Intermittent fasting is a dietary pattern that involves voluntary energy intake restriction. In this article we explore the different types of intermittent fasting, potential benefits and more.
What is fasting?
Fasting is not eating. It is common in normal life, for example: we don’t eat between bedtime and breakfast and prior to blood tests or some procedures (e.g. colonoscopy, surgery).
What is not fasting?
Technically, any form or food or drink intake breaks a fast. In practice, however, water and potentially some energy-free (i.e. non-caloric) beverages such as unsweetened black coffee do not seem to make an impact in fasting status.
Below are some examples of what some people do during their “fasting” periods that unfortunately break the fast:
- Eating very little food
- Having a calorie-containing beverage (e.g. a latte)
- Having a ketogenic meal or drink (e.g. Bulletproof coffee)
What is intermittent fasting?
The term intermittent fasting (IF) describes periods of voluntary abstinence from food and drink (1). Even though you might recognise it as a trendy “diet” or eating pattern, IF has been practised for many years, mainly for religious reasons (e.g. Ramadan fasting).
Types of intermittent fasting
There are 4 main types of IF: alternate day fasting, modified fasting, time-restricted eating and religious fasting (1). As it often happens with nutrition science, most of the studies for the 3 first types has been done in animals and there is limited evidence for the application of those protocols in humans. The effects are even more uncertain in free-living individuals.
Note: I’ve included some examples of who each protocol is not suitable for but make sure you check out the Who should not try intermittent fasting? below for a more comprehensive list.
Alternate day fasting
This protocol involves alternating fasting days with days of eating ad libitum (i.e. without any restriction).
- People who have a lot of control over their schedule and eating patterns such as single people with a regular work schedule
- People with limited time to cook
- People who don’t enjoy cooking, eating, or food in general
Not suitable for
- People with families, especially young children as it can be hard to not eat while the family is having meals and it can be detrimental to children to display dieting behaviour in front of them
- People with unpredictable work/study schedules
- People who have to eat or taste food for a living such as chefs, recipe testers, etc.
- Modest weight loss (~2.5%, i.e. 2.5 kg for a 100 kg person)
- Improvement in HDL-cholesterol (“good” cholesterol) and triglycerides levels
- Improvement in inflammatory markers (1) (as seen before, inflammation occurs in multiple chronic conditions)
This modality of fasting does not involve real fasting, but rather a reduced energy intake (typically 20-25% of normal intake) on “fasting” days (1).
Some examples of modified fasting include:
- 5:2 diet
Possibly the most popular diet in this category created by Dr Michael Mosley. It prescribes 5 days of normal energy intake and 2 non-consecutive days of 500-600 kcals (for women and men, respectively).
- Fast 800 diet
Also created by Dr Michael Mosley. As detailed in my book review of The Fast 800, this diet includes 3 stages: rapid weight loss (800 kcals/day), the new 5:2 (800 kcals on 2 non-consecutive days), and maintenance.
- Prolon diet
Created by Prof Valter Longo, author of The Longevity Diet. This very low energy, supplement-based diet is meant to be followed for 5 days per month over a few months.
- People with control over their food intake
- People who are not very physically active every day of the week
Not suitable for
- People with no control over their food intake
- People who are physically active every day of the week (e.g. labourers who do intense exercise on their day off work)
- Modest to moderate weight loss (3.2-8.0%)
- Decrease in fasting insulin (but not fasting glucose)
- Improvement in blood lipids (i.e. cholesterol levels)
- Improvement in inflammatory markers (1)
A.k.a. time-restricted feeding in animal studies, this type of IF involves eating within a narrower period than usual, normally up to 12 hours. This protocol does not involve energy restriction.
Popular examples of time-restricted eating include:
This is currently the most popular time-restricted eating protocol, which consists of 16 hours of fasting and 8 hours of eating. Some people implement this by skipping breakfast, some people by having an early dinner. This choice is normally based on work schedules, family commitments and personal preference.
- One meal a day (a.k.a. OMAD)
As its name implies, this style of eating consists on eating one big meal per day. This style of eating is favoured by busy individuals who prefer to consume all their kilojoules in one sitting. Needless to say, this meal tends to be quite large, so OMAD is not recommended for individuals with reduced digestive capacity. In addition, due to the extended fasting period, OMAD is not recommended for individuals with impaired sugar regulation.
- People not willing to count calories
- People with flexible meal times
Not suitable for
- People with set meal breaks that don’t fit in their chosen eating window
- People who tend to binge eat and/or don’t follow hunger cues as they may overeat during the eating window
- Modest weight loss (1.3-4.1%)
- Decrease in fasting glucose
Improvement in LDL- and HDL-cholesterol (1)
This is possibly the oldest way of intermittent fasting as per the definition mentioned in the What is intermittent fasting? section.
