Intermittent Fasting: From Digestive Rest to Metabolism and the Gut Microbiome

Farzollah Gholizadeh and Fariba Bayrami

A guide to 12, 14, 16, 18 and 20-hour fasting patterns, choosing one by temperament, and eating well around your fasting hours

Introduction: does when we eat matter as much as what we eat?

The health of the digestive system does not depend only on what we eat. When we eat, the gaps between meals, the quality of our sleep and how well our eating fits the body’s internal clock all matter too.

Intermittent fasting is a way of limiting the hours in which we eat. It can affect weight, insulin sensitivity, metabolism and the make-up of the gut microbiome.

Some studies have reported changes in bacteria such as Akkermansia muciniphila during fasting, but the results are not the same for everyone.

1. How does fasting affect the gut?

Autophagy and cell renewal

Autophagy is the cell’s natural process of recycling its damaged parts. Limiting energy intake can influence the pathways that regulate it.

However, it cannot be claimed that autophagy switches on in every person after exactly 16 or 18 hours of fasting, or that more autophagy necessarily leads to repair of the gut lining.

Rest for the digestive system

Between meals, a pattern of movement called the Migrating Motor Complex helps move contents through the digestive tract, especially the stomach and small intestine. Avoiding constant snacking gives this natural activity more chance to happen.

Insulin sensitivity and inflammation

In some people, intermittent fasting comes with weight loss, better insulin sensitivity and lower levels of some inflammatory markers. These changes may indirectly affect the gut environment and the microbiome.

Akkermansia and the gut lining

Some studies have reported more Akkermansia with time-restricted eating. However, the evidence is not strong enough to conclude that fasting reliably increases this bacterium or rebuilds the gut lining.

2. Fasting patterns and the temperament view

In humoral (traditional Persian) medicine, the choice of eating pattern and dietary restraint depends on a person’s temperament, digestive strength, age, physical condition and food reserves. The classification below combines that traditional view with modern fasting patterns. It should not be taken as a fixed, proven prescription for everyone of a given temperament.

• 12:12 (12 hours fasting, 12 hours eating): a gentle pattern to start with. Within the temperament framework it may suit bilious (safravi) and melancholic (sawdawi) people better, especially those with smaller body reserves.
• 14:10 (14 hours fasting, 10 hours eating): the next step for people who manage 12:12 well and eat enough during the day.
• 16:8 (16 hours fasting, 8 hours eating): for example, eating from 10 am to 6 pm. In the temperament view it is suggested for sanguine (damawi) and phlegmatic (balghami) people without severe obesity.
• 18:6 (18 hours fasting, 6 hours eating): a more restrictive pattern suggested for overweight phlegmatic people. Getting enough protein, energy and fluids becomes even more important.
• 20:4 (20 hours fasting, 4 hours eating): a very restrictive pattern that can make it hard to meet nutritional needs. It has not been shown to be better for gut repair or for increasing autophagy.

In the temperament view, bilious and melancholic people usually do best with the gentler 12:12 to 14:10 patterns, while sanguine and phlegmatic people may use longer patterns, depending on their individual situation.

An important note about water: traditional medicine recommends drinking less water during fasting for phlegmatic people, to strengthen the effect of the fast. In other temperaments, especially bilious and melancholic people, dehydration can cause low blood pressure, kidney problems and constipation. Always drink water with electrolytes while fasting. People with kidney disease or low blood pressure, older adults, and anyone fasting in hot weather or doing heavy physical activity should not restrict water.

3. Fasting in tune with daylight

Circadian fasting focuses on matching meal times to the body’s internal clock.

For example, finishing your last meal at 7 pm and eating again at 7 or 8 am gives a fasting period of 12 to 13 hours.

Eating earlier in the day, avoiding heavy dinners and cutting out night-time snacks can help regulate blood sugar, sleep and the gut’s microbial rhythms.

4. What to drink during fasting hours

Drinks suitable for a calorie-free fast

• Plain water: the best choice for staying hydrated and keeping the gut working normally.
• Unsweetened green and black tea: contain catechins and polyphenols, including EGCG.
• Plain black coffee: contains chlorogenic acid and other phenolic compounds.
• Unsweetened herbal teas: such as chamomile, peppermint, hibiscus, saffron and weak ginger tea.

Water with a little salt is also used in some fasting patterns, especially for people with a phlegmatic temperament. But drinking salt water every day, for example a quarter teaspoon of salt, is not necessary or suitable for everyone. People with high blood pressure, kidney disease or heart disease should talk to their doctor before adding salt.

What breaks a calorie-free fast

Milk, cream, butter, Bulletproof coffee, honey, sugar, fruit juice, whey protein and amino acid supplements that contain energy all break a calorie-free fast.

