Intermittent Fasting For Weight Loss And Longevity

From anti-ageing to weight loss, could intermittent fasting be the answer to a long, healthy life?

Intermittent Fasting For Weight Loss And Longevity  photo
The pros and cons of intermittent fasting

The recent surge in popularity of intermittent fasting and the claimed positive effects on our physiology, cells, aging, disease and overall health has spurred an increase in scientific research around the subject. Do the research findings truly live up to the claims, and are there caveats and cautions surrounding intermittent fasting?

What is intermittent fasting?

Throughout the history of mankind, fasting has been used in various religious and spiritual practices1. In fact, it’s probably the one practice that all religions seem to agree on. Hippocrates, Socrates and Plato all recommended fasting for health recovery. It has been used therapeutically since the 1900s to treat obesity, epilepsy and diabetes2. More recently, it’s become a popular tool for weight loss, especially alongside a ketogenic diet.

It’s not until recent years that eating three meals a day everyday became mainstream. This is supported by research which shows that modern hunter-gatherers go between intermittent periods of feeding and fasting depending on food availability3. For example, modern bushmen living in Africa go through frequent periods of food deprivation ranging from one to ten days. The research suggests that modern man has actually evolved to be able to withstand periods of fasting3.

Intermittent fasting is simply the deliberate practice of abstaining from food and certain drinks for a period of time and eating only within a compressed eating window. Rather than thinking of intermittent fasting as a "diet", think of it as a pattern of eating.

There are two fundamental types of Intermittent Fasting - Time Restricted Eating and Periodic Fasting:
  • Time restricted eating simply decreases the window of time in which you eat each day. An example of time restricted fasting would be no intake of food for 16 consecutive hours and eating within the remainder 8 hours of the day, typically eating two meals in a day. According to researchers at the Vanderbilt University, eating earlier in the day maximises fat burning by ensuring increased activity of metabolism during the night4. Dr. Fung, author of The Obesity Code, states that opening up your eating window for only one hour a day benefits one’s metabolic health and may in fact be the answer to type 2 diabetes. He calls this “OMAD”, or one meal a day5.
  • Periodic fasting typically calls for a person to choose a period of time (most commonly two days per week) and greatly reduce or completely forgo food consumption over that period. Frequency is totally determined by your schedule, lifestyle and personal preferences.
What changes happen in our bodies when we fast?

Most of us are heavily reliant on carbohydrates for our supply of fuel for energy production. This reliance on carbohydrates is positively correlated to the increase in metabolic disorders which include insulin resistance and diabetes6. Incorporating intermittent fasting strategies has been shown to help stabilise blood sugar while ultimately improving the body’s ability to burn fat as an energy source and use carbohydrates more efficiently when they are consumed7.

When we refrain from eating for 12-16 hours, the glycogen stores in the liver become depleted7. Glycogen is the stored form of glucose. The body then switches from using sugar as a source of energy to using fatty acids from stored fat instead. The fatty acids from the fat are converted into ketones for use in energy production and in the brain.

This metabolic state is known as ketosis and it brings about several changes in the body:

  • At 12 hours of fasting the body increases its production of growth hormone, which is the main fat-burning hormone and is also involved in protein synthesis. Of course, exercise will also increase levels of growth hormone8.
  • At about 16-18 hours of no food intake, autophagy kicks in8. The word autophagy translates directly to ‘self-eating’, and is a process whereby the body recycles dysfunctional components of our cells. Essentially, it’s the body’s way of cleaning out damaged cells and protein in order to regenerate new ones. Studies show that consistently fasting for 16 hours a day, while eating a healthy diet during the remaining 8 hours, helps your body to become more sensitive to insulin.
  • At about 24 hours of consistent fasting, you are starting to stimulate intestinal stem cells and the repair of the inner lining of the gut9.

What does the research say on intermittent fasting?

Intermittent fasting has gained popularity recently due mainly to the effortless weight loss that people experience, but also because there have been many reported health and longevity benefits to this system of eating.

A 2016 study focusing on 16:8 intermittent fasting for 8 weeks showed individuals significantly reduced fat mass while retaining muscle mass10.

The most popular meta-analysis study printed in the New England Journal of Medicine in December 2019, reviewed 1,500 different peer-reviewed journals. Analysis suggested that, based on human and animal studies, intermittent fasting seems to help those with11:

  • Obesity and diabetes
  • Cardiovascular disease
  • Certain cancers
  • Neurodegenerative disorders
  • Asthma
  • Multiple sclerosis
  • Arthritis
  • Surgical and ischemic tissue injury

According to the study, the key to all the benefits of intermittent fasting is the metabolic switch. The switch from burning glucose to burning fat as fuel starts a cascade of biological signalling which reduces inflammation, starts to repair damaged cells and brings multiple other benefits on a cellular level11.

