Intermittent fasting has garnered a lot of attention over the past decade, appearing everywhere from medical research journals to podcasts and talk shows. Reported health benefits include weight loss, reduction in insulin resistance and blood sugar levels, reduction in oxidative stress and inflammation, improvement in several markers for heart disease, enhanced cellular repair, improved brain health, and the potential to extend lifespan.
Sounds like a golden ticket, right?
Not so fast. Like any other diet plan, intermittent fasting is not one-size-fits-all. Many of the purported benefits of IF are related to its potential influence on hormones, cellular function, and gene expression. However, evidence on these health benefits is limited, and further research is needed on the diet’s overall impact.
Additionally, the potential risks of intermittent fasting such as energy, vitamin, mineral, and protein deficiencies, as well as the risk of disordered eating/eating disorders, should be considered before adopting this way of eating.
A registered dietitian can help you evaluate the benefits and drawbacks of intermittent fasting based on your health goals and history. If you choose to proceed, they will provide guidance to safely incorporate the diet plan. Finally, a dietitian will assist with customizing meals to increase satiety, which will enhance the sustainability of your fasting schedule.
Have we sparked your curiosity? Keep reading to learn what intermittent fasting (or IF) entails, what the research says, and whether you should (or should not) give it a try.
Intermittent fasting is a dietary approach that focuses on when you eat rather than what you eat. Put simply, individuals alternate between periods of eating and periods of fasting. The theory behind this method is that your body undergoes metabolic changes during fasting periods. Insulin levels drop, promoting fat burning, and human growth hormone increases, aiding muscle gain and fat loss. Additionally, cellular repair processes are enhanced, and there’s improved regulation of blood sugar levels and inflammation. Supporters say that these changes can support weight management, improve metabolic health, and even extend longevity.
Fasting periods vary, as intermittent fasting is actually not a single diet but an umbrella term for several different fasting regimens. The three most popular approaches are alternate-day fasting, the 5:2 diet, and time-restricted eating (also called time-restricted feeding).
The alternate-Day Fasting (ADF) regimen goes back and forth between a fasting day and an unrestricted day. The non-fasting day is known as an ad libitum eating day (eating as much as you want and stopping when you’re full — aka mindful eating).
There are two primary variations of alternate-day fasting: zero-calorie ADF and modified alternate-day fasting (MADF). In the zero-calorie regimen, no foods or caloric beverages are consumed on fasting days. In contrast, the MADF regimen allows for a caloric intake of 20% to 30% normal consumption on fasting days.
This method confines fasting to just two days per week (hence the name). Individuals eat their usual diet five days out of the week, then significantly limit intake to somewhere around 500 total calories on two (consecutive or nonconsecutive) days.
A variation on 5:2 is called the Eat Stop Eat method, where individuals consume zero calories for one or two 24-hour fasting periods per week.
Time-restricted eating (TRE) is a popular version of intermittent fasting where individuals fast for somewhere around 14 to 16 hours daily and eat freely within a 4- to 10-hour window. The TRE regimen is highly variable, with fasting windows ranging from 12 to 21 hours per day. For example, the “Warrior Diet” is a spin on intermittent fasting that generally limits food consumption to four hours a day in the evening. Unlike the 5:2 diet and alternate-day fasting, time-restricted eating emphasizes a consistent daily eating window rather than energy restriction, aiming to achieve other potential health benefits.
Some studies indicate that intermittent fasting can aid in weight loss, improve metabolic health, reduce body fat, and enhance brain function. In addition, research shows reductions in blood sugar levels, insulin resistance, and inflammation, which are key factors in preventing chronic disease. Finally, reviewers find that IF may promote longevity by triggering cellular repair processes and boosting human growth hormone levels.
Proceed with caution, however, as this research has several limitations. Self-reported dietary intake, commonly used in these studies, can introduce inaccuracies that affect data reliability. Most studies are short-term, typically under a year, limiting the understanding of long-term health effects and sustainability. Additionally, there is a lack of diversity in participant demographics such as age, gender, ethnicity, and health status. This seriously constrains the generalizability of findings.
The variability in fasting protocols (e.g. ADF, 5:2, TRE) also complicates comparisons and general conclusions about the efficacy and safety of intermittent fasting. Few studies consider the nutritional quality of diets during eating windows, which can significantly impact health outcomes. Moreover, there is a lack of long-term data. Most studies do not follow participants beyond a short period.
These limitations highlight the need for more comprehensive, long-term research to fully understand the health implications of intermittent fasting.
