From intermittent fasting to juice cleanses, fasting has taken the health world by storm, promising weight loss, mental clarity, and longevity. But is it actually the key to unlocking better health, or just the latest passing craze?

Fasting requires abstaining from food — or, in some cases, both food and drinks — for a set period of time. While different types of fasting exist, the most popular include intermittent fasting (where eating windows are restricted) and prolonged fasting (often lasting 24 hours or more). 

But despite its growing popularity, there’s still debate about its true health benefits. While fasting can offer a range of health benefits for many people, it’s not a one-size-fits-all solution and should be approached with caution, especially for those with specific health concerns.

Health Benefits of Fasting

While the results can vary depending on the type and duration of fasting, studies have shown significant improvements in various aspects of health.

Reduce Inflammation

Fasting is like hitting reset on your body’s systems, particularly helping reduce inflammation. A January 2024 study in Cell Reports found that fasting for 24 hours reduced markers of inflammation in both humans and animals, potentially helping to lower the risk of conditions linked to chronic inflammation like arthritis and heart disease.

Balance Blood Sugar

Keeping your blood sugar steady can help prevent those mid-day energy crashes, while also helping prevent or manage type 2 diabetes. A January 2022 study in Nutrients found it may help reduce risk factors of metabolic syndrome that increase the likelihood of type 2 diabetes, heart disease, and stroke. Additionally, an August 2023 review in Nutrients found that fasting can increase insulin sensitivity, making it an efficient therapy for the medically supervised management of type 2 diabetes.

Support Heart Health

Your heart health is key to everything else — take care of it now, and it’ll take care of you later. Fasting has been linked to improvements in heart health and reduced risk of heart disease, including reductions in blood pressure, LDL “bad” cholesterol, and triglycerides, according to a November 2020 review in Frontiers in Nutrition.

Help Optimize Growth Hormone

When your hormones are in harmony, you’re in the sweet spot for energy and mood. Fasting can help regulate hormones, including those responsible for metabolism and fat storage. An August 2021 review in Endocrinology and Metabolism found that fasting triggers the release of growth hormone, which is involved in metabolism, weight loss, and muscle growth.

Combat Obesity

Effectively managing weight goes beyond calories — it’s about making smarter lifestyle choices. Intermittent fasting has proven to be an effective strategy for weight loss and obesity management. A December 2021 review in JAMA Open Network concluded that intermittent fasting can help reduce body weight, body mass index, and cardiometabolic risk factors (ex. cholesterol, triglycerides, and blood pressure) in adults with overweight or obesity.

Improve Body Composition

Working on your body composition can help you shed fat and maintain lean muscle. Fasting promotes fat loss while preserving lean muscle mass. A November 2024 study in the Journal of Translational Medicine found that 16-hour fasting can improve body composition in healthy and non-trained individuals.

Protect Your Brain

Forgoing food for a short period of time just might help improve your focus and sharpen your thinking. An August 2024 study in Cell Metabolism found intermittent fasting improved cognition, memory, and executive function in 40 older adults with obesity and insulin resistance. 

Additionally, a January 2018 review in Nature Reviews Neuroscience that looked at intermittent metabolic switching — cycling between ketosis (a metabolic state induced by fasting and/or exercise) and recovery (eating, resting, and sleeping) — can improve neuroplasticity (the brain’s ability to change and adapt).

Though research is still emerging on the effect of fasting on the brain of healthy adults, an October 2019 review in Nutrients reports that, in animal studies, fasting has shown the potential to improve cognition, slow cognitive decline associated with aging, and slow neurodegeneration. However, further research is required to determine how fasting affects human brain function.

Potentially Increase Longevity

Slowing the aging process could be as simple as supporting your body’s natural repair systems. Evidence suggests fasting may promote longevity by triggering cellular repair processes like autophagy and reducing inflammation. 

A February 2021 study in the International Journal of Molecular Science found that periodic fasting increased the diversity of helpful gut bacteria, including a type related to longevity. Furthermore, a March 2022 review in Nature Aging states intermittent fasting has “positive effects on risk factors for aging, diabetes, autoimmunity, cardiovascular disease, neurodegeneration and cancer.”

