Fasting - a complete guide

Written By Laura Andreli

Fasting has become one of the most talked-about approaches for weight loss, metabolic health and longevity.

You may have heard that fasting can help with blood sugar, insulin sensitivity, inflammation, fat burning, autophagy and even brain health. Perhaps you have also heard that you need to fast for 16, 18 or even 24 hours to see meaningful benefits. But how much of this is supported by evidence? And is a longer fast necessarily a better fast?

The answer is more nuanced than many social media posts suggest.

For some people, reducing the amount of time they spend eating can be a useful way of improving meal structure, reducing evening snacking and supporting metabolic health. For others, particularly those with certain medical conditions, a history of disordered eating, high training demands or inadequate energy intake, fasting can be counterproductive.

There is also an important difference between going 12 hours overnight without eating and regularly undertaking 24-, 36- or 72-hour fasts. These approaches are often grouped together under the umbrella of "intermittent fasting", despite having very different physiological and practical implications.

So, what does the evidence actually tell us?

Key Takeaways

  • Intermittent fasting is an eating pattern that alternates periods of eating with periods of little or no calorie intake.

  • Common approaches include 12:12, 14:10, 16:8, 5:2 and alternate-day fasting. Prolonged fasting usually refers to fasting for 24 hours or longer, with some protocols lasting several days.

  • Fasting can support weight loss and some markers of metabolic health, but current evidence does not suggest that intermittent fasting is consistently superior to a well-designed calorie-controlled diet.

  • When you eat may matter as well as how long you fast. Research increasingly suggests that earlier eating windows may be more metabolically favourable than concentrating food later in the evening.

  • You do not necessarily need a 16-hour fast to gain benefits. A consistent 12-hour overnight fasting window can be a sensible starting point for many people.

  • Fasting is not appropriate for everyone, particularly during pregnancy and breastfeeding, with a history of eating disorders, or when certain medications or medical conditions are present.

  • Women may need a more individualised approach because energy availability, menstrual function, fertility, pregnancy and hormonal health need to be considered. Current evidence on fasting and female reproductive hormones remains limited and mixed.

  • Longer fasts are not automatically better. Prolonged fasting can result in loss of lean mass, electrolyte disturbances, dizziness and other adverse effects, and should not be undertaken casually.

Contents

  1. What is fasting?

  2. What happens in your body when you fast?

  3. The different types of fasting

  4. Is 12 hours of fasting enough?

  5. The 16:8 method

  6. The 5:2 diet

  7. Prolonged fasting

  8. What are the potential benefits of fasting?

  9. Fasting and the gut

  10. Fasting, the brain and mental health

  11. Fasting for women: is it different?

  12. Fasting and fertility

  13. Who should avoid fasting?

  14. How to approach fasting safely

  15. FAQs

  16. Conclusion and next steps

What is fasting?

Fasting simply means going without food for a defined period of time.

We all fast naturally. When you finish dinner and then eat breakfast the following morning, you have completed an overnight fast.

The difference with intermittent fasting is that the fasting period is deliberately extended or structured.

For example:

  • 12:12 - 12 hours fasting, 12 hours eating

  • 14:10 - 14 hours fasting, 10 hours eating

  • 16:8 - 16 hours fasting, 8 hours eating

  • 18:6 - 18 hours fasting, 6 hours eating

  • 5:2 - eating normally on five days and substantially reducing food intake on two days

  • Alternate-day fasting - alternating between fasting or very low-calorie days and normal eating days

  • Prolonged fasting - fasting for 24 hours or longer, sometimes for several days

Time-restricted eating (TRE) is slightly different from some other forms of intermittent fasting because the emphasis is primarily on when we eat, rather than prescribing specific foods. This distinction matters. Fasting does not automatically make an otherwise poor-quality diet healthy. If someone eats highly processed foods, very little protein and inadequate fibre during their eating window, simply compressing those foods into eight hours does not necessarily create a nutritionally adequate diet.

The quality, quantity and composition of food still matter.

What happens in our body when we fast?

Our metabolism is not static throughout the day. When you eat, nutrients from the meal become available for energy and storage. Insulin rises, particularly in response to carbohydrate and protein, and the body is in a more "fed" state.

As time passes without food, insulin levels generally fall and the body increasingly draws on stored energy. Initially, the body uses stored glycogen - particularly liver glycogen - to help maintain blood glucose. As fasting continues, fat mobilisation increases and the liver produces more ketones from fatty acids. This is sometimes described as a metabolic switch between using recently consumed glucose and relying more heavily on stored fat and ketones.

