Intermittent fasting is a dietary protocol that has gained popularity in recent years.
But far from simply being the latest fad, fasting has long been a feature in various global cultural and religious traditions.
And there is growing evidence that fasting practices may have multiple benefits for health, including hunger management, insulin sensitivity and blood pressure.
However, while it has its pros, intermittent fasting does not work for every lifestyle and fitness goal.
So, given the wealth of confusing and conflicting information online, how can you tell whether intermittent fasting is right for you or how to apply it?
Read on for our third instalment in our Diets Reviewed series for the full lowdown on intermittent fasting.
What is intermittent fasting?
Fasting is defined as the abstinence from consuming food and/or beverages for different periods, lasting from several hours a day right up to a few weeks at a time,. While there is no clear clinical definition regarding when fasting begins after consuming food or drink, it is a lifestyle strategy that appears to improve several chronic, non-infectious diseases,.
Intermittent fasting is an approach where food is not consumed for a specific period during a day or week. The common theme in all types of fasting is energy restriction, whether or not this is a daily occurrence.
Intermittent fasting can be further categorised into three types:
1. Intermittent calorie restriction
Also referred to as whole-day fasting, this fasting approach entails fasting for over 24 hours two or three times a week. On non-fasting days, observers can eat calories ad libitum (i.e. as much as they want). This fasting cycle is usually followed by at least one week of normal eating.
Intermittent calorie restriction can be further split into two sub-types:
- The 2:5 method: caloric restriction for two days followed by normal eating for five days.
- The 3:4 method: caloric restriction for three days, followed by normal eating for four days.
However, it’s not uncommon for these protocols to allow around 25% of total daily expenditure on fasting days, which typically works out at around 400-600 kCal per day.
2. Alternate day fasting (ADF)
ADF is an increasingly popular method that involves alternating between non-fasting days, where you can eat ad libitum, and fasting days, where you would only consume 25% of your normal dietary intake (generally under 500 kCal),. However, some ADF approaches do not permit any caloric intake on fasting days.
3. Time-restricted feeding (TRF)
Time-restricted feeding is an approach with daily periods of complete fasting, with eating only permitted outside of these hours. There are three main sub-types of TRF, including:
- 16:8 – 16 hours of fasting, followed by an eight-hour feeding window.
- 18:6 – 18 hours of fasting, followed by a six-hour feeding window.
- 20:4 – 20 hours of fasting, followed by a four-hour feeding window.
Ramadan, a period of religious fasting that takes place throughout the ninth month in the Islamic calendar, also falls under the umbrella of TRF. Observing Muslims abstain from food and water between dawn and sunset, which varies according to geographical location and season, although generally, the fast lasts around 18 hours.
One of the main proposed benefits of TRF stems from research into ‘chrono-nutrition’, which refers to the interaction between meal timing and the body’s circadian rhythm, which regulates human physiology, metabolism and behaviour. It has been theorised that the metabolic benefits of TRF may stem from better alignment with the natural pattern of feeding and fasting found in most organisms that have a circadian rhythm.
The main differences between these three approaches lie in the degree to which calories are restricted, the amount of food or drink permitted, and how often fasting occurs. For example, most TRF approaches involve fasting for a given period, such as 16 or 20 hours, therefore shortening the remaining eating window. In contrast, ADP methods involve alternating days eating ad libitum with fasting days, normally consisting of just one lunchtime meal with 25% of baseline calories.
How does intermittent fasting work?
Fasting is a lifestyle management strategy that appears to improve outcomes for several chronic, non-infectious diseases. This specifically includes ‘metabolic disease’ conditions, such as type 2 diabetes, cardiovascular disease (CVD), and high cholesterol. And fasting methodologies provide several health benefits when it comes to improving blood lipid profiles, insulin sensitivity and blood pressure, even without weight loss , ,.
