Intermittent vs 24-Hour vs 72-Hour Fasting: A Fitness-Focused Guide

Fasting has become popular among fitness enthusiasts, from casual gym-goers to bodybuilders. Many turn to fasting protocols – skipping meals each day or doing multi-day fasts – to help with fat loss, metabolic health, and mental clarity. In this article, we’ll break down three common types of fasting – intermittent fasting, 24-hour fasting, and 72-hour fasting – explaining what they are, why people use them for fitness goals, their potential benefits (backed by recent studies), and important safety considerations. We’ll also include a comparison table and visual aids to help you quickly grasp the differences.

What is Intermittent Fasting?

Intermittent fasting (IF) refers to an eating pattern that alternates between periods of not eating (fasting) and eating. Unlike a specific diet that tells you what to eat, IF focuses on when you eat. Common intermittent fasting schedules include:

🫵    Time-Restricted Feeding (e.g. 16/8): Fasting for ~16 hours each day and eating only during an ~8-hour window (for example, eating only from 12pm to 8pm, and fasting overnight until noon the next day).

🫵      5:2 Diet: Eating normally 5 days of the week and consuming very low calories (about 500 calories) on 2 non-consecutive “fast” days.

🫵     Alternate-Day Fasting: Every other day is a fast day (either complete fasting or ~500 calories), while on the “feeding” days, you eat normally.

🫵    OMAD (One Meal A Day): A form of IF where you fast ~23 hours and eat one large meal in a 1–2 hour window daily.

How it works: In a daily IF regimen like 16/8, your body has ~12–16 hours without new calories, enough time to burn through much of your stored blood sugar (glycogen) and start tapping into fat stores for energy. However, the fasting period is not so long that it severely stresses your system – it’s short enough to do every day. During the fasting hours, insulin levels drop, and fat-burning hormones like norepinephrine may increase, helping mobilise fat for fuel. Many people also report a suppressed appetite over time as the body adapts to the schedule, making it easier to maintain a caloric deficit for weight loss. 

Why fitness enthusiasts use IF: Intermittent fasting is popular in the fitness world largely for its convenience and fat-loss potential. By skipping breakfast or snacking less often, it can be easier to reduce overall calorie intake without strictly “dieting.” Weight loss is the most common reason people try fasting. Additionally, some find that working out in a fasted state (e.g. morning cardio before eating) may enhance fat burn, although total daily nutrition and training are still key for performance. IF is also thought to improve metabolic flexibility – your body’s ability to switch between burning carbs and fat, which is useful for endurance and overall metabolic health.

BENEFITS OF INTERMITTENT FASTING

·       Effective Weight Loss Strategy: Intermittent fasting can be as effective as standard calorie-restriction diets for losing weight and body fat. For example, in one 12-month clinical trial, an 8-hour time-restricted eating plan produced similar weight loss to regular calorie cutting – about 8 kg on average. The take-home message is that IF helps people eat fewer calories, but it’s the calorie reduction itself that primarily drives weight loss. Some studies even suggest alternate-day fasting may produce slightly more weight loss than constant dieting, likely because some people find it easier to adhere to.

·       Fat Loss with Muscle Retention: Anecdotally, many weightlifters use IF (notably the 16/8 Lean gains method) to cut body fat while keeping muscle. Fasting naturally increases human growth hormone (HGH) levels, which can help preserve lean muscle mass during calorie deficits. Research backs this hormonal effect: one study found that a 24-hour fast increased HGH levels five-fold (500% increase) in participants. Higher HGH during fasting may support muscle maintenance and recovery. (Note: Adequate protein intake and resistance training are still critical for preserving muscle.)

·       Improved Insulin Sensitivity and Blood Sugar Control: Going without food gives your pancreas and insulin receptors a break. Intermittent fasting tends to lower fasting insulin levels and improve insulin sensitivity, meaning your cells become more efficient at absorbing and using blood sugar. In a study, people who practised routine fasting had higher insulin sensitivity and more stable blood sugar levels than non-fasters, even when eating the same total calories. Better insulin sensitivity can translate to steadier energy levels and less fat storage over time. Some individuals also report fewer sugar cravings after adapting to IF.

