the latter to a lower extent. Glycogenolysis decreases. Stage 2: the gluconeogenic phase occurs ~1–10 days after beginning fasting. Here, protein catabolism is used to feed glucose to the CNS while other tissues feed on ketones and fat. Lipolysis and ketogenesis increase and then plateau, gluconeogenesis on the other hand begins to decrease and no glycogenolysis occurs. Stage 3: is a protein conservation phase that occurs when fasting extends beyond 10 days. Protein catabolism is decreased to a minimum, fatty acids are used ubiquitously and ketones are utilized as fuel in the CNS. Lipolysis and ketogenesis plateaus while gluconeogenesis decreases and then plateaus but to a much lower extent when compared to ketogenesis [33
2.2. Prolonged Fasting-Some Historic Examples
Early studies in 1915 by Francis Benedict looking into chemical and physiological alterations in a lean man fasting thirty-one days demonstrated significant declines in body weight (−12.4 kg with a rate of −0.84 kg/day at Day 1 declining to 0.32 kg/day by Day 31), (Figure 1
). Levels of various biological markers such as body temperature and blood pressure were maintained [35
In 1916, Spriggs reported various cases of fasting used as a method to treat diabetes whereby fasting was ‘continued in bed until the urine has been sugar-free for twenty-four hours, unless there is some definite contraindication, such as nausea, vomiting, insomnia, or faintness’ [37
]. Early studies also indicated a progressive decrease in daily urinary nitrogen excretion suggestive of an increase in conservation of body protein [38
] and that urine output gradually decreased throughout the fasting period [39
In 2006, a study on prolonged absolute fast (44-days) on a healthy non-obese man shed light on changes in various metabolic parameters [40
]. The TEE was not measured, but was estimated to be 1638–2155 kcal/day of which 13.0–17.1% was from protein oxidation. Total weight loss was 24.5 kg and body mass decreased by 25.5%; a quarter to a third was fat mass and the remainder to fat-free mass which was predominantly muscle and approximately 20% was total body protein.
More recently, in 2015, Müller and colleagues investigated effects of caloric restriction (CR) and weight loss on 32 subjects aged between 20–37 years old in a controlled environment. Patterns of habitual food intake, resting energy expenditure and physical activity were assessed. The 10 week (week) dietary intervention period duration included 1 week of overfeeding (at +50% of daily energy requirements; 4059 ± 52 kcal/day) followed by 3 weeks of CR (at −50% of energy requirements; 1353 ± 154 kcal/day) and a subsequent 2 weeks of re-feeding (at +50% of energy requirements; 4059 ± 452 kcal/day). Protein intake was 97 ± 11 g/day (baseline); 146 ± 17 g/day (overfeeding), 49 ± 6 g/day (CR), and 146 ± 17 g/day (re-feeding), respectively. The study reports a +1.8 kg weight gain (overfeeding), −6.0 kg (CR), and +3.5 kg (re-feeding). CR reduced fat mass and fat-free mass from skeletal muscle (−5%), liver (−13%), and kidneys (−8%) by a total of 114 and 159 g/day, respectively. CR also led to reductions in resting energy expenditure (−266 kcal/d) and respiratory quotient (−15%). The study concluded that during early weight loss, adaptive thermogenesis is associated with a fall in insulin secretion and body fluid balance [41
3. The Ramadan Fast: A Shift from Normal Eating Patterns
A typical eating pattern in most cultures includes three main meals, often accompanied by snacks in between (Figure 2
). Alterations in this ‘normal’ pattern can have important implications to energy balance. Some of the more common fasting regimens include intermittent fasting (IF), periodic fasting (PF) and time restricted fasting (TRF) [8
Ramadan fasting and Ramadan-type fasting are somewhat different from other forms of fasting mentioned above. Ramadan, the ninth month in the Islamic Calendar, requires Muslims to fast daily from dawn to dusk and the criteria are clearly defined in the Holy Quran [2
]. No food or drink is allowed after suhoor until iftar. The fast is traditionally broken with something sweet such as dates. This is followed by the main meal which tends to be heavy and carbohydrate-rich. Between iftar and suhoor, food can be taken without any restriction. Ramadan is a lunar month and as such lasts 29-30 days. The fast is a religious obligation for all adult Muslims. Exempt groups include the sick and also women during their menstrual period. Many people who are religiously exempt opt to fast, often for social and cultural reasons.