Can intermittent fasting be the key to better health for the right person? Let’s break down some of the potential benefits identified by researchers so far.
There is evidence that the three primary forms of intermittent fasting—alternate-day fasting (ADF), the 5:2 diet, and time-restricted eating (TRE)—are effective weight-loss strategies, leading to body weight reductions of 1% to 13% over periods ranging from 2 to 52 weeks.
However, when compared to caloric energy restriction, longer-term trials and other studies show no significant differences.
Results from a recent systematic review and meta-analysis published in the journal Obesity found that alternate-day fasting/modified ADF had the highest efficacy for weight loss compared with the 5:2 diet and time‐restricted eating. ADF/MADF showed the highest range of weight loss from 0.77% to 12.97% over 3 to 26 weeks as compared with the other regimens. The 5:2 diet produced weight loss ranging from 1.7% to 7.97%, whereas TRE ranged from 0.95% to 8.60% body weight loss.
However, the authors also concluded that more research is needed to determine how successful people are at sticking to IF versus traditional calorie restriction (CER).
Studies indicate that most people find it harder to maintain intermittent fasting after 3 months. This means that those attempting longer-term fasting protocols often struggle to adhere to the plan. In general, adherence to any weight-loss regimen (including IF) tends to decline after 1 to 4 months.
Currently, most of the research on IF and weight loss is limited to short-term studies with small sample sizes. Additionally, differences in meal timing, fasting duration, overall caloric intake, and background diet vary widely among studies, making it hard to confirm study findings.
If you’re on the fence, know that research shows other healthy diets (think: the Mediterranean diet, a well-rounded plant-based diet) are just as effective for weight loss as intermittent fasting — and they don’t (all) require strict food rules. Something to consider.
What happens to energy metabolism during fasting? When you begin fasting, your body first uses stored glucose (glycogen) from the liver and muscles for energy, a phase known as glycogenolysis. This lasts about 12-24 hours. Once glycogen is depleted, the body switches to creating glucose from non-carbohydrate sources like proteins, in a process called gluconeogenesis. As fasting continues, the body starts burning fat for energy, known as fat oxidation. The liver converts fatty acids into ketones, which serve as an efficient energy source for the brain and other organs.
This switch to fat burning and ketone production, called ketogenesis, is crucial to the health benefits of intermittent fasting. Ever heard of the ketogenic diet? This is exactly that process: ketosis.
This metabolic switch is thought to improve insulin sensitivity, reduce inflammation, and promote weight loss. Fasting also triggers a cellular cleanup process called autophagy, which removes damaged cells and proteins, potentially contributing to better overall health.
There’s evidence that intermittent fasting may bolster the body’s stress response by forcing cells to be more adaptive. This phenomenon harkens back to when our ancestors regularly experienced periods of starvation. As a result, cells became more adaptive, revving up their antioxidant activity and DNA repair and decreasing inflammation.
Due to these potential benefits, IF has been studied for its potential to prevent and manage conditions like cardiovascular disease, type 2 diabetes, cancer, and Alzheimer’s disease.
The evidence on whether intermittent fasting has a clinically beneficial effect on glucose and lipid metabolism, cancer, or Alzheimer’s is still inconclusive.
One recent study suggests that intermittent fasting for 12–24 hours might help reduce the risk of Alzheimer’s disease pathology and its clinical manifestation of dementia. However, most current research in this area is based on animal studies. There are limited human trials on the effects of intermittent fasting on cognitive function and dementia. This is most likely due to the challenges in maintaining a fasting regimen and ensuring trial adherence amongst prospective participants.
The impact of intermittent fasting (IF) on human cancer incidence and prognosis is still unclear due to a lack of high-quality randomized clinical trials. Early studies indicate that prolonged fasting might be safe for some cancer patients and could reduce chemotherapy-related toxicity and tumor growth. However, more research is needed to understand the risks and benefits of fasting for cancer patients. Therefore, it is not recommended for those undergoing active cancer treatment to try IF outside of clinical trials. While IF may help with weight management and potentially offer cancer prevention benefits, its direct effects on cancer-related pathways remain unknown.
We don’t yet know the effects of intermittent fasting on major health outcomes like death, heart attacks, and heart failure. This is simply due to insufficient data. Some studies suggest that IF can help reduce weight compared to unrestricted eating, making it as effective an approach as continuous calorie restriction. However, weight changes are often small and not clinically significant in the short term. The quality of the current evidence is low, leaving many uncertainties. More research is needed to determine which patients might benefit from IF and to understand its long-term effects on overall mortality and heart disease.