Types of Fasting

Fasting comes in many forms, each with its own approach and potential benefits. Here’s a breakdown of the most common types of fasting:

• Water Fasting: consuming only water for a set period, typically 24 to 72 hours, often used for detoxification or spiritual reasons

• Dry Fasting: avoiding both food and water, usually for shorter periods (12 to 24 hours) but risks dehydration

• Juice Fasting: only fresh fruit and vegetable juices for a period of time, eliminating solid foods but providing more nutrients than water/dry fasting

• Intermittent Fasting (IF): eating windows are restricted, like Alternate-Day Fasting (switching between days of fasting and eating), 16:8 Method (fasting 16 hours, eating during an 8-hour period), and 5:2 Method (eating 5 days a week, fasting for 2 days)

• Eat-Stop-Eat: fasting for a full 24 hours once or twice a week, typically from dinner one day to dinner the next day

• One Meal a Day (OMAD): consuming all your calories in one meal each day, fasting for the remaining 23 hours

• Partial Fasting: limits specific foods or food groups (such as meat, dairy, or processed foods) while allowing others, usually for several days or weeks

• Fasting Mimicking Diet (FMD): a low-calorie, plant-based diet designed to mimic the effects of fasting while still providing some nutrients

Is Intermittent Fasting Healthy?

Whether Intermittent Fasting is a good fit for you depends on your health status and goals. It may be an effective strategy for weight loss and improving metabolic health for some, but others may find the approach too restrictive. 

Hunger, low energy, and irritability are common during fasting periods, especially in the beginning. Also, fasting may disrupt hormones, particularly in women, potentially leading to menstrual irregularities, according to the Cleveland Clinic.

If you’re considering Intermittent Fasting, it’s essential to listen to your body, start slowly, and consult a healthcare provider to ensure it’s safe and sustainable for your lifestyle.

Side Effects of Fasting

If you’re new to fasting or pushing your limits, these are some side effects you might experience, according to Harvard Health Publishing:

• Hunger and overeating (in recovery periods)

• Fatigue and lethargy

• Constipation

• Metabolic issues

• Nausea (especially if you’re on medication taken with food)

• Electrolyte imbalance

• Headaches

• Irritability and crankiness

• Unhealthy weight loss that affects your bones, immune system, and energy level

If these symptoms become severe or persist for an extended period, it’s important to stop fasting and consult with your doctor to ensure your health isn’t at risk.

Who Should Not Fast?

While fasting can benefit many people, it’s not suitable for everyone. Certain individuals should avoid fasting or consult a healthcare provider before trying it:

• Children and teens under 18

• People who are pregnant or breastfeeding

• Individuals with type 1 diabetes who take insulin

• Those with a history of eating disorders

• People with certain chronic health conditions like heart or kidney disease

• Individuals with weakened immune system

Frequently Asked Questions

What can I eat while fasting?

During fasting periods, you should avoid consuming any food. However, some types of fasting, such as intermittent fasting, may allow non-caloric beverages like water, tea, or black coffee. Be sure to check the specific fasting method you're following for guidelines.

How long is it safe to fast?

The length of a fast should depend on the method you're following and your health. Generally, intermittent fasting methods like 16:8 are considered safe for most people. Extended fasting (more than 48 hours) should be done under medical supervision.

How long do I need to fast for it to be effective?

For intermittent fasting to be effective, you typically need to follow a consistent fasting schedule, with methods like 16:8 showing results after several weeks. Longer fasts (24 hours or more) may offer additional benefits but should be approached with caution.

What is dirty fasting?

Dirty fasting refers to a more relaxed version of fasting where you may consume small amounts of calories (like a splash of milk in your coffee or a sugar-free drink) during fasting hours, while traditional fasting requires no caloric intake at all during fasting periods.

Whether you’re cutting, leaning out, or trying to get shredded, your real goal may be more specific than just losing weight—you’re likely wondering how to lose weight without losing muscle.

There are numerous benefits to focusing on body recomposition rather than just weight loss. Losing fat while keeping your muscle allows you to maintain strength and energy and even supports your overall health, all while helping you get that lean, toned, and defined look that so many people are after.
But cutting down your body fat mass while keeping your muscles the same can be a bit confusing since they are actually two different goals.

In this guide, we’ll break down five science-backed strategies to help you burn fat, retain muscle, and reach your goals more effectively, plus a few expert tips on supplements, recovery, and nutrition timing.

1. Cut down on your calories, but prioritize protein

If you want to lose weight, you need to be in a calorie deficit. When you are in a calorie deficit (in other words, when you are eating fewer calories than your body uses for energy), it has to use the energy that it has stored in your body from previous meals.

Much of this extra energy is stored as fat tissue, which ultimately means that your body will “burn” through this fat tissue and help with weight loss!
Unfortunately, fat tissue isn’t the only thing that can be burned when you’re in a calorie deficit. You can also lose muscle in the process since some of your energy is also stored there in addition to in your fat tissue.

