Can a 36-hour fast help with weight loss? Here’s what research shows

Fasting is one of those wellness trends that never really goes away. One month it’s 16:8. The next it’s OMAD (one meal a day), 24-hour fasts, or longer fasts like 72 hours. A 36-hour fast sits somewhere in the middle, which can make it confusing: Is it basically a 24-hour fast? Is it closer to a 48-hour fast? And does that extra half-day actually matter?

A 36-hour fast usually means finishing dinner one evening, skipping food the next full day, and eating again the following morning. In other words, it’s a full day without food, plus the overnight fasting hours on both sides. That makes it more intense than common intermittent fasting approaches like 16:8 or the 5:2 diet, but not as long as a true multi-day fast.

Does it make it more effective, less effective? First of all, no one needs to fast for 36 hours (or any amount of time) to be healthy or lose weight. There are plenty of ways to improve blood sugar, support fat loss, and build better habits without skipping food for a day and a half. 

But 36-hour fasting does have some research behind it because it overlaps with alternate-day fasting, or ADF, a better-studied form of intermittent fasting. Some alternate-day fasting protocols use a full fasting day followed by a feeding day, which can create fasting windows of roughly 36 hours depending on meal timing.

In this article, you’ll learn what a 36-hour fast is, why people try it, and how it compares to other fasting approaches. To get into the science, we’ll also explore what happens in your body during the fast, the evidence-based benefits, and what risks to consider before trying it. 

What is a 36-hour fast?

A 36-hour fast means going without food and, usually, without calorie-containing drinks for 36 consecutive hours. Most people stick with water, black coffee, or unsweetened tea.

The number 36 didn’t come out of nowhere; it’s connected to alternate-day fasting (ADF). ADF means alternating your fasting days with your eating days. A 36-hour fast is a modified ADF schedule because it includes the overnight fasting hours before and after a full fasting day. So, the fasting window stretches to about 36 hours.

In practice, the timing might look like this: 

• Sunday: You finish dinner at 7 p.m.
• Monday: Skip all meals. 
• Tuesday: Eat at 7 a.m. 

Because the fast includes Sunday night, all of Monday, and Monday night, it stretches well beyond the 24-hour mark.

That timing matters because some of the metabolic changes people associate with fasting—like using more fat for energy and the energy switch to ketones—become more likely with longer periods. 

Learn more: 11 things that don’t break a fast (and 4 that do)

Why do people try 36-hour fasts?

Many people are talking about fasting in general—whether it’s for weight loss, better blood sugar control, or simply creating more structure around eating.

Part of the appeal is that fasting changes how your body uses energy. When you go long enough without eating, insulin levels drop, and your body starts shifting away from using glucose, or sugar, as its main fuel. Instead, it starts relying more on stored energy. Researchers often describe this shift as the “metabolic switch”.

A 36-hour fast builds on that idea. It’s long enough that your body moves beyond a typical overnight fast and further into fat-based metabolism. During this time, your body produces more ketones, which are compounds made from fat that can be used for energy. The thinking is that spending more time in this fat-burning, higher-ketone state might support weight loss and metabolic health. 

There’s also a practical reason people consider this specific timeframe. If you time it around sleep, a 36-hour fast can feel like “just” one full day without eating. For example, you might finish dinner Sunday, skip meals Monday, and eat breakfast Tuesday. Two chunks of that time happen overnight, which can make it feel more manageable.

Learn more: Can a 48-hour fast help me lose weight and live longer?

What happens during a 36-hour fast?

During a longer fast, your body shifts fuel sources: insulin drops, fat burning increases, and ketones rise as glycogen gets lower. Here are the steps your body goes through for this process to happen.

1. Your body uses stored carbohydrates

When you eat carbohydrates, your body stores some of that energy as glycogen in your liver and muscles. During the first 12 to 24 hours of a fast, your body relies heavily on those glycogen stores.

How long they last varies from person to person. It depends on things like what you ate beforehand, your activity level, body size, and metabolism.

As glycogen, your stored carbohydrate, gets lower, the body starts relying more on fat for energy and producing more ketones. Ketones are compounds your body makes from fat when carbohydrate availability is low. They can be used as fuel, especially by the brain, and they also act as signaling molecules in the body.

This is the shift people often mean when they talk about fasting to change your metabolism. 

2. Insulin drops and fat burn increases

As glycogen stores decline, insulin starts to fall, and your body begins increasing fat breakdown. This is often the hardest stretch because hunger, headaches, irritability, and low energy can show up. This broader transition is often called the “metabolic switch”, when the body begins moving from glucose-based energy toward fat- and ketone-based energy.

