Intermittent fasting is a type of diet that focuses on when you eat rather than what you eat. People turn to this method for many reasons — from managing their weight and blood sugar to supporting cellular health and longevity. A poor diet can affect your cells and every aspect of your health, but what’s the best way to eat for your body based on your individual needs, lifestyle, and health status?
Instead of cutting out food groups, intermittent fasting creates specific windows for eating and fasting throughout the day or week. This guide breaks down how intermittent fasting works, the different ways to do it, what actually happens in your body when you fast, and the science behind its potential benefits.
Key Takeaways
- Intermittent fasting is about when you eat, not what foods you eat. You'll pick specific times for eating and fasting that fit your schedule, with options ranging from daily 16-hour fasts to twice-weekly low-calorie days.
- Beyond weight loss, fasting triggers important changes in your body, including better blood sugar control, increased fat burning, and cellular repair processes that may help slow aging.
- Your body adapts to fasting by cleaning up damaged cells and reducing inflammation - think of it like giving your cells time to tidy up when they're not busy processing food.
- Most people can try intermittent fasting safely, but it's not for everyone. Pregnant women, teens, diabetics, and those with a history of eating disorders should avoid it.
- The first few weeks are often the most challenging due to your body adjusting. However, most side effects, like hunger and fatigue, are temporary. Starting with a gentler schedule, like 16:8, makes the transition easier.
What Is Intermittent Fasting?
You’ve likely heard about it if you’re interested in various diet protocols, but what is intermittent fasting? Simply put, intermittent fasting involves alternating periods of eating with predetermined periods of fasting. The fasting window is generally longer than the time spent eating. You can eat meals as you usually would during your eating window, although some people eat less simply because there isn’t time for three meals and snacks.
How Does Intermittent Fasting Affect Your Body?
Intermittent fasting (IF), an increasingly popular wellness practice, is usually advertised as a weight-loss tool. But the benefits go far beyond caloric reduction. From gut health to metabolic support, the evidence behind intermittent fasting continues to grow. Keep reading to learn about the benefits of intermittent fasting.
- Hormonal changes: During fasting periods, your body increases the production of human growth hormone (HGH), which can help burn fat and repair muscle tissue.[1] At the same time, insulin levels drop significantly, helping reduce the risk of type 2 diabetes by improving insulin sensitivity.[2]
- Metabolic benefits: When you fast, your body starts to burn fat for energy.[3] This process, called lipolysis, helps with weight management and can improve cholesterol levels and heart health markers.[4]
- Brain health: Research suggests fasting may promote neurogenesis — the growth of new brain cells. These new neurons play a key role in learning and memory, potentially explaining why some people report better mental clarity during fasting.[5]
- Digestive health: Regular breaks from processing food allow your digestive system to repair and reset while influencing the gut microbiota. This can improve overall gut health.[6]
Intermittent Fasting Schedules
One of the best things about intermittent fasting is its flexibility — there's no single "right" way to do it. Here are the most popular intermittent fasting schedules you can choose from:
16:8 Fast
This is often the easiest schedule to follow, especially for beginners, because you fast for just 16 hours and eat during an 8-hour window. For example, eating between 12 pm and 8 pm, then fasting until noon the next day. Most people naturally fast overnight, so you're just extending that time period a bit longer. You might skip breakfast or dinner, depending on what works better for your schedule. Many people find this pattern sustainable enough to do every day, while others prefer to do it just a few days per week.
Eat-Stop-Eat
This approach involves a full 24-hour fast once or twice a week. For instance, you might eat dinner at 6 pm on Monday, then not eat again until 6 pm on Tuesday. While it's more challenging than the 16:8 method, you only have to do it once or twice a week. The key is eating normally on non-fasting days and not trying to make up for the fasted day by overeating.
20:4 Fast
Also known as the Warrior Diet, this is a more intense version of daily fasting. You eat during a 4-hour window and fast for the other 20 hours. For example, you might eat only between 4 pm and 8 pm each day. This method requires more adjustment and planning since you'll need to fit most of your daily nutrition into that shorter window. It's usually better suited for experienced fasters who have already adapted to shorter fasting periods.
5:2 Method
Rather than complete fasting, this approach uses very low-calorie days (500-600 calories) twice a week while eating normally the other five days. These "fasting" days don't have to be consecutive. You might choose Monday and Thursday, for example. Some people find this easier than complete fasting since you can still eat small meals throughout the day.
Benefits of Intermittent Fasting
So why do people fast? Is fasting just the latest health trend? Is fasting good for you? The answer is, intermittent fasting can definitely be valuable. There is certainly no shortage of IF benefits listed in medical journals and on various websites. In our appearance-obsessed American culture, the intermittent fasting benefit usually discussed is weight loss. Just by virtue of eating less frequently, people typically consume fewer calories when intermittent fasting, which can result in weight loss.
