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  • Writer's picturePhoebus Tian

Intermittent Fasting: What is it, and how does it work?

In recent years, intermittent fasting seems to have become a new favourite, a method that is believed to lose weight and increase energy and even prolong life. In Chinese medicine, Bigu (辟谷) is a Taoist fasting technique developed in the context of Taoist philosophy and has been used in ancient China to seek longevity. But does sufficient scientific evidence exist to support these effects?

What is intermittent fasting?

Many diets focus on what to eat, but intermittent fasting is all about when you eat. With intermittent fasting, you only eat during a specific time. Fasting for a certain number of hours each day or eating just one meal a couple of days a week, can help your body burn fat. And scientific evidence points to some health benefits, as well.

Johns Hopkins neuroscientist Mark Mattson, PhD, has studied intermittent fasting for 25 years. He says that our bodies have evolved to be able to go without food for many hours, or even several days or longer. Before humans learned to farm in prehistoric times, they were hunters and gatherers who evolved to survive and thrive for long periods without eating. They had to: It took a lot of time and energy to hunt game and gather nuts and berries.

How Does Intermittent Fasting Work?

A series of studies have shown that alternating between fasting and eating contributes to cellular health, possibly by triggering a metabolic switch, an ancient form of adaptation in response to periods of food shortage. This switch occurs when cells deplete their reserves of sugar-based energy that they can access quickly and begin the slower metabolic process of converting fat into energy. (R, 2020)

To understand how intermittent fasting leads to fat loss, we first need to understand the difference between the fed state and the fasted state.

Your body is in the fed state when it is digesting and absorbing food. Typically, the fed state starts when you begin eating and lasts for three to five hours as your body digests and absorbs the food you just ate. When you are in the fed state, it's very hard for your body to burn fat because your insulin levels are high.

After that timespan, your body goes into what is known as the post-absorptive state, which is just a fancy way of saying that your body isn't processing a meal. The post-absorptive state lasts until 8 to 12 hours after your last meal when you enter the fasted state. It is much easier for your body to burn fat in the fasted state because your insulin levels are low.

When you're in the fasted state, your body can burn fat that has been inaccessible during the fed state.

Because we don't enter the fasted state until 12 hours after our last meal, our bodies are rarely in this fat-burning state. This is one of the reasons why many people who start intermittent fasting will lose fat without changing what they eat, how much they eat, or how often they exercise. Fasting puts your body in a fat-burning state that you rarely make it to during a regular eating schedule. Source:

Intermittent Fasting Benefits

According to research published in the journal Obesity, alternate day fasting reduces fasting insulin and insulin resistance levels more than restricting calorie intake in overweight or obese adults with insulin resistance.

Preliminary findings suggest that intermittent fasting may be more effective than daily caloric intake restriction in reducing insulin resistance in adults at risk of developing diabetes. (Gabel, 2019)

In a review article published in Nature Reviews, Cancer, Professor Longo VD of the University of Southern California, USA, describes the cellular-molecular biological mechanisms by which fasting enhances the efficacy of radiation/chemotherapy and improves the quality of patient survival. (Nencioni, 2018)

  1. Overall, fasting or FMD leads to a decrease in blood levels of glucose, insulin-like growth factor 1 (IGF1), insulin and leptin; an increase in levels of the lipocalin adiponectin and ketone bodies; and inhibition of the energy metabolism pathways IGF1R-AKT-mTOR-S6K and cAMP-PKA.

  2. Starvation induces differences in stress resistance between normal and cancer cells. Stimulated by starvation, normal cells down-regulate the rate of cell proliferation, forcing cells into "maintenance mode" and thus more "tolerant" of radiotherapy/chemotherapy.

  3. Cancer cells proliferate in an uncontrolled manner, with uncontrolled nutritional demands and a lack of oxidative stress, resulting in increased sensitivity to radiotherapy/chemotherapy.

  4. Fasting stimulates the expansion of lymphocyte precursor cells and promotes the body's immune response against cancer cells.

A study found that patients with heart disease can live longer with proper "starvation". In this case, "starvation" means intermittent fasting. The study, which analysed a total of 2001 heart attack patients, found that those who were properly 'starved' had a higher survival rate and were rarely diagnosed with heart failure over a 4.5-year follow-up period.

After taking into account demographic characteristics, socio-economic factors, cardiovascular risk factors, comorbidities, medications, and lifestyles such as smoking and alcohol consumption, the researchers continued to find that patients who were properly "starved" regularly over a long period of time had a higher survival rate and a lower risk of heart failure.

