Is Intermittent Fasting a Weight Loss Diet? (The Ultimate Guide to Intermittent Fasting)

Written by Andy Mobbs
featured image for article on intermittent fasting

What is the origin of intermittent fasting? Well, fasting is one of the oldest known treatments for disease. Historians found it first mentioned by the Greek physician Hippocrates in the 5th Century BC (Hippocrates is often considered the father of modern medicine), where he recommended forgoing food and drink to treat certain diseases.

It’s since been talked about by a huge range of well-known historical figures from Plato to Paracelsus. Fasting is also a part of virtually every religion and spiritual practice with Jesus, Buddha and Mohammed all extolling the virtues of it. It has been used to treat or improve a huge range of diseases and health issues including improving concentration, losing weight, improving insulin resistance and cognitive problems such as dementia and Alzheimer’s, and even reversing ageing itself!

Why Is Intermittent Fasting So Popular?

One specific type of fasting, intermittent fasting, is currently very fashionable. It almost seems as if everyone is talking about it, and if you’re not doing it, you could be forgiven for thinking that you’re the odd one out, and probably should be.

However, despite the hype, intermittent fasting is not right for everyone. In fact, if it’s done wrongly it can actually hurt you. Unfortunately, there is not enough good information out there that explains how to understand the difference.

So in this guide, we are going to look at how to do intermittent fasting properly to gain the potential benefits of intermittent fasting, but also to make sure you avoid the risks. This should give anyone the tools they need to attempt intermittent fasting in a way that makes it relatively easy and safe. If you’re finding it very challenging, and are feeling like giving up, you’re probably doing it wrong.

Unfortunately, simply following the advice of a ‘fitness guru’, or anyone else for that matter, might not be the right way to do it for you. As we’ll soon see, everybody’s biology is different and understanding this is the key to success for everyone. This is true whether you want to do intermittent fasting for yourself or create programs for your clients.

Are You Fasting or Starving? How to Start Intermittent Fasting?

Don’t Starve Yourself

The first rule of intermittent fasting is the most important, and that is don’t starve yourself!

At any given time, the human body is using both glucose and fat to make energy. In fact, at rest, around 50% – 70% of the energy we need comes from fat (1). However, we always need glucose available because our red blood cells that carry oxygen around the body cannot use fat to make energy, only glucose. Also, glucose is vital for the brain to run. The brain can use ketones, a fat-based molecule, for energy as well as we will discuss later, but even when there is a large supply of ketones available, the brain still needs at least 33% of its energy to come from glucose (2).

We need sugar in our blood at all times!

Because of this, we need sugar in our blood at all times, so that we have fuel for our RBCs and our brains. On average there are 4 grams (0.14 oz) of sugar circulating in the blood stream, which is about 1 teaspoon full, and 60% of this is used by the brain for energy (3), and the body will go to great lengths to keep these 4 grams stable. 4 grams is a tiny amount when you think about it compared to how much a person weighs, but it’s vital for life. If it goes too low then all of our metabolic functions start to go wrong. And if it continues to drop, the central nervous system will start to shut down, and then we’ll eventually go into a seizure and then death.

During our evolution, blood sugar drops were a bigger risk factor to life than having too high blood sugar. In other words, starvation was more likely than our chance of finding boxes full of donuts to feast on.

Because of this we have four hormones that can raise blood sugar:

  • glucagon
  • adrenaline
  • cortisol
  • and growth hormone.

However, we have only one to lower it — insulin.

infographic on intermittent fasting

The Role of Liver in Intermittent Fasting (It Keeps Blood Sugar Stable)

At rest, whenever blood sugar goes lower, the brain senses the drop and signals for the production of the hormones glucagon, adrenaline, cortisol, and growth hormone. This signals the liver and blood sugar is increased (4).

The liver does this in two ways:

  1. The first is by releasing glycogen from its glycogen stores (glycogen is simply the name for stored glucose in the body).

2. The second way is through the production of more glucose in a process called gluconeogenesis. The liver stores about 80 – 100 grams of glycogen. Relatively, this is a huge amount and equals roughly 5%-6% of the liver’s weight (compared to 1%-2% in muscle) (5).

Whilst the liver keeps blood sugar stable, the body also starts to get more energy from fat. It does this by increasing the breakdown of fat cells (adipocytes) in the body to release fatty acids into the blood and then increasing the burning of the fatty acids for energy in a process called beta-oxidation. At the same time, more of the fatty acids arrive at the liver from the bloodstream, and it uses them to produce more of a fat-based molecule called ketones.

