Types of Intermittent Fasting

Key takeaways

• Intermittent fasting is a type of meal scheduling technique which requires alternating between set periods of eating and fasting, with the goal of improving health and longevity. 

• Fasting patterns include the limiting of eating hours to certain windows per day, whole days of fasting or even longer fasts. 

• Benefits may include weight loss, improved cell integrity, disease prevention and enhanced gut and brain health. 

Intermittent fasting is a popular ‘non-dietary’ pattern of eating that’s garnered increasing attention due to its potential health benefits, including weight management, improved metabolic health and positive effects on cellular processes and longevity. Some people find it easier to adopt than traditional nutrition plans because it focuses on when you eat, rather than what you eat. 

What is intermittent fasting?

Numerous religions and cultures have practised forms of fasting for centuries for health, spiritual and religious purposes (1). Intermittent fasting is an approach that involves cycles of fasting and eating on a daily or weekly basis. 

This dietary practice is rooted in the idea that our ancestors did not have continuous access to readily available foods, unlike today's markets, restaurants and refrigerators. Unlike traditional diets that focus on limiting or avoiding specific foods or calories, intermittent fasting primarily emphasises when to eat. 

While there are no strict guidelines on what to include or exclude from your meals during the designated eating period, it's important to consider eating nutritious foods. Not eating for  extended periods makes it even more important to consume nourishing foods within your eating window.  

Types of intermittent fasting explained

Time-restricted fasting 

This is the most widely embraced form of intermittent fasting. It involves restricting the time restricted eating- window to a specific timeframe. One of the most popular TRF methods is the 16:8 plan - which involves eating for 8 hours or less per day while fasting for sixteen hours (2). However, times can be tailored according to fasting experience, bodily needs and goals. Other time-restricted methods include 12:12 or 14:10.

Alternate-day fasting (ADF)

Alternate-day fasting consists of eating normally one day then alternating to an intake of fewer than 500 calories on fasting days. This approach involves fasting may aid in rapid weight loss, however, it’s not recommended for beginners and may lead to overeating due to the stimulation of appetite hormones such as ghrelin (5). Ghrelin increases hunger levels following weight loss, in order to prevent starvation (6).

The 5:2 diet

This plan involves regular eating for five days each week, with two non-consecutive days designated for fasting. During these two fasting days, the recommended calorie intake is between 500-600 calories (3). This approach requires a very low-calorie intake so is more suitable for experienced fasters.  

Eat-Stop-Eat (ESE)

This approach requires fasting for one or two 24-hour periods per week (4). It differs from the 5:2 diet which allows consumption of food during the two fasting days. This fasting method offers significant benefits to individuals aiming to enhance their digestion, prevent and manage diseases, and achieve weight loss through a caloric deficit. However, this approach is also a better option for experienced fasters without health issues or medical concerns. 

Benefits of intermittent fasting

Weight loss 

Approximately 12 to 16 hours into a fast, your body experiences a decline in glucose (the main energy source for cells) and amino acid (protein building blocks) levels. This process leads to a reduction in insulin levels (which decreases blood sugar) and elevated glucagon levels  (increases blood sugar). This metabolic change kick starts your body into burning fat and producing ketones (19). The timing of this metabolic switch depends on how much glucose is stored in the liver, and is influenced by when and what you last ate, as well as your physical activity level (9). 

This metabolic change allows the body to function while doing fat loss and minimising muscle breakdown during the fasting periods because of intermittent fasting schedules (22). So a great plus of an intermittent fasting program is that it has the potential to decrease your body weight and fat mass during fasting period while maintaining muscle integrity.

Cell integrity and renewal  

Autophagy, derived from Greek, meaning ‘self-eating’, is a natural bodily process that involves the removal and recycling of damaged or dysfunctional cell components. This process helps maintain cellular functionality and integrity, which in turn contributes to overall health and longevity (11). Autophagy typically begins at about 16 hours into a fast, with maximum benefits experienced between two to four days (10). 

Lower levels of inflammation and disease prevention 

Chronic inflammation may increase the risk of chronic diseases, such as diabetes, heart disease and arthritis (12). Intermittent fasting can play a key role in reducing levels of inflammation (20).