·       Cardiometabolic Health Markers: Besides weight loss, IF has shown benefits for other health metrics. Studies have noted improvements in cholesterol and blood pressure with certain fasting routines. For instance, intermittent fasting can reduce triglyceride levels (blood fats) and increase HDL (“good”) cholesterol in some cases. Blood pressure reductions are common, especially in individuals with hypertension – one review found that fasting interventions significantly lowered blood pressure in people with high BP. These changes support heart health and are valuable for fitness enthusiasts aiming for overall health, not just aesthetics.

·       Reduced Inflammation: Short fasting periods may lower inflammation in the body. Research suggests that IF can decrease markers like CRP (C-reactive protein) in some individuals, indicating a reduction in systemic inflammation. Lower inflammation can aid recovery from workouts and improve general well-being.

·       Convenience and Mental Benefits: Many appreciate the simplicity of eating fewer meals. Meal prep and planning become easier when you only eat 2 meals a day instead of 5 or 6. Some also report enhanced mental clarity and focus during the fasting period, which they attribute to stable blood sugar and the presence of ketones (an alternative fuel for the brain). In fact, fasting has been linked to increased levels of brain-derived neurotrophic factor (BDNF), a protein that supports brain health and can improve mood and focus. This might explain the “sharp” feeling people get after fasting ~16+ hours. Many find that once they adapt, they feel alert, not sluggish, in the mornings, even without breakfast. (Of course, individual experiences vary.)

What is a 24-Hour Fast?

A 24-hour fast means abstaining from food for a full day. For example, you might finish dinner at 7pm on Monday and then not eat until 7pm Tuesday – essentially skipping breakfast and lunch the next day. Water, black coffee, or plain tea are typically allowed (and encouraged to stay hydrated), but no significant calories. This approach is also known as the “Eat Stop Eat” method, popularised by some fitness experts, or simply a one-day water fast. Some people do a 24-hour fast once or twice per week as a strategy to control weekly calorie intake and promote certain health benefits. Others might do it occasionally (e.g. after an indulgent holiday, as a “reset”).

What happens in your body: A 24-hour fast is long enough to push your body into a deeper fasting state than a typical overnight fast. By the 20–24 hour mark, most of your glycogen (stored glucose) is depleted, forcing your body to switch primarily to burning fat for energy. This metabolic transition is sometimes called the “metabolic switch” – you go from running on carbohydrates to running on fats and ketones. At this point, ketone bodies (byproducts of fat metabolism) begin to rise in your bloodstream as an alternative fuel, especially for your brain. Many people will enter a state of nutritional ketosis somewhere just after the 24-hour fast mark. Hormones respond as well: for instance, the hunger hormone (ghrelin) tends to peak and then drop, and as mentioned earlier, growth hormone (HGH) surges to help maintain muscle and break down fat.

Benefits of a 24-Hour Fast: Fasting for a full day confers many of the benefits of shorter intermittent fasts, but often to a higher degree. Some noted benefits include:

🫵      Pronounced Fat Burning: By 24 hours, your body is firmly in fat-burning mode. One study measuring blood markers found that circulating fatty acids increase during a 24-hour fast, indicating the body is pulling more fuel from stored fat. This can help break fat-loss plateaus. Anecdotally, people often notice a drop of a pound or two after a day fast (though some is water weight). More importantly, periodic day-long fasts can improve your body’s metabolic flexibility, making it more efficient at switching to fat as fuel when needed​.

🫵       Spike in Human Growth Hormone: As noted earlier, fasting causes HGH to rise. A 24-hour fast can substantially boost HGH levels – research has shown HGH can increase by 300–500% (4-5× baseline) after one day of fasting. Some reports (in individuals with low baseline HGH) even saw over 1000% increases. This hormone spike during fasting helps preserve muscle tissue and tells your body to use fat for fuel. For fitness enthusiasts, that means a well-managed 24-hour fast is unlikely to burn away your muscles – your hormones are actually working to protect muscle and provide energy from fat.