However, it is important not to turn this into a simplistic "fat-burning starts at exactly X hours" message. There is no magic fasting threshold at which your metabolism suddenly switches from unhealthy to healthy.

Your physiology is continuously adapting.

Fasting may also influence pathways involved in cellular stress responses, insulin signalling and nutrient sensing. Autophagy (our body’s natural cellular recycling system that breaks down and reuses old damaged cells, literally derived from the Greek meaning “self-eating”) is often discussed in this context, although much of the impressive mechanistic evidence comes from laboratory and animal research rather than long-term human clinical trials.

This is one reason I am cautious about claims that fasting "detoxes" the body or guarantees longevity. The biology is interesting. The clinical evidence is still developing.

The Different Types of Fasting

Not all fasting is created equal.

The potential benefits and risks depend on how long you fast, what you eat during your eating periods, your overall energy intake, your health status and your individual circumstances.

12:12 fasting

A 12:12 pattern means you have a 12-hour eating window followed by a 12-hour overnight fast. For example:

7.30am breakfast → 7.30pm dinner → breakfast at 7.30am

This is often overlooked because it sounds too simple. But for someone who currently eats from 7am until 10pm, moving towards a consistent 12-hour eating window may eliminate several hours of late-night eating without requiring extreme restriction.

One small randomised trial in adults with metabolic syndrome found that a 12-hour time-restricted eating pattern was associated with modest weight loss, although it was not significantly better than standard dietary advice for weight loss. That is an important finding. You do not need to jump straight to 16:8.

For many people, a consistent overnight gap of around 12 hours is a sensible place to start. The goal is not to see how long you can tolerate hunger. It is to find an eating pattern that supports your health, lifestyle, sleep, training and nutritional needs.

The 16:8 Method

The 16:8 approach involves fasting for 16 hours and eating within an 8-hour window. For example:

10am-6pm eating window → 6pm-10am fasting window or

12pm-8pm eating window → 8pm-12pm fasting window

The 16:8 approach is popular because it is relatively straightforward and can reduce opportunities for grazing and snacking.

Research suggests that time-restricted eating can improve body weight and some cardiometabolic markers, particularly in people with overweight or metabolic risk factors. However, when compared with continuous calorie restriction, the differences are generally modest.

If fasting helps you naturally eat less, reduce evening snacking and create a more consistent eating rhythm, it may be useful. But the fasting window itself is not necessarily responsible for all of the benefit. In many studies, total energy intake decreases as a consequence of having fewer opportunities to eat.

The 5:2 Diet

The 5:2 diet involves eating normally on five days of the week and substantially restricting energy intake on two non-consecutive days. Traditionally, the "fasting" days provide around 500-600 calories, although versions vary. Technically, this is not a complete fast. It is better described as intermittent energy restriction.

The attraction is obvious: rather than restricting food every day, you only have to consciously restrict intake on two days.

But it can also be challenging.

Some people find the structure liberating. Others become extremely hungry, preoccupied with food or compensate by overeating on non-fasting days.

When researchers compare intermittent energy restriction, including 5:2, with continuous calorie restriction, intermittent approaches generally do not produce dramatically greater weight loss. They can work, but they are not inherently superior.

The best dietary pattern is often the one you can follow consistently while still meeting your nutritional needs.

Prolonged Fasting: 24 Hours, 48 Hours and Beyond

This is where I would encourage considerably more caution. A prolonged fast generally refers to going without food for 24 hours or longer. Some fasting protocols extend to several days. Physiologically, a multi-day fast is very different from simply finishing dinner at 7pm and eating breakfast at 7am.

During prolonged fasting:

  • Glycogen stores become increasingly depleted

  • Fat mobilisation increases

  • Ketone production rises

  • Insulin levels fall

  • Protein breakdown can contribute to energy production

  • Fluid and electrolyte balance can change

  • Hormonal adaptations occur

There can be measurable short-term changes in weight, blood pressure and some metabolic markers during prolonged fasting. However, the evidence base is relatively small.

One review of prolonged water fasting found that 5-20 day fasts produced substantial weight loss, but approximately two-thirds of the weight lost was estimated to be lean mass rather than fat. Headaches, hunger, insomnia and metabolic disturbances were also reported in some studies. More recent research also highlights potential concerns including electrolyte imbalance, orthostatic intolerance, loss of lean tissue and challenges associated with refeeding.