3. Ketones rise

Between 24 and 36 hours, the shift becomes more likely to happen because the timeline is longer than other fasts. In one controlled trial, a 36-hour fast increased fat oxidation by about 44% compared with a 12-hour fast. Beta-hydroxybutyrate, or BHB, a key ketone, also rises significantly, a sign that the body has moved further into a fat-burning metabolism at this time period. 

Benefits of a 36-hour fast: What the science shows

A 36-hour fast can create real changes in the body, but the benefits are often more limited—and more temporary—than fasting headlines make them sound. Most of the strongest evidence comes from ADF-style research, where people repeat fasting cycles over time, not from one isolated 36-hour fast. Here’s a look at how 36-hour fasting can affect weight loss and overall health.

Can a 36-hour fast help you lose weight?

A 36-hour fast can lead to short-term weight loss. That’s partly because you’re eating less, and partly because your body starts using stored energy during the fast.

In the first day or so, some of the drop on the scale comes from glycogen and water. Glycogen holds water in the body, so when glycogen is used up, water weight drops too. That can make the scale move quickly, but it doesn’t mean all of the weight lost is fat.

With repeated fasting, fat loss can happen. In a 22-day alternate-day fasting study, participants lost about 2.5% of body weight and 4% of fat mass. The study also found increased fat burning.

That’s meaningful, but it needs context. Weight loss from fasting depends on what happens after the fast, too. If eating patterns aren’t changed in a sustainable way, weight often returns.

There is also an important muscle-related caveat. Fasting can increase fat use, but your body doesn’t only lose fat during extended or repeated fasting. A review of prolonged fasting studies found that both fat mass and lean mass can decline during extended fasting, with the amount depending on the duration of the fast and how people eat afterward.

That doesn’t mean one 36-hour fast will cause major muscle loss. But if longer fasts become frequent and eating days are low in protein, low in calories, or not paired with resistance training, lean mass can start to decline, too. For long-term health, the goal is usually not just to lose weight. It’s to lose fat while keeping as much muscle as possible.

Does a 36-hour fast improve blood sugar and insulin?

Insulin is the hormone that helps move sugar from your blood into your cells. When you are not eating, your body has less incoming sugar to manage, so insulin levels tend to drop.

One study that directly tested a 36-hour fast did so in adults with type 1 diabetes. Under monitored conditions, researchers found higher fat burn and higher ketones after 36 hours compared with 12 hours. The study also reported a low risk of serious blood sugar complications in that supervised setting. 

It’s important to note that this was a monitored study in carefully selected participants. Fasting, especially for people with diabetes, can change blood sugar and medication needs, which can become risky without medical guidance.

In the 22-day ADF study, fasting insulin dropped substantially. But that result reflects repeated fasting cycles, not one isolated 36-hour fast.

A single 36-hour fast can temporarily lower insulin and raise ketones, but longer-term changes depend on whether the overall pattern is maintained—and whether it supports healthy eating, activity, sleep, and body composition.

Does a 36-hour fast reduce inflammation?

Inflammation is part of your immune system. In the short term, it helps your body respond to injury, infection, or stress. That kind of acute inflammation is normal and useful. Chronic inflammation is different. When inflammation stays elevated over time, it can be linked with conditions like heart disease, type 2 diabetes, and obesity-related metabolic issues. That’s why people are interested in whether fasting can reduce chronic inflammation. 

Some research suggests intermittent fasting may improve certain inflammatory markers but not others. One review found that ADF may reduce one inflammation marker when weight loss is greater than 6%, but ADF did not meaningfully lower other inflammation markers. 

Longer fasts may also negatively affect inflammation.  A scoping review of prolonged fasting found that fasts of 48 hours or more are associated with increased inflammatory markers. That’s likely a temporary stress response, indicating that fasting can put serious strain on the body.

For a 36-hour fast specifically, the evidence is limited. It is reasonable to say fasting may influence inflammation over time, especially when it contributes to weight loss or improved metabolic health. But a single 36-hour fast is unlikely to produce meaningful or lasting anti-inflammatory effects on its own.

Does a 36-hour fast put you into ketosis and trigger autophagy?

Most people have heard of ketosis in the context of low-carb or keto diets. Ketosis simply means your body is making ketones from fat and using them as an energy source. This can happen when carbohydrate availability is low and during fasting.

By 36 hours, people may reach ketosis. Ketosis is the metabolic state where the body relies on ketones for energy instead of glucose A case report found that ketones quadrupled after a 36-hour fast compared with an overnight fast.

