When someone tells me they’re “doing fasting”, my first question is usually: What kind? Because fasting is not one single protocol.

Finishing dinner at 7 pm and eating breakfast at 7 am is technically a fast. So is eating within an eight-hour window. So is eating normally five days a week and substantially reducing food intake on another two. Alternate-day fasting is another approach again. And then we have traditional naturopathic fasting, which predates the modern intermittent fasting conversation and comes from quite a different therapeutic philosophy. 

They all get put under the same fasting umbrella, but physiologically and practically, they are not the same thing. As a naturopath and sports nutritionist, I think this matters enormously.

Rather than asking “Which fasting protocol is best?”, I find it much more useful to ask:

What are we trying to achieve, who are we working with, and what else is happening in that person’s life and physiology?

So, let’s look at five common approaches to fasting, what the evidence currently tells us, and whether they suit you (or not).

First, what actually counts as intermittent fasting?

Modern intermittent fasting generally refers to eating patterns that alternate periods of food intake with periods of little or no energy intake.

The three approaches most frequently studied are:

1. time-restricted eating
2. the 5:2 diet or other whole-day fasting approaches
3. alternate-day fasting.

Research has found that all three can produce weight loss and improvements in some cardiometabolic markers, particularly in adults with overweight, obesity or metabolic dysfunction. But when intermittent fasting is compared with conventional continuous energy restriction, it generally isn't dramatically superior.

So, yes, fasting can be useful... But it isn't necessarily magical. And different forms of fasting may achieve similar outcomes through slightly different routes.

1. The 12-hour overnight fast

This is probably the least glamorous form of fasting, which may also be why it gets overlooked.

This is where your least meal finishes by 7pm and your first meal is at 7am. (example)

That's it.

No elaborate app. No stopwatch. No heroic declarations about surviving until lunchtime.

For many people, this creates a natural overnight break from eating without dramatically restricting the time available to meet nutritional requirements.

From a practical perspective, I often think this is a useful place to begin when the real issue is something like:

• continual evening grazing
• eating very late at night
• irregular meal patterns
• eating from the minute someone wakes until the minute they go to bed.

A moderate overnight fast can create structure without necessarily turning the entire day into a nutritional maths problem.

When might it make sense?

For someone wanting to improve meal structure or reduce unnecessary late-night eating, a 12-hour overnight fasting period can be relatively easy to fit around normal life. It also leaves plenty of opportunity to eat enough protein, carbohydrate, fats, fibre and micronutrients during the day. Which, for active people, that flexibility is particularly useful.

What does the research say?

Most clinical fasting research investigates narrower eating windows than 12 hours, so we cannot simply assign all of the findings from 8-hour time-restricted eating studies to a 12-hour overnight fast.

A 12-hour overnight fasting period is better thought of as a meal-timing structure rather than assuming it will reproduce every metabolic effect reported in more restrictive fasting studies. 

And sometimes that is enough. Not every nutrition intervention has to be extreme to be useful.

2. Time-restricted eating: 14:10, 16:8 and similar approaches

Time-restricted eating (TRE) such as 14:10 or 16:8 is what many people now mean when they talk about intermittent fasting. What it means is that rather than restricting food on particular days, time-restricted eating (TRE) limits food intake to a daily window.

Examples include:

• 14:10: fasting for 14 hours and eating within 10.
• 16:8: fasting for 16 hours and eating within eight.

Some research protocols use even shorter eating windows.

TRE has become popular partly because it is relatively simple. There is no requirement to calculate specific calories on fasting days. You simply create a defined period in which food is eaten.

What does the evidence show?

A recent umbrella review found high-quality evidence linking TRE with reductions in body weight, fat mass, fasting insulin and HbA1c among adults with overweight or obesity. However, the authors also highlighted the need for more research into long-term effects, adverse outcomes and how fasting needs to be individualised across different populations.

A large systematic review and network meta-analysis has also found that TRE can improve several metabolic outcomes compared with usual eating patterns, including body weight, waist circumference, fasting glucose and fasting insulin. Importantly, when the eating window occurs may matter as well as how long it is. Earlier eating windows appear to show some metabolic advantages over eating predominantly later in the day.

This brings circadian biology into the fasting conversation.

It is not necessarily just: How many hours did you fast?

It may also be: When did you eat those meals?

When might TRE make sense?

TRE may be useful for someone who:

• likes clear meal structure
• tends to graze throughout the evening
• can comfortably meet their nutritional requirements within the eating window
• has metabolic health goals for which TRE may be appropriate
• doesn't have training demands that make the eating window impractical.

Where I become more cautious

This is where my sports nutrition hat comes on.

An 8-hour eating window sounds simple until somebody is:

• doing an early morning training session
• training again after work
• trying to consume enough carbohydrate
• trying to distribute protein appropriately
• attempting to recover between sessions.

