How fasting can benefit your metabolic health

Abby Courtenay
Abby Courtenay
June 9, 2021

You’ve seen the Instagram adverts, you have heard success stories, but is fasting a fad diet or is there some sturdy science behind the claims. Is it possible that when you eat is as important (if not more important) as what you eat, or even how much you eat? In general, I tell my patients that if it sounds too good to be true, it probably is. Does that hold true for this diet? Before we go any further, let’s take some time to lay down some definitions.

Intermittent fasting vs time restricted eating

Firstly, intermittent fasting (IF) is when there are days of regular intake interspersed with days of severely reduced intake (<500kCal/ day). This usually happens every other day (e.g. alternate day fasting) or fasting on two non-consecutive days (e.g. 5:2 diet where you fast for 1 day, twice a week with normal eating days in between).

Time restricted eating (TRE), is where you reduce your eating window every day. It is usually achieved by breaking your fast (i.e. having breakfast) a bit later and having dinner a little earlier. A popular split is known as the 16:8 diet where you fast for 16 hours and then eat for 8. When eating in this manner, you could, for example, start eating at 11h00 and finish at 19h00. In research that is currently available on humans, food choices and quantities are not restricted during this eating time (but more on my thoughts about that later).

So, which fasting method is better? It really depends on the individual’s preferences and lifestyle. It appears that IF and TRE garner similar health and weight loss effects, and that essentially you need to choose the option that you are able to stick with in the long run. Some may find the very low calorie days unbearable, whilst others may thrive off them.

For the purpose of this article, we will use the term ‘fasting’ to refer to both IF and TRE.

Many of you may be thinking, well, if I have fewer hours to eat, I’m probably going to eat less. Eating less will probably result in weight loss and related benefits. It’s as simple as that! This is partially true, weight loss does usually accompany fasting. However, there also seems to be a systemic (or whole body) effect that occurs during fasting that we are only now starting to understand.

Circadian rhythm

The term circadian rhythm is gaining popularity. Essentially, it is the changes experienced by our bodies (physical, mental, or behavioural) that follow a 24-hour cycle. Two main examples of this include our sleep-wake cycle and the related fasting-feasting cycle. Our biological clocks are our natural timing devices that regulate your circadian rhythms. They are genetically coded to produce certain proteins that interact with all the cells in your body. Your circadian rhythm is important for a variety of reasons, but with respect to eating, it can influence the release of nutrient sensitive hormones like insulin, leptin, ghrelin and adiponectin. These hormones activate your nutrient sensing pathways which help your body oscillate between storage and mobilisation of nutrients, depending on its needs. Fasting works hand in hand with your circadian cycle as it promotes eating in the waking hours and fasting in the sleeping hours.

Your biological clocks anticipate reoccurring and predictable daily changes (e.g. eating a meal). The body thrives off the regularity of nutrient availability, so when deciding to try fasting, regularity is key. If you eat at an unanticipated time (i.e. not on your regular schedule), then the nutrient sensing pathways act on your circadian clocks to change them to anticipate food at this new time. Essentially, each time you eat out of your schedule, your clocks need to reset themselves, putting you back a step and making it difficult for your body to keep up with what you are feeding it. With regards to fasting, this means that you need to stick to a schedule to get your body into a routine. If you break your fast at 11h00, try to do this everyday. A fallacy to overcome in this regard, is ‘eat when you are hungry’. If you are always eating, you will always be hungry.

Let’s explore what happens when we fast vs when we eat.

Fasting:

When you are energy restricted for 10-14 hours (or more) a few key things happen in your body. Your glycogen stores (or glucose storage in your muscles and liver) are used up and your body starts to tap into your fat store by converting triglycerides to glycerol and free fatty acids.

Free fatty acids activate certain transcription factors which cause your DNA to code for a protein known as fibroblast growth factor. This protein has widespread effects and may be one of the reasons why fasting has a notable effect on glucose and lipid metabolism.

