FAQ: What is Intermittent Fasting?

Intermittent fasting (IF) is an ancient practice that has become more popular in recent years. It is exactly how it sounds: essentially you take a break from the fed state (eating) and spend more time in the fasted state (not eating). Some of the most touted benefits of IF are better brain health/memory, increased weight loss, growth hormone and metabolism. It’s also been shown to reduce insulin resistance, improve blood glucose control and reduce blood pressure, as well as decreasing triglycerides and LDL cholesterol.

What are the metabolic effects of IF?

The theory is that intermittent fasting can stimulate fat loss and positive metabolic effects due to harnessing the effects of the hormone insulin. Insulin works as a chemical messenger in the body, alerting cells to take in glucose, fats and protein from the bloodstream when the body digests food. (Carbohydrate ingestion has the biggest effect on insulin levels, though fats and protein also increase insulin levels to a lesser degree.) Carbohydrates are broken down into glucose or fructose molecules which are then converted by the body into energy. If our cells do not require glucose, it is stored in the liver for later as glycogen (long chains of glucose molecules). Once the liver stores are full (our livers store around 24-36 hours of stored glucose or around 100 grams), glucose is converted into triglycerides and stored in our fat cells.

Think of insulin as a storage hormone, a cellular gatekeeper whose job it is to tell the body to store energy, rather than burn energy. As long as we have adequate glucose in the blood stream and stored in the liver, there is no need to access the stored fat for energy.

Over time, many individuals develop insulin resistance, which occurs when cells become resistant to the chemical messages sent by the insulin. This causes the cells to not be as receptive to taking up glucose from the bloodstream effectively and blood sugar and insulin levels remain elevated, making weight loss difficult. Thus, the body needs to continually produce more and more insulin to achieve the regulation of blood sugar from the bloodstream. The more sensitive the body is to insulin, the more our metabolism can function properly.

Insulin levels remain low in the fasted state, allowing the body to break down stored glucose and fat for energy versus relying on energy from a meal.

Putting IF into Practice

The basic goal of intermittent fasting is to allow the body to rely more on stored energy in the liver and fat cells instead of from food being eaten. This occurs more easily when insulin levels are low (remember, insulin is a fat storage hormone). Proponents of intermittent fasting suggest that the effects of fasting overnight are only compounded by increasing the duration of the fast. There are several approaches to intermittent fasting. Two of the most popular are the 5:2 fast and the 16/8 fast. The 5:2 option allows for five days of eating “normally” and two days eating a reduced number of calories. Another option that seems to be more realistic for most people is the 16/8 fast. This way of fasting allows for an 8-hour eating window of “normal” eating, and a longer fasting window of 16 hours. The easiest way of trying this is to simply extend your overnight fast. For example, you might fast from 7 PM to 11 AM.

However, to really get the benefits of fasting, it is important to still focus on eating well balanced and minimally processed meals within your eating window. We recommend consuming lots of non-starchy vegetables, healthy lean protein, and monounsaturated fats/high-quality saturated fats and avoiding heavily refined and processed starchy carbohydrates for the most part to help keep blood sugar and insulin levels balanced after your break you fast.

Common Myths about Fasting

Myth #1: Fasting will cause fatigue and low blood sugar  

The truth is that the body is still being fueled during a period of fasting, but the fuel is coming from stored glycogen (glucose) in the liver and fat. Additionally, as triglycerides (fat) are broken down, the glycerol molecule is converted to glucose, which fuels the red blood cells and the brain and keeps blood sugar levels from dropping too low. Fatigue, dizziness or lightheadedness is more likely due to needing more electrolytes/sodium or being dehydrated than low blood sugar. In fact, many people report feeling more energized during a period of fasting. However, it’s very important to work with a trusted professional before trying IF.

Myth #2: Fasting is the same as a low-calorie diet

Metabolism decreases during low calorie diets. Studies have found that basal metabolic rate decreases 25-30% during a long term 1500 calorie diet. Eating within a certain caloric range does not take into account blood sugar or insulin levels. Insulin may well remain elevated in a low-calorie diet, effectively locking our fat stores in place and causing weight loss resistance in the long term. During a fast, adrenaline, growth hormone and adiponectin increase, thereby increasing metabolism and breaking down stored glycogen and fat for energy.

Myth #3: Fasting is a stressor on the body

We know that from an evolutionary standpoint, we are designed to be able to withstand and adapt to times of feast and times of famine. Studies show that cortisol (our main stress hormone) does not increase meaningfully during IF. Cortisol is a big driver for high blood pressure and blood sugar. Since fasting does not result in low blood sugar or hypoglycemia, cortisol levels remain in normal range. One important note: while the physiological aspect of fasting does not result in elevated cortisol, if fasting seems psychologically stressful for you, then it probably is raising cortisol! Tune in and listen to your body, and always remember to work with an experienced professional to find out if fasting feels good to you.

Who Should Not Fast?

  • Those with gastro-esophageal reflux disease (GERD)
  • Those taking daily medications that need to be taken with food
  • Those with Type 1 Diabetes or Type 2 Diabetes who are taking insulin
  • Pregnant or breastfeeding women
  • Those who are underweight
  • Those who have a history of disordered eating behaviors or diagnosed with an eating disorder

Proceed with caution and work with your healthcare provider if:

  • You have known adrenal dysregulation (either too much cortisol or too little) or dysautonomia. Just as a reminder:While research does not show meaningful increases in cortisol levels in those who practice IF, if it feels stressful to you, it probably is.
  • Those with hypothyroidism that is not well controlled or whose root cause not been determined

As with all things that relate to nutrition and health, it’s important to recognize that IF is not a one size fits all approach and will not be suitable for everyone. It can be a powerful too, but its important to know how to use it properly. Always work with a trusted healthcare professional before embarking on a new way of eating.




“The Complete Guide to Fasting” by Jason Fung, MD

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