Suddenly, an eight-hour window can become quite crowded.

Research looking at TRE combined with exercise suggests it may reduce body weight and fat mass without necessarily compromising physical performance, but the research base is still relatively small, and participants do not necessarily represent athletes with large energy requirements.

So I would never take an eight-hour window that works beautifully for one person and automatically apply it to everyone.

3. The 5:2 approach

The 5:2 diet works differently. Rather than fasting for a set number of hours every day, people eat relatively normally on five days of the week and substantially restrict energy intake on two days. Those two days may involve very low energy intake rather than complete fasting.

So although 5:2 is regularly grouped with intermittent fasting, it is really a form of intermittent energy restriction.

What does the evidence show?

Research suggests 5:2 can produce reductions in weight and several cardiovascular or metabolic risk markers among people with overweight or obesity. However, when intermittent energy restriction approaches such as 5:2, TRE and alternate-day fasting are compared with conventional daily energy restriction, the overall differences are often fairly small.

Again, this brings us back to an important point: The benefit of fasting may sometimes come partly from making it easier for someone to reduce overall energy intake.

That doesn't make it ineffective. It simply means we need to be cautious about attributing every outcome to the fasting period itself.

When might 5:2 make sense?

Some people prefer having two clearly defined lower-energy days rather than restricting themselves slightly every day. For the right person, psychologically and practically, that can feel easier.

What about athletes?

For athletes, scheduling becomes important when implementing a TRE fasting approach.

Putting a very low-energy day alongside:

• a long run
• a high-intensity session
• heavy strength work
• a competition
• or a double-training day

doesn't make much sense from a fuelling perspective.

If 5:2 is being used by an active person, the lower-energy days need to be considered alongside the training programme rather than randomly dropped into the week.

A rest day is one thing. A 3-hour training day is another.

4. Alternate-day fasting

Alternate-day fasting, or ADF, usually involves alternating between fasting or substantially energy-restricted days and normal eating days. Depending on the protocol, a fasting day might contain no food or a small proportion of someone's normal energy intake.

This is a considerably more aggressive intervention than simply stopping eating after dinner.

What does the research say?

ADF has received quite a lot of research attention. A major 2025 systematic review and network meta-analysis including 99 randomised trials found that intermittent fasting strategies generally reduced body weight compared with unrestricted eating. When compared with continuous energy restriction, ADF was the only fasting approach in that analysis to show an additional reduction in body weight — although the difference was modest, at roughly 1.3 kg. Longer-term evidence remains more limited.

Other reviews have also found potential improvements in weight and cardiometabolic markers with ADF, although results vary depending on the population, duration and comparison diet.

So yes, it can work. But the more important clinical question is: At what cost to practicality, hunger, training and adherence for this particular person?

Who might find ADF difficult?

I would be particularly thoughtful about ADF in someone with:

• high training loads
• significant energy requirements
• difficulty meeting protein requirements
• poor recovery
• a tendency towards restrictive eating
• a complicated relationship with food.

A protocol can demonstrate efficacy in a controlled trial and still be completely impractical for the person sitting in front of you. Clinical nutrition lives in that gap.

5. Traditional naturopathic fasting and Nature Cure

Long before modern intermittent fasting research, periods of fasting and restricted or simplified diets were used within European Nature Cure traditions that contributed to the development of modern naturopathy. The nineteenth-century European Nature Cure movement incorporated approaches including hydrotherapy, physical activity, nutrition and other lifestyle practices. Historical naturopathic facilities also incorporated fasting and dietary therapies.

Traditional Nature Cure approaches came from a broader therapeutic philosophy involving rest, diet and other natural therapies such as:

• water fasting
• juice fasting
• green juice fasting
• mono-food fasting
• simplified diets
• periods of reduced food intake.

Some historical explanations of Nature Cure — particularly around concepts such as detoxification or “foreign matter” causing disease — reflect the medical philosophies of their time rather than contemporary physiology. That doesn't mean we need to dismiss the whole practice. It means we need to separate the historical rationale from what we can reasonably explain using modern physiology and research.

How does traditional therapeutic fasting work?

Unlike time-restricted eating, where somebody may still consume their normal amount of food within a shorter window, traditional therapeutic fasting generally involves a meaningful reduction in overall energy intake for a defined period. Some forms involve no energy at all, such as water-only fasting. Other therapeutic fasting traditions allow small amounts of energy from vegetable broths, juices or similar foods.

For example, modern Buchinger therapeutic fasting, which developed from European fasting traditions, is generally defined as a fluid-based fasting programme providing up to around 500 kcal per day for at least five days, with a preparation period beforehand and gradual food reintroduction afterwards. Contemporary consensus definitions also emphasise that therapeutic fasting needs to be individualised according to factors such as age, sex, body composition, physical activity, health status and the intended duration of the fast.