One explanation behind this finding could be that fasting triggers a series of coordinated metabolic changes that allow the body to conserve carbohydrates and increase its reliance on fat for energy. This phenomenon, known as ‘metabolic switching’, sees glucose levels remain elevated for around six hours after a meal before decreasing for around 16 hours before eating again. Circulating insulin lowers while levels of free fatty acids and ketones rise (a substance produced by the body when you do not have enough insulin to convert glucose into energy). As a result, intermittent fasting could be beneficial for those for whom improving insulin sensitivity is a priority but who struggle to follow a low-carbohydrate diet, e.g., vegetarians or vegans.
However, plenty of evidence shows that weight loss (which we can achieve through multiple means) is the strongest predictor of improved insulin sensitivity, which improves with regular physical activity. In most cases, the most tangible benefit of fasting approaches lies in their capacity to reduce average calorie intake while improving satiety. By shortening your eating window, you effectively reduce the wait between meals. So, if you’re following a calorie-controlled diet, IF may help you feel less hungry, thereby increasing weight loss and its accompanying benefits.
Fasting is one of the ways to improve insulin sensitivity, alongside weight loss and exercise and physical activity.
Claims under the spotlight
Intermittent fasting has become a highly popular yet contentious area of debate in recent years. So, which of the claims can you believe, and which should you take with a pinch of salt?
1. ‘Intermittent fasting boosts fat loss.’
Research shows that ADF has beneficial effects for reducing body weight as well as fat mass,. In particular, observational studies have shown that obese individuals and women tend to see greater reductions in weight and body fat percentage changes after fasting, which has been shown to occur regardless of changes in calorie intake,. On the flip side, other evidence shows that weight loss outcomes are, on average, largely similar with both continuous energy restriction and IF methodologies. Therefore, it’s likely that intermittent fasting’s benefits stem from its ability to reduce average energy intake across the week rather than special ‘fat burning’ effects.
2. ‘Intermittent fasting improves metabolic health.’
Insulin resistance and elevated blood sugars are hallmarks of metabolic disease. Together they form a lethal cocktail for CVD and atherosclerosis and damage the crucial beta cells of the pancreas, which produce insulin , , . As a result, very advanced type 2 diabetics often require insulin injections to prevent blood sugars from reaching dangerous levels.
In this vein, there is some substance to IF’s health benefits as evidence indicates it has a largely positive effect on these markers. Most IF research focuses on early time-restricted feeding, limiting eating to a four-to-eight-hour window in the early part of the day (e.g. 8 am – 4 pm) with a fast for the remaining period. This methodology has been found to decrease blood pressure, improve insulin sensitivity, and restore the beta cells of diabetic patients even without weight loss or calorie restriction, . One study even found that this fasting variant lowered blood pressure as much as gold-standard medications.
However, no single dietary or exercise approach works for everyone, and losing around 10% of total body weight appears to have largely the same positive effects . So, while IF might have a slight edge over other methodologies, it’s certainly not the only tool in the box.
3. ‘Intermittent fasting causes muscle loss.’
While IF approaches bring benefits in terms of fat oxidation and insulin sensitivity, which may help improve body composition, it’s important to remember that insulin plays an important ‘anti-catabolic’ role in the body, i.e. preventing tissue breakdown. A critical component preventing the breakdown of muscle tissue is circulating levels of amino acids, which come primarily from protein intake.
The body is in a constant state of ‘turning over’ muscle tissue through breaking down and resynthesising new proteins, a process amplified by resistance training. Protein synthesis is a metabolic process that is highly sensitive to our nutritional environment, which significantly impacts muscle mass quality and health. During periods of caloric restriction, the two primary means of signalling to the body to retain lean body mass are protein intake and resistance training. Because fasting inevitably involves going for long periods without food, it has been hypothesised that it is suboptimal for maintaining and enhancing muscle mass.
However, further research has shown that, where increased muscle loss does occur due to IF, such results are due to the severity of the deficit, the speed of weight loss and reduced total protein intake,. Therefore, unless you’re planning on stepping on the Olympia stage, as long as your calorie and protein intake match your goal and body composition, muscle loss shouldn’t be a huge concern.
Even when following a fasting protocol, protein intake and resistance training are the primary ways to signal to the body to retain lean muscle.