🫵      Autophagy and Cellular Cleanup: Going a bit longer without food triggers a process called autophagy (a cellular recycling and repair mechanism). Autophagy is like your cells taking out the trash – cleaning up old or damaged cell components. Research suggests that autophagy ramps up significantly somewhere around 24–30 hours of fasting. By fasting for a full day, you likely tap into this cellular cleansing phase more than a shorter fast would. This is thought to have benefits for ageing and cellular health, potentially helping with muscle recovery and reducing inflammation. (Most direct evidence is from animal studies, but it’s a compelling area of human research.) 

🫵       Improved Metabolic Health Markers: Even short-term 24-hour fasts can improve certain health markers. In one study, 30 healthy adults did a single 24-hour water fast and saw significantly lower triglyceride levels (blood fats linked to heart disease) afterwards. Other studies have found that 24-hour fasting can reduce insulin levels and insulin resistance (making your body more insulin-sensitive) independent of weight loss. There is also evidence of reduced oxidative stress and inflammation with intermittent 24-hour fasts.

🫵       Gut Rest and Possible Microbiome Benefits: A day without food gives your digestive system a chance to rest. Some researchers speculate this can lead to a healthier gut lining and even beneficial shifts in gut bacteria. While more research is needed, longer fasts have been associated with changes in the gut microbiome and reduced gut inflammation.

🫵       Mental and Disciplinary Benefits: Many people report that completing a 24-hour fast gives them a sense of accomplishment and a “reset” of their relationship with food. It can recalibrate your hunger signals – you realise you won’t die if you skip lunch, and that real hunger comes and goes in waves. This can lead to better appetite control on normal days. Some also experience heightened mental clarity during the fast (often noticing it on the second morning of not eating), possibly due to elevated ketones and adrenaline. It’s not uncommon for fasters to feel a bit euphoric or deeply focused by the end of a 24-hour fast, once the initial hunger passes.

Despite these benefits, a 24-hour fast is still relatively short-term and generally safe for healthy individuals. In fact, researchers recently concluded that a 24-hour water-only fast is “not an onerously long period” and can trigger meaningful metabolic changes (like HGH release) without major downsides. Most people find the first half of the day easy (if you’re used to skipping breakfast), and the real challenge is the usual lunch or snack time, where you have to push through hunger. Staying busy and hydrated helps. By dinner time, many report that the hunger feeling subsides again due to hormonal adjustments.

What is a 72-Hour Fast (3-Day Fast)?

A 72-hour fast is an extended fast lasting roughly three full days (typically 72 consecutive hours without caloric intake, only water and non-caloric fluids). This is a form of prolonged fasting that goes well beyond the daily intermittent fast. People sometimes undertake 48- to 72-hour fasts for deeper health benefits such as cellular regeneration or as a “metabolic reset.” It’s much less frequent – one might do a 3-day fast only a few times a year, or even just once, as it is quite demanding. Because of the duration, 72-hour fasts should be approached with preparation and caution (more on safety later). Some individuals follow guided programs or do it under medical supervision, especially if they have any health conditions.

What happens during 72 hours of fasting: An extended fast puts your body through several distinct stages:

🫵       Hour 0–12: You’re in the normal fed state initially, then the post-absorptive stage once you finish digesting your last meal. Blood glucose is used for energy, and any excess is stored as glycogen. No significant changes yet aside from slightly dropping blood sugar and insulin.

🫵       Hour 12–24: Glycogen stores in the liver start depleting. By ~18–24 hours, the liver has released most of its stored glucose, and your insulin levels drop to low baseline levels. The body begins the metabolic switch to fat burning. Fatty acids are released from fat tissue, and the liver converts fats into ketone bodies for fuel. Many people reach mild ketosis by the end of Day 1. You might experience some hunger pangs, irritability, or headache as this transition happens – common side effects as your body adapts.