Keto and Intermittent Fasting – the Role of Ketones

The increase in ketone production during periods of fasting evolved to help humans survive periods when food was not freely available. Ketones give us another source of energy so we could prioritize glucose stores for the brain and RBCs (5, 6).

Unfortunately, fatty acids can only be used by the brain in very small amounts (7). However, the brain loves ketones, so producing and using ketones allows us to spare glucose. It gives the brain a good energy source, so it doesn’t panic and flood the body with stress hormones.

The problem is not everyone produces ketones efficiently. Studies have shown that people who are physically fit and train regularly have higher levels of ketones. They also have a better ability to use those ketones to produce energy (8, 9). They can much more easily break down stored fat into free fatty acids, so that the liver has access to a larger amount of fat, and this is what allows it to produce more ketones (8). Trained individuals can also better use ordinary fatty acids to produce energy as well (through beta oxidation).

This means that some people will find it relatively easy to fast, and be able to easily switch to a higher level of fat burning, as well as ketone synthesis and use.

However, people who are overweight, untrained, or have other health issues may struggle with intermittent fasting. A good way to picture the problem is that fasting is basically stressing the body. For healthier people, this is a stress they can handle, and it’s this healthy stress that produces the positive potential benefits of fasting (10, 11). Stressing cells, healthy cells at least, creates a positive response making the cells ‘stronger’ and more able to resist stress in the future. It works in the same way that physical training makes someone stronger, faster, or builds greater endurance.

Intermittent fasting times

But What Happens When The Cells Are Not Healthy?

Stress that doesn’t cause a positive response causes damage.

People who try intermittent fasting need to have the metabolic flexibility to be able to switch between using more glucose and using more fat for energy.

This flexibility is vital, and I often cringe when I hear fit and healthy nutritionists or personal trainers recommending intermittent fasting to their clients without considering their metabolic flexibility.

So How Do I Know If I (or My Clients) Can Handle Intermittent Fasting?

There are two warning signs that you can’t handle intermittent fasting.

The first one is rapid weight loss.

Now that might sound a bit strange because surely one of the benefits of fasting is weight loss? It is, but only if the weight lost is fat.

If you are starving yourself, your body will use all of its glycogen supplies for energy. Whilst the liver only has 80 – 100 grams of glycogen stores, we also keep glycogen in other parts of the body. The biggest storage being in the muscles, but it’s also in other parts of the body, even the brain!

On average, people have around 600 grams of glycogen stored throughout the body, although it can go up to 1kg. For every 1 gram of glycogen we burn for energy, we also lose another 3-4 grams of water with it (12). So using up 600 grams of glycogen could mean an extra 2400 grams of water lost, meaning 3kg in total weight could be lost very quickly and none of it would be fat. We can also break down protein for energy, the biggest source being our skeletal muscle, and for every one gram of muscle protein broken down, we also lose another 4 grams in water weight (13). So people can easily see significant weight loss without any of it being fat.

The second warning sign — intermittent fasting should not be a struggle.

I Feel Like I’m Dying, but I’m Losing Weight!

The second thing, which when you have an appreciation of the physiology of fasting is very obvious, and that is you feel bad. Intermittent fasting should NOT be a struggle.

Low blood sugar raises hormones associated with stress. When glycogen levels at the liver also start to decrease, the brain will further increase hormone levels, unless it’s getting an adequate supply of ketones for fuel to take over from the lack of glucose.

However, when it doesn’t have an adequate supply of ketones, then it will increase stress hormone levels and create a whole range of negative side effects (14).

That’s when people experience intermittent fasting side effects, which will include:

  • hunger cravings
  • lightheadedness
  • dizziness
  • anger
  • weakness
  • irritability
  • anxiousness
  • depressive feelings, or anything similar.

It’s OK to have some slight feeling of hunger, but if you actually feel bad, there is a reason for it. Your brain is receiving a strong message that there is no food available in the environment. And is therefore doing everything it can to stop you from moving around and burning extra calories, basically by making you feel ‘like crap’. It’s a completely logical response from an evolutionary point of view.

So if you feel bad and have rapidly lost lots of weight — stop. When you are starving, motivation can only take you so far. Eventually, it will fail, and when that happens people very often have rebound weight gain. This is because they haven’t actually burnt any fat, just muscle, and glycogen. Your brain also now believes there is a shortage of food. So when you finally give in and start eating again, you will have a huge appetite. And your body will quickly rebuild glycogen stores to even greater levels than before and store any excess calories as fat as a safety measure in case food becomes scarce again (14, 15). You, therefore, end up with more fat and glycogen than before you started and you weigh more as a result.

tired man in the gym with low energy

So If I Feel Bad I Shouldn’t do Intermittent Fasting At All?

