The concept of fasting – having periods of not eating – is an emerging topic in health circles with more and more people opting to try this technique to help shed a few undesired pounds. Like many in the health profession, this concept initially concerned me with fears of metabolic slow down and system wide shut down, but I now view the concept of taking breaks from nutrient intake to be one of the most powerful tools we have in our arsenal for healing, improving healthspan and longevity and reversing metabolic conditions like obesity and diabetes.

WHAT IS FASTING?
Strictly speaking, fasting is simply the state of not consuming food – by choice. Technically, we all fast in between meals and snacks and while we sleep.

The term ‘intermittent fasting’, which we are hearing more and more, actually has no precise definition aside from referring to having a period of not eating for part of a day. It can mean many things and is synonymous to the concept of ‘time-restricted feeding’ – where we aim to eat all our meals within a certain time window.

Generally, people would think of an intermittent fast as anything from 12-24 hours of consecutively not eating. Conversely, a longer fast would be over 24 hours in length.

There are many variations to fasting protocols. The bottom line is that fasting involves lengthening the duration between the last meal of one day and the first meal of the next.

WHY FAST?
For the vast majority, we simply eat too much and too often. The explanation for this is complex and ranges from years of incorrect dietary advice to evolutions in food supply and manufacturing practices. Regardless, we have become a culture of overabundance and over nourishment. North Americans are fatter than ever, and we have lost the balance in our relationship to food. Our bodies need a break from the feasting.

In providing such breaks from food, the body can reset hormones, improve metabolism, burn fat, improve the immune system and most likely improve many other conditions.

We need access to our fat stores in order to lose weight and the only way for this to happen efficiently is to have periods of not eating long enough for our body to gain access these stores.

HOW THE BODY STORES ENERGY
When we consume food, we either use that food energy or we store it. We exist in either the fed (high insulin state) or the fasted (low insulin) state.

When we eat, insulin is secreted and signals to the body that it needs to process incoming energy. It is a storage hormone and helps glucose get into cells and also primes the body to convert excess energy into storage. Energy can be stored as a carbohydrate (glycogen) or as fat. Think of glycogen as stored glucose in the liver – it is used for easy access to energy in between meals but it has limited capacity.

Once glycogen stores are full, the body will convert excess glucose into fat (triglycerides) in the liver – termed ‘de novo lipogenesis’ (new fat production). This fat is then exported into fat cells (adipose tissue) for longer term storage. One advantage to fat is that the body has virtually limitless potential to store it. A frightening thought!

WHAT HAPPENS WHEN WE FAST?
One of the principal reasons to fast is to allow the body to use up stored energy. To do this, we must allow insulin to drop as much as possible – and the longer the better. This is the only way we can gain significant access to these fat stores. Recall that insulin is a storage hormone and if it is circulating, we don’t have easy access to our fat supplies.

In the first few hours of a fast (ie. after a meal), there is still abundant glucose for your body to use as energy and the body is still typically adding to glycogen and fat stores.

By about 4-16 hours after your last meal, your glucose from your meal is no longer available and now you must start to utilize your stored energy. Glycogen is the easiest to access and so this is used up first. Glycogen is converted to glucose for use in the body.

By about 16-36 hours, glycogen stores run low and the body will primarily burn fat as fuel. While the machinery necessary for fat burning (oxidation) is ramping up, the body will temporarily break down protein to convert to glucose. Muscle protein is not broken down for this purpose – a common myth.

Once the body is adapted to fat as fuel and with insulin low, it has unlimited access to fat stores. Each pound of fat roughly equates to 3500 calories – for most, an ample supply of energy for quite a while. Due to this easy access to energy, metabolic rate does not decrease while fasting – another common myth.

Most organs and muscle can actually use fat directly as fuel, but the brain requires fat to be converted into ketones first in order to cross the blood brain barrier. Part of the kidney (called the renal medulla), red blood cells and part of the brain do require glucose and the body is able to convert fat to glucose for this purpose.