🫵       Hour 24–48 (Day 2): Ketones in your blood continue rising sharply as fat breakdown accelerates. By the second day, ketosis is in full swing – your body is now getting the majority of its energy from stored fat. You may even notice the telltale signs of ketosis (like a fruity smell on your breath). Autophagy mechanisms are well activated at this point, helping to clean up cellular debris. Many people report that hunger diminishes on Day 2; energy and mental clarity might improve compared to Day 1, thanks to a steady fuel supply from ketones. However, you might feel physically weaker if you try strenuous activity, since glycogen is low. Hydration and electrolytes become very important now to prevent dizziness (the body is shedding a lot of water and salt as it empties carbohydrate stores).

🫵      Hour 48–72 (Day 3): By the third day, your body enters a “survival mode” equilibrium. Fat is now the exclusive fuel – your metabolism has fully shifted to burning fat and ketones for all energy needs. Ketone levels typically peak around this time. You might be producing 3–5 mmol/L of ketones in your blood (versus <0.5 mmol/L when not fasting). To put it simply, your body is running on fat power. Interestingly, your body may intentionally induce a form of temporary insulin resistance in your muscles at this stage – this means muscles take up less glucose, sparing blood sugar for the brain, while muscles happily oxidise fatty acids and ketones. This is a protective mechanism during starvation (not the harmful insulin resistance associated with type 2 diabetes). Autophagy likely reaches high gear, removing damaged cells and possibly stimulating regeneration. Some fasters on Day 3 describe a state of profound mental clarity or even mild euphoria, as ketones and certain neurotransmitters (like BDNF) elevate. Of course, others simply feel low-energy and ready to eat – experiences vary. By 72 hours, you have achieved a deep fasting state that triggers a host of physiological changes not seen in shorter fasts.

Blood ketone levels typically remain very low during short fasts (e.g. 12–16 hours) but rise significantly during prolonged fasting, reflecting the body’s shift to fat-burning mode. After about 24 hours, ketosis begins in earnest, and by 48–72 hours, ketone levels can climb several-fold. (Ketone values are approximate and vary by individual.)

Benefits of a 72-Hour Fast: Extended fasts are undertaken for more therapeutic or advanced benefits beyond what shorter fasting can do. Some key potential benefits include:

🫵      Maximised Autophagy and Cellular Repair: While even 16-24 hour fasts may induce some autophagy, a 72-hour fast really amplifies this process. By two to three days, your body is breaking down old immune cells, misfolded proteins, and precancerous cells at an accelerated rate. Think of it as a deep cleanse at the cellular level. Pioneering research at USC found that fasting for 2–3 days can trigger stem cells to regenerate new immune cells, essentially “rebooting” the immune system. In mice, repeated 2-3 day fasts cleared out old, damaged immune cells and grew new ones, and a small human trial suggested similar immune-regenerative signs. This suggests a 72-hour fast could have benefits for immune health, disease resistance, and longevity that shorter fasts might not achieve.

🫵       Profound Insulin Sensitivity and Blood Sugar Reset: By the third day of fasting, insulin is at rock-bottom levels, and your insulin receptors get a chance to “reset.” Some research noted that in lean individuals, a 72-hour fast led to a beneficial reduction in insulin signalling in muscle (to spare glucose), accompanied by increased fat oxidation. Once you refeed, your insulin sensitivity is often higher than before. This improved insulin function can be great for metabolically “resetting” if one has been on a high-carb diet or struggling with pre-diabetic blood sugar levels. (However, caution: the first carb-rich meal after a long fast can cause a big insulin spike – so refeeding should be done gently, with balanced macros.)