In addition to Ramadan fasting, many Muslims practice the same dawn-to-sunset type of fast on other days of the year and this may include Mondays and Thursdays. Fasting some days may have some physiological differences from fasting an entire month as some physiological adaptations which may happen later during Ramadan may not occur in the short term.
4. The Ramadan Diet
Management of a healthy balanced diet is necessary not only for the maintenance of a healthy weight, but for the maintenance of the overall nutritional health of individuals too. Energy intake plays a central role [42
]. Nonetheless, multiple factors influencing energy intake such as cultural and lifestyle differences, make it difficult to maintain healthy balanced diet long-term. During non-fasting periods, recent statistics indicate that average daily adult energy intake is: (1) 2250 kcal/day (female 2000 and male 2500 kcal/day) in the UK [43
], (2) 2300 kcal/day (female 2000 and male 2600 kcal/day) in the USA [44
] and 2255 kcal/day (female 2010 and male 2600 kcal/day) in Australia [45
]. Collectively, an average adult consumes ~2268 kcal/day (female 2003 and male 2533 kcal/day) (Figure 3
A) with an additional margin for genetic (e.g., predisposition to overweight/obesity) and environmental influences (e.g., daily activity and feeding habits).
Ramadan nutrition planning (RNP) is encouraged as per DaR guidelines, which take into consideration variations in cultural food choice and calorie consumption (range of 1200 kcal/day for weight reduction for females to maximum of 2000 kcal/day weight maintenance for males) [46
]. Due to the inevitable changes in feeding patterns and associated physiological shifts in circadian rhythms, hormone levels fluctuations and overall daily lifestyle, Ramadan meal planning becomes an essential component for healthy Ramadan fasting. This is of particular importance for patients with chronic conditions, such as diabetes. A ‘Ramadan Plate’ is recommended to contain a balanced selection of carbohydrates (40–50% of total daily calorie intake (TDCI) of low glycaemic index and high-fibre containing foods), protein (20–30% of TDCI of non-red meat sources and legumes) and reduced fat intake (35% of TDCI of mostly mono- and poly-saturated fatty acids). Suhoor, the pre-dawn meal, is recommended to constitute 30–40% energy intake for the day, iftar 40–50% and snacks 10–20% as necessary.
In theory, in terms of energy intake, skipping one main meal in a 24-h period should be associated with a major reduction in food content and energy intake. This is the principle in the intermittent 5:2 fasting diet where fasting can be up to 18 h (Figure 2
(AIII,BIII)). Therefore, during Ramadan, in addition to eating healthily, this reduction in energy intake could lead to weight loss but in practice this does not occur in most cultures (Figure 3
B). Many studies indicate a great variability in Ramadan diets [30
] in different cultures, age groups, geographical locations and duration of fasting hours as well as the impact of physiological and pathological conditions (e.g., diabetes) and associated with modest reduction of energy intake in most but not all groups studied.
El Ati and colleagues investigated a group of 16 healthy female volunteers fasting during Ramadan and reported 84% of total daily energy intake was taken at the evening meal, and the remaining 16% was taken between 8 p.m and midnight. This is in contrast to periods before Ramadan where breakfast, lunch and dinner constituted 9.4, 41.6 and 21.8% of total daily energy intake. Although the findings of this small study cannot be generalized to the larger population of fasting Muslims, the observation of a disproportionately large meal at iftar time is a common finding [31
]; often reflected in feeding patterns (Figure 2
) and in glycaemic profiles.
5. Weight and Body Composition Changes During Ramadan Fasting
There seems to be much inter-individual variability in weight trends with Ramadan fasting and as with other modalities of weight change; one would expect these to be determined by individual, cultural and social factors as well as genetic, epigenetic and other factors such as gut microbiome. Several small studies (with participants between 16–81 years old in most) have examined the effect(s) of Ramadan on body weight and reported a modest weight loss of 1–2 kg by the end of Ramadan, with some other studies reporting weight gain [18
]. A meta-analysis of the older studies (by Kul et al., 2014) showed a small weight loss of around 0.7 kg in fasting men, but no significant change in fasting women [18
]. The largest study of 202 participants (Hajek et al., 2012) recruited at mosques in East London showed a net weight loss of around 0.8 kg by the end of Ramadan [49
]. As in some other studies that had post-Ramadan weight recorded, this study showed that all the lost weight was regained 4–5 weeks after Ramadan [18
]. In terms of satiety and hunger, the levels remained the same for males during Ramadan while for females more hunger was experienced earlier in the month and then decreased as the Ramadan month progressed [30
In a more recent excellent meta-analysis, Fernando and colleagues showed that the mean weight loss with Ramadan fasting was 1.34 kg and that most of the weight was regained a few weeks post-Ramadan [51
]. It has also been shown that weight loss is greater among Asian populations compared with Africans and Europeans [19
] and that there does not appear to be any gender difference in the absolute magnitude of weight loss with Ramadan fasting.
