Guide to Intermittent Fasting

July 05, 2019

Have you ever felt that no matter how hard you exercise or how diligent you are with your nutrition, the weight just keeps slowly creeping up? The weight increases and slows you down, causing fatigue, pain and inflammation. You are not alone in the fight against weight gain, and there is a smarter path to walk down.

Functional medicine, including techniques like intermittent fasting, helps you re-envision the weight-loss process as a set of skills you can develop to combat chronic weight-gain. These skills are focused on shifting our environment, and not fighting against our amazing bodies. Done correctly, lifestyle skills like intermittent fasting can save time and energy, promote productivity and ultimately lead to longevity.

While intermittent fasting is a very popular topic right now in the wellness community, it is important to remember that the practice of fasting dates back to our earliest human cultures and spans most religious and spiritual practices. Fasting is not new, but due to the flood of new evidence on the benefits of fasting, we have a new way of talking about it.

As a Naturopathic Doctor with more than 12 years in clinical practice helping 1000s of professionals lead with their health, I can help you understand the benefits of intermittent fasting so that you can consider using it as a tool for your success.

This guide is for information purposes only; always check with your primary care provider before you make changes to your health plan.

Why do I care so much about intermittent fasting (IMF)?

Your energy matters: it is the fuel for your success. IMF can help you make more energy by optimizing your metabolism.

Here’s how it works:
  • Mitochondria are the energy manufacturing machines that live in every single cell of your body. They are highly concentrated in your brain, liver and muscle tissue.
  • Your muscle mass is like an energy investment with a quick and high return, the more you have the more energy you can make efficiently and easily.
  • Body fat on the other hand is like an investment that is very difficult to access, it is very difficult to get your energy out and the more you accumulate the more inflamed, tired and sluggish you feel.
  • For longevity and optimal health, we want to work to increase our ratio of muscle mass to body fat, which naturally declines with age. Fasting can help us do that.

Intermittent fasting is like a genetic switch that:

  • Improves insulin sensitivity, which is a key component of an increased metabolism, putting you in the fat burning versus fat storing mode. This is how we combat those long hours at a desk that promote our growing belly fat.
  • Increases growth hormone, which is a powerful anti-aging hormone that does amazing work for our body. It repairs tissue and helps us grow stronger and more resilient. Growth hormone dramatically declines after the age of 25. The more we can promote it naturally as we age the younger our cells will be.
  • Increases cellular repair and waste elimination. IMF promotes autophagy. Autophagy is the body’s natural process of identifying old and dysfunctional cells, breaking them down and removing them from the body. It is the ultimate detox process, like the body’s natural “KonMari” method. Let go of all of the old stuff that no longer serves us.
  • Optimizes gene expression. Our DNA is not exactly our destiny. Functional medicine can support optimal gene expression, which is like putting your best genetic blueprint forward for replication. There are many negative environmental influences on our gene expression like stress and pollution. IMF can help tip the scale in favour of our best health.

The health benefits of IMF are significant, especially as we face a growing number of metabolically-linked diseases like type 2 diabetes, Alzheimer’s, dementia, cancer and heart disease. Fighting metabolically-linked disease with drugs alone is not enough to win the war. The pressures of the modern world have moved us away from our optimal rhythm, and functional medicine can help to bring us back.

What does IMF look like?

Fasting simply means not eating any energy-containing foods. You should drink water and can drink herbal tea while you fast. We already fast every night while we are sleeping. Research shows that we need to be mindful of the number of hours we spend in a fasting state. Our culture of late-night dinners, wine before bed and stress eating after long hours at the office have decreased the amount of time we spend fasting.

How long do I need to fast?

There is a large body of evidence to support the “13 hour fast”. This would mean for example eating breakfast at 7 am and having dinner by 6pm. Long term repetition of this pattern is very promising for fat lost and metabolic balance over the course of the lifespan.

  • Your “feeding window” would be 11 hours.
  • Your “fasting window” would be 13 hours.

Other studies show that fat loss may be more significant with a 16-18 hour fast. This might look like eating your first meal at 10 am and your last meal at 6pm.

  • Your “feeding window” would be 8 hours
  • Your “fasting window” would be 16 hours
Do I fast every day?


While some people may feel the benefit of doing a 13 hour fast regularly, the intention behind longer periods of fasting is not meant to be daily but intermittent.

The frequency of fasting should be determined by your specific health goals. While most of us are not fasting enough, some of those who have caught on to the popular health trend are fasting beyond what the research is showing is beneficial.

A good analogy for how much fasting is right for you is to compare against running for exercise. What is your end goal and where is your starting point?

Training for a 13 hour fast is like training for a “couch to 3-5km”: it’s doable and it’s sustainable. You can do it most days to feel good, but you don’t need to do it every day to reap the health benefits on your metabolism.

Training for a 16-18 hour fast is like training for 7-10km. At first it is going to feel uncomfortable but over time your body will adapt, grow stronger and be better able to regulate. If you completed this fast every day you would certainly be “over training” and stressing your body. This type of fasting is best done 1-2 times a week.