Through a process known as ketogenesis, the free fatty acids are converted in the liver to ketone bodies, which can then be used to fuel the body and the brain. However, fuel is not their only function, they also challenge your cells and organs to engage in a coordinated adaptive stress response. This response bolsters the following processes:

  • Mitochondrial function: The mitochondria is the powerhouse of the cell and its job is to release energy from energy molecules (ATP) for fuel. When we suffer from metabolic disturbances and obesity, our mitochondrial function is reduced.
  • Stress resistance: During these times of energy restriction, the cells adopt a stress resistance mode through a reduction in insulin signalling and overall protein synthesis.
  • Antioxidant defenses: There is a well-established link between oxidative stress and inflammation. A variety of these inflammatory mediators are involved in several chronic diseases. When antioxidant defence systems are enhanced, we see a possible decrease in oxidative stress and related consequences.

The bottom line is that you need to fast (or drastically reduce your calories) enough so that your body switches to ketones for energy for a predetermined amount of time.

Recovery (eating):

When you eat, the body is forced to switch from fasting to feasting and adapt to energy repletion. The body goes from using ketones for energy, back to glucose and this is known as ‘metabolic switching’.

During this time, there will be an increase in glucose and insulin (depending on what you eat) and your ketone levels will decrease. In the recovery period, there is an increase in the formation of proteins and in this phase we see cell growth (specifically mitochondrial growth) and cell plasticity (or the ability of your cells to adapt to changes in its environment). This adaptability is vital to form long term benefits for your health.

The bottom line is that you shouldn’t stay in ketosis. The key to the health benefits associated with fasting is switching between ketones and glucose as an energy source, giving you the best of both worlds.

Long term adaptations

Clearly, both of these processes have important effects on the body and together they lead to long term adaptations such as:

  • Increased insulin sensitivity and glucose regulation
  • Improved lipid metabolism
  • Reduced inflammation
  • Reduced abdominal fat and
  • Reduced blood pressure and improved heart rate

What does the research say?

A plethora of research has been conducted in rodents and other animals with regards to the benefits of fasting on various disease states. The number of human studies conducted is much lower and so there is still much work to be done before we fully understand the effects of fasting on different groups of people. It appears that there may be a marked difference in the effects depending on your sex, diet and genetic predispositions. One of the limitations of current human studies is that they often have small sample sizes and they do not continue for very long (and dropout rates may be pretty high). The reason why a diet like this may not be very popular is that it takes some time before you get used to the side effects of fasting and having symptoms like hunger, irritability and loss of concentration can be inhibitory for some. What the research tells us, is that these types of symptoms only last for 2 weeks to 1 month and that if participants are warned of these symptoms beforehand (i.e. they know what to expect), then they are less likely to drop out. It is vital to understand that your organ systems respond to fasting by overcoming the challenge and then restoring balance and that repeated exposure will lead to a lasting adaptive response.

However, to date there are a handful of human studies that show that fasting may help to decrease weight, increase insulin sensitivity and decrease dyslipidemia, blood pressure as well as inflammation. In one promising study, when alternate day fasting was adopted, cardio-protective effects were noted within as little as 2-4 weeks.

Can I eat whatever I want during the eating phase?

With all this in mind, one of the key factors to remember when deciding to fast, is that nutrient requirements are still incredibly important. If you do decide to eat within a specified time frame, eating ultra-processed junk food will not give your body what it needs and even though there may be some weight loss, ultimately your health may deteriorate in the long run.

A real-life example of fasting presents itself in Okinawa, in Japan. This is a community that has consistently been found to have low levels of obesity and chronic metabolic diseases like diabetes and many of its inhabitants reach their 100th birthdays. The Okinawans eat an energy-poor, nutrient rich diet made up of whole, unprocessed or minimally processed food like sweet potatoes, vegetables and legumes. Choosing foods in their most natural and unrefined state is paramount to consuming all the nutrients you need, not just to survive, but also to thrive. Variety is king in a healthy diet. Not only will this help to reduce taste fatigue, but eating a variety of foods means that your body is receiving a variety of nutrients, with each type of nutrient offering a specific health benefit.

Beyond the metabolic advantages of fasting, research looks very promising with regards to cancer (prevention and treatment), reduction in neurodegenerative diseases (like Alzheimer’s and Parkinson’s) and even disorders like asthma, multiple sclerosis and rheumatoid arthritis. It is quite clear that fasting offers a number of broad spectrum benefits for multiple health conditions. This being said it is certainly not suitable for everyone, those who should not be fasting include:

  • Children (<18 years) and the elderly (>70 years)
  • Pregnant and breastfeeding women
  • Diabetics (especially those controlling blood sugar levels with medication)
  • Those with disordered eating patterns

Changing your diet and lifestyle (no matter who you are), should always be undertaken with the supervision of a healthcare professional and it is vital that you not fall for supplements and medications that promise to mimic some of the effects of fasting. These may come with some hefty side effects and are usually inferior to fasting itself.