Physiologically, once energy intake falls substantially, the body begins shifting away from its usual fed-state metabolism. Initially, stored glycogen helps maintain blood glucose. As fasting continues and glycogen availability falls, fat mobilisation increases and the liver produces more ketone bodies to provide an alternative fuel source. There are also changes in insulin, glucagon and other metabolic signalling pathways.

That is quite different from the older explanation that fasting simply allows the body to “detox”. We have organs doing detoxification rather effectively already — particularly the liver, kidneys, lungs and gastrointestinal tract. So from a modern naturopathic perspective, I prefer to talk about fasting in terms of energy restriction, digestive rest, metabolic adaptation and changes in fuel utilisation, rather than making claims that the evidence cannot support.

Who might traditional therapeutic fasting work best for?

There is growing research into prolonged and therapeutic fasting, but it is nowhere near as extensive as the modern evidence base for general nutrition interventions. We certainly don't have evidence that everyone needs to periodically fast for health.

The populations in which therapeutic fasting has been investigated most often tend to include adults with issues such as:

• overweight or obesity
• metabolic dysfunction
• some cardiometabolic risk factors
• people specifically seeking a structured therapeutic fasting intervention.

There may also be people who find a short period of simplified eating useful because it creates a clear break from their usual eating habits and encourages them to reassess appetite, food choices and meal structure.

But I would distinguish between “this person may tolerate a therapeutic fast” and “this person needs a therapeutic fast.”

Research into prolonged water-only fasting, for example, demonstrates substantial metabolic changes, but we are still learning about the longer-term significance of many of those changes. A recent meta-analysis found that lipid responses vary according to fasting duration, illustrating why we cannot simply assume that a longer fast automatically produces greater health benefits.

For me, traditional fasting makes most sense when there is a clear rationale, careful screening and an appropriate therapeutic context — not simply because fasting sounds cleansing or because somebody wants to see how long they can go without food.

Who might Nature Cure fasting not suit?

I would be particularly cautious about therapeutic or prolonged fasting in:

• pregnant or breastfeeding women
• children and adolescents
• people who are underweight or already undernourished
• anyone with current or previous eating-disorder behaviours
• frail older adults or people at risk of sarcopenia
• people with osteoporosis or significant bone-health concerns
• athletes with high training loads
• anyone showing signs of low energy availability or REDs
• people taking glucose-lowering medication
• people with medical conditions where fasting could alter medication requirements, hydration, electrolytes or blood glucose.

People who already have high energy requirements or limited nutritional reserves may have much less room for error. Recent reviews identify pregnancy, breastfeeding, childhood, malnutrition risk, eating-disorder history and frailty as situations in which fasting may be inappropriate or require particular caution. For people with diabetes or other conditions affected by food intake, fasting may also require medication adjustment and clinical monitoring rather than being something to experiment with independently.

And what about athletes?

Traditional Nature Cure fasting and hard training generally have competing priorities. One is deliberately reducing energy intake. The other is deliberately creating a physiological stress that requires energy and nutrients for adaptation and recovery. Which doesn't make them absolutely incompatible, but I would want a very good reason before combining prolonged fasting with substantial training.

An athlete doing a deliberate therapeutic fast is not in the same situation as someone performing an easy morning session before breakfast.

With a prolonged fast, we need to think about:

• total energy availability
• glycogen availability
• protein intake
• muscle preservation
• recovery
• hydration and electrolytes
• training quality
• immune function
• hormonal health.

So if someone is in a heavy training block, preparing for competition, trying to gain muscle, recovering from injury or already struggling to eat enough, that is generally not where I would be looking to introduce an aggressive fasting intervention. 

Is one fasting approach better than the others?

Not really.

And this is one of the problems with the way fasting is often discussed - especially on social media.

Research does not currently point towards one universally superior intermittent fasting method. Across randomised trials, TRE, 5:2 and ADF can all produce improvements in weight and some metabolic markers, particularly when compared with unrestricted eating. But many benefits are broadly comparable with conventional energy restriction.

That means the decision cannot simply come down to: Which one causes the greatest weight loss?

We also need to consider:

• metabolic health
• training requirements
• body composition goals
• age
• sex
• life stage
• meal quality
• protein intake
• carbohydrate requirements
• medication
• lifestyle
• hunger
• sleep
• adherence
• relationship with food.

This is where individualisation becomes far more important than choosing the most fashionable protocol.

Before choosing a fasting strategy, ask these 5 questions:

1. What are you actually trying to achieve?  

This sounds obvious, but it is often skipped.

Why are you fasting?