WHAT ARE SOME BENEFITS OF FASTING?
– Ability to use stored energy
– Weight and fat loss
– Improves insulin sensitivity
– Improves metabolism and fat burning
– Improves fatty liver
– Improves inflammation
– Likely reduces type 2 diabetes
– Likely improves cholesterol markers
– Likely improves energy
– Likely improves mental focus and concentration
– Likely reduces risk for certain dementias
– Likely reduces risk for autoimmune disease
– Likely promotes cellular repair + cleansing (autophagy)
– Possibly can lead to cancer prevention
– Provides for gut rest
– Simple, free, saves time,
– Can add to any diet or situation. You can be paleo, vegan, gluten free, nut allergic, unable to cook and still fast.

WHAT ARE THE DIFFERENT TYPES OF FASTS?
There are endless fasting protocols and they all offer benefit. The key is to allow the body more time in the non-fed state. Some common fasts:

Shorter
– 16:8 – This intermittent fast involves fasting for 16 hours and then eating during an 8-hour window. This is often done daily and a typical eating window would be
11am-7pm. One would typically eat 2-3 meals during this window. This form of intermittent fasting could be adapted longer or shorter (ie. 20:4 or 14:10) based on preference and goals.

Longer fasts >24 hours
– 24-hour – This fast involves a 24-hour gap in eating. It’s often done by eating dinner on one day then skipping the next meals until dinner the next. This can be stretched out to 36-42 hours.
– 5:2 – This is a well-studied protocol that involves eating regularly for 5 days per week and consuming 500 calories on the other 2 days per week. Technically, this isn’t a true fast as calories are consumed during the fasting segment.
– Extended fasting – involves fasting greater than 48 hours and should be done under medical supervision. Often a few vitamins and minerals are consumed. There are also extended fasting mimicking diets (FMD) with growing evidence showing efficacy.

WHAT CAN YOU EAT WHILE FASTING?
There are various protocols as mentioned above. There may be benefits to only consuming water or non-caffeinated beverages during a fast. For the most part, I recommend consuming amble fluids and allow for black coffee, unsweetened teas, and broths.

WHO SHOULD NOT FAST?
Fasting is generally a very safe strategy and has been used throughout human history for various reasons by men and women alike. These groups, however, should not fast:
Underweight (BMI <18.5) or have an eating disorder history
Pregnant or breastfeeding women
Children under 18
If you have serious medical conditions, you should be under medical supervision.

CAN I EXERCISE WHILE FASTING?
It is safe to exercise during a fast. It likely improves efficacy as you will deplete glycogen stores faster and get into fat burning sooner. However, be cautious. Stay hydrated and go slow as you start easing into fasting. If you feel tired, dizzy, lightheaded, do not exercise.

WHAT ARE POSSIBLE SIDE EFFECTS?
The most common side effects include:
– Hunger
– Constipation
– Headaches
– Dizziness
– Heartburn
– Muscle cramps

MORE ABOUT HUNGER
It’s normal to be hungry – and I think good to experience from time to time. Part of the problem in our culture is we experience it less and less as we eat almost constantly. Hunger will pass and comes in waves. If you ignore it and drink some fluids, it will often pass quicker than you think. It helps to keep busy.

WHAT ABOUT BREAKFAST?
Technically, this is the first meal after a fast. Eating breakfast in the morning isn’t as important as we have been led to believe and likely is beneficial to skip or delay in order to extend your fast.

WILL I BURN MUSCLE?
No. And it wouldn’t make evolutionary sense. Fat is there by design to be used when energy is low. You won’t burn muscle unless you run out of fat. It will use other proteins (skin, immune cells) first and will rebuild these with refeeding.

TOP TIPS
– Stay hydrated and drink water – a lot of our hydration comes from food. So important to drink additional fluids during a fast.
– Stay busy
– Drink coffee or tea
– Ride out hunger waves
– It’s ok if people you know disagree with fasting
– Eat low carb in between fasts; will reduce hunger and improve weight loss
– Don’t binge after fasting

FINAL THOUGHTS
Our physiology was designed with the ability to store energy as fat for times when we have no food to consume and conversely, it was designed to thrive during periods of fasting. Quite simply, we need more time in the unfed state to allow our body to burn through our energy stores. Metabolically, this was our design. I tell my patients that I want them to enjoy life to its fullest while feasting! But I tell them equally to balance that by having periods of fasting. I think the health benefits are profound and may well be one of our most powerful medical interventions available.

RECOMMENDED RESOURCES
The Complete Guide to Fasting by Jason Fung, MD (and his online articles).