So while prolonged fasting may be an interesting area of research, I would not recommend DIY multi-day fasting as a general health strategy. Longer is not necessarily better.

What Are the Potential Benefits of Fasting?

Research into intermittent fasting suggests several potential benefits.

1. Weight management

Intermittent fasting can support weight loss, particularly where it helps reduce overall energy intake. Longer-term studies suggest that intermittent fasting and continuous calorie restriction produce broadly comparable reductions in body weight and adiposity (fat tissue).

This means fasting is best viewed as one possible structure for achieving an appropriate energy intake, rather than a metabolic loophole that allows you to eat unlimited calories.

2. Blood sugar regulation

Time-restricted eating may improve fasting glucose and insulin sensitivity, particularly in people with metabolic dysfunction.

A 2026 systematic review and network meta-analysis of 41 randomised trials found that time-restricted eating was associated with improvements in body weight, fasting glucose, fasting insulin and triglycerides compared with usual diets. Earlier eating windows generally performed better than later ones. This is particularly interesting because it suggests that meal timing may matter, not simply the number of hours you fast.

3. Metabolic health

Fasting may influence:

  • Insulin sensitivity

  • Blood glucose

  • Triglycerides

  • Blood pressure

  • Waist circumference

  • Body fat

However, these effects vary between individuals and fasting protocols. The strongest evidence tends to be in people who have overweight, obesity or metabolic risk factors, rather than assuming everyone needs to fast to optimise their health.

4. Appetite and eating behaviour

Some people find that a defined eating window simplifies their day. Instead of constantly deciding whether to have another snack, they have clearer boundaries around meals. For others, the opposite happens. They become increasingly focused on food, experience intense hunger or compensate during the eating window.

This is why your psychological response to fasting matters just as much as the theoretical metabolic benefit.

5. Reduced inflammation?

This is an area where the marketing often gets ahead of the science. Fasting may influence inflammatory and metabolic signalling pathways, and weight loss itself can reduce some inflammatory markers.

However, the evidence does not support the idea that prolonged fasting is a guaranteed way to "flush out inflammation".

In fact, a 2025 scoping review of prolonged fasting lasting at least 48 hours found that inflammatory markers such as CRP often stayed the same or temporarily increased during fasting, with some changes normalising after refeeding. This is a good example of why human physiology is more complicated than "fasting = less inflammation".

If inflammation is a concern, I would be much more interested in the bigger picture:

  • Sleep

  • Blood glucose regulation

  • Metabolic health

  • Gut health

  • Nutrient status

  • Physical activity

  • Stress and recovery

  • Dietary quality

  • Alcohol intake

  • Smoking

  • Underlying inflammatory or autoimmune conditions

Fasting may be one piece of that puzzle. It is rarely the whole puzzle.

Fasting and the gut

One proposed benefit of fasting is giving the digestive system a break.

There is some biological plausibility here, particularly because a structured eating pattern creates longer periods without food entering the gastrointestinal tract. But "resting your gut" is not the same as healing a gut condition.

If you have IBS, SIBO, inflammatory bowel disease, reflux or chronic bloating, fasting may affect symptoms differently depending on the underlying driver. For example, someone with significant reflux may find a longer gap between dinner and bedtime helpful. Someone with IBS who becomes extremely hungry and then eats a very large meal may experience the opposite.

And if you already struggle to eat enough because of nausea, early satiety or digestive symptoms, extending the fasting window may simply make your nutritional intake worse. This is where personalised nutrition becomes important.

The question is not simply: "Can fasting improve gut health?"

It is: "What is happening in your gut, what are you currently eating, and would changing your meal timing actually address the underlying problem?"

Fasting, the brain and mental health

Fasting has attracted considerable interest in relation to brain health because ketones can be used as an alternative energy substrate by the brain, while fasting also influences cellular stress-response pathways. However, this does not mean that fasting automatically improves cognitive function or mental health.

Some people report improved mental clarity when fasting. Others experience:

  • Brain fog

  • Irritability

  • Poor concentration

  • Headaches

  • Low energy

  • Anxiety around food

  • Difficulty sleeping

A 2024 meta-analysis of randomised trials in adults with overweight or obesity found no significant overall increase in fatigue or headaches with intermittent fasting compared with control diets, although dizziness was numerically higher in some non-early TRE groups. Your response matters.

