Intermittent Fasting: A Beginner’s Guide
By Dani Matthies, RD, CD, LD
Instead of focusing on what you eat, it’s all about when you eat. That’s intermittent fasting (IF) in a nutshell. Not a new concept (people have practiced fasting for centuries for religious and cultural reasons), but recently, IF has made waves as a tool more specifically for weight management and overall health.
The concept is pretty simple: restrict eating to a certain window of time (your “eating window”) o encourage a metabolic switch toward burning fat. This often also results in a calorie deficit, which can support weight loss.
Where it can get complicated is the variety of ways to intermittently fast:
- 16/8 Method: Fast for 16 hours, eat within an 8-hour window. For instance, eating between 11 a.m. to 7 p.m. Variations may also include 14/10 or 18/6 windows.
- 5:2 Diet: Eat normally five days a week, then cut down to about 500–600 calories on two non-consecutive days.
- Eat-Stop-Eat: Choose one or two days a week to fast for a full 24 hours.
- Alternate-Day Fasting: Eat one day, then fast (or eat very little) the next.
Note: there is not no one-size-fits-all approach!
Why People Try Intermittent Fasting
The research on IF is still evolving, but studies show some interesting potential benefits, including:
Weight Loss: A review of 27 trials found that participants lost anywhere from 0.8% to 13% of their body weight with fasting. That said, it’s not always more effective than regular calorie restriction.
Better Blood Sugar Control: IF may improve insulin sensitivity and help regulate blood sugar, especially in people with prediabetes or type 2 diabetes.
Heart Health: Some evidence points to lower blood pressure, improved cholesterol, and reduced body fat. Still, one large study hinted that very tight eating windows (under 8 hours per day) could actually raise heart risks in some populations.
Early research is also looking at fasting’s impact on inflammation, brain health, gut function, and even longevity, but findings aren’t yet consistent.
Thinking of Trying Intermittent Fasting?
It’s important to work with a healthcare provider to determine if IF is right for you. Extended fasting can cause headaches, fatigue, dizziness, irritability, dehydration (especially if fluids aren’t consumed, though no-calorie beverages are usually allowed), nausea, and constipation.
Intermittent fasting is not recommended for people with a history of eating disorders, type 1 diabetes (or type 2 without medical guidance), weakened immune systems, or those who are pregnant or breastfeeding.
If you do try IF, choose an approach that feels sustainable and fits your lifestyle. A more flexible option like the 12/12 method (12 hours fasting, 12 hours eating) can be a gentle way to start. Pay attention to how you feel, not just the number on the scale, but energy levels, lab results, and body composition are all important markers of health. During eating windows, prioritize lean proteins, whole grains, healthy fats, fruits, and vegetables. And always check in with your healthcare provider if you have questions or concerns!
Sources:
- Nutrients. Intermittent fasting and metabolic health.
- Canadian Family Physician. Intermittent fasting and weight loss.