Is it for:

• weight management?
• glucose control?
• metabolic health?
• reducing late-night eating?
• appetite structure?
• digestive comfort?
• convenience?
• athletic performance?
• because someone online told you it was healthy?

Different goals do not automatically call for the same protocol. And sometimes fasting is not actually the intervention required.

If the problem is eating half a packet of biscuits while watching Netflix at 10 pm, we may not need an 18-hour fasting protocol. We may simply need to address the biscuits.

2. How much exercise are you doing?  

Someone exercising three times per week for 30 minutes has very different nutritional requirements from somebody training for a marathon.

The greater the volume and intensity of exercise, the more opportunities we need to provide:

• energy
• carbohydrate
• protein
• fluid
• micronutrients.

This doesn't automatically rule out fasting, but it makes timing far more important.

3. Can you still eat enough?  

This might be the most overlooked question in fasting because it's entirely possible to fast and still meet your nutritional needs, AND it is also entirely possible to use fasting as a very effective way of under-eating.

This becomes especially relevant when somebody combines: fasting + weight loss + low carbohydrate + high exercise volume.

You can end up stacking restriction upon restriction.

If the eating window is so small that somebody struggles to consume enough food, protein or carbohydrate, the issue is no longer fasting. We may be creating low energy availability. And for athletes, that's a very different clinical conversation.

4. What is happening to your energy, recovery and performance?  

I always want to know what is happening after someone begins fasting. Are they:

• energetic?
• sleeping well?
• training well?
• recovering normally?
• maintaining strength?
• concentrating?
• managing hunger comfortably?

Or are we seeing:

• fatigue
• poor recovery
• declining performance
• persistent soreness
• disturbed sleep
• food preoccupation
• menstrual changes
• repeated injuries?

An eating strategy doesn't get a free pass because it has the word “fasting” attached to it. If someone's health and performance are deteriorating, we need to investigate why.

5. Can you live with it?  

Yes, you may discover the theoretically perfect eating window, but if it means:

• never eating breakfast with your family
• avoiding social dinners
• constantly watching the clock
• refusing food when you're genuinely hungry
• reorganising your entire life around when you're “allowed” to eat

Then perhaps that fasting approach it isn't entirely healthy for you.

Nutrition has to exist in real life. 

What about fasting for metabolic health?

Intermittent fasting has shown potential benefits for weight, glycaemic control and other cardiometabolic markers, especially among adults with overweight, obesity or metabolic dysfunction. But we need to be precise about the population.

Evidence in an overweight adult with insulin resistance cannot automatically be transferred to:

• an endurance athlete
• a teenager
• a pregnant woman
• somebody already under-fuelling
• a lean athlete trying to maximise performance.

This is one of my biggest frustrations with nutrition research being translated onto social media.

A study shows an intervention benefits one population, and suddenly we're told everybody should do it. 

No.

Always ask: Who was actually studied?

And what about fasting for fat burning?

Fasting increases periods where insulin is relatively low and can increase reliance on stored fuels. But “burning more fat” at a particular moment is not automatically synonymous with losing more body fat over weeks and months. Total energy balance, dietary intake, exercise, hormonal factors and adherence still matter. Likewise, an endurance athlete may become very efficient at fat oxidation and still need carbohydrate to perform high-intensity work well.

From a sports nutrition perspective, I am far more interested in: Can this person perform, recover and adapt to training?

than in chasing maximum fat oxidation every hour of the day.

So which fasting strategy makes sense for you or your client?

Rather than beginning with the protocol, begin with the person. Because the goal is to use nutrition in a way that actually supports the outcome you're trying to achieve.

If I could leave you with one thing, it would be this:

Don't choose a fasting strategy because it is popular. Choose it because it fits the person, the goal and the circumstances.

The fasting window is only one piece of the nutritional picture.

We still need to consider:

1. What are they eating?
2. Are they eating enough?
3. When are they exercising?
4. How are they recovering?
5. What stage of life are they in?
6. What is happening metabolically?
7. And can they actually sustain this approach?

That's where fasting moves from being another dietary trend into something we can think about properly.

Want to understand fasting beyond the protocols?

In my two-part webinar series, The Science & Practice of Fasting, I take a much deeper look at both sides of this conversation.

In Traditional Fasting & Nature Cure, we explore the history and philosophy behind naturopathic fasting approaches, including water, juice and mono-food fasting, and consider where traditional ideas may fit within modern practice.

Then, in Intermittent Fasting & the Modern Evidence, we look at contemporary fasting styles, time-restricted eating, current research, metabolic effects and the factors that can make a fasting approach useful for one person and completely inappropriate for another.

The series is primarily designed for health practitioners and students, but it is also useful for anyone wanting to understand fasting properly rather than simply following another protocol.

🔖 Learn more about The Science & Practice of Fasting