But it’s the hormonal changes that can really make a weight loss difference for many people—the impact to insulin, human growth hormone, and norepinephrine levels all drive augmented fat burning. In addition to weight management, a review article published in the New England Journal of Medicine found that intermittent fasting is associated with many health benefits, including:[7]
- Longevity: Research suggests that periods of fasting can trigger cellular repair processes and changes in gene expression that may help slow the aging process. These changes appear to protect against age-related diseases and potentially extend lifespan.[8]
- Cognitive health: Fasting appears to boost brain function by increasing the production of a protein called BDNF (Brain-Derived Neurotrophic Factor), which supports the growth of new nerve cells. This may help protect against cognitive decline and improve memory and focus.[9]
- Blood sugar balance: Regular fasting periods can improve insulin sensitivity and help maintain steady blood sugar levels. This makes intermittent fasting particularly interesting for people concerned about metabolic health and diabetes prevention.[10]
- Cardiovascular protection: Studies show that intermittent fasting may improve various heart health markers, such as blood pressure, cholesterol levels, and inflammation.[11] These changes may help reduce the risk of heart disease.
- Improvements in oxidative stress and inflammation: Fasting triggers several repair mechanisms in the body that help reduce harmful inflammation and oxidative damage to cells. This protective effect may help prevent chronic diseases.[12]
- Chemoprotection: Early research suggests that fasting might help protect cells from damage during cancer treatments while potentially making cancer cells more vulnerable to therapy.[13]
While researchers are still attempting to understand precisely why fasting may be so helpful, many of these advantages are likely related to the cellular adaptations that occur when the body is in a fasted state.
What Happens to Your Cells When You Fast?
While nutrition research usually examines diet and how the body responds when you eat certain nutrients, the power of intermittent fasting happens when there is a lack of nutrients, so the body is forced to adjust.
When you eat, your body is in a cycle of growth, or anabolism. Nutrients from your diet are used to build molecules in the body. While this is an essential physiological process, your body also needs time to balance anabolism with periods of repair.
In these times of rest, the body can focus on clearing out cellular debris, waste, and free radicals that otherwise lead to oxidative stress and damage in the body. Many of these restorative and protective processes only happen during periods of nutrient scarcity (most commonly when you sleep)[14] or while fasting.
As research tells us that oxidative stress is intimately connected with chronic health conditions, especially those associated with aging, fasting may be a tool to help. Some of the known cellular adaptations that may protect your body against oxidative damage include:
Repair and Waste Removal Through Autophagy
A well-studied cellular adaptation to fasting is autophagy. Autophagy is the body’s way of removing damaged cells, cellular debris, and waste products to make room for healthier, stronger cells.[15] It’s like the body’s programmable robotic vacuum that turns on at night when you are sleeping, moving from room to room, cleaning up the dust and mess created during your daily activities. We like to call it cellular hygiene.
The absence of nutrients during fasting and low insulin levels (also seen in ketogenic diets) drives autophagy. As seen in a review paper from the journal Cell Death and Differentiation, autophagy has been well-studied as a protective measure against oxidative damage.[16]
A review published in Frontiers in Cell and Developmental Biology also suggests that autophagy is a potential tool to fight back against the aging process.[17]
Inflammation and Immune Response
Acute inflammation is a normal part of the immune response, but chronic inflammation is associated with significant health concerns. Fasting may help downregulate inflammation by impacting the pro-inflammatory immune cells that communicate messages to turn on the inflammatory process.
Researchers from Mount Sinai discovered that intermittent fasting may reduce the number of circulating monocytes (inflammatory immune cells) in your blood.[18] Further, this study found that the monocytes found in blood from the IF group had less inflammatory activity than those found in fed subjects.
As researchers are interested in the association between monocytes and certain chronic health conditions, reductions through fasting could be a simple, non-invasive approach to drive down inflammation.[19]
Mitochondrial Health
Mitochondria are often called the powerhouses of the cell because they generate energy throughout the body. When nutrients are converted to energy, they also create reactive oxygen species (free radicals) as a normal byproduct.
Mitochondrial health can also be negatively affected by various lifestyle factors or simply as a regular part of metabolism. Mitochondrial dysfunction is associated with accelerations in aging and many chronic health concerns.[20]
Fasting can support mitochondrial function and cellular nutrition, cleaning up free radical byproducts through autophagy. Research found that fasting also increases NAD+,[21] which stimulates sirtuin activity.
Sirtuins are proteins associated with many healthy aging benefits, including the support of mitochondrial function and adaptations to stress.[22] They may also play a role in the regulation of autophagy.[23]
Downregulation of mTOR Activity
The mammalian target of rapamycin (mTOR) is a signaling pathway that helps with cellular growth and metabolism.[24] Fasting turns off mTOR activity, which is one form of autophagy.
Autophagy can allow for the growth of new, healthy cells. Once nutrients are reintroduced during the feeding window, mTOR can turn back on and regenerate new healthy cells, but only after the body has had a chance to clear out any that are damaged. One caveat about turning off mTOR through fasting — this can also disable muscle protein synthesis. For older adults (50+), deactivating mTOR can pose some potential risks if not monitored properly, as mTOR is important to building and maintaining muscle as you age.