This is where autophagy often enters the conversation. Autophagy is the body’s process of breaking down and recycling damaged cellular components. Fasting proponents are often “chasing” autophagy because, in theory, better cellular cleanup could support healthy aging and metabolic health.

The reality is more cautious. Animal research suggests that autophagy may increase somewhere between 24 and 48 hours of fasting, which puts a 36-hour fast in a plausible window. But human autophagy is difficult to measure directly in relevant tissues, and the 36-hour studies above did not directly prove autophagy benefits. So it is fair to say a 36-hour fast may begin activating related pathways, but not fair to market it as a proven cellular “reset.”

The downsides of a 36-hour fast

A 36-hour fast can produce measurable metabolic changes, but it also has downsides that are easy to overlook if you focus only on its potential benefits.

Hunger

One of the biggest challenges is hunger. In the 22-day alternate-day fasting study, participants lost weight and fat mass, but hunger on fasting days did not decrease over time. That’s an important counterpoint to the common claim that fasting quickly becomes effortless.

A longer-term case report tells a more hopeful but much more limited story. In one person practicing 36-hour fasting twice weekly for 82 weeks, hunger decreased over time, and weight loss was maintained. That suggests some people may adapt. But it is still one case report, not a guarantee.

In real life, that means your experience matters. If fasting leaves you distracted, overly hungry, or thinking about food all day, that’s useful feedback. It may not be the right fit for you.

Muscle loss

Lean mass is another tradeoff. Fasting can cause the breakdown of fat into energy, but the body doesn’t exclusively burn fat when there aren’t enough calories coming in. If fasting is repeated often and eating days are low in protein, low in calories, or not paired with resistance training, lean mass can decline, too.

That matters because muscle supports strength, metabolism, mobility, and long-term health. If someone chooses to fast, the surrounding habits matter: enough protein, enough total nutrition, and some form of resistance training.

The benefits may be temporary

Most of the metabolic changes from a single 36-hour fast happen because you are fasting. Insulin drops, ketones rise, and fat burn increases during the fast. Those changes don’t automatically translate into long-term health improvements unless the larger pattern is sustainable.

This is one of the biggest misunderstandings around fasting. A strong short-term signal doesn’t always mean a strong long-term outcome.

Is a 36-hour fast safe?

For healthy adults, a 36-hour fast may be well tolerated. But it still places stress on the body and isn’t risk-free.

One study tested a single 36-hour fast in adults with type 1 diabetes and found it was safe under monitored conditions, with low risk of serious blood sugar complications. That is encouraging, but it should not be taken as permission for people with diabetes to try this alone. The study was supervised, participants were selected, and glucose was monitored.

Medical guidance matters, especially if you take insulin or other diabetes medications, take medications that need to be taken with food, are pregnant or breastfeeding, have a seizure disorder, have heart or kidney disease, or have a history of eating disorders. The NIH recommends talking with a health care provider before fasting, especially for people with medical conditions or who are taking certain medications.

Some discomfort can happen during fasting, but dizziness, confusion, fainting, heart palpitations, severe weakness, or persistent nausea are signs to stop and seek medical guidance.

How often should you do a 36-hour fast?

You don’t have to do 36-hour fasts at all. That point is worth saying clearly. Many of the goals people hope to get from fasting—weight loss, better blood sugar, improved metabolic health—can also come from more moderate calorie reduction, higher protein intake, regular movement, strength training, better sleep, and a routine that is easier to keep.

ADF studies often use repeated fasting days, and the 22-day study showed short-term weight and fat loss with this pattern. The single-person case report followed twice-weekly 36-hour fasting for 82 weeks and found sustained weight loss, reduced hunger over time, higher ketone levels, and stable energy expenditure.

But a case report is still just one person, and a short controlled study doesn’t tell us what will feel sustainable for everyone. 

More fasting isn’t automatically better. If fasting days are frequent and eating days are low in protein or nutrients, or chaotic, the pattern can increase the risk of muscle loss, nutrient gaps, rebound overeating, and burnout.

For most people, if a 36-hour fast is used at all, it makes sense to approach it gradually and sparingly. Start with shorter fasting windows first, pay attention to energy and mood, and get medical guidance before making it a regular habit.

How does a 36-hour fast compare to other fasts?

Fasting approaches differ in structure and intensity. Some, like a 12-hour overnight fast or 16:8 fasting, still include eating every day. Others involve going one or more full days without food.