So, a calorie deficit isn’t the only thing you need to think about when you’re trying to lose fat while keeping your muscle — you’ll also need to pay attention to the quality of your diet and eat enough protein, so that muscles have what they need.

Protein is often called a “building block”, because it makes up the structure of important components of our body, including muscle.

In fact, research suggests that eating plenty of protein can even promote increases in lean body mass during a calorie deficit. Aim for 1.2–1.6 grams of protein per kilogram of body weight per day, spread evenly across meals.

Focus on lean sources like:

• Chicken breast

• Greek yogurt

• White fish

• Lentils and beans

• Low-fat dairy

• Tofu and tempeh

These foods help you stay full, fuel recovery, and prevent your body from breaking down muscle tissue.So, your nutritional goals are twofold: you should eat at a moderate calorie deficit to encourage fat loss, but also fill up on plenty of protein sources to maintain muscle.

Note: No foods are strictly “off-limits.” What matters most is your overall calorie intake, nutrient balance, and consistency. The foods listed under “avoid” are often higher in calories, added sugars, or processed fats, which can make fat loss and muscle preservation more difficult if eaten in excess. However, enjoyed occasionally and in moderation, they can still fit into a healthy, goal-supporting diet.

2. Combine Resistance Training and Cardio

To burn fat without losing muscle, your workout routine needs more than cardio. Cardio exercises like running, swimming, or cycling help create the calorie deficit you need. But resistance training—like weightlifting or bodyweight movements - is what tells your body, “keep the muscle.”Aim for:

• 2–3 sessions of resistance training per week (target all major muscle groups)

• Cardio on alternate days or after strength sessions

• Recovery days to prevent overtraining

3. Try HIIT

If you want to attack both your fat loss and resistance training goals with one intense, heart-pounding exercise, adding a few days of HIIT training to your workout routine is a great idea!High-intensity interval training (HIIT) involves doing high-intensity exercises for short periods of time, with brief periods of rest in between. These intense workouts are designed to accelerate your heart rate and can include repetitive whole-body movements like jumping jacks, sprints, lunges, burpees, and more.Because these workouts are so intense, HIIT is highly effective at burning calories. But even better, HIIT also seems to help with maintaining muscle mass at the same time!One study found that young overweight adults were able to better maintain muscle mass on a hypocaloric diet when they added HIIT to their training regimen. Try:

• 20-minute HIIT workouts 2–3 times per week

• Mix of upper/lower body movements

• Progress gradually to avoid burnout

4. Eat Satisfying, Nutrient-Dense Foods

One of the biggest obstacles that people tend to run into when attempting to lose weight is battling with their appetites. If you’re trying to lose fat, the key to maintaining a good calorie deficit is to fill up your diet with foods that keep you feeling full while still allowing you to stay within your calorie limits.Fill your plate with:

• Lean proteins for satiety

• High-fiber foods like beans, lentils, spinach, and oats

• Non-starchy vegetables like broccoli, zucchini, kale, and carrots

• Healthy fats like avocado and nuts (in moderation)

Research even links higher legume consumption with lower body fat percentages and improved BMI.Avoid ultra-processed snacks that offer quick calories but no lasting fullness.

5. Strategize the best meal and snack times

Because you’re in a calorie deficit, it can be extremely helpful to strategize the ideal mealtimes for yourself, to determine what works best for you and will keep you from feeling deprived throughout the day.

Ultimately, the most important thing for fat loss is to burn more calories than you take in.
However, experimenting with your mealtimes and finding out what makes you feel the most satisfied can help make restriction much more comfortable and tolerable throughout your fat-loss journey.

For example, eating several small, frequent meals during the day is a popular strategy that has been linked to fat loss and increases in fat-free mass (otherwise known as muscle mass).

Some people also swear by intermittent fasting (IF) since it can help you stay in your calorie deficit.

Current research suggests that IF can be an effective means of losing primarily body fat.

Do some experimenting to figure out which strategy helps you stay the most comfortable while working on your goals.

6. Don’t Skip Recovery and Sleep

Muscles grow during recovery - not workouts.Getting enough rest between sessions and prioritizing 7–9 hours of sleep each night is essential for hormone regulation, muscle retention, and fat burning.Strategies to support recovery:

• Active recovery (walking, yoga)

• Rest days between training muscle groups

• Cold showers or compression for soreness

Lack of sleep raises cortisol, which can increase fat storage and reduce muscle.

