The Science of Intermittent Fasting – Does It Work?
Dr. Eric Berg
10/23/2023Intermittent fasting has become a popular tool for weight loss and improving metabolic health. But does intermittent fasting work?
Yes, intermittent fasting works—and research confirms that time-restricted eating promotes a healthy body weight, supports hormonal balance, increases energy, and is linked to a lower risk of diabetes and heart disease.
Discover the science behind intermittent fasting and learn why a time-restricted meal plan may be the key to longevity.
What is intermittent fasting?
Intermittent fasting is a time-restricted meal plan that’s characterized by cycling from fasting periods to periods of eating.
Caloric restriction during the fasting period pushes your metabolism to utilize stored body fat instead of sugar and carbs for energy. As your liver breaks down fat, it produces ketones, an organic acid and a highly efficient energy source.
When ketones enter your circulation and start to fuel your cells, your body enters a metabolic state known as ketosis.
“Ketosis triggers profound metabolic changes and impacts how your body utilizes energy,” says Dr. Berg. “This explains why intermittent fasting is one of the most effective weight loss methods and is associated with improved energy levels, mental clarity, and diminished hunger and cravings.”
Types of intermittent fasting
There are several popular types of intermittent fasting, including:
Time-restricted eating (TRE) refers to eating only during a specific daily eating window. The 16:8 method is a widely practiced fasting method characterized by 16 hours of fasting followed by an eight-hour eating period.
Alternate-day fasting (ADF) refers to alternating between days of normal eating and days of consuming fewer calories, typically around 500 to 600 calories daily.
One meal a day (OMAD) is a type of intermittent fasting characterized by eating only one large meal during a 24-hour period and fasting for the remaining 23 hours.
The 5:2 method involves consuming a regular diet five days a week and restricting calorie intake to around 500 to 600 calories on the other two days.
Extended intermittent fasting refers to fasting for one to five days, followed by resuming your usual mealtime schedule. The 24-hour and 48-hour fasting are popular examples of prolonged intermittent fasting.
Watch the video below to learn about the most important intermittent fasting basics for beginners.
Does intermittent fasting work?
Yes, research supports that intermittent fasting works.
Intermittent fasting keeps blood sugar and insulin levels low, which profoundly benefits metabolic health and significantly lowers the risk of metabolic imbalances such as obesity, insulin resistance, diabetes, and metabolic syndrome.
In addition, a study published in Nutrition Research found that an intermittent fasting eating pattern stimulates metabolic and hormonal effects associated with healthy aging and longevity.
It’s believed that caloric restriction triggers autophagy, a set of biochemical processes that break down and recycle damaged or dysfunctional cellular components.
Autophagy is critical for maintaining cellular health and may play a role in extending lifespan by reducing the risk of various age-related diseases.
Intermittent fasting may also benefit hormonal balance, including increased growth hormone production linked to tissue repair, muscle growth, body composition, and overall health.
Critics of intermittent fasting point out that eating fewer meals can lead to nutrient deficiencies. That’s why it’s crucial to combine intermittent fasting with Healthy Keto®, a nutritious high-fat diet that enhances the metabolic benefits of ketosis with balanced nutrition.
By focusing on non-GMO organic vegetables, full-fat organic dairy, pasture-raised eggs, game meats, wild-caught fish, and grass-fed beef, Healthy Keto provides the nutrients you need to maintain a healthy body.
Eight science-backed benefits of intermittent fasting
Intermittent fasting patterns can be traced back to our early human ancestors, who didn’t always have consistent access to food.
“The human body evolved and thrived in an environment characterized by feast and famine,” explains Dr. Berg. “Intermittent fasting mimics this metabolic state and counteracts the adverse health effects of caloric abundance widespread in modern societies.”
Various fasting routines have been practiced for cultural, religious, or health reasons for centuries, and researchers noticed that individuals who regularly fasted generally had better health and a lower risk of elevated blood pressure, obesity, and heart disease.
During the last four decades, countless studies have been conducted to investigate the effectiveness and long-term safety of intermittent fasting.
Here are eight science-backed benefits of intermittent fasting.