7. Discussion and Concluding Remarks
Calorie restriction and different forms of fasting have been shown to have major physiological effects; from health benefits to longevity [6
]. Ramadan fasting has also been shown to have beneficial effects including positive changes in body composition with reported reduction in body fat as well as weight loss which is a common although not universal consequence [50
]. Similar to calorie-restricting diets targeting calorie reduction at ~500–800 kcal/day [55
], skipping a meal during fasting, such as in the context of Ramadan, can theoretically lead to weight loss. However, dietary changes during Ramadan vary and often include an increase in carbohydrate intake [56
Weight loss strategies including many dietary interventions are often unsuccessful in the medium and the long term. One explanation for this is the phenomenon of adaptive thermogenesis. This occurs by promoting optimization of energy reserves while preserving protein pools via reduction in basal metabolism, decrease in secretion of anabolic factors (e.g., insulin) and increase in catabolic hormones (e.g., adrenaline and glucagon) [3
]. Along with protein loss, weight loss also occurs; initially at a higher rate (~1kg/day) which then decreases (~0.7 kg/day by 24 h, 0.5 kg/day by day 6 and 0.3 kg/day from day 21 onwards) [33
]. Importantly, the few small studies of energy expenditure in the context of Ramadan fast have found no evidence of a metabolic adaptation [24
]. This finding needs to be investigated in larger studies and if confirmed, may have important implications on Ramadan and IF as potential weight loss strategies. Admittedly, overcompensation with an increase in energy intake at the evening meal is common practice in observers of the Ramadan fast [31
]. Although the increased appetite at the end of the fasting day [49
] is the main drive for this phenomenon, this is in many ways voluntary. With appropriate education and a shift in food choices it may be possible to limit this increase in intake of energy dense food and make the prospect of weight loss with the Ramadan fast more realistic.
Aside from weight changes, Ramadan fasting induces a plethora of physiological and metabolic alterations. The impact of Ramadan on sleep alone includes decreased total sleep time, delayed sleep, decreased sleep period time (decreased REM sleep duration, decreased proportion of REM sleep) and increased proportion of non-REM sleep [13
]; also reported with high inter-individual variation.
An important issue on interpretation of Ramadan studies is the potential hypohydration that would be expected towards the end of the Ramadan fasting day. A study investigating the effects of prolonged fasting and fluid deprivation reported a loss of body weight of around 1.5 kg in individuals fasting between 10 pm and 4 pm the next day; the weight loss was presumed to be due to loss of body water [39
]. Fluid homeostasis during Ramadan fast has been investigated in several studies and has been reviewed elsewhere [58
]. Water turnover has been shown to increase during Ramadan fast with concomitant increases in indicators of body hydration including haematocrit, serum urea and creatinine and urine osmolality. However, total body water appears to be conserved and aside from potentially contributing to weight loss that might be observed in Ramadan, no detrimental effects on health have been directly attributed to negative water balance and hypohydration at the levels experienced during Ramadan [58
]. Furthermore, hypohydration has been shown to have no significant effect on RMR and blood glucose in healthy subjects [59
Studies of Ramadan fasting in general need to be interpreted carefully and with consideration for certain factors such as the timing of previous meal, methodological differences and also hydration status. An important and relevant factor in studies of Ramadan fasting is the duration of the fast, and hence geographical location; the impact tends to be most marked in countries at higher altitudes and with more daylight hours [60
]. Fasting hours also include the seasonal changes whereby fasting Ramadan during winter months for instance would have physiologically different effects when compared to fasting Ramadan during summer months. Although the literature specifically pertaining to energy expenditure changes during Ramadan is steadily mounting, it is currently small in number. Therefore, future studies need to address these variables to tackle the inter-variability issues that continually arises in the current literature.
In conclusion, although the metabolic consequences of Ramadan fast are complex, there is potential for using this month as a weight reduction model provided the fasting is carried out mindfully; balancing food type, quantity and levels of physical activity. Pre-Ramadan planning (nutrition plans, medication and health checks) is necessary; more so for individuals with chronic conditions such as diabetes who need specialist advice should Ramadan fast be deemed suitable in the first place. The long-term effects are thus of interest and studies are necessary for elucidation.