Take home messages

A fasting eating pattern may be beneficial for both weight loss and have lasting, beneficial health outcomes. Whilst this style of eating is not for everyone, if you do choose to do it this way be sure to remember the following:

  1. Choose a fasting method appropriate to your preferences and lifestyle
  2. Stick to a routine with regards to meal timing
  3. Metabolic switching is necessary for fasting benefits
  4. Allow 2 weeks - 1 month for side effects to subside
  5. When eating, choose a variety of whole and minimally processed foods
  6. Do not fast if you are in an at-risk group
  7. Chat to your doctor before making any drastic changes to your diet

References

  • Chaix A, Manoogian E, Melkani G, Panda S. Time-Restricted Eating to Prevent and Manage Chronic Metabolic Diseases. Annual Review of Nutrition. 2019;39(1):291-315.
  • de Cabo R, Mattson M. Effects of Intermittent Fasting on Health, Aging, and Disease. New England Journal of Medicine. 2019;381(26):2541-2551.
  • Circadian Rhythms [Internet]. Nigms.nih.gov. 2021 [cited 28 April 2021]. Available from: https://www.nigms.nih.gov/education/fact-sheets/Pages/circadian-rhythms.aspx
  • Gnocchi D, Bruscalupi G. Circadian Rhythms and Hormonal Homeostasis: Pathophysiological Implications. Biology. 2017;6(4):10.
  • Xie T, Leung P. Fibroblast growth factor 21: a regulator of metabolic disease and health span. American Journal of Physiology-Endocrinology and Metabolism. 2017;313(3):E292-E302.
  • Glick D, Barth S, Macleod K. Autophagy: cellular and molecular mechanisms. The Journal of Pathology. 2010;221(1):3-12.
  • Hussain T, Tan B, Yin Y, Blachier F, Tossou M, Rahu N. Oxidative Stress and Inflammation: What Polyphenols Can Do for Us?. Oxidative Medicine and Cellular Longevity. 2016;2016:1-9.


You’ve seen the Instagram adverts, you have heard success stories, but is fasting a fad diet or is there some sturdy science behind the claims. Is it possible that when you eat is as important (if not more important) as what you eat, or even how much you eat? In general, I tell my patients that if it sounds too good to be true, it probably is. Does that hold true for this diet? Before we go any further, let’s take some time to lay down some definitions.

Intermittent fasting vs time restricted eating

Firstly, intermittent fasting (IF) is when there are days of regular intake interspersed with days of severely reduced intake (<500kCal/ day). This usually happens every other day (e.g. alternate day fasting) or fasting on two non-consecutive days (e.g. 5:2 diet where you fast for 1 day, twice a week with normal eating days in between).

Time restricted eating (TRE), is where you reduce your eating window every day. It is usually achieved by breaking your fast (i.e. having breakfast) a bit later and having dinner a little earlier. A popular split is known as the 16:8 diet where you fast for 16 hours and then eat for 8. When eating in this manner, you could, for example, start eating at 11h00 and finish at 19h00. In research that is currently available on humans, food choices and quantities are not restricted during this eating time (but more on my thoughts about that later).

So, which fasting method is better? It really depends on the individual’s preferences and lifestyle. It appears that IF and TRE garner similar health and weight loss effects, and that essentially you need to choose the option that you are able to stick with in the long run. Some may find the very low calorie days unbearable, whilst others may thrive off them.

For the purpose of this article, we will use the term ‘fasting’ to refer to both IF and TRE.

Many of you may be thinking, well, if I have fewer hours to eat, I’m probably going to eat less. Eating less will probably result in weight loss and related benefits. It’s as simple as that! This is partially true, weight loss does usually accompany fasting. However, there also seems to be a systemic (or whole body) effect that occurs during fasting that we are only now starting to understand.