Tips for starting out with intermittent fasting
  • Intermittent fasting can raise levels of cortisol, so it’s best to choose a period to start fasting when you are not stressed. Once you have started intermittent fasting, it’s a good idea to incorporate some cortisol-lowering exercises like deep breathing.
  • Choose a time: Consider which part of the day would be the easiest time for you to refrain from eating for a few hours. For some people, pushing breakfast down the morning a little is a good way to start. Similarly, having a cut-off point at which you finish eating in the evening can be useful so that your overnight fast is gradually extended.
  • Reduce your sugar: Take the time to prepare for intermittent fasting by reducing the amount of sugar in your diet over a couple of weeks. This will help your body to adapt to burning fat for fuel. Focus on planning your meals around protein-rich foods with vegetables and some healthy fat, taking care not to make drastic dietary changes without the advice of your doctor if you are taking medications. See our articles on the keto diet here and on carbohydrate restriction here.

Maintaining healthy eating
  • While fasting is about refraining from eating, what you eat when you are not fasting is important:
  • Unlike most other eating plans, with intermittent fasting there is no need to count calories. In fact, it’s vitally important to consume enough calories to avoid your metabolism slowing down.
  • Skipping meals should be no excuse for eating processed food with ‘empty calories’.
  • Focus on including vegetables daily along with a choice of protein such as local or grassfed meat, free-range chicken or wild-caught fish.
  • Every meal should also contain a portion of healthy fats such as half an avocado or guacamole, some olive oil, ghee or grassfed butter.
  • Low sugar fruits such as blueberries or raspberries are a great treat after a meal.
  • Eating a diet high in sugar and starch can make it harder to sustain periods of fasting.
Who should definitely not be fasting or should limit fasting?

Remember! A one size fits all approach doesn’t exist when it comes to nutrition.

There are some individuals who should refrain from intermittent fasting.

Pregnancy & breastfeeding

Pregnant ladies, those trying to become pregnant or breastfeeding, should not fast. When fat burning kicks in, the release and mobilisation of toxins takes place, redistributing those toxins to the foetus or breastmilk12.


Individuals under stress or with adrenal dysfunction need to be cautious. It’s not to say that intermittent fasting is a complete no for individuals with hypothalamic-pituitary-adrenal dysfunction or thyroid dysfunction, however it is important to be slow and methodical about the way that you fast in order to avoid further stress on the body12.


Individuals with gallstone disease should be aware that fasting may increase the risk of further gallbladder problems12.

Underweight/eating disorders

Underweight individuals or those with disordered eating should not fast. Restricting calories increases the risk of falling back into old habits or feeding into the disorder’s habits12.

Other considerations

If you intend on fasting for longer than 72 hours, it is advisable to limit physical activity.


Intermittent Fasting can be more complex for individuals who suffer from hypoglycaemia or have diabetes of any kind. People in this category should consult with their doctor or nutritional therapist before changing their eating schedule.


Due to the fluctuations in hormones across the month, women may be more sensitive to the effects of fasting. Fasting at certain times of the month could send some sex hormones into a depleted state. Between Day 21 and Day 28 of a lady’s cycle, it’s best to avoid fasting so as not to deplete hormones. If you’re already in menopause, timing may still matter and your nutritional therapist can order a DUTCH hormone test to help determine this. Fasting according to your cycle is recommended, with some women benefitting from only doing intermittent fasting a few days a week rather than every day.

When to know it’s time to break a fast

Fasting safely and considering fasting parameters should be the main priority when implementing a fasting lifestyle. Ignoring the warning signs of when you are forcing the fast and pushing on through them is certainly not a good idea. So, when do you know it’s time to break a fast?

Watch your blood sugar and ketones

If you are going into a longer fast of more than 48 hours, it’s important to measure your blood glucose and ketone levels. Two readings a day are advised. If your blood glucose drops below 2.3mmol/L you should stop the fast. Similarly, if your ketones rise above 7.0, it’s time to eat.13

Watch for warning signs

Becoming extremely fatigued, limbs feeling heavy, greater than usual hair loss and experiencing severe dizziness are signs that you should end the fast.