No, you can still fast, but you need to do a fat adaption program first. This means training your body to get better at breaking down fat and using it directly for energy and for producing ketones. It’s the same program that people who are starting a ketogenic diet do and lasts about 3 weeks. After doing this your body will be producing adequate ketones to fuel your brain throughout the fasting periods.

I have covered it in depth in this article on the ketogenic diet, so please do read it if you need to. However, a summary of the steps that need to be taken are:

  • Start off eating 75% – 80% of calories coming from fat, 5% of calories coming from carbs, 15% – 20% of calories coming from protein.
  • Measure your ketone levels, using an electronic ketone meter (available online). You are trying to get to a blood ketone level of 0.5mmol – 3mmol which is known as nutritional ketosis.
  • Take the supplements C8 oil, and/or BHB ketones if you are not in nutritional ketosis of 0.5mmol – 3mmol to give you brain fuel as you become keto-adapted.
  • If you are still not in nutritional ketosis, increase fat and reduce carbs and/or protein.
  • When you are in nutritional ketosis, continue there for 3 weeks. After 3 weeks, you are ‘keto adapted’ and can safely try intermittent fasting again.

Intermittent Fasting 16-8 Method

Once you know you are fat / keto-adapted, I recommend the 16-8 method. This means fasting for 16 hours each day.

So if dinner is at 8 pm, then the next meal would be at midday the next day. During the initial 2 weeks of starting intermittent fasting this can be adjusted to 14-8, so fasting for 14 hours a day. So again if dinner is at 8 pm, then the next meal can be brought forward to 10 am. The 16 hour fast benefits have been studied extensively and have shown benefits for weight loss, heart disease, diabetes, blood pressure, blood cholesterol, and the reduction of inflammation (15, 16).

Autophagy and Intermittent Fasting

Many of the benefits of intermittent fasting are caused by a cellular process called autophagy. This is a process where the cell uses certain structures called autophagosomes which can be thought of as cellular ‘nets’ that go around collecting things in the cell that are either old or damaged. It then fuses with a lysosome which contains lots of enzymes that can break them all down.

It’s a little bit like fixing your house up after one too many wild parties. If we are eating all the time, then it’s like a party for our cells. But fasting allows the opposite to happen, where we replace, repair, and renew any damage. Autophagy starts to happen in our bodies about 12 hours after our last meal, so the 16/8 intermittent fasting hours are a good method for increasing autophagy (17).


Intermittent Fasting and Working Out

Exercising while fasting can increase autophagy, and some people following the 16/8 method like to add this to their routine (18). Whilst this can bring added benefits, people who want to try it need to be aware that the extra energy burnt during a workout could limit the availability of ketones for the brain.

In other words, you shouldn’t notice any negative side effects when you workout fasted, beyond the normal feelings you would get from the workout itself. If you find you are experiencing negative side effects when exercising, don’t try and push through. Instead, ease back on exercise intensity to a load where there are no side effects. Then if you want to you can slowly build up the intensity.

Can Children and Teens Try Intermittent Fasting?

I don’t recommend fasting for kids or teens going through puberty, as they have elevated nutritional and caloric requirements (19). Also, a study on 9 – 11-year-olds found that skipping breakfast had negative effects on the children’s late morning problem-solving performance (20).


Intermittent fasting can produce definite health benefits and can be used in the short term, or over the longer term safely. However, it is important to make sure it is being done properly, and that anyone trying the diet has developed the metabolic flexibility to switch between glucose and fat / ketone use. When people taker the time to adapt properly fasting can produce really good health benefits and it shouldn’t be a struggle. Once fully adapted participants can also choose to fast just for a few days a week, instead of every day, depending on their lifestyles.


(1) Nicola Longo, Marta Frigeni, and Marzia Pasquali, Carnitine Transport and Fatty Acid Oxidation, Biochim Biophys Acta. 2016 Oct; 1863(10): 2422–2435.

(2) Berg JM, Tymoczko JL, Stryer L., Biochemistry. 5th edition, New York: W H Freeman; 2002.

(3) David H. Wasserman, Four grams of glucose, Am J Physiol Endocrinol Metab. 2009 Jan; 296(1): E11–E21.