7. Supplements That May Help

While not necessary, a few well-studied supplements can support fat loss and muscle maintenance:

• Whey protein – helps meet protein goals conveniently

• Creatine – supports strength and muscle performance

• Omega-3s – may reduce muscle breakdown

• Caffeine – boosts training intensity and fat oxidation

Always consult with a health professional before adding supplements to your routine!

Supplements Cheat Sheet

Key Takeaways

You don’t have to sacrifice your hard-earned muscle to get lean.By combining:

• A moderate calorie deficit

• High protein intake

• Smart strength and cardio training

• Rest, sleep, and nutrient-dense food

You’ll not only lose fat - you’ll maintain your energy, strength, and definition. Track your body fat percentage and measurements, not just the number on the scale. And remember, real progress isn’t always visible overnight—but consistency pays off.

Sometimes, it seems like you’re doing everything right when it comes to diet and exercise, yet you still aren’t making progress on your body composition goals. 

If you’ve ever had trouble seeing the results of your hard work at the gym, what you may be observing is the outcome of your body’s internal struggle with your hormone levels. 

Hormones can be thought of as your body’s “messengers.” These chemicals are made by your endocrine glands and travel throughout your body via your bloodstream, regulating important processes, including your growth and metabolism. 

Because they’re such an integral part of your body’s functioning, having too much or too little of any hormone can have a very real impact on how well your body’s systems work. 

In this article, we’ll go over some of the most common hormones that affect body composition. 

We’ll also talk about some ways to bring any imbalanced hormone levels back to normal so that your body will begin responding more positively to interventions designed to improve your body composition. 

Of course, you should consult your medical provider before implementing any of these suggestions!

5 Hormones That Are Important for Your Body Composition

1. Insulin 

Insulin is a crucial hormone for your metabolism. It helps your body to properly digest and store glucose, a kind of sugar. 

However, because insulin is such a crucial player in your metabolism, this also means that having irregular insulin levels can negatively impact your body composition, not to mention your health as a whole.

Why? Well, one of the key roles of insulin is to bring the sugar that’s circulating in your bloodstream into your cells, where it can be used as an energy source. 

If you are consistently eating a diet that is high in fat and sugar, your body needs to release more and more insulin to bring sugar from your bloodstream into your cells for energy. 

Over time, these high insulin levels can make your cells less responsive to insulin, a state that is referred to as insulin resistance. 

In cases like these, since your cells are less efficient at bringing sugar inside them to use for energy, the sugar remains in your bloodstream and is thus more likely to be stored for later use as fat tissue.

Insulin resistance is linked to a variety of conditions. It can often occur alongside obesity, and it’s also associated with long-term health concerns like diabetes and cardiovascular disease. 

As a result, your insulin levels can help or hinder your fitness goals. For example, a weight loss intervention study on healthy women with different levels of insulin resistance put all the women on a similar calorie restriction but instructed some of them to follow an at-home exercise program. 

The researchers found that although the insulin-resistant women lost more weight over time, exercise promoted weight loss only in women who did not show any insulin resistance.

How to balance insulin levels

Dietary changes like increasing your consumption of fiber (found in whole grains, non-starchy vegetables, and raw fruit) and minimizing your consumption of simple sugars may help improve insulin resistance. But it’s best to talk to a doctor if you believe that your insulin levels are imbalanced, or if you’re experiencing insulin resistance, since it can be linked to serious issues like diabetes. In these cases, you might even need to take exogenous insulin as a medication to reach optimal levels. 

2. Cortisol 

If there’s one hormone that has gotten a reputation as a potential saboteur of fitness goals in the past few years, it’s cortisol, “the stress hormone.”

Cortisol is directly responsible for your “fight or flight” instinct—the feeling that kicks in when you’re faced with a potentially dangerous situation. 

One of cortisol’s main modes of operation is to act on your liver, breaking down your stored glycogen into glucose (aka blood sugar), so that your cells can access a quick and efficient source of energy in the event that you need to spring into action.  

In the short term, this makes cortisol highly beneficial (for instance, if you need to generate quick energy to run away from a life-threatening danger). 

Unfortunately, many people are under chronic, ongoing levels of high stress that can contribute to consistent metabolic changes, which can have a negative impact on your health and your fitness goals over the long term. 

For example, in a population-based study of Japanese individuals, researchers found that higher levels of cortisol were associated with decreased insulin secretion. 

As we’ve already discussed, insulin is important for managing your metabolism, and low levels can lead to metabolic inefficiency (a potential factor in weight gain).  

