Intermittent Fasting

Intermittent fasting has become quite popular over the last several years. After understanding the science, it all made sense. Frequent meals (4+ meals) and a prolonged eating window (13+ hours) do not promote longevity. 

Fasting is a great way to cleanse and rejuvenate your body. It's been used for centuries as an effective form of dieting, but many people find it hard going on their first time or even after four days without food!

With easy access to food, most people are over-nourished nowadays. The frequent meals and prolonged eating window keep the body in a growth state - do you feel like your size is just about right? 

The human metabolism slows down when we stop growing at adulthood due to lifestyle choices like consistently consuming more calories than necessary or not exercising enough!

Have you become carbohydrate dependent?

The human body prefers to burn carbohydrates because it’s quick and easy energy. However, when one becomes “dependent” on carbohydrates, the body has a difficult time transitioning to the fat-burning state. 

The reason why humans have body fat is for times of famine, which is something we don’t experience anymore. Fat is simply stored energy.

Being able to switch from burning carbs to fat is called metabolic flexibility. Metabolic flexibility is especially important for endurance athletes. What fuel source do you think you are using during a marathon? Fat. You run out of carbohydrate stores (glycogen) after about 2 hours of exercise.  

If you cannot go 2 hours without a snack, or have intense sugar cravings, this may be a sign of carbohydrate dependency.

Why Fast?

There are plenty of health reasons to practice intermittent fasting. 

Here are the top 6 reasons to fast:

  • Metabolic flexibility 
  • Increase insulin sensitivity 
  • Decrease body fat or maintain weight 
  • Longevity (live longer by decreasing risk of disease) 
  • Give your digestive system a break 
  • Mental clarity 

The eating pattern has also been linked with other improvements in health, such as:

  • lowering blood pressure
  • improving blood sugar
  • repairing damaged cells
  • protecting brain health

What happens in our bodies when we fast?

About 12 hours after your last meal, your body begins entering the fasted state. At this point, your glycogen stores are depleted. (Remember, glycogen is stored carbohydrate.) Because your body needs energy at all times to function, it begins to break down body fat for energy. Several other physiological responses occur:

  • Lipolysis – Fat breakdown for energy
  • Autophagy – The recycling of cells (the breakdown of old cells so your body can regenerate newer cells). Autophagy promotes longevity. 
  • Low fasting insulin – Increases insulin sensitivity and promotes the breakdown of fat.  
  • Ketones – An energy source when glucose is low – Ketones may play a part in the increase of mental clarity experienced during the fasted state.
  • Stem cell production – This occurs during longer fasts (5+ days). Stem cells are unique cells that can develop into many different cell types. In some cases, they can also fix damaged tissues.

Electrolyte excretion also occurs while fasting. It is important to replenish your electrolytes with meals/drinks. Specifically, sodium, potassium, and magnesium.

Popular ways to intermittent fast:

The following are listed in order of popularity. Most studies on intermittent fasting utilize the Alternate Day Fasting method.

  • Time-restricted eating – A prolonged overnight fast lasting anywhere from 13-18 hours. 
  • Leangains method 16/8 or 18/6 – Fasting for 16 hours with an 8-hour eating window. Example: 11 am to 7 pm eating window.
  • Early time-restricted eating consists of an earlier eating window. Example: 8 am to 4 pm eating window.
  • Circadian Cycle Fast – The eating window lasts from sunrise to sunset. After sunset, the overnight fast begins. It mirrors the day/night cycle, AKA circadian rhythm. 
  • One meal a day (OMAD) – Fasting for the majority of the day, to eat one meal. OMAD tends to be a 22-hour fast with a 2-hour eating window. 
  • Full-day fast (usually done once a week) – Fasting for a full day can equate to 36+ hours. For example, you consume your last meal on Sunday evening (7 pm) and break the fast on Tuesday morning (7 am). 
  • Alternate Day Fasting (full-day fast every other day) – The 5:2 falls under this type. Fasting occurs two days of the week. Some practice a low-calorie diet (less than 500 calories) instead of completely fasting.
  • Fasting Mimicking Diet (A 5-day low calorie, low protein diet which tricks your body into thinking it’s fasting despite eating) – You can purchase this program at
  • Prolonged Fast (4+ days) – An advanced form of fasting in which no food is consumed for more than 4-5 days. 