Zero-calorie sweeteners such as stevia, sucralose, aspartame and erythritol differ in their effect on insulin and the microbiome; it cannot be said that all of them definitely stop autophagy.

A few drops of lemon juice or apple cider vinegar usually contain very little energy, but they may cause discomfort in people with reflux or a sensitive stomach.

5. How to break your fast

How you open the eating window matters for digestive comfort and appetite control.

The best choice is to start with natural fats such as cold-pressed olive oil, ghee, coconut oil, sesame oil or black seed oil. Then have boiled water cooled to lukewarm, green tea or another herbal tea. Protein or cooked vegetables with olive oil are other options, depending on what the person tolerates.

The main meal should include enough protein, healthy fat and fibre: for example eggs or meat, vegetables, olive oil and a suitable amount of cooled wholegrain bread or cooled brown rice, which contains type 3 resistant starch (RS3).

Avoiding a large amount of sweets and refined carbohydrates at the start of the meal helps keep blood sugar under better control.

In longer fasts, getting enough protein, healthy fats, fibre and energy in the eating window is important to prevent extreme hunger and muscle loss.

6. Fasting, resistant starch and polyphenols work together

Fasting sets the timing of eating, but during eating hours the beneficial bacteria need to be fed.

Type 3 resistant starch in cooked and cooled rice, bread and potatoes, together with plant fibres, helps the gut produce short-chain fatty acids.

Polyphenols from pomegranate, tea, olives, grapes and berries are also transformed by the microbiome and can influence the gut environment.

This combination makes biological sense, although the exact timing of polyphenols after a fast has not yet been proven as a reliable way to increase Akkermansia.

7. The best time for green tea and coffee

• Green tea: can be drunk in the middle of the fasting period or at the start of the eating window. Drinking it one to two hours after a meal containing resistant starch is also fine, but no specific benefit of that timing has been proven. Brewing with water at about 80 °C preserves the flavour and helps extract the catechins.
• Black coffee: mid-morning, or one to two hours after waking, suits many people. Coffee on an empty stomach makes reflux or stomach discomfort worse in some people. Caffeine can stimulate movement of the large intestine, but this should not be confused with directly triggering the small intestine’s cleansing waves.

Unsweetened tea and coffee are better for a calorie-free fast. Adding milk, cream or honey adds calories.

Tea polyphenols can reduce absorption of plant (non-haem) iron, so leaving one to two hours between tea and iron-rich meals is sensible, especially for people prone to iron deficiency.

8. Choosing a fasting pattern: what matters

A longer fast does not necessarily mean a bigger effect. Age, physical activity, body composition, energy reserves, blood sugar, sleep quality and the ability to meet nutritional needs should all be considered.

Fasts of 18 to 20 hours can lead to low energy, muscle loss or extreme hunger in some people. Long fasts are not suitable for children, pregnant or breastfeeding women, or people with a history of eating disorders, and people taking blood-sugar-lowering medicines need their doctor to adjust them.

In humoral medicine, temperament, physical strength, age and season can be part of a personal assessment, but they should never replace checking a person’s actual health.

Conclusion

Intermittent fasting, especially gentle patterns that follow the body’s internal clock, can help regulate metabolism and keep the digestive system in order.

For many people, 12:12, 14:10 and 16:8 are more practical than very long fasts. Alongside good timing, food quality matters most: enough protein, fibre, healthy fats, polyphenols and resistant starch.

The goal is not to stretch hunger at any cost, but to find a balance between nourishment, digestive rest, a healthy microbiome and keeping your strength.

To choose the right pattern, first find out your temperament with our free temperament test.

References

• Qin J, et al. A human gut microbial gene catalogue established by metagenomic sequencing. Nature. 2010;464:59–65.
• Belzer C, de Vos WM. Microbes inside—from diversity to function: the case of Akkermansia. ISME J. 2012;6:1449–1458.
• Koh A, et al. From dietary fiber to host physiology: short-chain fatty acids as key bacterial metabolites. Cell. 2016;165:1332–1345.
• O’Callaghan A, van Sinderen D. Bifidobacteria and their role as members of the human gut microbiota. Front Microbiol. 2016;7:925.
• Lockyer S, Nugent AP. Health effects of resistant starch. Nutr Bull. 2017;42:10–41.
• Sonnenburg ED, Sonnenburg JL. Starving our microbial self. Cell Metab. 2014;20:779–786.
• Dao MC, et al. Akkermansia muciniphila and improved metabolic health during a dietary intervention in obesity. Gut. 2016;65:426–436.
• Birt DF, et al. Resistant starch: promise for improving human health. Adv Nutr. 2013;4:587–601.