You have probably heard of intermittent fasting as a way to lose body fat or change body composition.

Is it a suitable approach for those goals? What should you consider?

Read on to find out.

What is intermittent fasting (IF)?

First, let us define fasting.

The simplest definition is a period without food. Physiologically, the fasted state develops after the post-meal period, as nutrient absorption subsides, and continues until the next intake.

Intermittent fasting alternates periods without food with eating periods in which nutritional needs should still be met.

Benefits of Intermittent Fasting

Research into intermittent fasting has explored a range of possible health effects. Proposed mechanisms are not the same as established benefits in humans.

Changes in pathways involving IGF-1 and SIRT3 have been studied in relation to ageing. They do not prove that intermittent fasting extends human life or reduces mortality.
It can help reduce body fat when it produces a sustainable energy deficit; increased fat oxidation alone does not guarantee fat loss.
Some studies report improved insulin sensitivity, but results vary and are influenced by weight loss and meal timing.
Some people find the structure helps manage eating habits and appetite, while others find it difficult.
Fasting does not automatically prevent loss of muscle mass during weight loss. Adequate protein, energy planning and resistance training remain important.
It may change food shopping costs, but eating less variety is not a nutritional advantage.

Why do people try intermittent fasting?

The answer depends on the person. Some frequent snackers prefer a predictable meal schedule. Appetite signals, including ghrelin secretion, interact with habits, sleep and circadian rhythms, but fasting does not reliably “reprogramme” appetite for everyone.

People seeking a diet for muscle gain while reducing fat also sometimes consider it. Alternate-day approaches may use a single meal on restricted days, but changes in growth hormone or blood insulin do not prove increased metabolism or muscle gain. Any fat loss still depends largely on the overall energy deficit. Some find fewer eating decisions easier; others become too hungry or struggle to fuel training.

Types of intermittent fasting

Many people think first of 16/8: fasting for 16 hours and eating within 8. Other protocols have also been studied. Three broad approaches are described below.

Alternate-day fasting (ADF)

Some studied versions alternate unrestricted-intake days with days containing a single meal providing around 25% of daily energy needs. Unrestricted does not mean that food quality stops mattering.

Restricted days can cause considerable hunger. The fasting-day pattern is not automatically suitable for athletes. A plan might combine maintenance intake, higher-energy training days and a deficit on other days, but recovery and muscle gain are not guaranteed and severe restriction may be counterproductive.

Whole-day fasting (WDF)

This intermittent fasting approach restricts or omits food on 1 or 2 days a week, often non-consecutively. Some protocols exceed 24 hours, but longer fasting is not a routine recommendation and needs careful assessment.

Time-restricted eating (TRE): 16/8 or 20/4 fasting

Eating is confined to a 4–8-hour daily window, leaving 16–20 hours without food. One example is eating between 14:00 and 22:00, although earlier schedules may suit some people better.

A regular window can reduce grazing for some people. For someone training once daily and hoping to reduce fat or gain muscle, the main question is whether the schedule allows adequate fuelling, recovery and total nutrient intake.

Busy work schedules may lead to reliance on convenient ultra-processed foods, but skipping food is not automatically better than eating. Accessible, nutritious meals and snacks are another option.

16/8 intermittent fasting

This pattern leaves 16 hours without food and an 8-hour eating window. Water and other suitable non-caloric drinks are generally allowed in dietary fasting protocols.

20/4 intermittent fasting

This more restrictive pattern groups meals into 4 hours, leaving 20 hours without food. Meeting nutritional needs may be harder.

Here is a summary of the three approaches provided by Miguel Troyano.

When might intermittent fasting be considered?

Some adults consider it when aiming to change body composition, but it is not appropriate at every life stage or training phase. Assess contraindications and energy needs first.

Does intermittent fasting work for weight loss?

Its rules may seem simple, but effectiveness should be judged against other approaches rather than assuming that a shorter eating window has special effects.

One study cited in the original article limited eating to 6 am–7 pm and reported a statistically significant reduction in weight of nearly half a kilogram per week compared with no guidance.

That intervention lasted only 2 weeks. Other research examined intermittent fasting over 12 weeks.

In the study described, fasting from 6 pm to 10 am was associated with around 3 kg average weight loss compared with the usual-diet group.

