Intermittent Fasting: Everything You Need to Know for Success (eBooks, FAQs, Myths, Health Benefits, How-To, Delicious Recipes and Studies Included)
Expert Insights from an Integrative Doctor & Nutritionist
FAQs
What is the best schedule of intermittent fasting?
Should I vary my fasting window or stick to a regular schedule?
How many hours of fasting does it take to activate autophagy?
Does fasting increase the risk of cardiovascular death?
Will I lose muscle mass?
Do I need to adjust fasting if I’m in the perimenopause, menopause, or postmenopause?
Should I adjust fasting to my menstrual cycle?
Do I need to adjust fasting if I travel?
Do supplements break a fast?
How should fasting be broken?
Does cortisol increase with fasting?
Hi! 👋 I’m Sara Redondo, Medical Doctor, author (Delicious Longevity: Practical, Science-Based Guide to Holistic Wellness and Lasting Health through Nutrition), and integrative and preventive health advocate.
For years, intermittent fasting has been a cornerstone of my healthy habits.
Today, I’ll share with you its proven health benefits, answer these and much more frequently asked questions, and guide you on how to do it (eBook included!).
With delicious recipes to do so—brought to you by my dear colleague
, Nutritionist and integrative health practitioner.Let’s dive in!
The Timeless Power of Fasting
Fasting has long brought physical, mental, and spiritual benefits.
Ancient Greeks fasted to purify body and soul. Hippocrates and Seneca promoted it for healing and strength. Islam (Ramadan), Judaism (Yom Kippur), Christianity (Lent), Buddhism, and Hinduism all embrace fasting as sacred.
Fasting shaped human evolution. Faced with food scarcity, our ancestors adapted to survive, making fasting part of human resilience.
The Science behind Fasting
After eating, insulin rises, helping cells use glucose for energy and store the excess as glycogen in muscles and liver or as fat.
During fasting, glucagon, cortisol, and adrenaline increase, breaking down glycogen and fat. When liver glycogen depletes—usually within 12 to 24 hours—the body shifts to fat for energy.
This shift produces ketone bodies, fueling the body in ketosis, a metabolic state triggered by fasting and the ketogenic diet.
In ketosis, fat becomes the main energy source, delivering proven health benefits.
Intermittent Fasting
Intermittent fasting alternates between fasting and eating periods.
The main types are 5:2 fasting, alternate-day fasting, and time-restricted eating (TRE), the most common and easiest to follow.
TRE, based on chrononutrition, limits eating to a set window each day. The primary benefit appears to be its effect on circadian rhythms, optimizing the timing of meals to better align with our natural biological cycles.
The 16:8 method—16 hours of fasting, 8 hours of eating—allows three meals daily, making it easy to meet nutritional needs. Its simplicity makes it ideal for beginners.
A systematic review published in the journal Nutrients revealed that, unlike traditional diets, TRE improves numerous health markers regardless of calorie intake or weight and fat loss.
For example, a recent clinical trial showed that eating between 12 p.m. and 8 p.m. was more effective for weight loss and reducing HbA1c levels (a marker of blood glucose over the past 2-3 months) in adults with type 2 diabetes without the need for calorie counting, compared to calorie restriction.
Moreover, according to a 2024 systematic review and meta-analysis, eating earlier in the day within TRE provides greater benefits for some glycemic parameters compared to delaying meal times.
In one of the analyzed studies, for participants who consistently adhered to the schedule at least 5 days a week, early TRE (7 a.m. to 3 p.m.) proved valuable for improving body weight, body fat, cardiometabolic health, and mood.
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Myths
In 2024, a study published in Nature based on multiple clinical trials addressed four common myths about intermittent fasting.
The findings revealed that, contrary to popular belief, intermittent fasting:
Does NOT worsen diet quality
Does NOT trigger eating disorders in individuals without a history of them
Does NOT negatively impact sex hormones
Does NOT cause excessive muscle loss compared to other weight loss methods
Muscle loss is a significant concern for many. However, previous studies suggest that it is possible to maintain muscle mass during intermittent fasting, even with a restricted four-hour eating window, as long as adequate calorie and protein intake is ensured and combined with resistance training.
Health Benefits
The growing body of scientific evidence has positioned fasting as an effective and natural tool for improving holistic well-being.