1. Weight loss
Intermittent fasting pushes your liver to burn fat, which has profound metabolic benefits and explains why intermittent fasting for weight loss has become increasingly popular.
The impressive weight-loss effects of time-restricted eating have been confirmed by a randomized controlled trial published in Nutrition Journal that studied the weight-loss effects of one day of extended intermittent fasting (36 hours) per week compared to two days of extended intermittent fasting (60 hours) per week in obese adults.
After four weeks, all participants experienced significant weight loss and improved body composition and reported no significant increase in hunger or cravings despite a caloric reduction of around 40%.
Participants who fasted for 60 hours a week lost, on average, 29% of body weight and reduced waist circumference by as much as 38%.
Another study published in Obesity found that obese adults who practiced ADF for 12 weeks reduced body weight and fat mass by around 6 pounds (3 kg) and that combining ADF with exercise three times a week resulted in a significant weight loss of about 12 pounds (6 kg).
This highlights that increasing energy expenditure during fasting forces your liver to tap into stored body fat to generate energy, making intermittent fasting a powerful tool that helps lose weight and reduce fat mass.
Some suggest that the weight-loss effects of intermittent fasting are due to consuming fewer calories, not a result of the metabolic changes caused by ketosis. However, this claim has long been refuted.
A large meta-analysis published in Nutrients reviewed 11 randomized controlled trials comparing the weight-loss effects of intermittent fasting and continuous caloric restriction (CCR), also known as a low-calorie diet.
The analysis concluded, “In summary, we found that intermittent fasting was more effective than CCR for weight loss and is metabolically superior in obese people.”
2. Metabolic health
Metabolic imbalances such as elevated blood glucose and insulin resistance are the leading causes of obesity, diabetes, and metabolic syndrome.
Intermittent fasting is one of the most effective tools to maintain or restore metabolic health and significantly lowers the risk of metabolic diseases.
A study published in The International Journal of Obesity compared the metabolic benefits of intermittent fasting with continued energy restriction (CER) in overweight women.
Participants were randomly assigned to CER of 25% calorie restriction for seven days a week or an intermittent fasting protocol consisting of a very low-calorie diet for two days a week with no restrictions for the other five days.
While both CER and intermittent fasting were beneficial for weight loss, participants in the intermittent fasting group showed better improvements in blood glucose levels and other markers of metabolic health.
The table below illustrates the changes in fasting insulin and insulin sensitivity (HOMA) over six months and shows that intermittent fasting had a more significant impact on insulin balance than CER.
Table 1. Changes in insulin and insulin sensitivity (HOMA) over six months
This highlights that the health benefits of intermittent fasting are not merely due to weight loss but that the changes in energy-making pathways triggered by a time-restricted eating pattern can significantly improve insulin sensitivity, a hallmark of metabolic health.
A meta-analysis published in The Journal of Clinical Endocrinology & Metabolism reviewed findings from seven clinical trials and summarized that intermittent fasting is an effective non-pharmacological approach to support weight loss and improve metabolic health markers.
3. Prevention and management of diabetes
Obesity, insulin resistance, elevated blood glucose, and belly fat are risk factors for type 2 diabetes, a chronic health condition that affects almost 12% of all U.S. adults.
Diabetes has serious health consequences and significantly increases the risks of heart and kidney disease, impaired vision, nerve damage, and premature death.
Because of intermittent fasting’s benefits for metabolic health and body composition, time-restricted eating has been extensively studied as a therapeutic tool for metabolic diseases and could replace insulin for diabetes.
A randomized controlled trial published in Diabetes Care investigated the efficacy and safety of intermittent fasting in individuals with type 2 diabetes compared to a standard healthy diet.
Participants in the intermittent fasting group maintained 18-hour fasting periods three days a week with no restrictions on the remaining days.
The intermittent fasting group significantly reduced HbA1c, a measurement of long-term blood sugar control and therapeutic marker for diabetes treatment after 12 weeks. The standard diet group shows no changes in HbA1c.
The researchers summarized that “Intermittent fasting over 12 weeks is safe, reduces HbA1c, body weight, and lowers daily insulin dose in people with insulin-treated type 2 diabetes.”