In addition, all of that stress can also take a toll on your behavior and habits. 

For some people, stress may trigger overconsumption, or, “stress eating,” which can get in the way of hitting your body composition targets if you’re trying to reduce body fat.  

How to balance cortisol levels

Stress can sometimes feel inevitable depending on your lifestyle. However, finding healthier ways to manage that stress may help. Various studies have found that meditation may decrease cortisol levels in at-risk populations. Additionally, exercise might also help improve cortisol levels in some populations, especially people dealing with long-term conditions. Finally, if you find that you tend to put your sleep on the back burner, you may find that prioritizing high-quality sleep can help, since cortisol is released based on your natural circadian rhythms. 

3. Testosterone

Testosterone is another important player in your fitness level, because it can directly influence your body’s ability to build muscle. Testosterone is perhaps best known as a male reproductive hormone. However, testosterone is an important driver of muscle growth and maintenance for everyone and can influence lean mass in women as well as in men. In addition, testosterone is also thought to play a role in body fat. For instance, in men, testosterone deficiency is associated with increased fat mass. Testosterone is especially associated with increased belly fat. 

How to balance testosterone levels 

Testosterone levels largely depend on your genetics and your age, and it’s fairly normal for testosterone levels to decrease as you get older. But you can also increase your testosterone levels with lifestyle changes. For example, studies have shown that testosterone levels increase immediately after moderate-intensity and high-intensity training, although this result is temporary. There’s also some evidence that certain herbs can increase testosterone in healthy young men, but talk to your doctor first to determine whether this is the right course of action for you. Avoid over-the-counter steroids, which are marketed as supplements that increase your testosterone but can have dangerous symptoms and side effects like heart and liver damage. Finally, some circumstances may warrant testosterone replacement therapy. If you think testosterone replacement therapy might be appropriate for your needs, reach out to a medical professional. 

4. Ghrelin  

If you find that your appetite is often high, and you’re eating more than your body really needs for energy, it may be in part due to the hormone ghrelin. Ghrelin is a hormone that’s responsible for triggering hunger. Along with leptin, another hormone that plays a role in inhibiting your appetite, ghrelin can influence your eating habits. Having high levels of ghrelin in your system naturally increases your appetite and cravings, which may drive calorie overconsumption. Because your body’s ability to maintain its weight depends largely on the amount of energy that you consume versus the energy that you spend during the day, eating more calories than necessary can ultimately lead to fat storage and/or having a harder time losing weight.   In addition to the behavioral aspect, ghrelin may also play roles in influencing how your body stores fat. Plus, ghrelin can influence how your body releases growth hormone, a hormone that regulates various aspects of body composition—including muscle growth. So, you can see what a strong impact ghrelin can have on body composition!

How to balance ghrelin 

To manage your appetite, start by filling your plate with nutrient-dense and filling foods that trigger fullness. Foods that have plenty of dietary fiber and/or lean protein are good choices, since they can fill you up while delivering the nutrients your body needs for growth and fat loss. It’s also a good idea to prioritize your sleep if you find that your appetite is larger than your caloric needs. Studies show that ghrelin levels can increase with sleep deprivation. 

5. Thyroid Hormones

Finally, if you find that your body is holding onto fat even if you’re doing everything else right, it’s worth looking at your thyroid levels. Your thyroid is an organ that produces several hormones that are important for regulating the speed of your metabolism (in other words, how quickly your body uses energy). As a result, if there is something wrong with your thyroid, and it can’t produce enough hormones, it can cause your metabolism to slow down, ultimately leading to unexplained weight gain. On the other hand, you can also have thyroid levels that are too high, which will speed up your metabolism to abnormal levels. This may contribute to unexplained weight loss. 

How to balance thyroid hormones

If your thyroid is underproducing or overproducing hormones, your best bet is to talk to your doctor. While there is evidence that eating a healthy diet with micronutrients like iodine, selenium, iron, zinc, and Vitamins B12, D, and A may help reduce symptoms of thyroid issues or prevent thyroid risks in the future, an over- or under-active thyroid is a medical issue. Treating it may require medication to supplement low hormone levels or inhibit overactive thyroid activity. 

Conclusion

Your diet and exercise routines are absolutely the most crucial pieces of the puzzle when it comes to improving your body composition.However, if your hormone levels are not where they should be, it can make your progress much slower. By investigating and addressing any hormone imbalances, you can help your metabolism become a more efficient system that helps you reach your goals.