Circadian rhythm

The term circadian rhythm is gaining popularity. Essentially, it is the changes experienced by our bodies (physical, mental, or behavioural) that follow a 24-hour cycle. Two main examples of this include our sleep-wake cycle and the related fasting-feasting cycle. Our biological clocks are our natural timing devices that regulate your circadian rhythms. They are genetically coded to produce certain proteins that interact with all the cells in your body. Your circadian rhythm is important for a variety of reasons, but with respect to eating, it can influence the release of nutrient sensitive hormones like insulin, leptin, ghrelin and adiponectin. These hormones activate your nutrient sensing pathways which help your body oscillate between storage and mobilisation of nutrients, depending on its needs. Fasting works hand in hand with your circadian cycle as it promotes eating in the waking hours and fasting in the sleeping hours.

Your biological clocks anticipate reoccurring and predictable daily changes (e.g. eating a meal). The body thrives off the regularity of nutrient availability, so when deciding to try fasting, regularity is key. If you eat at an unanticipated time (i.e. not on your regular schedule), then the nutrient sensing pathways act on your circadian clocks to change them to anticipate food at this new time. Essentially, each time you eat out of your schedule, your clocks need to reset themselves, putting you back a step and making it difficult for your body to keep up with what you are feeding it. With regards to fasting, this means that you need to stick to a schedule to get your body into a routine. If you break your fast at 11h00, try to do this everyday. A fallacy to overcome in this regard, is ‘eat when you are hungry’. If you are always eating, you will always be hungry.

Let’s explore what happens when we fast vs when we eat.

Fasting:

When you are energy restricted for 10-14 hours (or more) a few key things happen in your body. Your glycogen stores (or glucose storage in your muscles and liver) are used up and your body starts to tap into your fat store by converting triglycerides to glycerol and free fatty acids.

Free fatty acids activate certain transcription factors which cause your DNA to code for a protein known as fibroblast growth factor. This protein has widespread effects and may be one of the reasons why fasting has a notable effect on glucose and lipid metabolism.

Through a process known as ketogenesis, the free fatty acids are converted in the liver to ketone bodies, which can then be used to fuel the body and the brain. However, fuel is not their only function, they also challenge your cells and organs to engage in a coordinated adaptive stress response. This response bolsters the following processes:

  • Mitochondrial function: The mitochondria is the powerhouse of the cell and its job is to release energy from energy molecules (ATP) for fuel. When we suffer from metabolic disturbances and obesity, our mitochondrial function is reduced.
  • Stress resistance: During these times of energy restriction, the cells adopt a stress resistance mode through a reduction in insulin signalling and overall protein synthesis.
  • Antioxidant defenses: There is a well-established link between oxidative stress and inflammation. A variety of these inflammatory mediators are involved in several chronic diseases. When antioxidant defence systems are enhanced, we see a possible decrease in oxidative stress and related consequences.

The bottom line is that you need to fast (or drastically reduce your calories) enough so that your body switches to ketones for energy for a predetermined amount of time.

Recovery (eating):

When you eat, the body is forced to switch from fasting to feasting and adapt to energy repletion. The body goes from using ketones for energy, back to glucose and this is known as ‘metabolic switching’.

During this time, there will be an increase in glucose and insulin (depending on what you eat) and your ketone levels will decrease. In the recovery period, there is an increase in the formation of proteins and in this phase we see cell growth (specifically mitochondrial growth) and cell plasticity (or the ability of your cells to adapt to changes in its environment). This adaptability is vital to form long term benefits for your health.

The bottom line is that you shouldn’t stay in ketosis. The key to the health benefits associated with fasting is switching between ketones and glucose as an energy source, giving you the best of both worlds.

Long term adaptations

Clearly, both of these processes have important effects on the body and together they lead to long term adaptations such as:

  • Increased insulin sensitivity and glucose regulation
  • Improved lipid metabolism
  • Reduced inflammation
  • Reduced abdominal fat and
  • Reduced blood pressure and improved heart rate

What does the research say?

A plethora of research has been conducted in rodents and other animals with regards to the benefits of fasting on various disease states. The number of human studies conducted is much lower and so there is still much work to be done before we fully understand the effects of fasting on different groups of people. It appears that there may be a marked difference in the effects depending on your sex, diet and genetic predispositions. One of the limitations of current human studies is that they often have small sample sizes and they do not continue for very long (and dropout rates may be pretty high). The reason why a diet like this may not be very popular is that it takes some time before you get used to the side effects of fasting and having symptoms like hunger, irritability and loss of concentration can be inhibitory for some. What the research tells us, is that these types of symptoms only last for 2 weeks to 1 month and that if participants are warned of these symptoms beforehand (i.e. they know what to expect), then they are less likely to drop out. It is vital to understand that your organ systems respond to fasting by overcoming the challenge and then restoring balance and that repeated exposure will lead to a lasting adaptive response.