There is no “best” type of intermittent fasting – it’s ultimately up to you and your lifestyle. Things you want to consider are your daily schedule, social life, and health goals. You can follow one regimen, or you can incorporate a mixture of all of them. 

How to Start Intermittent Fasting for the Newbie: 

  1. Trial a 12 hour overnight fast. Finish dinner by 7 pm and eat breakfast no earlier than 7 am. Follow this 12/12 plan for a week or two.  
  2. Shorten your eating window by an hour. Bring breakfast up to 8 am or finish dinner by 6 pm. This step is when you start to personalize your fasting regimen to fit your lifestyle. You may stop here if you are comfortable. 
  3. If you find a 12-14 hour overnight fast easy, you can attempt a longer fast. Experiment with different eating windows (9 am-5 pm, 10 am-6 pm, 11 am-7 pm) – Customize to your lifestyle and schedule. 

Intermittent fasting is a great way to lose weight and feel fuller faster. The only problem? You have to work really hard in the beginning, so you don't give up on it before reaping the benefits! Allow yourself 2-4 weeks after starting your new routine for everything to get easier, including those hunger pangs that might come along with it.

Is Longer Better?

If you are trying to lose weight, a longer fast will keep you in a fat-burning state. A 16-hour and 20-hour fast differ by spending an extra 4 hours in the fasted state, primarily using fat for fuel. 

If your goals include longevity, longer is also better. Stem cell production occurs on the fifth day of a water-only fast or FMD. This regimen is not recommended for a newbie faster, but it can be something to consider down the road. 

What’s allowed during a fast?

  • Water
  • Sparkling water (without sweetener or sugar alternatives). I recommend La Croix. 
  • Black coffee
  • Plain tea 
  • Salt or electrolytes (without sweetener or sugar alternatives)

Water is extremely important when you are fasting. If you find that you are experiencing headaches while fasting, try drinking more water with added electrolytes.

Can you eat whatever you want during the eating window?

Intermittent fasting’s focus is not on WHAT to eat, but rather on WHEN to eat. The primary reason for fasting is entering into the fasting state. With that being said, most people do eat whatever they want during their eating window. However, it’s essential to clarify your goals for fasting. I would argue that refeeding is just as important as fasting.

It’s crucial to replenish the nutrients lost during the fast. Also, choosing foods that nourish your body will be beneficial for long-term health. If fasting is an excuse for you to binge on nutrient-poor foods (low fiber, processed), you may be causing more harm than good. 

Fasted Exercise

The safety and efficacy of exercising in the fasted state are ultimately dependent on the type of workout you are doing. Reflect on your current lifestyle, including your exercise regimen and goals, before choosing a fasting method. 

  • Lose weight: Beneficial to exercise in the fasted-state (low-moderate intensity such as walking or jogging)
  • Endurance: Beneficial to exercise in the fasted state 
  • Build mass: Safe to weightlift in the fasted state; however, you may feel weaker. Human growth hormone is significantly increased during the fasted state. Refeeding is critical to repairing muscles after weightlifting in the fasted state. 
  • Strength/Speed: Not beneficial to exercise during the fasted state as you will need your glycogen stores filled for explosive workouts. 

People who should not fast (more than 12 hours):

  • children
  • pregnant/lactating women
  • type 2 diabetics on insulin (MUST be medically supervised and working with endocrinologist)
  • those with an eating disorder
  • low BMI (<18)
  • gallbladder disease

The bottom line

Intermittent fasting is a weight loss tool that works for some people, but not everyone.

It’s not recommended for individuals who once had or currently have an eating disorder. It may also be unsuitable for children, people with underlying health conditions, and people who are pregnant or breastfeeding.

If you decide to try intermittent fasting, remember that just like with any eating pattern, diet quality is key.

To gain the most from intermittent fasting, be sure to eat a variety of nutrient-dense whole foods during your eating window and limit ultra-processed foods.

Furthermore, before embarking on an intermittent fast, be sure to consult a trained healthcare professional to ensure that it’s safe for you to do so.