The biochemical response to fasting affects many of the hallmarks of ageing, including mitochondrial dysfunction, telomere attrition, genomic instability, loss of proteostasis and deregulated nutrient sensing (21). Periodic fasting has the potential to slow down ageing and prevent or delay chronic conditions. 

Learn how fasting affects the hallmarks of ageing: What you need to know about the 12 hallmarks of ageing.

Improved gut health 

Fasting has shown to promote favourable change in the mix of bacteria, archaea, fungi, and viruses in the gut, which collectively comprise the gut microbiome (21) Fasting tends to boost both the presence of beneficial species and the overall diversity of the microorganisms living in the gut. At the same time, it can decrease the levels of harmful species of microorganisms. These positive shifts in the gut microbiome are associated with improved health (15).

Fasting can induce a notable increase in bacteria known as Lachnospiraceae. These bacteria play a crucial role in promoting the production of short-chain fatty acids such as butyrate, known for its anti-inflammatory properties. Short-chain fatty acids also have positive metabolic effects and anti-ageing properties (14). Fasting can also increase the abundance of the bacteria Akkermansia muciniphila, which has been associated with decreased gut inflammation and a stronger gut lining (16). 

Thinking and memory 

Fasting may assist in improving mitochondria (cells’ powerhouse) function and can protect brain cells by providing ketones as fuel. Ketones are produced when the body uses up glucose as fuel and switches to breaking down fat as fuel. Ketones play a key role in promoting the production of brain-derived neurotrophic factor (BDNF), a remarkable compound known for encouraging the regeneration of brain cells, both shielding them against stress and inflammation, and enhancing cellular longevity and productivity. 

Some studies suggest that regular fasting could even slow the progression of neurodegenerative conditions such as Alzheimer’s by improving the connections between the brain cells (synaptic plasticity) and decreasing inflammation (17). 

Possible risks associated with intermittent fasting

Nutrient deficiency

Depending on the chosen fasting regimen, it can prove challenging to get all essential nutrients into a reduced eating window. This could lead to deficiencies in vitamins, minerals and other important dietary components if not managed carefully. For some people, it may be useful to consider supplements while they fast. 

Negative symptoms 

Some people may experience bothersome symptoms, especially during the initial phases of intermittent fasting. Common side effects include headache, irritability, fatigue, dizziness and mood swings (18). These symptoms are exacerbated if unhealthy foods are consumed during the eating window, dehydration occurs or if the fasting plan is too extreme. 

Social implications

Intermittent fasting can make it difficult to share meals with friends or family during typical eating windows or hours. Adhering to a specific hour eating window schedule may also be hard to align with work schedules. In some cases there can be a tendency to compensate for fasting periods by over consuming during the eating or fasting time restricted eating or window, both negating potential benefits and possibly leading to weight gain.

Intermittent fasting is not suitable for everyone

Intermittent fasting may not be a healthy diet suitable for those with certain health conditions, during pregnancy or breastfeeding; a history of eating disorders, or those with diabetes. Fasting may also exacerbate certain digestive disorders or conditions that require consistent food intake. If you’re an athlete or are frequently active, intermittent fasting may not be right for you as there could be a decline in performance.

How to start intermittent fasting 

• Start slowly - consider beginning with a 14:10 or even 12:12 plan, and allow time for your body to adjust.  

• Find a fasting pattern that works for you and your schedule. 

• Have a healthy pre-fast meal that's nutrient-rich and balanced. 

• Ensure sufficient intake of fluids and electrolytes, including sodium, calcium, potassium, magnesium and chloride. 

• Avoid excessive reliance on caffeinated beverages as they may reduce hydration levels.

• Break your fast with a modest, easily digestible meal containing low glycemic load carbs (oats, lentils, beans, dairy, vegetables) and proteins. 

Final thoughts: types of intermittent fasting

Intermittent fasting provides a versatile way of eating that may result in a number of health advantages. Just like any dietary modification, seeking advice from healthcare professionals is recommended, particularly if you have an existing health concern or condition. 

References

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3. Hajek, Peter, Dunja Przulj, Francesca Pesola, Hayden McRobbie, Sarrah Peerbux, Anna Phillips-Waller, Natalie Bisal, and Katie Myers Smith. 2021. “A Randomised Controlled Trial of the 5:2 Diet.” PloS One 16 (11): e0258853. https://doi.org/10.1371/journal.pone.0258853.

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