(4) D G Johnston, A Pernet, A McCulloch, G Blesa-Malpica, J M Burrin, K G Alberti, Some hormonal influences on glucose and ketone body metabolism in normal human subjects

(5) Bob Murray, Christine Rosenbloom, Fundamentals of glycogen metabolism for coaches and athletes, Nutr Rev. 2018 Apr; 76(4): 243–259.

(6) Louise M. Burke, Luc J. C. van Loon, and John A. Hawley, Postexercise muscle glycogen resynthesis in humans, Journal of Applied Physiology, 01 MAY 2017

(7) Peter Schönfeld, and Georg Reiser. Why does brain metabolism not favor burning of fatty acids to provide energy? – Reflections on disadvantages of the use of free fatty acids as fuel for brain. J Cereb Blood Flow Metab. 2013 Oct; 33(10): 1493–1499.

(8) Askew, E. W.; Dohm, G. L.; Huston, R. L. (1975). Fatty Acid and Ketone Body Metabolism in the Rat: Response to Diet and Exercise1. The Journal of Nutrition, 105(11), 1422–1432.

(9) Johnson, R. H., Walton, J. L., Krebs, H. A. & Williamson, D. H. (1069) Post exercise ketosis. Lancet 2, 1383-1385.

(10) Duan W. and Mattson M. P. (1999) Dietary restriction and 2-deoxyglucose administration improve behavioral outcome and reduce degeneration of dopaminergic neurons in models of Parkinson’s disease. J. Neurosci. Res. 57, 195–206.

(11) Mark P. MattsonT, Ruiqian Wan, Beneficial effects of intermittent fasting and caloric restriction on the cardiovascular and cerebrovascular systems, Journal of Nutritional Biochemistry 16 (2005) 129 – 137

(12) S N Kreitzman, A Y Coxon, K F Szaz, Glycogen storage: illusions of easy weight loss, excessive weight regain, and distortions in estimates of body composition. The American Journal of Clinical Nutrition, Volume 56, Issue 1, July 1992, Pages 292S–293S

(13) Guyton, Arthur C. (1976). Textbook of Medical Physiology (5th ed.). Philadelphia: W.B. Saunders. pp. 284, 424. ISBN 0-7216-4393-0.

(14) N Mazurak, A Gunther, FS Grau, ER Muth, M Pustovoyt, SC Bischoff, S Zipfel and P Enck, Effects of a 48-h fast on heart rate variability and cortisol levels in
healthy female subjects, European Journal of Clinical Nutrition (2013) 67, 401–406

(15) Bartosz Malinowski, Klaudia Zalewska, Anna Węsierska, Maya M. Sokołowska, Maciej Socha, Grzegorz Liczner, Katarzyna Pawlak-Osińska, and Micha Wiciński, Intermittent Fasting in Cardiovascular Disorders—An Overview, Nutrients. 2019 Mar; 11(3): 673.

(16) Dylan A Lowe, Nancy Wu, Linnea Rohdin-Bibby, A Holliston Moore, Nisa Kelly, Yong En Liu, Errol Philip, Eric Vittinghoff, Steven B Heymsfield, Jeffrey E Olgin, John A Shepherd, Ethan J Weiss, Effects of Time-Restricted Eating on Weight Loss and Other Metabolic Parameters in Women and Men With Overweight and Obesity: The TREAT Randomized Clinical Trial, JAMA Intern Med. 2020 Nov 1;180(11):1491-1499.

(17) Flipping the Metabolic Switch: Understanding and Applying Health Benefits of Fasting, Stephen D. Anton, Keelin Moehl, William T. Donahoo, Krisztina Marosi, Stephanie Lee, Arch G. Mainous, III, Christiaan Leeuwenburgh, and Mark P. Mattson.
Obesity (Silver Spring). 2018 Feb; 26(2): 254–268.

(18) Nina Brandt, Thomas P. Gunnarsson, Jens Bangsbo, and Henriette Pilegaard, Exercise and exercise training‐induced increase in autophagy markers in human skeletal muscle. Physiol Rep. 2018 Apr; 6(7): e13651

(19) Das, J. K.; Lassi, Z. S.; Hoodbhoy, Z.; Salam, R. A. Nutrition for the Next Generation: Older Children and Adolescents. Ann. Nutr. Metab. 2018, 72(3), 56–64.

(20) E Pollitt, N L Lewis, C Garza, R J Shulman, Fasting and cognitive function, J Psychiatr Res. 1982-1983;17(2):169-74. doi: 10.1016/0022-3956(82)90018-8.

Claim 15% Off Your First Order

Enter your email address to receive your 15% discount coupon