Intermittent fasting: The pros
1. IF can help reduce hunger
Research shows that people often report feeling fuller when following intermittent fasting approaches. This is likely due partly to decreases in circulating levels of the hunger hormone ghrelin, which appears to occur independently of weight loss. Other than physiological changes, IF’s primary benefit for hunger management is that it shortens the eating window, and once you break your fast, you have to wait less time between meals. So, if you’re someone that struggles to stay away from the fridge, IF could improve your adherence.
2. IF can help you reduce your calorie intake without tracking or weighing
Research shows that intermittent fasting can cause people to restrict their calorie intake by 20% or more without intentionally changing their diet. This is likely due to IF’s satiating effects, which reduces the likelihood of overeating post-fast, thereby eliminating any net deficit. As a result, IF could be a useful approach for people who don’t enjoy meticulously tracking every calorie. However, remember that energy balance is always the overriding factor for weight loss, so even if you don’t track, you still need to ensure you burn more calories than you consume.
3. IF could be beneficial if you have a busy morning schedule
Meal timing is relatively low on the priority list behind total daily calorie intake, protein intake and nutritional density. Therefore, unless you’re an elite-level athlete, it won’t make much difference from a fat loss perspective whether you eat all your calories in a single sitting or across 10 meals. But if you have a busy schedule, such as a morning training session or dropping the kids at school, eating your first meal slightly later could help you feel fuller and a bit less frazzled in the morning.
4. IF could be helpful if you have high blood pressure, diabetes or cholesterol
IF has proven benefits for insulin sensitivity, lipid profiles and blood pressure, ,. If you have been diagnosed with any of these conditions, fasting could be a means to improve your health outcomes quickly. However, remember that improving your body composition should be the number one priority, so choose whichever method allows you to do that most consistently for long enough.
Fasting has proven benefits for insulin sensitivity, lipid profiles and blood pressure.
Intermittent fasting: The cons
1. IF may not always fit your preferences and lifestyle
The success of any dietary strategy lies in whether you can adhere to it, and even the ‘best’ plan in the world is useless if it doesn’t fit your lifestyle. If you like to eat first thing in the morning upon waking or struggle with long periods without eating, IF may not be for you.
2. IF could impact training performance
What you eat around your training window appears to impact training performance, muscle growth, and strength, , . It’s important to remember that total energy intake and macronutrient breakdown are the main determinants of growth and recovery. However, if you choose IF, you may want to ensure that you don’t schedule your training right in the middle of the fasting window.
3. IF may negatively affect female hormonal health
Some research shows that fasting can increase the likelihood of menstrual cycle disorder,. Therefore, if you have a hormonal imbalance or an irregular menstrual cycle, IF might not be advisable for you.
4. IF works well for fat loss approaches but may make muscle gain phases more challenging
Intermittent fasting can make dieting easier through its beneficial effects on hunger management and total calorie control. But if you’re following a muscle gain phase in a calorie surplus, IF could make it hard to eat all the calories you need for growth.
Fasting can impact your performance in the gym.
Intermittent Fasting: The Takeaways
The evidence undoubtedly shows that intermittent fasting approaches are beneficial for improving adherence to a given weight loss strategy and generating specific improvements for health markers. However, it’s important to understand that the main determinant of success for any dietary approach is its application. Therefore, if intermittent fasting is an approach that suits your preferences and lifestyle, it could give you a leg up when it comes to achieving your goals. But, if long periods without food aren’t for you, other methods are just as effective at improving long-term health outcomes.
- Intermittent fasting (IF) is a dietary protocol that involves abstaining from food or drink for specific periods.
- Varieties of intermittent fasting include intermittent caloric restriction, alternate-day fast and time-restricted feeding, of which Ramadan fasting is one type.
- Fasting has proven benefits for metabolic health independently of weight loss through its effects on blood lipids, insulin sensitivity and blood pressure.
- One of the main benefits of fasting is that it may help improve your adherence to a calorie deficit through improving appetite and satiety.
- While there are benefits to IF, other methodologies can achieve similar outcomes if it doesn’t work for you or your lifestyle.