These comparisons may reflect reduced calorie intake as well as timing. The linked research also discusses morning versus evening intake under equal-calorie conditions, but findings vary. Researchers at the United States Department of Agriculture also tested a 5 pm–9 pm window against meals spread through the day, reporting around 2 kg average difference after 8 weeks in the study cited. Such individual trials do not establish universal superiority.

Overall, intermittent fasting with 12–20-hour fasting periods can be one weight-management strategy for suitable adults. It is generally not clearly better than a comparable sustainable calorie-restriction plan. Choose based on safety, preferences, food quality and adherence, with professional support where useful.

How to approach intermittent fasting: basic considerations

If you are considering intermittent fasting, the following points may help assess its suitability.

Consider daytime eating

Meal timing is not the sole determinant of weight loss. Trials comparing different schedules have also looked at outcomes beyond weight.

The linked research reported higher cholesterol and blood pressure with a 5 pm–9 pm eating window despite weight loss, which in itself often improves these markers. Other trials of earlier intake report improvements in blood pressure, oxidative stress markers, fullness, insulin secretion and sensitivity. Results should be interpreted within each study’s design.

In this linked study, participants practising intermittent fasting from 13:00 until 08:00 the next morning had some improvements in biochemical markers without losing weight, with food and quantities controlled.

Earlier eating may therefore be worth considering where practical, but claims of maximum fasting benefits remain uncertain. Suitability, sleep, medication and training schedules still matter.

Keep variety

Each food has a different nutritional composition.

Variety helps provide protein, carbohydrates, fats, vitamins, minerals, fibre and plant compounds.

Fewer meals can make it harder to include enough different foods. Use varied ingredients and colourful fruit and vegetables for nutrients and fibre; colour alone is not a complete measure of nutritional value.

A poke-style bowl with 10 ingredients may offer more variety than chicken and mashed potato alone, though both can fit a balanced plan.

Choose nutritious foods

The body obtains energy by metabolising macronutrients: protein, carbohydrate and fat.

Calories alone do not describe nutritional quality; alcohol, for example, supplies energy without meeting key nutrient needs.

Many ultra-processed foods are relatively low in fibre or micronutrients, although this broad category includes different products.

Base the diet on foods that help meet your needs, especially when intermittent fasting reduces eating opportunities. This involves more than simply eating less.

Food quality matters too.

Maintain appropriate hydration

Water is essential for normal bodily processes.

Pay attention to hydration while fasting, especially around exercise. Drink appropriately without forcing excessive fluid.

Plan satisfying meals

Changing meal times may initially increase hunger. This does not mean you should ignore severe discomfort or signs of under-fuelling.

Water, black coffee and unsweetened infusions may fit the chosen protocol, but should not replace needed nutrition.

Include fibre, protein and suitable fats, mainly unsaturated, in meals to support fullness and nutrient intake.

Appetite and habits

Hunger reflects both biological needs and learned routines, rather than being mostly irrational or unnecessary.

Changing schedules can affect appetite, but our genetic background does not mean discomfort should be ignored.

The old “reptilian brain” versus rational neocortex explanation is an oversimplification. Use a routine that supports your health and adequate intake; stop and reassess if it causes distress, dizziness or an unhealthy preoccupation with food.

Is intermittent fasting safe for women?

Suitability for intermittent fasting depends on individual health, energy availability, life stage and eating history. It should not be assumed safe solely because calories or food choices appear reasonable.

Can it be used throughout the menstrual cycle?

Some concerns originated in rat studies reporting changes in hormones involved in the menstrual cycle.

Human evidence on intermittent fasting and sex hormones is limited, and the small number of trials cannot establish safety for every cycle phase or person.

A substantial energy deficit can disrupt menstrual function. Irregular or missed periods require assessment rather than being accepted as part of the diet.

Intermittent fasting and pregnancy

Do not begin restrictive intermittent fasting during pregnancy for weight loss. An energy deficit and inadequate nutrient intake can jeopardise maternal and fetal health.

Concerns include inadequate fetal growth and low birth weight, although specific outcomes should not be presented as inevitable. Seek pregnancy-specific clinical advice.