If fasting makes you feel mentally sharp and well nourished, it may be compatible with your physiology and lifestyle. If you spend half the morning thinking about food and the afternoon feeling exhausted, it may not be.

Fasting for Women: Is It Different?

This is one area where I would be particularly cautious about applying blanket fasting rules. A lot of the early fasting research was conducted predominantly in men or mixed populations, and female reproductive physiology has not always been adequately represented.

The current evidence does not support the simplistic idea that fasting is inherently harmful to women. But it also does not support the idea that every woman should follow a strict 16:8 or longer fasting protocol.

Female physiology is particularly sensitive to energy availability.

If energy intake is chronically too low relative to expenditure, the body may downregulate reproductive function. This is particularly relevant for women who are:

  • Very physically active

  • Training for endurance events

  • Naturally lean

  • Losing weight rapidly

  • Eating a restrictive diet

  • Experiencing irregular or absent periods

  • Trying to conceive

  • Pregnant or breastfeeding

Research specifically examining time-restricted eating and female reproductive hormones remains limited. A recent systematic review found that randomised trials in women with obesity generally did not show significant changes in key reproductive hormones, while evidence in PMOS (formerly PCOS) was more promising but largely came from smaller or non-randomised studies.

So I would not tell every woman: "Never fast."
Nor would I tell every woman: "16:8 is the optimal fasting schedule."

Instead, I would consider the whole picture and your personal response.

Fasting and Fertility

If you are trying to conceive, this deserves particular attention.

There is emerging research into intermittent fasting in women with PMOS (formerly PCOS), including potential improvements in insulin sensitivity, weight and some reproductive markers. However, the evidence is still relatively limited and larger, higher-quality studies are needed.

For someone with insulin resistance and PCOS who is carrying excess weight, an appropriately designed eating pattern may be useful.

But fertility nutrition is not simply about lowering insulin. We also need adequate:

  • Energy

  • Protein

  • Essential fats

  • Folate

  • Iron

  • Iodine

  • Choline

  • Vitamin D

  • Zinc

  • Other micronutrients

And once pregnant, intentional fasting is not appropriate. The goal is to create the best nutritional environment for conception and a healthy pregnancy.

Who Should Avoid Fasting?

Intermittent fasting is not suitable for everyone. You should seek individual medical or nutritional advice before fasting if you:

  • Are pregnant

  • Are breastfeeding

  • Have a current or previous eating disorder

  • Have diabetes, particularly if using insulin or medications that can cause hypoglycaemia

  • Are underweight or struggling to maintain weight

  • Have significant nutritional deficiencies

  • Have a medical condition requiring regular food intake

  • Are taking medications that need to be taken with food

  • Have a history of problematic restrictive eating

  • Are undertaking high-volume endurance training

  • Are experiencing menstrual irregularity associated with low energy availability

For people with diabetes, fasting can alter blood glucose and medication requirements. Diabetes UK recommends discussing fasting with your healthcare team, particularly for people using insulin or medications that increase the risk of hypoglycaemia.

How To Approach Fasting?

For most healthy adults, I would start much more gently than social media would have you believe.

1. Start with your overnight fast

Try finishing dinner at a reasonable time and leaving around 12 hours before breakfast.
For example: 7pm dinner → 7am breakfast
This creates a natural fasting period without requiring you to skip meals.

2. Reduce late-night eating

Ask yourself whether you are genuinely hungry at 9.30pm or whether the habit is driven by:

  • Stress

  • Boredom

  • Alcohol

  • Screen time

  • Habit

  • Under-eating earlier in the day

Sometimes the most useful "fasting intervention" is simply stopping unnecessary evening grazing.

3. Prioritise meal quality

During your eating window, focus on:

  • Adequate protein

  • Fibre-rich carbohydrates

  • Vegetables and fruit

  • Healthy fats

  • Omega-3-rich foods

  • A diverse range of plant foods

  • Sufficient micronutrients

  • Adequate hydration

4. Consider meal timing

If possible, avoid pushing the majority of your food intake into the late evening. A slightly earlier dinner may be more metabolically favourable than simply trying to make the fasting window as long as possible.

5. Protect muscle

If body composition is your goal, make protein and resistance training priorities. A fasting protocol that produces weight loss but also compromises muscle mass is not necessarily a successful long-term strategy.