There are a few groups that definitely need to steer clear of IF: individuals with advanced type 2 diabetes, pregnant or breastfeeding women, and those with a history of disordered eating. People prone to stress and anxiety may also want to avoid intermittent fasting.
Diabetics should eat at regular intervals to maintain stable blood sugar levels. Fasting periods can lead to hypoglycemia (dangerously low blood sugar) if medications such as insulin or sulfonylureas are not adjusted appropriately. Additionally, extended fasting may result in hyperglycemia (high blood sugar) if the body starts breaking down muscle tissue for energy. This can complicate the management of diabetes and lead to other health issues.
Pregnancy and breastfeeding require a steady intake of calories and nutrients to support the growth and development of the baby and to maintain the health and well-being of the mother. Intermittent fasting can lead to a calorie deficit and nutrient deprivation, which may affect milk production, energy levels, and overall nutritional status. Furthermore, fasting can cause fluctuations in blood sugar levels, which can be detrimental to both the mother and the baby. It’s crucial for pregnant and breastfeeding individuals to have a balanced healthy eating pattern with regular meals to ensure they are meeting their increased nutritional requirements.
When it comes to individuals with a history of disordered eating, IF can be dangerous territory. Why? The restrictive nature of intermittent fasting diets can lead to a preoccupation with food, rigid control over eating times, and feelings of guilt or anxiety around meals, all of which can be harmful for those recovering from or prone to eating disorders. Additionally, the fasting periods can contribute to binge eating during eating windows, creating a cycle of restriction and overconsumption. Plus, fasting can mess with natural hunger and satiety cues. For individuals with a history of disordered eating, a more flexible and balanced approach to healthy eating, focusing on regular, nutritious meals, is generally safer and more supportive of long-term recovery and mental health.
Fasting increases levels of the stress hormone cortisol in the body. This makes sense: not eating for an extended period of time eventually triggers the body’s stress response. If you’re someone who already struggles with anxiety, elevated cortisol levels related to IF may further exacerbate physiological symptoms of stress, like rapid heart rate and breathing, high blood pressure, and high blood sugar. Additionally, prolonged fasting periods can lead to irritability, mood swings, and difficulty concentrating, further aggravating stress and anxiety.
Even if you don’t fall into any of the above categories, IF still may not be for you if it doesn’t match your lifestyle. For instance, if you’re someone who lives for breakfast, there’s no reason to subject yourself to time restricted eating. Similarly, if you know you get light-headed when you haven’t eaten in a few hours, fasting may pose a serious threat to your health. If that’s the case, consider IF “not worth it” and consult with a registered dietitian to find a healthy meal plan that works better for your body.
If you want to try intermittent fasting, it’s important to find a method that aligns with your lifestyle and health needs. Start by considering your daily schedule, work commitments, and personal preferences. Time-restricted eating, where you fast for 14 to 16 hours daily and eat during an 8- to 10-hour window, is popular for its flexibility and ease of integration into most routines. If you prefer a less frequent fasting pattern with majority non-fasting days, the 5:2 method might be a better fit. If alternate-day fasting appeals to you, just consider whether you will be able to maintain your normal activity levels on restricted eating days.
Begin by slowly increasing the duration of your fasting periods, allowing your body to adapt to each new eating pattern without feeling overwhelmed. For instance, if you choose alternate-day fasting, you might start by incorporating one fasting day per week and gradually increase it to every other day. Listen to your body’s cues for hunger and fullness, and don’t disregard signs of fatigue, irritability, or excessive hunger. If you experience any discomfort or adverse effects, consider adjusting your approach.
There’s a misconception about IF that non-fasting or ad libitum periods give carte blanche to eat endless unhealthy foods. “Ad libitum” is Latin for “to one’s pleasure.” When referring to eating, this phrase means you can eat as much and as often as desired, simply stopping when you’re full. But it’s important to note that the positive (or negative) health effects of specific foods will still impact you even if you’re practicing periods of intermittent fasting.
Maintaining a balanced diet during your eating windows is crucial for the success and safety of an intermittent fasting eating plan. Focus on consuming a variety of nutrient-dense foods that provide essential vitamins, minerals, and energy. Incorporate plenty of fruits and vegetables, whole grains, lean proteins, and healthy fats into your meals. A balanced approach will ensure that your body receives the necessary nutrients to function optimally on both fasting and non-fasting days. This will support sustained energy levels and promote overall well-being.
Lastly, it’s essential to stay hydrated — especially when fasting. Make sure you drink plenty of water!
