Intermittent fasting is the healthcare and nutrition craze that everybody’s been talking about over the last few years. Many people swear by leaving bigger gaps between mealtimes as a go-to way to control their weight. Besides weight, there are other surprising health benefits as well, such as reduced insulin and blood glucose levels. To find out exactly what effects intermittent fasting can have on our body, how to do it safely, and how to measure that it’s actually working, THE INSIDER spoke to Prof. Dr. Stephan Herzig, Director and Head of the Division for Diabetic Complications at the Helmholtz Center in Munich.

1.9 billion people worldwide were classified as clinically obese by the World Health Organization in 2019, meaning that they are more than 20% over their ideal weight. These numbers lead Prof. Dr. Herzig to describe the situation as a “pandemic”. The origins of this are simple to trace: we are eating more food and moving less than ever before.

“These days, basically everyone in most countries of the Western world has access to food at all times of the day, and plenty of it. And this is not balanced out by an increased activity level for most people.” As Prof. Dr. Herzig points out: “In prehistoric times, food was not always available.”

Yet Prof. Dr. Herzig reminds us that the obesity pandemic is unlike that of coronavirus or any other pandemic that we might be used to in the modern era. Obesity, he tells us, is: “not like a viral infection or so, but a long-term chronic situation that at the end of the day can lead to really long-term problems such as type 2 diabetes, and even cancer.”

The practice of intermittent fasting was inspired by looking to prehistoric times for solutions to a modern problem. Thousands of years ago, when food was not available in the abundance that many of us have grown accustomed to today, the human body adapted to be able to continue functioning in hours, days or even weeks that it might take our ancient ancestors to find food.

When we don’t eat, our blood glucose level drops, which could be a problem as our brain relies on blood glucose in order to function. However, the human body has developed defense mechanisms to prevent this from impeding our cognitive and metabolic functions, and to allow us to continue to search for (and eventually find) food to survive. 

Fasting mode activates alternative energy sources, such as fat, stored in the body, and triggers the production of Ketone bodies to supply the brain with fuel. In overweight people, fasting benefits the reduction of insulin levels, which ultimately leads to weight loss. While fasting, your body will also have an increased sensitivity for certain hormones, which means that your body will have a better reaction and greater chance of efficiently absorbing the nutrients in the food that you eat when you eventually break the fast.

The good news is that whether you realized it or not, you are probably already enjoying some of these benefits by practicing intermittent fasting when you sleep! If you eat your final meal a few hours before going to bed, and then your morning meal within an hour or two of getting up, that leaves a 12-13 hour fasting period.

Intermittent fasting in this post, however, is about more than just skipping late night snacks. Here, by intermittent fasting we mean the voluntary application of additional fasting times in which overweight or type 2 diabetes patients abstain from food during the day to reduce insulin and blood glucose levels and also aid weight loss. Here’s how it works.

The most common model for intermittent fasting is to create the 16:8 ratio. That means that you set aside 16 hours in which you don’t eat, while maintaining an 8 hour window in which you have all of the food and drinks you would normally eat throughout the day.

Many people interpret this in a way to mean they can indulge in any high caloric snacks. Or they believe that 16:8 means that you should simply skip breakfast. However, Prof. Dr. Herzig points to studies showing that this approach of skipping breakfast may not be the best way to practice intermittent fasting, although it will still bring results:

For most people, replacing dinner with a late afternoon meal is harder than skipping breakfast because dinner is a social event that you share with friends and family. It could be awkward if someone invites you over for dinner and you just sit there with a glass of water, politely refusing to touch any of the food. Prof. Dr. Herzig says that although starting your 8 hour eating window later in the day so you in effect skip breakfast is not the optimal way to practise intermittent fasting, it is “still beneficial”.

Bear in mind, though, that intermittent fasting is not about skipping meals or reducing your calorie intake. The idea is that you still consume the same number of high-quality calories that your body requires each day, but within a smaller, defined window of time, rather than eating and snacking throughout the day.

If you want to start intermittent fasting to lose weight, Prof. Dr. Herzig has good and bad news. The good news is that intermittent fasting should help most people to lose weight. Some people, however, might not see any change.

If you don’t see yourself losing weight while practicing intermittent fasting, don’t be discouraged! Overweight and type 2 diabetes patients will still benefit in other really important ways. Prof. Dr. Herzig says that you should still see a reduction in insulin levels in your blood, and your body’s sensitivity to insulin should increase. This is a good thing, because it means that you will need less insulin to transport glucose into your cells.  The best way to see whether that’s happening, is to check your blood glucose levels, using a Continuous Glucose Monitoring device (CGM).

Checking your blood glucose levels is important to make sure that intermittent fasting is working, but it’s also important to keep an eye on whether your glucose levels are dropping too much. If that’s the case, you should make changes to your intermittent fasting regime to help them stabilize. As always, if in doubt it’s always best to consult your doctor.