6. Pay attention to your body’s response

Watch for:

  • Dizziness

  • Persistent fatigue

  • Poor sleep

  • Irritability

  • Headaches

  • Poor training performance

  • Increased food preoccupation

  • Menstrual changes

  • Bingeing or overeating when the fast ends

These are signals to reassess rather than push through.

FAQs

  • Fasting can be beneficial for some people, particularly when it helps improve eating structure, reduce overall energy intake and support metabolic health. Research suggests intermittent fasting can improve body weight and several cardiometabolic markers, but it is not consistently superior to conventional calorie restriction.

  • A 12-hour overnight fast is a reasonable starting point for most healthy adults. We do not necessarily need to progress to 16:8 to benefit from a more structured eating pattern. Evidence comparing different fasting durations is still evolving, and the quality and timing of meals remain important.

  • Not necessarily. Some studies show benefits from shorter eating windows, but there is no universal fasting duration that is optimal for everyone. Research increasingly suggests that when you eat may be at least as important as extending the fasting window, with earlier eating patterns generally looking more favourable for metabolic health.

  • Fasting can contribute to overall weight and fat loss if it helps create an appropriate energy deficit. However, it does not specifically target belly fat. Reductions in waist circumference and visceral fat can occur alongside overall weight loss, but fasting is not a mechanism for selectively burning abdominal fat.

  • It may. Time-restricted eating has been associated with improvements in fasting insulin, fasting glucose and insulin sensitivity, particularly in people with metabolic risk factors. Earlier eating windows may have additional metabolic advantages. However, individual results vary and fasting should not replace appropriate medical management of diabetes or prediabetes.

  • Fasting can be appropriate for some women, but it should be individualised. Adequate energy availability is particularly important for menstrual function, fertility, pregnancy and athletic performance. Current research on intermittent fasting and female reproductive hormones is limited and mixed, so there is no evidence-based reason to prescribe the same fasting protocol to every woman.

  • It can potentially affect menstrual function if fasting contributes to insufficient overall energy intake, particularly in women who are lean, highly active or already experiencing low energy availability. Research directly examining intermittent fasting and female reproductive hormones remains limited, so menstrual changes should not simply be dismissed as part of "adapting" to fasting

  • This depends on your individual circumstances. Some research suggests time-restricted eating may have metabolic and reproductive benefits in women with PCOS, but the evidence is still limited. If you are trying to conceive, the priority should be adequate energy and nutrient intake rather than maximising fasting duration.

 

Conclusion.

Fasting can be a useful tool. But it does not need to become another health rule that you feel you are failing if you cannot follow perfectly.

For many people, simply creating a consistent 12-hour overnight break from food, reducing late-night grazing and paying more attention to meal timing may be a more sustainable starting point than jumping straight into 16:8 or multi-day fasting.

And if you have gut symptoms, hormonal concerns, fertility goals, metabolic health issues or a demanding training schedule, the right question is not:

"How long can I fast?"

It is:

"What eating pattern best supports my physiology, health goals and life?"

That answer will look different from person to person.

If you are unsure whether fasting is appropriate for you, or you are experiencing symptoms such as fatigue, digestive problems, menstrual changes, difficulty managing your weight or blood sugar fluctuations, I can help you look at the bigger picture - joining the dots between your nutrition, metabolism, hormones, gut health and lifestyle to create a plan that is genuinely personalised to you.

Book a call

About the author

Laura Andreli, Nutritional Therapist

Laura helps clients unlock the powerful connection between the gut and the brain. She specialises in IBS, SIBO, digestive disorders, food sensitivities, and the gut–brain axis - particularly where symptoms such as brain fog, anxiety, low mood, fatigue, and poor concentration may be linked to underlying microbiome imbalance and metabolic stress.

Laura uses evidence-informed nutrition strategies and targeted lifestyle interventions to support digestive function, calm neuroinflammation, and improve energy, mood, and cognitive performance.

Laura’s path into nutritional therapy is personal. While studying at Cambridge, she was diagnosed with Polyendocrine Metabolic Ovarian Syndrome (PMOS; formerly termed PCOS). The experience of receiving a label without clear, actionable guidance shaped her philosophy: translate complex science into practical steps that genuinely help people feel and function better. A former England-level long jumper, she understands first-hand how hormones, metabolism, and nutrition can influence performance, recovery, and cognition.

Laura Andreli https://lauraandreli.com
Next

Read this before starting a juice cleanse