Intermittent fasting can be beneficial in reducing hypertension by altering the composition of the gut microbiota in animal models. Gut dysbiosis contributes to hypertension by altering bile acid signalling.

Fasting can have a potential clinical application by manipulating the microbiota to affect the host. Many bacteria in the gut microbiota are involved in producing compounds that have been shown to have beneficial effects as they enter the circulation and help regulate the host's physiology. One day, fasting programmes may help regulate the gut microbiota activity, thus providing health benefits naturally." (Shi, 2021)

How to intermittent fasting and where to start?

Consider an easy form of fasting when starting.

If you want to try fasting, start by cutting out dinner and snacks, limiting your eating time each day. Gradually, you can increase your fasting window. Speak with your doctor or a nutritionist before starting IF.

There are many ways of doing intermittent fasting: the 16/8 method, the 5:2 diet, eat stop eat and alternate-day fasting.

  • The 16/8 method: Daily fasts of 16 hours and have 8 hours eating window.

  • The 5:2 diet: 2 days in a week eating 500–600 calories and eating normally the other 5 days.

  • Eat stop eat: Two 24-hour fasts per week.

  • Alternate-day fasting: Fast every other day.

Is this the fastest way to lose weight?

Intermittent fasting is very good, But do not only rely on intermittent fasting to lose weight. A balanced lifestyle is more important.

Who Should Be Careful Or Avoid It?

An opinion piece in JAMA highlights that intermittent fasting is not suitable for patients with type 2 diabetes, leading to severe blood glucose fluctuations.

The authors noted insufficient evidence on the benefits and safety of intermittent fasting in patients with diabetes and that restricting caloric intake may be a better option for weight loss in patients with diabetes. Intermittent fasting may not be safe, and diabetics may be at increased risk of dehydration, hypotension and hypoglycaemia. (Horne, 2020)

Can I Work out While Fasted?

Yes, it is OK to work out while fasting. You might not perform as well during fasting. A study suggests that exercise training in the fasted state may enhance muscle growth and endurance. (Anton, 2018)

But both IF and exercising can lower blood pressure (Malinowski, 2019). Be careful when you are exercising if you feel dizzy or lightheaded. And if the sugar levels drop too fast, this can cause faint. It is also vital to keep hydrated during exercise.

Can I Take Supplements While Fasting?

Yes, you can use most of the supplements while you're fasting. But If anything upset your stomach, probably you want to take it in the eating state.

de Cabo R, Mattson MP. Effects of Intermittent Fasting on Health, Aging, and Disease. N Engl J Med. 2019 Dec 26;381(26):2541-2551. doi: 10.1056/NEJMra1905136. Erratum in: N Engl J Med. 2020 Jan 16;382(3):298. Erratum in: N Engl J Med. 2020 Mar 5;382(10):978. PMID: 31881139.

Gabel, K., Kroeger, C.M., Trepanowski, J.F., Hoddy, K.K., Cienfuegos, S., Kalam, F. and Varady, K.A. (2019), Differential Effects of Alternate‐Day Fasting Versus Daily Calorie Restriction on Insulin Resistance. Obesity, 27: 1443-1450.

Nencioni A, Caffa I, Cortellino S, Longo VD. Fasting and cancer: molecular mechanisms and clinical application. Nat Rev Cancer. 2018 Nov;18(11):707-719. doi: 10.1038/s41568-018-0061-0. PMID: 30327499; PMCID: PMC6938162.

Shi H, Zhang B, Abo-Hamzy T, Nelson JW, Ambati CSR, Petrosino JF, Bryan RM Jr, Durgan DJ. Restructuring the Gut Microbiota by Intermittent Fasting Lowers Blood Pressure. Circ Res. 2021 Apr 30;128(9):1240-1254. doi: 10.1161/CIRCRESAHA.120.318155. Epub 2021 Feb 18. PMID: 33596669; PMCID: PMC8085162.

Horne BD, Grajower MM, Anderson JL. Limited Evidence for the Health Effects and Safety of Intermittent Fasting Among Patients With Type 2 Diabetes. JAMA. 2020 Jul 28;324(4):341-342. doi: 10.1001/jama.2020.3908. PMID: 32614382.

Anton SD, Moehl K, Donahoo WT, et al. Flipping the Metabolic Switch: Understanding and Applying the Health Benefits of Fasting. Obesity (Silver Spring). 2018;26(2):254-268. doi:10.1002/oby.22065

Malinowski B, Zalewska K, Węsierska A, et al. Intermittent Fasting in Cardiovascular Disorders-An Overview. Nutrients. 2019;11(3):673. Published 2019 Mar 20. doi:10.3390/nu11030673

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