However, to date there are a handful of human studies that show that fasting may help to decrease weight, increase insulin sensitivity and decrease dyslipidemia, blood pressure as well as inflammation. In one promising study, when alternate day fasting was adopted, cardio-protective effects were noted within as little as 2-4 weeks.

Can I eat whatever I want during the eating phase?

With all this in mind, one of the key factors to remember when deciding to fast, is that nutrient requirements are still incredibly important. If you do decide to eat within a specified time frame, eating ultra-processed junk food will not give your body what it needs and even though there may be some weight loss, ultimately your health may deteriorate in the long run.

A real-life example of fasting presents itself in Okinawa, in Japan. This is a community that has consistently been found to have low levels of obesity and chronic metabolic diseases like diabetes and many of its inhabitants reach their 100th birthdays. The Okinawans eat an energy-poor, nutrient rich diet made up of whole, unprocessed or minimally processed food like sweet potatoes, vegetables and legumes. Choosing foods in their most natural and unrefined state is paramount to consuming all the nutrients you need, not just to survive, but also to thrive. Variety is king in a healthy diet. Not only will this help to reduce taste fatigue, but eating a variety of foods means that your body is receiving a variety of nutrients, with each type of nutrient offering a specific health benefit.

Beyond the metabolic advantages of fasting, research looks very promising with regards to cancer (prevention and treatment), reduction in neurodegenerative diseases (like Alzheimer’s and Parkinson’s) and even disorders like asthma, multiple sclerosis and rheumatoid arthritis. It is quite clear that fasting offers a number of broad spectrum benefits for multiple health conditions. This being said it is certainly not suitable for everyone, those who should not be fasting include:

  • Children (<18 years) and the elderly (>70 years)
  • Pregnant and breastfeeding women
  • Diabetics (especially those controlling blood sugar levels with medication)
  • Those with disordered eating patterns

Changing your diet and lifestyle (no matter who you are), should always be undertaken with the supervision of a healthcare professional and it is vital that you not fall for supplements and medications that promise to mimic some of the effects of fasting. These may come with some hefty side effects and are usually inferior to fasting itself.

Take home messages

A fasting eating pattern may be beneficial for both weight loss and have lasting, beneficial health outcomes. Whilst this style of eating is not for everyone, if you do choose to do it this way be sure to remember the following:

  1. Choose a fasting method appropriate to your preferences and lifestyle
  2. Stick to a routine with regards to meal timing
  3. Metabolic switching is necessary for fasting benefits
  4. Allow 2 weeks - 1 month for side effects to subside
  5. When eating, choose a variety of whole and minimally processed foods
  6. Do not fast if you are in an at-risk group
  7. Chat to your doctor before making any drastic changes to your diet

References

  • Chaix A, Manoogian E, Melkani G, Panda S. Time-Restricted Eating to Prevent and Manage Chronic Metabolic Diseases. Annual Review of Nutrition. 2019;39(1):291-315.
  • de Cabo R, Mattson M. Effects of Intermittent Fasting on Health, Aging, and Disease. New England Journal of Medicine. 2019;381(26):2541-2551.
  • Circadian Rhythms [Internet]. Nigms.nih.gov. 2021 [cited 28 April 2021]. Available from: https://www.nigms.nih.gov/education/fact-sheets/Pages/circadian-rhythms.aspx
  • Gnocchi D, Bruscalupi G. Circadian Rhythms and Hormonal Homeostasis: Pathophysiological Implications. Biology. 2017;6(4):10.
  • Xie T, Leung P. Fibroblast growth factor 21: a regulator of metabolic disease and health span. American Journal of Physiology-Endocrinology and Metabolism. 2017;313(3):E292-E302.
  • Glick D, Barth S, Macleod K. Autophagy: cellular and molecular mechanisms. The Journal of Pathology. 2010;221(1):3-12.
  • Hussain T, Tan B, Yin Y, Blachier F, Tossou M, Rahu N. Oxidative Stress and Inflammation: What Polyphenols Can Do for Us?. Oxidative Medicine and Cellular Longevity. 2016;2016:1-9.