As well as weight loss, intermittent fasting has been proven to be an effective tool in reducing insulin resistance in people who are overweight^[1] or have type 2 diabetes. However, if you are already in your healthy weight range and don’t have type 2 diabetes^[2], some studies have shown that intermittent fasting may actually lead to unhealthy metabolic consequences.^{[3][4][5][6][7]}

Although healthy participants did lose weight, intermittent fasting also resulted in increased insulin resistance, fasting glucose and lipids. 

There is therefore limited evidence that intermittent fasting is any more effective for healthy people within their desired weight range than maintaining a healthy, nutritionally balanced and calorie-conscious diet.

If you stop intermittent fasting, your body returns to its original state almost immediately. That means that the benefits of intermittent fasting are only applicable while you are actively practicing it. On the flip side, if you stop intermittent fasting and then start again, you will see the benefits returning very quickly once more.

This means that it’s very easy to take a break from intermittent fasting, for example a weekend or a week off, and get back into it whenever you feel ready and start feeling the benefits again. As Prof. Dr. Herzig puts it: “It’s a very responsive thing that happens, and that’s why the longer and the more continuously you do it, the better.”

Intermittent fasting is currently the most easily available and scientifically proven way for people to reprogram their cells. Reprogramming in that sense means to respond differently to the hormones and nutrients in their body. Yet Prof. Dr. Herzig can imagine a future in which scientists have worked out how to achieve this through the use of simple medication, or a pill:

“Once we understand exactly how intermittent fasting works in a particular tissue or a particular organ, then, of course, we can also try to derive certain tools and new compounds that exactly mimic the situation.

So at the end of the day, we would then have something that would mimic this natural response. And by then indeed, one day you could take a pill instead of doing intermittent fasting, but there is still a long way to go, and many, many caveats along the way. But I think we’re, we’re getting a little bit closer by understanding more how things actually work.”

If you haven’t tried intermittent fasting yet:

Start with a schedule that’s realistic for you and your routine, and then gradually increase the intensity and duration. There’s no need to dive in head-first right away.

Keep hydrating yourself with non-caloric fluids during your fasting period. These can include water, herbal teas, and calorie-free flavoured drinks.

While you may be tempted to treat yourself to your favorite snacks and comfort foods as soon as your fasting period is up, try to stick to a healthy diet with proteins, fruit and vegetables.

Keep an eye on how your body is responding to any dietary changes you’re making by monitoring your blood glucose level with a continuous glucose monitoring device (CGM). Remember, intermittent fasting may not be the solution for you if you are already in a healthy weight range or do not have type 2 diabetes. 

Find out more about intermittent fasting on the HELLO INSIDE Science Talk Podcast with Prof. Dr. Herzig and our host, Chief Medical Officer Dr. Anne Latz.

^[1] Harris L, Hamilton S, Azevedo LB, et al. Intermittent fasting interventions for treatment of overweight and obesity in adults: a systematic review and meta-analysis. JBI Database System Rev Implement Rep. 2018;16(2):507-547. doi:10.11124/JBISRIR-2016-003248

^[2] Borgundvaag E, Mak J, Kramer CK. Metabolic Impact of Intermittent Fasting in Patients With Type 2 Diabetes Mellitus: A Systematic Review and Meta-analysis of Interventional Studies. J Clin Endocrinol Metab. 2021;106(3):902-911. doi:10.1210/clinem/dgaa926

^[3] Heilbronn LK, Civitarese AE, Bogacka I, Smith SR, Hulver M, Ravussin E. Glucose tolerance and skeletal muscle gene expression in response to alternate day fasting. Obes Res. 2005;13(3):574-581. doi:10.1038/oby.2005.61

^[4] Moller L, Stodkilde-Jorgensen H, Jensen FT, Jorgensen JO. Fasting in healthy subjects is associated with intrahepatic accumulation of lipids as assessed by 1H-magnetic resonance spectroscopy. Clin Sci (Lond). 2008;114(8):547-552. doi:10.1042/CS20070217

^[5] Carlson O, Martin B, Stote KS, et al. Impact of reduced meal frequency without caloric restriction on glucose regulation in healthy, normal-weight middle-aged men and women. Metabolism. 2007;56(12):1729-1734. doi:10.1016/j.metabol.2007.07.018

^[6] Betts JA, Richardson JD, Chowdhury EA, Holman GD, Tsintzas K, Thompson D. The causal role of breakfast in energy balance and health: a randomized controlled trial in lean adults. Am J Clin Nutr. 2014;100(2):539-547. doi:10.3945/ajcn.114.083402

^[7] Ravussin E, Beyl RA, Poggiogalle E, Hsia DS, Peterson CM. Early Time-Restricted Feeding Reduces Appetite and Increases Fat Oxidation But Does Not Affect Energy Expenditure in Humans. Obesity (Silver Spring). 2019 Aug;27(8):1244-1254. doi: 10.1002/oby.22518. PMID: 31339000; PMCID: PMC6658129.