Activation of Nrf2
Nrf2 is a protein that turns genes on or off, activated in response to oxidative stress to protect against free radical damage.
A review article found that while Nrf2 levels naturally decline with age, fasting activates Nrf2, contributing to the clean-up of reactive oxygen species in the body.[25]
It also plays a vital role in healthy detoxification and is associated with neuroprotective benefits through its antioxidant activity.[26]
Is Intermittent Fasting Safe?
Many people enjoy the benefits of intermittent fasting without side effects, but there are some groups that should avoid it:[27]
- Children and teens
- Pregnant or breastfeeding women
- Insulin-dependent type 1 diabetics
- Anyone who has suffered from an eating disorder
- Active women who are peri- or postmenopausal should check with their OB/GYN before implementing IF
While intermittent fasting can be safe and utilized long-term, side effects could include headaches, nausea, fatigue, or anxiety. A healthcare professional should be consulted before implementing IF.
While the previously discussed fasting schedules are regarded as viable and beneficial, fasting for extended periods may not necessarily result in greater benefit and may actually be physically dangerous.
Also, when first starting IF, there is an average ramp-up period of roughly two to four weeks, during which people can feel hungry or moody.[27] Once people make it past this hurdle, they typically feel the benefits of intermittent fasting and continue with it.
Common Challenges With Intermittent Fasting
Starting an intermittent fasting schedule can feel overwhelming at first, and most people face some common hurdles in their first few weeks. The good news is that these challenges typically fade as your body adjusts to its new eating pattern. Here's how to handle the most common obstacles you might encounter:
- Hunger and cravings: The first week or two of fasting often brings increased hunger, especially during your usual meal times. You can combat this by staying hydrated with water, black coffee, or tea. When you do eat, focus on protein-rich foods and fiber to help you feel fuller longer. Start with a less challenging schedule and work your way up to longer fasts.
- Energy dips and fatigue: Feeling tired is expected as your body adjusts to using fat for energy instead of constant glucose from food. Help manage your energy by scheduling your fasting window when you're naturally less active (like overnight). During your eating window, choose complex carbs and healthy fats instead of processed foods that can crash your energy.
- Social situations and dining out: One of the trickiest parts of intermittent fasting is navigating social events that don't align with your eating window. Consider being flexible with your schedule on special occasions. You can always return to your regular fasting pattern the next day. For regular social events, try scheduling them during your eating window when possible. If you're dining out, most restaurants won't mind if you just order a coffee or sparkling water while others eat. Being open with friends and family about your fasting schedule can also help them understand and support your choices.
Intermittent Fasting FAQs
Can you exercise while fasting?
Yes, you can work out during your fasting window, but you'll want to listen to your body, especially when you're first starting out. Without immediate fuel from food, you might notice your energy isn't quite what it usually is.
Many people prefer to schedule intense workouts right before their eating window or save their harder training sessions for feeding days. Light exercises like walking, yoga, or gentle strength training can work well during fasts. If you start feeling dizzy or unusually fatigued, that's your cue to either eat something or scale back your workout intensity.
Can you drink coffee or tea during fasting periods?
Black coffee and plain tea are perfectly fine during your fast. In fact, many people find these beverages helpful for managing hunger and maintaining focus. Just skip the cream, sugar, and other additives since these contain calories that could break your fast. Sparkling water is another good option if you're looking for something other than plain water.
While we're on the topic of beverages, it's worth noting that alcohol can affect your cells in ways that counter the cellular benefits of fasting, so it's best to avoid alcohol during both your fasting and eating windows if you're trying to get the most from your intermittent fasting practice.
Can you take supplements when fasting?
This depends on the supplement. Some vitamins and minerals are fat-soluble, so they need to be taken with food containing fat to be properly absorbed. Others, like Tru Niagen NAD+ boosting supplements, can be taken with or without food, making them convenient to take during fasting periods.
What is an NAD+ supplement? NAD+ is a molecule that supports your cells’ health and ability to produce energy and activate repair processes. Some people combine NAD+ supplements with intermittent fasting since both support cellular health.
When in doubt, check with your healthcare provider about the timing of your specific supplements. If a supplement tends to upset your stomach, it's usually better to take it with food during your eating window.
Intermittent Fasting Is a Flexible Wellness Tool
Intermittent fasting may be a way to support your health through the cellular adaptations that happen when the body goes without nutrients. A conversation with your healthcare practitioner is always a good idea before starting, but intermittent fasting can provide a flexible framework to benefit from these changes without longer-term caloric deprivation. It’s important, also, to continue to meet the appropriate caloric intake requirements for your body during your eating window if you do practice IF. While everyone responds differently, and some will benefit from shorter or less frequent fasts, intermittent fasting can be a simple, effective tool to add to your wellness practice.
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