Intermittent Fasting or Intermittent Energy Restriction is a dietary pattern based on WHEN one eats in comparison to the more conventional way of restricting calories based on WHAT one eats. It alternates periods, or patterns, of when to eat and when to fast. One eats only during a specific time frame and fasts for the remainder of the hours in a 24 hour period.
The intent of intermittent fasting is to create longer hours of fasting in comparison to eating hours with the goal to burn more body fat by “flipping the metabolic switch.” This means triggering a shift towards using fats as fuel once initial glucose stores are used up. Fasting for long hours forces the body to use up the stored form of glucose called glycogen, rather than storing unused glucose as fat.
With empty glycogen stores, the liver breaks down fats into fatty acids for energy. A byproduct of fatty acid breakdown are ketones.The kidneys remove ketones when the levels are high. As ketones build up, the body starts to remove them through sweat, urine and breath.
The science behind intermittent fasting
When one eats, sleeps, and wakes affects blood glucose and weight.
- The method by which the body breaks down and uses energy is regulated by the circadian system. The circadian system is a rhythm the body follows based on light and dark hours. Several hormones play a role in making our bodies feel hungry or more alert at certain hours of the day, and less so at other hours of the day.
- Human trials do show that exposure to light at the wrong time (think screen time until late at night), mis-timed sleep or food intake can worsen glycemic control and increase the risk of diabetes and obesity.
Types of intermittent fasting
There are many types of intermittent fasting. Below are the most widely studied
- Alternate-day fasting
- 5:2 intermittent fasting (fasting on 2 non-consecutive days/week and eating normally for the other 5 days/week)
- Daily time-restricted feeding
- 16:8 = Fast for 16 hours and eat within an 8 hour window
- 14:10 = Fast for 14 hours and eat within a 10 hour window
It is far more difficult to study the effects of intermittent fasting in humans than animals (there are a lot more variables to control for!). As such, most human studies last no longer than 3-5 months, and much of the long term data comes from animal studies. The longest human clinical trial done was for 1 year.
Many, but not all, human studies suggest that time restricted feeding may
- Improved insulin absorption
- Increased fat burning
- Improved after meal blood glucose
- Reduced hunger levels and thus reduced food intake
- Improved blood pressure
- Glycemic control may improve with an eating window that starts early (first meal at 7am) and ends early (last meal at 3pm), as compared to one that starts late (eat at 11am) and ends late (7pm)
- Time restricted feeding such as the 16:8 is the most popular and quite possibly sustainable with a healthy meal plan followed during the eating hours
Animal studies have suggested
- Increased lifespan
- Improved physical function/endurance
- Enhanced memory
- Improved insulin sensitivity
- Reduced obesity
- Improvements in cardiovascular health
- May help reset hunger levels by decreasing the appetite stimulating hormone- Ghrelin and increasing the appetite reducing hormone- Leptin, regardless of a change in weight
Intermittent fasting is not recommended for
pregnant women and people with eating disorders
- Fatigue, headache, depression, and irritability during the fasting period
- Incorrect ways of eating during the fed hours can cause
- Vitamin deficiencies
- Binge eating or choice of high caloric foods when breaking a fast
- No significant difference has been found in weight loss or blood glucose change in studies conducted over a period of 6 months
- Benefits may not last once a normal diet is resumed
- Diabetes medications may require adjustment
- Alternate day modified fasting, which recommends fewer calories for 2 days a week and normal eating the rest of the days was not only hard to maintain as seen in a 1 year human trial but also showed no benefit over a normal eating plan with restricted calories for weight loss.
Intermittent fasting has become popular at a fast pace! If this is a meal plan you choose to follow, we suggest you take it slow. It is not only about how long one fasts but also how one nourishes during the eating period (what, how much, why and when one eats). Most importantly listen to your body and know that any meal plan that feels forced, restrictive and difficult to sustain in the long run might not be worth the effort. It’s better to be the tortoise than the hare and take a slow, gradual approach while attempting this form of eating practise with guidance from your care team.