A significantly larger study published in The American Journal of Clinical Nutrition confirms that 12 months of intermittent fasting for at least two days of the week markedly improved fasting insulin and insulin resistance in overweight women with a history of gestational diabetes.
Gestational diabetes and being overweight are significant risk factors for type 2 diabetes, and these findings suggest that intermittent fasting can be a valuable tool in improving markers of blood sugar control, which significantly lowers the risk of insulin resistance and the development of diabetes.
A recent review published in Diabetes Research and Clinical Practice analyzed several studies that investigated intermittent fasting regarding diabetes management and prevention and concluded that time-restricted feeding should be considered as a therapeutic intervention for individuals with diabetes and those at risk.
4. Cardiovascular Health
Metabolic and cardiovascular health are closely linked, and poor metabolic function greatly increases the risk of cardiovascular disease, including hypertension, atherosclerosis, heart attacks, and stroke.
Excessive weight, insulin resistance, and poor blood sugar control are linked to elevated blood lipids, a primary risk factor for heart disease, and intermittent fasting’s metabolic benefits may help improve and maintain cardiovascular health.
A study published in The Journal of Nursing compared the effects of a 16:8 intermittent fast, ADF, and a standard diet on cardiovascular risk factors in overweight or obese adults.
After three weeks, participants in the 16:8 and ADF groups had significant reductions in body weight and waist circumference and a markedly improved lipid profile compared to the standard diet.
Both fasting protocols improved heart-healthy high-density lipoprotein (HDL) cholesterol levels and lowered low-density lipoprotein (LDL) cholesterol linked to cardiovascular disease and atherosclerosis.
These findings align with a meta-analysis published in Nutrients that analyzed existing evidence on the impact of intermittent fasting on lipid levels and cardiovascular risk.
The analysis found that intermittent fasting is consistently associated with a significant improvement in total cholesterol and lower levels of LDL cholesterol and triglycerides and that time-restricted eating schedules can enhance lipid profiles linked to a lower risk of cardiovascular disease.
5. Liver health
The liver is your primary metabolic organ, and the metabolic benefits of intermittent fasting have been found to promote liver health and may play a role in preventing and even reversing fatty liver disease.
Caloric restriction enhances the production and activity of hepatic enzymes needed to break down excessive fat stores in the liver. This explains why intermittent fasting is an excellent mealtime schedule for individuals with poor liver function or fatty liver disease.
A clinical trial published in Nutrition compared the effects of a 16:8 intermittent fasting protocol combined with a low-sugar diet to a standard diet in individuals with non-alcoholic fatty liver disease (NAFLD).
A significant difference in liver enzyme concentrations, including alanine aminotransferase and aspartate aminotransferase, was noted between the intermittent fasting group and the group following a standardized healthy eating plan, indicating improved liver function.
In addition, intermittent fasting also appeared to reduce fibrosis scores, a set of metabolic markers that indicate liver health. The authors concluded, “Intermittent fasting plus a low-sugar diet can improve liver, lipid, and inflammatory markers in patients with NAFLD.”
Interestingly, this study focused on intermittent fasting with a low-sugar diet similar to a ketogenic diet. This highlights that practicing time-restricted eating in combination with a high-fat diet can enhance the metabolic benefits of ketosis and promote healthy liver function.
Another study published in Cell Metabolism investigated the impact of three months of AFD with or without aerobic exercise on markers of NAFLD.
After three months, AFD with exercise five days a week reduced liver fat by 5.5% compared to only 1.3% in the exercise-without-fasting group.
AFD, combined with aerobic exercise, also improved liver enzyme concentrations, demonstrating that caloric restriction can be an effective approach for individuals with fatty liver disease.
Watch the video below to discover how intermittent fasting helps reverse fatty liver disease.
6. Lowers inflammation
While inflammation plays a critical role in immune system function and wound healing, chronic inflammation is a significant risk factor for heart disease, cancer, autoimmune diseases, liver damage, musculoskeletal conditions, and mental health issues.
During periods of caloric restriction, the body produces fewer metabolic by-products, such as reactive oxygen species and free radicals linked to oxidative stress and inflammation.