Longer fasting windows can drive a greater shift toward using stored energy, fat burning, and ketone production. But that doesn’t mean they are better for your body or right for you as an individual. Longer fasts are harder to sustain, may pose greater risks, and don’t necessarily lead to better long-term results.

Here’s how some common fasting approaches compare:

Fasting approach	Typical structure	What’s happening metabolically	Difficulty/sustainability
Intermittent fasting (16:8)	Daily eating window (8–12 hrs eating, 12–16 hrs fasting)	Mostly using glycogen; small rise in fat use; minimal ketosis	Low, easiest to maintain and start with
24-hour fast (ADF)	Eat → fast a full day → eat again	Glycogen depletion begins; early fat burning; small ketone increase	Moderate, will experience noticeable hunger
36-hour fast (ADF-style)	Dinner → skip full day → breakfast next day	Shift to fat burning; meaningful ketone production; lower insulin	Moderate, may be harder to sustain
48-hour fast	Two full days without food	Deeper ketosis; greater reliance on fat; more pronounced metabolic stress	High, difficult for most people
72-hour fast	Three full days without food	Sustained ketosis; further metabolic adaptation; higher physiological stress	Very high, not practical for most, requires experience

Learn more: Can a 72-hour fast really reset your metabolism?

Frequently asked questions about 36-hour fasts

Is a 36-hour fast the same as alternate-day fasting?

Not exactly. ADF means alternating fasting days with eating days. A 36-hour fast is an ADF-style or modified ADF schedule because it includes the overnight fasting hours before and after a full fasting day. Instead of a simple 24-hour fast followed by 24 hours of eating, the fasting window often stretches to about 36 hours.

Will a 36-hour fast burn fat?

Fat burning likely increases during a 36-hour fast as your body enters ketosis. But some early weight loss also comes from water and glycogen and isn’t permanent. Long-term fat loss depends on the overall pattern, not one fast, and you are likely to regain some weight and fat once you go back to eating. 

Will a 36-hour fast cause muscle loss?

One fast on its own isn’t likely to cause meaningful muscle loss. But when fasting becomes frequent—especially without enough protein, calories, or resistance training—that’s where lean mass can start to drop along with body fat. Research on prolonged fasting shows this pattern clearly: weight loss often includes both fat and lean mass, not just fat.

In practical terms, it is less about avoiding fasting entirely and more about what surrounds it: getting enough protein, staying active, and giving your body what it needs to hold onto muscle while you’re losing weight.

Does hunger get easier while fasting?

It depends, and it isn’t the same for everyone. Some people do start to feel less hungry over time, but others don’t see much change. In a 22-day alternate-day fasting study, hunger on fasting days did not decrease, while a separate long-term case report of twice-weekly 36-hour fasts found that hunger did ease over time.

In real life, adaptation is possible, but not guaranteed. Your experience can depend on how often you fast, what you eat on non-fasting days, your sleep, your stress level, and your overall routine. If fasting consistently leaves you overly hungry or thinking about food all day, that’s useful feedback. It may not be the best fit for you right now.

Is a 36-hour fast better than a 24-hour fast?

Not necessarily. A 36-hour fast usually produces a stronger ketone response and a deeper shift toward fat burning than a shorter fast. But it also adds more stress and may be harder to sustain. For many people, a shorter fasting window or a more moderate calorie-reduction approach is more realistic.

The bottom line: Fasting is one of many wellness tools

A 36-hour fast can be one option for people who want a more structured way to reduce calorie intake or experiment with how their body responds to longer periods without food. Some people like the simplicity of “not eating at all” for a set window rather than constantly thinking about what or how much to eat.

The research shows that this approach can produce real short-term effects: increased fat oxidation, higher ketone levels, lower insulin, and potential weight and fat loss. 

But the tradeoffs are just as real. Hunger can be difficult, lean mass can be lost if nutrition and strength training aren’t in place, and many of the metabolic changes are temporary. 

Importantly, you don’t need to fast for 36 hours to see meaningful health improvements. Many of the same benefits can also come from more moderate, sustainable habits like eating balanced meals, increasing protein intake, moving regularly, strength training, and getting enough sleep.

If your goal is to improve your overall health—not just your weight—it can help to take a more holistic approach. Noom’s Proactive Health program is designed to give you a more complete picture of what’s actually changing in your body.

It tracks 17 key health markers, including heart and metabolic health, as well as other whole-body indicators, so you can see real progress beyond the scale. The Noom Biomarkers Test Kit is an at-home blood test that provides clear, actionable insights into what’s working and where you may need to make adjustments.