Training for a 24 hours fast or longer is like training for a marathon. You don’t run a marathon without training and preparation first. You need to ensure you are committed to supporting your overall health though out this practice of you risk over stressing your body. You are likely completing this type of fasting 1-4 times per year to achieve the health benefits and the research is not yet clear if more is better.

What do I need to do before I start fasting?

Let’s walk you through our functional medicine checklist:

  • You need to have your sleep cycle dialed in.

The benefits of fasting depend on your sleep: the two metabolic processes go hand in hand. Fasting can help to improve the quality of your sleep, but you are not going to experience the fat loss benefits if you are staying up late.

  • Your nutrition game needs to be strong.

Fasting is not about starving your body. It is about cleaning house and requires nutrient optimization. Your “feeding window” needs just as much attention as your “fasting window”. Your diet needs to be nutrient dense: you need to eat well, and you will need to plan ahead.

  • Your liver needs some love.

Your liver is the main controller of the fuel you use to make energy. It is an overlooked organ that plays a major role in your metabolism. Removing stressors to your liver like alcohol, sugar, hydrogenated and some saturated fats, and potentially even coffee, might be necessary to kick your metabolism into the gear it needs to be in to achieve your goals.

  • Your bowels need to move daily.

Remember that this process is also about elimination. If your bowels are not moving, you might not feel good while you are fasting, and long fasts might contribute to more constipation. Address this with your Naturopathic Doctor before you start.

  • You need to stay hydrated.

The process of burning fat for fuel requires water. The more you practice fasting the more your body’s need for water will increase. If your bowel movements slow down or you start feeling really tired your hydration levels are the first thing you should consider. Many people forget to drink plenty of water when they are focused on avoiding food.

  • You need a plan.

A good plan considers how fasting might contribute to your overall health and also considers how it might also take away. Each person has a unique set of metabolic circumstances that should be evaluated.

When intermittent fasting might not be a good idea:

  • If you are taking insulin or have been diagnosed with diabetes you should only fast under the direct supervision of your healthcare professional. Fasting will affect your blood sugar regulation and could be fatal in this circumstance if not monitored.
  • If you have a history of gallstones or constipation, fasting may aggravate your condition. You can address these factors with your Naturopath.
  • If you have battled eating disorders or are triggered by obsessive thinking around your nutrition or weight loss, then tracking your fasting window may exacerbate obsessive behaviour. There is certainly an argument that fasting can be a loving expression of eating in tune with our natural circadian rhythm. It can also be used as a tool for self -deprivation. A deep understanding of our motivations and how we approach selfcare should be explored before you start.
  • If you are pregnant or breastfeeding, your body has a different metabolic priority. It is focused on building and feeding your baby. This is a time to build, not a time to clean house.
  • If you are experiencing malnutrition or malabsorption for any reason. While fasting can play a role in the management of inflammation and autoimmunity, if you are currently experiencing malnutrition due to an IBD flare or your celiac disease is not well managed, correcting malnutrition and malabsorption must be accomplished before fasting could be beneficial. Fasting while malnourished will only add to your state of nutrient deprivation.

Intuitive fasting and mindfulness are the keys to long-term success. Working from a functional approach that considers your metabolic starting point and tracks your success using:

  • Body composition analysis (BIA)
  • Waist circumference and additional markers of insulin sensitivity (HBA1C)
  • Preliminary lab work to establish nutrient status and liver function
  • Sleep quality apps or heart rate variability monitoring as markers of stress reduction and optimal recovery

Working on a program and with a health coach will help you avoid the most common mistakes, including:

Giving up too soon:
  • Many people start and stop too soon without understanding the impact they have achieved, or the success they could have if they make fasting a long-term pattern.
Unnecessary and unproductive suffering:
  • Feeling angry and tired is not a badge of honour. It is a sign that you are not “fat adapted” and your liver needs support before you start fasting on a regular basis.
When fasting goes too far and becomes disordered eating:
  • It can be difficult to accurately assess our own behaviours. A coach can help you stay accountable to your highest self.
Not eating enough nutritious food:
  • Fasting requires nutrient optimization. Our program focuses on nutrient dense, easy to access foods that will help you nourish and recovery your best health. Our health coach will ensure the program is tailored for you based on your unique metabolic needs and activity level.

Like exercise, fasting is about improving our long-term metabolic health benefits it is not about short-term weight loss (although that is often a nice side effect).

The greatest influence on your health is you. It’s time to stop fighting your body and shift your environment in favour of your metabolism.

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The advice provided in this article is for informational purposes only.  It is meant to augment and not replace consultation with a licensed health care provider.  Consultation with a Naturopathic Doctor or other primary care provider is recommended for anyone suffering from a health problem or looking to optimize their health.

Dr. Erin Wiley is a Naturopathic Doctor with a strong focus on preventative and integrative medicine. She is the Owner and Clinic Director of the Integrative Health Institute, an integrative medical clinic located in downtown Toronto. Erin has a strong clinical emphasis on autoimmune conditions, stress related illness, anxiety, depression and hormone balance. As a naturopathic doctor, Erin is passionate about working with people to help them better understand their health and achieve their health goals.

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