In addition, intermittent fasting stimulates autophagy, which promotes cellular health and removes dysfunctional cell components and debris that can contribute to inflammation.
An elaborate laboratory study published in Cell Metabolism investigated the effects of time-restricted feeding in mice and found that animals that were fed on an intermittent fasting schedule had lower concentrations of proinflammatory compounds, including tumor necrosis factor-alpha (TNF-α) and interleukin-1 beta (IL-1β) compared to mice that where free to eat normally.
While animal studies can’t fully inform the effects of intermittent fasting on inflammatory markers in humans, several human studies confirm the anti-inflammatory effect of a time-restricted meal pattern.
Research published in The Journal of Translational Medicine found that eight weeks of time-restricted feeding (16:8) resulted in lower TNF-α and IL-1β compared to a standard diet that showed no changes in these inflammatory markers.
Interestingly, the study mentioned above was conducted in non-obese and resistance-trained males, indicating that intermittent fasting isn’t just advantageous for overweight individuals but also benefits those with a healthy weight.
This aligns with the findings of a randomized controlled trial published in Medicine and Science in Sport. This 12-month study compared the anti-inflammatory effects of intermittent fasting with an 8-hour eating window to a regular eating schedule in healthy, normal-weight adults.
Participants in the intermittent fasting group showed a 23% reduction in IL-1β and lowered TNF-α concentrations by 14%. In comparison, no changes in inflammatory markers were observed in the group that followed a regular mealtime schedule.
7. Diverse intestinal microflora
Intermittent fasting has been shown to promote a diverse intestinal microflora by favoring the growth of beneficial bacteria linked to improved immune defenses, digestive health, and balanced regulation of inflammatory pathways.
A study published in The Journal of Clinical Endocrinology & Metabolism found that eight weeks of intermittent fasting two days a week induced significant changes in the participants' microflora.
The researchers compared the bacterial diversity of the intermittent fasting group with a control group and found that intermittent fasting increased concentrations of Spirochaetes and Firmicutes. These beneficial gut bacteria are associated with improved digestion and enhanced metabolic function.
Immune functions are greatly influenced by gut microbes, which can play a crucial role in the development of autoimmune conditions.
Intermittent fasting may be an effective strategy to enhance bacterial diversity and help prevent or manage autoimmune diseases such as rheumatoid arthritis, multiple sclerosis, Crohn’s disease, or psoriasis.
A study published in Cell Metabolism found that mice with experimentally induced multiple sclerosis (MS) showed markedly improved intestinal microflora diversity and a significant reduction in MS symptoms after being fed on an intermittent fasting protocol.
Although more human clinical trials are needed to confirm the role of intermittent fasting in the management of autoimmune diseases, the authors summarized that intermittent fasting may be an effective strategy to improve intestinal microbial diversity in individuals with multiple sclerosis.
8. Longevity
Time-restricted eating is linked to improved metabolic, hormonal, neurological, and cardiovascular, and its role in longevity is of increasing interest to the scientific community.
Intermittent fasting mimics the metabolic effects of caloric restriction, which can slow the aging process and is consistently recognized as an effective tool for extending lifespan in animal studies.
Research on longevity in humans, especially concerning dietary interventions, is challenging to conduct as there isn’t a specific marker or measurement that can predict lifespan.
However, a clinical trial published in Cell Metabolism aimed to study the effects of ADF on molecular markers of aging in healthy, non-obese adults.
This study found that time-restricted eating periodically increases concentrations of the ketone β-hydroxybutyrate, which has potent cardioprotective effects and increases blood flow to the brain, which is associated with long-term healthspan.
The study also found that four weeks of ADF lowered levels of free triiodothyronine (fT3), a thyroid hormone that regulates various critical metabolic and cellular processes.
These findings may be of importance as low levels of fT3, without impaired thyroid gland function, are linked to healthy aging and longevity.
Is intermittent fasting safe?
Intermittent fasting is considered safe and generally well-tolerated by most adults.
Millions of people practice intermittent fasting worldwide, and much research has been conducted to evaluate the safety of time-restricted meal plans, and no evidence suggests that intermittent fasting has long-term adverse health effects.