🫵       Potential Weight Loss and Metabolic Gains: A 3-day water fast will certainly cause weight loss, though much of it is water and glycogen. Still, one can lose a few pounds of fat over the period. More importantly, some evidence shows fasting can reduce abdominal fat and improve metabolic syndrome markers beyond what standard calorie cutting does. Longer fasts may also lower blood pressure and inflammation significantly in the short term​ – for example, medically supervised fasts of a week or more have shown drastic blood pressure improvements. A 3-day fast might not be as potent as a 1-2 week therapeutic fast, but it tends to move these markers in the right direction, especially if someone has high blood pressure or high blood sugar.

🫵      Mental and Emotional Resilience: Completing a 72-hour fast is no small feat. Many who do it report a powerful psychological benefit, proving to yourself that you have the discipline and ability to do something very challenging. It can break psychological dependency on constant snacking or junk food. People often describe their sense of taste resetting so that healthy foods taste better afterwards. There are also reports of mental clarity and even spiritual benefits (extended fasting has been used for millennia in religious contexts). From a fitness perspective, this resilience can carry over into stronger willpower in training and nutrition after the fast.

🫵       Caution – Not a Regular “Diet” Tool: It’s worth noting that while a 72-hour fast has these potential benefits, it’s not something you do frequently for weight management. Extended fasts are more of an occasional intervention. Doing them too often (without medical need) could lead to muscle loss or nutritional deficiencies. Think of it as a once-in-a-while deep reboot for the body. For daily/weekly fat loss, intermittent fasting or 24-hour fasts are more practical. Some longevity experts suggest doing a 3+ day fast a few times a year to potentially lower long-term disease risk – but again, this should be individualised.

Exercise and 72-hour fasts: Most people do not attempt heavy workouts during a 3-day water fast – energy levels and strength can be quite low, especially by Day 2 and 3. Light activity like walking, stretching, or gentle yoga is usually fine and can even help you feel better. But intense weightlifting or endurance training on Day 3 of no food is not advised (you simply won’t have the fuel or recovery capacity, and you risk dizziness). If you plan to incorporate an extended fast, consider doing it during a de-load week or a break from intense training. After the fast, ease back into normal eating and training; your first workout after refeeding will feel amazing for some, but make sure you’re replenished.

Why Do People Fast for Fitness Goals?

We’ve touched on some reasons already, but let’s summarise why fitness-minded individuals incorporate fasting protocols:

🫵       Fat Loss and Body Composition: The primary motivation is usually to burn fat and lose weight. Fasting creates a calorie deficit and encourages the body to use stored fat for energy, which helps decrease body fat percentage. Many find it easier to manage their calorie intake with fasting (skipping meals) than with constant calorie counting. Moreover, fasting may target belly fat and improve metabolic health, which is appealing for getting lean.

🫵      Maintain Muscle while Dieting: When done correctly, fasting with adequate protein intake in eating windows can help retain lean muscle mass during cutting phases. The spike in growth hormone during fasting and overall hormonal environment can be muscle-sparing, as long as you’re not fasting for so long that your body starts breaking down muscle significantly (short fasts and occasional 24-48h fasts are generally fine in this regard). This makes fasting an attractive strategy for bodybuilders and athletes who want to drop fat but keep muscle.

🫵       Metabolic Health and Performance: Improving insulin sensitivity and blood sugar control through fasting can benefit athletes by providing more stable energy and reducing the risk of crashes during workouts. Some endurance athletes train in a fasted state to enhance fat utilisation, which can improve how the body performs in long-duration events (training low, competing high strategy). There’s also interest in fasting’s effect on mitochondrial health and reducing oxidative stress, potentially aiding recovery and stamina in the long run.

🫵       Convenience and Lifestyle Fit: Fitness enthusiasts with busy schedules appreciate that IF means fewer meals to prep and more time free in the morning or late night. It can simplify life – for instance, you might train at lunchtime and break your fast after, which consolidates you’re eating around workouts. Fasting can also help curb late-night snacking habits, aiding adherence to a nutrition plan.

🫵       Discipline and Mental Toughness: Pushing through the discomfort of hunger in a fast can build mental toughness. This discipline can translate to sticking to tough training regimens. The psychological aspect of “food control” is huge – once you know you can willingly delay gratification (food), sticking to a healthy diet or tough workout might feel easier.

