Nutrition
Intermittent fasting pros and cons: A beginner's guide
By Shetal Desai Rautela, RDN, CDCES
Published: Mar. 11, 2025
4 min read
The content in this article should not be taken as medical advice. Please consult with your healthcare provider regarding your individual health needs.
The primary focus of intermittent fasting (IF), also known as intermittent energy restriction (IER), isn’t counting calories or eliminating certain foods—it’s about when you eat.
Instead of focusing on what or how much you consume, intermittent fasting alternates between periods of eating and fasting within a 24-hour cycle. The goal? To extend fasting hours and shift the body’s energy source from glucose to stored fat—also known as “flipping the metabolic switch.” When this happens, your body starts burning fat for fuel, which can have a range of potential health benefits for some.
The science behind intermittent fasting
The timing of when we eat, sleep, and wake plays a major role in metabolism and glucose regulation. This concept, called chrononutrition, looks at how meal timing affects overall health.
Our metabolism follows the body’s natural circadian rhythm—an internal clock influenced by light and dark cycles. Several hormones regulate hunger and alertness throughout the day, meaning late-night snacking, poor sleep, and excessive screen time can negatively impact glucose levels and increase the risk of obesity and type 2 diabetes.
Here's how intermittent fasting works:
- When you start fasting, your body initially relies on glycogen (stored glucose) for energy.
- Once glycogen stores run out, the liver begins to break down stored fat into fatty acids. This process results in the creation of ketones.
- Ketones become the body's primary energy source. This shift may lead to health benefits, such as weight loss and increased energy.
For most people, the body carefully regulates glucose and ketone levels in the blood during fasting. However, for individuals whose bodies don't produce any or enough insulin, like those living with type 1 diabetes, glucose levels must be monitored and managed closely to ensure they remain in a safe range.
Different ways to start intermittent fasting
There are several popular approaches to intermittent fasting. Let’s take a look.
Alternate-day fasting
Cycling between days of fasting and regular eating.
The 5:2 method
Eating normally for five days a week and restricting calories on two non- consecutive days.
Time-restricted eating (TRE)
Limiting eating to a specific window each day:
- 16:8—Fasting for 16 hours, eating within an 8-hour window.
- 14:10—Fasting for 14 hours, eating within a 10-hour window.
Among these, daily time-restricted eating is often the easiest to maintain. Research suggests that early time-restricted feeding (eating earlier in the day) may be particularly beneficial for weight and glucose management. The 16:8 method is the most common and can be sustainable with a balanced diet.
Pros of intermittent fasting
While long-term human studies on intermittent fasting are limited, short-term research suggests potential benefits, including:
- Improved insulin sensitivity
- Increased fat oxidation (fat burning)
- Better post-meal glucose control
- Reduced hunger levels due to hormonal shifts (decreased ghrelin, increased leptin)
- Lower blood pressure
- Improved glycemic control, particularly with early eating windows (e.g., 7 a.m. to 3 p.m. as compared to 11 a.m. to 7 p.m.)
Animal studies also indicate possible additional benefits like increased lifespan, improved cognitive function, and better cardiovascular health, but more human research is needed.
Cons of intermittent fasting
Intermittent fasting isn’t for everyone. Some common challenges include:
- Fatigue, headaches, irritability, or mood disturbances during fasting periods.
- Risk of vitamin deficiencies or dehydration if nutrient intake is inadequate.
- Increased likelihood of binge eating or poor food choices when breaking a fast.
- Possible need for medication adjustments, especially for individuals living with diabetes.
Is intermittent fasting right for you?
Intermittent fasting has gained popularity for its potential benefits, but it’s not a one-size-fits-all solution. If you’re considering giving it a try, a gradual transition is best. Success depends not just on how long you fast but also on what, how much, why, and when you eat during your eating windows.
The key is to listen to your body, take a mindful approach, and consult with your healthcare provider and/or a registered dietitian (RD) to determine whether intermittent fasting aligns with your health goals and lifestyle.
The production of this article was sponsored by Stelo by Dexcom.
Author profile
SD
Shetal Desai Rautela, RDN, CDCES
Shetal Desai Rautela, RDN, CDCES, is a registered dietitian, diabetes care, and education specialist with 15+ years of expertise in diabetes, metabolic health, and child nutrition.