Potential side effects of intermittent fasting
While intermittent fasting has impressive benefits for health and longevity, it can also have downsides.
Intermittent fasting triggers significant changes in electrolyte and hormone levels, which can cause short-term side effects in some people.
Symptoms similar to the keto-flu, such as dizziness, dehydration, fatigue, and headaches, can develop during the early phases of intermittent fasting and typically improve once your body becomes adapted to utilizing fat as a primary energy source during fasting periods.
Who shouldn’t do intermittent fasting
Restricting eating times isn't suitable for everyone, and some people should avoid fasting.
Pregnancy and breastfeeding are periods of increased caloric needs, and pregnant or breastfeeding mothers are advised to abstain from intermittent fasting or any other fasting protocol.
Individuals with eating disorders and those who are underweight may lose weight with intermittent fasting, which could worsen their condition and increase the risk of adverse health effects.
Intermittent fasting significantly influences your body’s energy-making processes, and it’s crucial to consult with a healthcare provider if you have underlying medical conditions such as diabetes, hypoglycemia (low blood sugar), or kidney problems before you try intermittent fasting.
Key takeaways
Does intermittent fasting work? Yes! Research shows that time-restricted eating benefits metabolism and weight management, promotes cardiovascular health, lowers inflammation, and supports liver function.
Science confirms that intermittent fasting is highly effective for preventing and managing metabolic imbalances such as insulin resistance, obesity, and type 2 diabetes and may even help extend your lifespan.
FAQ
1. What are the benefits of intermittent fasting?
Intermittent fasting triggers profound metabolic changes linked to weight loss, improved metabolic and cardiovascular health, lower inflammation, improved liver function, intestinal microbial diversity, and longevity.
2. How much weight can you lose in a month with intermittent fasting?
How much weight you can lose in a month with intermittent fasting depends on your health status, body weight, metabolism, and age.
However, it’s not uncommon to lose between six to eight pounds of body weight per month with a consistent intermittent fasting routine.
3. Does intermittent fasting work for weight loss?
Yes, intermittent fasting works for weight loss.
Caloric restriction during fasting periods forces your liver to use stored body fat to generate energy, making intermittent fasting a highly effective weight loss tool that has helped many people achieve and maintain a healthy body weight.
4. How long does it take for intermittent fasting to work?
How long it takes for intermittent fasting to work depends on your diet, metabolism, and health status. Most people notice improved energy levels, weight loss, better sleep, and mental clarity within three to four weeks of practicing intermittent fasting.
5. What are the basic rules of intermittent fasting?
The basic rules of intermittent fasting are gradually transitioning into your chosen fasting hours, listening to your body and tweaking your schedule as necessary, and maintaining a consistent daily fasting routine.
During the fasting period, avoid eating food and only drink calorie-free beverages such as water, herbal teas, or black coffee to push your metabolism to utilize stored body fat as an energy source.
6. What are the best hours for intermittent fasting?
The best hours for intermittent fasting depend on your daily routine and your preferences.
Most people find it convenient to fast overnight and through the morning for 16 to 18 hours, followed by an eight to six-hour eating window, and resume their fast after dinner.
7. Is OMAD healthy?
OMAD—one meal a day—is a form of intermittent fasting that involves eating only one large meal during 24 hours and fasting for the remaining 23 hours. OMAD can be an effective weight loss tool and has been found beneficial in the management of insulin resistance and metabolic syndrome.
However, OMAD is restrictive and can lead to nutrient deficiencies if not combined with a nutritious diet rich in organic vegetables and dairy, grass-fed beef, wild-caught fish and seafood, and plenty of healthy fats.
8. How many hours should I fast per day for autophagy?
How long it takes to trigger autophagy with fasting depends on how metabolically flexible you are, meaning how quickly your body can switch to utilizing fats as a primary fuel source.
Some evidence suggests that it takes around 16 to 18 hours to initiate autophagy and that longer fasting periods, such as 24 or 48-hour fasts, enhance the profound benefits of this cellular repair and recycling process.
Sources
https://nutritionj.biomedcentral.com/articles/10.1186/1475-2891-11-98
https://www.sciencedirect.com/science/article/pii/S1550413114004987
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