Intermittent fasting is not for everyone. Before starting an IF program, check with your doctor to identify any underlying health conditions or medications that fasting might affect. Then, meet with a registered dietitian to create a balanced, nutritious, and sustainable intermittent fasting meal plan. Your RD will also monitor your progress and make necessary adjustments to ensure you’re meeting health goals without compromising your well-being.
Since you will not be receiving essential nutrients through food during fasting windows, it’s important to ensure maximum nutrition during non-fasting times. Registered dietitians have extensive knowledge of various eating plans, and they understand the science behind the different modes of intermittent fasting. A registered dietitian will help you determine what to eat when you’re not fasting to promote satiety and overall health.
An RD also has the experience to predict potential pitfalls, and can help you adjust your fasting plan before you experience negative impacts. One common issue is dehydration. A registered dietitian can give you hydration tips and teach you what to look out for to avoid becoming dehydrated.
As part of your care plan, a registered dietitian will also share sample recipes and food prep tips that are personalized to your fasting protocol, lifestyle, and tastes.
Finally, a registered dietitian can also advise you on supplements and vitamins to support intermittent fasting as needed.
Next, we’ll tackle some of the intermittent fasting questions we hear most often in our nutrition practice. Note that these are generalized responses. Consult with your registered dietitian for more personalized advice!
You can drink coffee and tea during fasting periods, but opt for black coffee or plain tea to avoid breaking your fast. Avoid adding sugar, cream, or any high-calorie additives. These can interfere with the fasting process by triggering an insulin response. If you prefer a little flavor, try adding a slice of lemon or a dash of cinnamon to your tea. Additionally, be mindful of your caffeine intake, as excessive amounts can lead to dehydration and poor sleep hygiene.
Balance is key. Make the most of your eating windows by focusing on nutrient-dense foods. You can still incorporate less nutritious treats into your regimen, as long as you consume them in moderation. Steer clear of excessive high-sugar and highly processed foods, as they can cause spikes in blood sugar levels and lead to cravings and energy crashes. Additionally, you may want to limit your intake of refined carbohydrates like white bread and pastries, which offer little nutritional value. Instead, prioritize whole foods that provide sustained energy and essential nutrients, such as fruits, vegetables, lean proteins, whole grains, and healthy fats.
Intermittent fasting can influence exercise performance in various ways, with effects differing from person to person. For some, fasting may enhance fat burning and improve metabolic efficiency, which can be beneficial for endurance activities. However, exercising on an empty stomach might also lead to decreased energy levels, reduced strength, and lower overall performance, particularly for high-intensity or resistance training workouts. It’s crucial to listen to your body and adjust your exercise routine accordingly. Some individuals find it helpful to schedule their workouts during their eating windows to ensure they have adequate fuel and recovery nutrients.
Common side effects of intermittent fasting include hunger, fatigue, irritability, and difficulty concentrating. Some people may also experience headaches, dizziness, or digestive issues like constipation or bloating. These symptoms often subside as the body becomes accustomed to fasting. Long-term or overly restrictive fasting can lead to nutrient deficiencies, reduced muscle mass, or metabolic slowdown. There is also a risk of developing a psychological condition such as binge eating disorder, which can be triggered or exacerbated by time restricted eating.
It’s important to monitor your health with a medical provider before and during intermittent fasting to navigate these potential side effects.
Experiencing extreme hunger or weakness during intermittent fasting is a signal that your body needs nourishment and energy. It’s important to listen to these cues and address them appropriately. It may be beneficial to adjust your fasting schedule or the types of foods you’re eating on non fasting days. Incorporating more nutrient-dense, high-fiber, and protein-rich foods can help keep you fuller for longer to maintain steady energy levels. Don’t hesitate to break your fast if necessary; your health and well-being are the top priorities.
Initial research into intermittent fasting shows that it could result in improved metabolic health, enhanced cellular repair, and weight loss for the right candidate. But there is also a risk of side effects, including malnutrition and disordered eating. That’s where professional support and guidance comes in.
Partner with a registered dietitian to determine whether intermittent fasting is a safe and effective approach for your needs. Whatever you decide, your dietitian will help you develop a nutritious and satiating way of eating that supports your long-term goals.
Any general advice posted on our blog, website, or application is intended for reference and educational purposes only and is not intended to replace or substitute for any professional medical advice, diagnosis, treatment, or other professional advice. If you have specific concerns or a situation arises in which you require medical advice, you should consult with an appropriately qualified and licensed medical services provider.