Try this simple hack

A tip for getting through the milder symptoms of intermittent fasting is to have a teaspoon of a healthy fat such as coconut oil, MCT oil, ghee or olive oil. It’s just enough to raise insulin ever so slightly and drop ketones slightly, but not enough to push you too far out of the fasted state. If, after taking the fat, your ketones remain climbing while blood glucose continues to drop, it’s time to heed the warning and break the fast.

Mind your minerals

An important concept to grasp in order to ensure fewer symptoms when fasting is to maintain your mineral intake. The three main minerals to maintain at good levels during a fast are potassium, magnesium and sodium. Add in a tasty electrolyte drink mix that replaces vital electrolytes, but ensure there are no hidden sugars. Breaking your fast with a good quality bone broth with added celtic sea salt will help replenish those lost electrolytes as well as ease your gut into digestion.

Combining Keto and intermittent fasting

There’s a common denominator with a ketogenic diet and intermittent fasting. Ketones! Reducing carbohydrates in your diet increases ketone production and fasting increases ketone production. The two work hand-in-hand harmoniously as they put your body in a hormonal state that favours fat burning.

The ketogenic diet mimics fasting, still with the desired outcome of the production of ketones, but with food coming in that is low in carbohydrate.

Combined, the ketogenic diet and intermittent fasting gives body fat a one-two-punch. By maintaining high ketones and lower insulin not only during the fasting window but also during the eating window ensures that the body becomes primed to tap into the body fat stores for fuel.


For the majority of people, intermittent fasting is safe, with an array of benefits beyond weight loss supported by various studies. As you fast, human growth hormone increases while insulin reduces, initiating some important physiological repair processes. It is important though to ensure you are maintaining adequate calorie intake alongside healthy food.

Safety should always be the priority no matter the benefits. Heed the warning signs of pushing the fast. If you’re unsure then consult with a nutrition & fasting expert to try intermittent fasting safely.

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  2. Anson, R. Jones, B. de Cabod, R. (2005), ‘The diet restriction paradigm: a brief review of the effects of every-other-day feeding’, Age (Dordr), 27(1), pp. 17-25.
  3. Mattson, M. Allison, D. Fontana, L. et al. (2014),’Meal frequency and timing in health and disease’, PNAS, 111 (47), pp.16647-16653
  4. Kelly, K. McGuinness, P. Buchowski, M. et al. (2020),’Eating breakfast and avoiding late-evening snacking sustains lipid oxidation’,PLOS Biology, 18(2): e3000622.5. Fung, J. 2016. The Obesity Code. Vancouver: Greystone Books.
  5. Feng, R. Du, S. Chen, Y. et al. (2015),’High carbohydrate intake from starchy foods is positively associated with metabolic disorders: a Cohort Study from a Chinese population’, Sci Rep, 5, pp.16919.
  6. Zubrzycki, A. Cierpka-Kmiec, K. Kmiec, Z. et al. (2018),’The role of low-calorie diets and intermittent fasting in the treatment of obesity and type-2 diabetes’, J Physiol Pharmacol, 69(5). doi: 10.26402/jpp.2018.5.02.
  7. Anton, S. Moehl, K. Donahoo, W. et al. (2018),’Flipping the Metabolic Switch: Understanding and Applying the Health Benefits of Fasting’, Obesity (Silver Spring), 26(2), pp.254-268
  8. Mihaylova, M. Cheng, C. Cao, A. et al. (2018),’Fasting Activates Fatty Acid Oxidation to Enhance Intestinal Stem Cell Function during Homeostasis and Aging’, Cell Stem Cell, 22(5), pp.769-778.
  9. Moro, T. Tinsley, G. Bianco, A. et al. (2016),’ Effects of eight weeks of time-restricted feeding (16/8) on basal metabolism, maximal strength, body composition, inflammation, and cardiovascular risk factors in resistance-trained males’, J Transl Med, 14(1), pp. 290.
  10. de Cabo, R. & Mattson, M. (2019),’Effects of Intermittent Fasting on Health, Aging, and Disease’, N Engl J Med, 381, pp. 2541-2551
  11. Phillips, M. (2019),’Fasting as a Therapy in Neurological Disease’, New Zealand Nutrients,11(10), pp. 2501.
  12. Blanco, J. Khatri, A. Kifayat, A. et al. (2019),’Starvation Ketoacidosis due to the Ketogenic Diet and Prolonged Fasting - A Possibly Dangerous Diet Trend’, Am J Case Rep, 20, pp. 1728-1731.