🫵      Health and Longevity Goals: Many fitness enthusiasts are also interested in long-term health. Fasting’s potential benefits for cellular repair (autophagy), inflammation reduction, and even longevity (as seen in animal studies) are attractive. Some view occasional extended fasts as a way to “clean house” in the body, potentially preventing diseases and improving biomarkers of health that matter for an active life. For example, intermittent fasting has been linked to lower risks of chronic diseases like diabetes and heart disease in some studies.

In short, people fast not just to “get lean”, but to feel healthier and more in control of their bodies. That said, it’s not a magic solution – it works best when combined with a nutritious diet and exercise. Fasting is simply one tool in the fitness toolbox.

Who Should Avoid Fasting or Be Cautious?

Fasting isn’t appropriate or safe for everyone. Experts advise that certain individuals should avoid fasting (or only do so under medical supervision):

🫵       Pregnant or Breastfeeding Women: Pregnancy and lactation greatly increase nutritional needs. Fasting during these times can be dangerous for both mother and baby and is not recommended.

🫵       Children and Teenagers: Kids and teens are still growing and typically have high nutritional requirements. They should not do restrictive fasting diets. (Note: Religious fasting in teens, such as Ramadan, is an exception under guidance; but for fitness/weight loss purposes, minors should not fast.)

🫵       Individuals with Eating Disorders (or History of Disordered Eating): Anyone with anorexia, bulimia, binge eating disorder, or a history of such conditions should avoid fasting protocols. Fasting could trigger harmful behaviours or relapses. It’s important to pursue a balanced approach to eating in these cases, with professional guidance.

🫵       Underweight or Malnourished Individuals: If someone is severely underweight or has a wasting condition, fasting will deprive them of needed calories and nutrients. Likewise, those with nutrient deficiencies should focus on nourishing the body, not withholding food.

🫵       People with Certain Medical Conditions: This includes: 

• Diabetes: Especially type 1 diabetics or type 2 on medications like insulin, fasting can lead to dangerous hypoglycaemia. Some type 2 diabetics, under doctor supervision, have used fasting to improve insulin sensitivity, but it must be done with medical guidance. If you have diabetes or any blood sugar condition, do not fast without talking to your healthcare provider.

• Heart Conditions: Those with serious heart disease or low blood pressure should be cautious. Fasting can lower blood pressure and alter electrolyte balance, which might be risky in people with heart conditions or on blood pressure meds.

• Other Chronic Illnesses: Anyone with liver or kidney disease, cancer (unless as part of a supervised treatment plan), or any condition that requires consistent nutrition or medication timing should avoid unsupervised fasting.

• Individuals on Medications: Some meds must be taken with food or at regular intervals – fasting could interfere or cause side effects. Always check with a doctor.

🫵       If You’re Sick or Recovering: Fasting during an illness (e.g., severe infection) or when recovering from surgery is generally not advised, since your body needs nutrients to heal, and immune function could be compromised.

🫵      Those Who Cannot Stay Hydrated: If someone has a job or lifestyle where they cannot drink water for long periods, fasting would risk dehydration. Water intake is crucial during fasts.

For healthy individuals, short-term fasting is generally considered safe​. But “safe” assumes you’re doing it wisely: staying hydrated, not pushing beyond your limits, and breaking the fast properly. It’s always a good idea to consult with a healthcare professional before starting a new fasting regimen, especially if you plan to do extended fasts or have any health concerns.