Weight Loss
In 2024, a systematic review and meta-analysis demonstrated that intermittent fasting induces fat mass reduction while preserving lean mass, making it a healthy and effective solution for weight loss.
While it offers benefits similar to continuous caloric restriction for weight loss and improving cardiometabolic risk factors, its popularity has grown immensely.
Below, you’ll discover the main reasons fasting has become such a popular method for weight loss.
Adherence
Intermittent fasting is easier to maintain long-term for weight loss.
Skipping constant calorie counting and strict restrictions helps individuals progress without feeling deprived or trapped in a diet, making consistency easier.
Unintentional Caloric Deficit
By reducing eating hours, you naturally lower calorie intake without counting portions or calories.
This makes achieving a caloric deficit straightforward and effortless. Fasting promotes fat burning by keeping insulin levels low, but real weight loss comes from the caloric deficit.
Whether eating three meals a day or following intermittent fasting, WEIGHT LOSS DEPENDS ON CONSUMING FEWER CALORIES THAN YOU BURN.
Intermittent fasting simplifies this by limiting eating hours, putting you in control.
To lose weight, maintaining a caloric deficit is key. Strategies like intermittent fasting or the ketogenic diet help, but results come only from reducing caloric intake.
Rebound Effect
A major drawback of continuous caloric restriction is rebound weight gain:
Sustained deficits make the body adapt by burning fewer calories, even during daily activities.
This increases the risk of regaining weight when returning to normal intake due to a slower metabolism.
Intermittent fasting, by alternating fasting with regular eating, helps prevent metabolic slowdown, maintains higher energy expenditure, and reduces long-term rebound weight gain.
Cardiovascular Disease and Metabolic Health
In 2024, widely disseminated news linked fasting to a nearly doubled risk of cardiovascular death, according to findings presented at an American Heart Association (AHA) conference.
However, this study had NOT been peer-reviewed—meaning experts in the field had not evaluated it before publication—because it was presented as a scientific poster at a conference. While relevant, its conclusions remain subject to further review.
The study relied on self-reported data from the National Health and Nutrition Examination Survey (NHANES) and mortality statistics from the CDC.
Observational studies like this cannot establish causation and are among the least robust scientific methodologies due to potential memory errors or participant dishonesty.
Another critical limitation was:
The fasting group had a higher body mass index and a greater proportion of smokers.
The study focused only on meal timing without evaluating critical factors such as diet quality, sleep, or stress levels.
Finally, these findings contradict rigorous studies showing that intermittent fasting improves cardiometabolic profiles.
In 2024, a systematic review of reviews and meta-analyses published in The Lancet demonstrated that intermittent fasting reduces waist circumference, fat mass, LDL cholesterol, triglycerides, total cholesterol, fasting insulin, and blood pressure while increasing HDL cholesterol and lean body mass.
Autophagy and Longevity
Autophagy, from the Greek “auto” (self) and “phagein” (to eat), means “self-eating.”
It is a process in which cells break down and recycle damaged components. Through autophagy, cells eliminate toxins and accumulated waste, maintaining an optimal cellular environment.
It could be compared to “selling old furniture” and using the “money” as a resource to sustain vital processes in the body.
A key benefit is longevity.
By removing defective cells, autophagy lowers the risk of chronic diseases like neurodegeneration, cardiovascular issues, and cancer, promoting a longer, healthier life.
Is Fasting the Best Strategy to Activate Autophagy?
Fasting is a popular tool for promoting longevity and activating autophagy, a crucial process for cellular health.
However, most studies on autophagy have been conducted on MICE, whose metabolism is MUCH faster than that of humans.
In mice, autophagy activates after short fasts, whereas in humans, it seems to require between 18 and 24 hours, peaking between 24 and 72 hours.
Prolonged fasting of this duration can pose risks:
Extreme fatigue
Nutritional deficiencies
Dehydration
Electrolyte imbalances
Hypoglycemia
Muscle loss
Increased cortisol (the stress hormone)
Refeeding syndrome if the fast is broken improperly
Making it not always the best option.
Conversely, moderate caloric restriction (20-30%) and regular physical exercise are well-recognized strategies for effectively stimulating autophagy.
Intermittent fasting, such as the 16:8 method, combined with these practices, can help you optimize autophagy sustainably and safely, promoting a longer life and a better quality of life.
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