Even though many religions implement some time of fasting, the most well-known and studied is Ramadan. During the month of Ramadan, observers fast from dawn to sunset. This can mean anywhere from 11 to 22 hours, depending on the season and country (1). Therefore, Ramadan falls into the time-restricted eating category.
- Modest weight loss (one meta-analysis found 1.24 kg weight loss during Ramadan, 0.72 kg weight regain post-Ramadan)
- Decrease fasting blood glucose
- Improvement in blood lipids and inflammatory markers (1)
Note: these benefits seem to be gender-specific and are likely to fade a few weeks after resuming normal eating patterns.
Potential mechanisms linking intermittent fasting with health
The circadian rhythm refers to the ~24-hour cycle that governs all processes in our bodies, including energy metabolism, hormonal secretion, physical coordination and sleep. The master clock is located in the hypothalamus (a region of the brain), but there are also other clocks in other tissues such as the liver (1).
Besides daylight, the ingestion of energy is a potent timing cue for our biological clocks (1). On the other hands, fasting periods allow for cellular repair processes to kick in.
Disrupted circadian rhythms (e.g. shift work, long-haul travel) are associated to increased risk of obesity, diabetes, cardiovascular disease (CVD) and certain cancers (e.g. breast) (1).
Gastrointestinal processes follow circadian rhythms, e.g. gastric emptying and blood flow are greater during daytime, while glucose metabolism is lower in the evening. Therefore, disturbed circadian rhythm may affect gastrointestinal function (1).
Intermittent fasting can also potentially affect the composition of the gut microbiome. Our gut bacteria is responsible for the extraction of energy and nutrients from the food we eat. Scientists have identified a “obese microbiome”, a mixture of bacterial species that are characteristic of obese individuals. These species may be able to extract more energy from food, leading to overweight and obesity. The “obese microbiome” can also increase gut permeability and translocation of bacterial products into the bloodstream (1), leading to systemic inflammation. By altering bacterial composition, intermittent fasting could potentially shift an “obese microbiome” into a “lean microbiome”.
Intermittent fasting can lead to unintentional lifestyle behaviour changes that can positively impact body weight and health.
Modified fasting protocols involve energy intake reduction by definition, provided energy intake in the non-fasting days remains around usual intake.
Time-restricted eating can also lead to involuntary reduced energy intake, as there are less opportunities to eat in less hours.
In addition, if the eating window is placed earlier in the day, there are less chances of eating energy-dense nutrient-poor foods (i.e. junk food). This is due to the fact that we tend to reach for those type of foods in the evening, when we are tired and have decision fatigue.
Finally, it has been observed that the levels of some hormones which regulate appetite (e.g. leptin and ghrelin) are altered in shift workers (1). This suggests that a disruption in circadian rhythms leads to increased energy intake.
While animal studies indicate that physical activity is regulated by circadian biology, there is no human evidence available (1). However, intuitively it makes sense that we are more willing to be physically active during the day than in the evening, regardless of shift work. This would suggest that regulating circadian rhythms by adhering to a daytime restricted eating window could lead to increased energy expenditure, and thus weight loss and other health benefits.
There is evidence to suggest that eating in the evening can negatively impact sleep in terms of quantity and quality (1). This, of course, is dependent on the size and composition of the evening meal(s). Disrupted sleep can, in turn, lead to insulin resistance and increased risk of obesity, diabetes, CVD and cancer (1).
Who should not try intermittent fasting?
- Individuals with a history of disordered eating or eating disorders
- Individuals with high anxiety or stress
- Currently malnourished individuals
- Children and elderly individuals
- Individuals with certain health conditions without consulting with their healthcare professional
- Pregnant women
- Athletes during training and competition
How to make it work
- Choose a protocol that works with your circumstances and lifestyle
- Follow hunger cues to adjust energy intake
- Ensure adequate nutrient intake, especially:
- 5 or more serves of vegetables
- Up to 2 serves of fruit
- Protein according to body weight and activity level
- Adequate fluids
- Alternate day fasting or modified fasting (e.g. 5:2)
- Time “fasting” days with days of low energy output
- Make sure “fasting” days are non-consecutive and ideally not the same every week
- Time-restricted eating
- Time eating window ideally within sunrise/sunset
- If physically active, time eating window so that there are meals consumed after intense training session(s)
- Unsweetened black tea or coffee can be consumed in fasting period
- Patterson RE, Laughlin GA, LaCroix AZ, Hartman SJ, Natarajan L, Senger CM, et al. Intermittent Fasting and Human Metabolic Health. J Acad Nutr Diet. 2015 Aug;115(8):1203–12.