Comparing Intermittent, 24-Hour, and 72-Hour Fasts

The table below highlights the key differences between these fasting approaches, so you can see which might suit you best:

Aspect	Intermittent Fasting (e.g. 16/8 daily)	24-Hour Fast (One Meal a Day or “Eat Stop Eat”)	72-Hour Fast (3-Day Extended Fast)
Fasting Duration	~14–20 hours fasting, every day (varies by protocol). Eating window each day (4–10 hours).	24 hours of continuous fasting (water only). Typically done 1–2 times per week, not daily.	72 hours (3 full days) of continuous fasting (water only). Typically done rarely (e.g. a few times per year).
Feeding Pattern	Daily pattern: You still eat every day, just restrict timing. Examples: Skip breakfast daily, or alternate day (fast one day, normal the next).	Often practised from dinner to dinner (skip breakfast & lunch). You eat normally on non-fasting days.	No food for 3 days straight. Requires preparation (easing in and out). Normal eating on all other days.
Primary Goals	Steady fat loss, convenience, and improved daily insulin sensitivity. Great for weight management as part of a lifestyle.	Enhanced fat loss and metabolic reset for the week. Good for breaking weight plateaus and stimulating autophagy more than daily IF.	Deep health benefits (cellular repair, possible immune reset, major insulin reset). Sometimes for rapid short-term fat loss, but mainly for longevity or metabolic health experimentation.
Physiological Impact	Does not fully deplete glycogen stores in one cycle. Partial metabolic switch – limited ketosis (in longer daily fasts) before refeeding. Hormones like HGH rise modestly. Manageable hunger for most after adapting.	Glycogen was largely depleted by the end of 24h. Enters ketosis by around the 24-hour mark. Notable spike in fat-burning hormones (e.g. HGH can surge 4–5× baseline). Moderate autophagy kicks in. Hunger comes in waves; cit an be challenging but achievable.	Glycogen is fully depleted; the body runs on fat/ketones almost exclusively by day 3​. Deep ketosis (blood ketones are highly elevated). High autophagy and cellular stress resistance. Metabolic rate may start to slow slightly to conserve energy. Significant hormonal shifts (e.g. thyroid may dip, cortisol can rise transiently).
Notable Benefits	- Sustainable long-term (can be done indefinitely). - Effective for fat loss and improving body composition when combined with exercise. - Improves insulin sensitivity and cholesterol/BP in many cases. - Easy to fit into daily routine (skip one meal). - Mild ketosis in longer fasts may give mental clarity.	- Strong fat-burning on fasting day (helps create a bigger weekly calorie deficit). - Significant increase in growth hormone promotes fat breakdown and muscle preservation. - Initiates autophagy (cell cleanup) beyond what daily IF does. - Can improve blood sugar, lower triglycerides, and reduce inflammation with repeated practice. - Gives the digestive system a rest.	- Maximal autophagy and cellular repair may clear out old immune cells and spur production of new ones. - Profound reset of insulin and leptin levels (greatly improved insulin sensitivity post-fast)​. - Possible large reductions in chronic inflammation and blood pressure (short-term)​. - Mental “reset” and potential clarity/euphoria from sustained ketosis. - Potential longevity benefits (mimics evolutionary fasting periods, activates stress-resistant pathways).
Main Drawbacks/Risks	- Hunger and irritability at first adaptation (first 1–2 weeks). - May not suit individuals with very high caloric needs (some athletes need more frequent meals to meet intake). - If the eating window is too short, the risk is not getting enough nutrients or protein. - Some (especially some women) may experience hormonal disruptions if fasting is too extreme or coupled with heavy training.	- Can be challenging on fasting day (low energy, headache, mood swings in some). - Not suitable for daily use – needs to be limited to once or twice a week max. - Risk of dehydration if you forget to drink fluids on a fasting day. - Some people may overeat or choose unhealthy foods when they finally break the fast (“rebound binging”), which can negate benefits.	- Difficult: significant fatigue, and difficulty concentrating or exercising by Day 2–3 for many. - Must be done carefully: risk of electrolyte imbalances, dehydration, or drops in blood pressure (possible dizziness/fainting). - Not eating for 3 days can be dangerous if you have any underlying health issues (should ideally have medical supervision). - Can lead to muscle loss if prolonged too far or done too often. - Refeeding after 72h needs to be gradual to avoid gastrointestinal upset or electrolyte swings (refeeding syndrome in extreme cases).
Who It’s Best For	Those who want a flexible, sustainable approach to lose fat or maintain weight. Great for people who can’t or don’t want to count calories but can stick to a schedule. Works for many healthy individuals and recreational athletes.	Individuals who need an extra “push” for fat loss or want the health boosts of a longer fast but without going multiple days. Often used by people who plateau with daily IF or want the discipline challenge. You should be generally healthy and experienced with shorter fasts first.

Table: A comparison of intermittent fasting, 24-hour fasting, and 72-hour fasting protocols, highlighting their durations, effects, benefits, and considerations

Key Takeaways

🫵       Intermittent Fasting (e.g. 16/8 daily) involves short daily fasts and is a popular, sustainable method for fat loss and metabolic health. It’s convenient for many and can improve weight control, insulin sensitivity, and cholesterol/BP over time. It’s generally safe for healthy adults as a long-term lifestyle, though consistency and proper nutrition in the eating window are crucial.

🫵       24-Hour Fasts are a more intense version done about once or twice a week. They push your body into a deeper fasting state (ketosis) that can amplify fat burning and trigger beneficial hormonal and cellular changes (like big HGH release and some autophagy)​. This can accelerate weight loss and “reset” your system, but doing it too often isn’t necessary. Ensure you stay hydrated and break a 24h fast sensibly to avoid overeating.

🫵       72-Hour Fasts (3-Day fasts) take fasting to an advanced level. By 3 days, you’re in full ketosis and autophagy, which may offer unique benefits such as immune system regeneration and profound insulin reset. However, this is not for everyone – it’s physically and mentally challenging and carries more risks if done incorrectly. It should be done sparingly and with preparation. Most fitness goals can be met with shorter fasts; 72-hour fasts are more about therapeutic or longevity goals than routine fat loss.

🫵       Why Fast for Fitness? In all its forms, fasting can be a useful tool for cutting body fat while preserving muscle (thanks to hormonal adaptations) and improving metabolic health. It aligns with many fitness enthusiasts’ goals: a leaner physique, better insulin sensitivity, improved energy control, and potentially healthier ageing. Plus, many appreciate the simplicity – fewer meals to plan – and the mental edge it can give.

🫵       Listen to Your Body & Stay Safe: Fasting can have great benefits, but it’s not a one-size-fits-all. Pay attention to how you feel. It’s normal to feel hungry and a bit low-energy when starting out or during longer fasts, but you should not feel terrible – dizziness, severe weakness, or mental confusion are signs to stop and refeed. Always stay well-hydrated, and consider supplementing electrolytes (sodium, potassium, magnesium), especially for fasts beyond 24 hours to help you feel stable. If you have any medical conditions or uncertainties, consult with a healthcare professional before fasting. Do not fast if you fall into high-risk groups like pregnant/breastfeeding, history of eating disorder, etc., as outlined earlier.

🫵       Combine Fasting with Smart Nutrition and Training: Fasting by itself isn’t a magic bullet – it works best alongside a healthy diet and exercise. When you do eat, focus on nutrient-dense foods: plenty of protein (critical for muscle), vegetables, healthy fats, and quality carbs as needed for your activity level. This ensures you reap the benefits of fasting without nutrient shortfalls. Align your fasting schedule with your workouts in a way that you still have energy to train and protein to recover (for example, many do well eating right after a workout). Remember, consistency is key – the best fasting protocol is one you can stick with long-term while still enjoying life and making progress toward your fitness goals.

Fasting, in its various forms, can be a powerful ally for fitness enthusiasts when used wisely. It taps into our body’s ancient adaptation to periods of feast and famine, with modern science now confirming many of the health benefits. Whether you choose to adopt a daily eating window or occasionally challenge yourself with a full-day (or even multi-day) fast, always prioritise listening to your body. When done correctly, fasting can help you not only look leaner but also feel healthier and more energised – all without requiring fancy diets or supplements. As always, balance and moderation are important: fasting is just one strategy among many on the road to fitness and well-being.