Intermittent fasting: contraindications and precautions

The following circumstances require avoidance or medical assessment before considering fasting; this is not an exhaustive list:

Underweight or inadequate energy intake
Porphyria
Active illness or infection
Uncontrolled thyroid disease
Diabetes treated with medication
Advanced kidney or liver disease
Pregnancy or breastfeeding

Also avoid restrictive fasting with a current or previous eating disorder, and do not apply adult protocols to children. Discuss suitability with a healthcare professional.

What are the risks of intermittent fasting?

Risks depend on duration, nutritional adequacy, medication and health. Fasts beyond 24 hours can increase concerns about undernutrition and muscle loss as glycogen availability falls.

Possible problems include changes in mood or concentration, hunger and digestive discomfort. Fasting for 16–20 hours is not automatically free of risk, particularly with hypoglycaemia-prone medication or low energy availability.

Is intermittent fasting suitable for everyone?

No single protocol suits everyone because needs and circumstances differ.

It may be unsuitable for people who are underweight or who struggle to eat enough for muscle gain and training.

People with diabetes medication or kidney or liver disease need medical assessment; pregnancy, breastfeeding and eating disorders are important reasons to avoid restrictive fasting.

What should an intermittent fasting diet contain?

As with other eating patterns, variety and plenty of plant foods matter.

Food selection should match your needs and goals.

If you struggle to maintain weight because you cannot fit enough energy into the window, nut or seed pastes and olive oil can add energy. Widening the window or abandoning fasting may be more appropriate.

For appropriate weight loss, lower-energy-density foods such as vegetables and fruit, alongside wholegrains and pulses, provide fibre and can help with fullness without neglecting protein or total needs.

Example intermittent fasting menu

Meal choices depend on the schedule. The source labels this 16/8 fasting but lists eating from 7 am to 2 pm, a seven-hour window. Treat it as an illustrative menu, not an exact eight-hour protocol.

7 am: oat porridge with soya drink, banana, peach, blueberries, walnuts, cinnamon and ground ginger.
10:30 am: a tuna wrap with lettuce, tomato, red onion, sweetcorn and homemade guacamole, plus Greek yoghurt. The original “pm” was a timing error.
2 pm: teriyaki chicken noodles with sautéed vegetables and sesame seeds, plus a piece of fruit.

Intermittent fasting: frequently asked questions

Here are answers to common questions about intermittent fasting.

How does intermittent fasting differ from Ramadan?

Studies sometimes discuss dietary fasting alongside Ramadan.

A key difference is that Ramadan fasting excludes both food and drink during daylight, whereas most dietary intermittent-fasting protocols allow water and other non-caloric drinks.

Consequently, Ramadan studies cannot be directly applied to every intermittent-fasting schedule. Changes in sleep and hydration may affect endurance performance alongside food timing.

Which organs can intermittent fasting affect?

Research examines the pancreas, insulin responses, skeletal muscle and cardiovascular markers. Some studies report improvements, but effects vary and do not establish a universal organ-health benefit independent of energy intake and weight change.

Which drinks do not break a dietary fast?

Most protocols allow water, black coffee and unsweetened infusions. Rules concerning non-caloric sweeteners vary. “Not breaking a fast” is a protocol definition, not proof that a substance has no digestive or metabolic effects. Follow specific medical-test instructions if fasting for a procedure.

Why am I not losing weight with intermittent fasting?

The eating window may still provide as much or more energy than you expend. Meal timing does not remove the importance of overall intake.

Lower-energy-density foods and adequate fibre can support fullness where weight loss is appropriate.

They may help manage portions during eating periods and hunger during fasting periods. Avoid pushing the deficit so far that nutritional needs, health or training suffer.

What happens if I fast for 12 hours every day?

A 12-hour overnight interval is quite common: dinner at 9 pm and breakfast at 9 am, for example. It suits many adults, but no duration is automatically safe for every medical situation, and risks do not begin only after 24 hours.

What if I fast from Monday to Friday?

A continuous five-day fast is very different from a daily eating window. Fasts exceeding 24 hours can pose substantial risks and should not be self-prescribed. Taking vitamins and minerals does not make prolonged fasting nutritionally adequate or safe.

When might effects become noticeable?

Some short studies report changes in insulin resistance and other markers within about a month, especially in groups with metabolic disease.

This does not guarantee the same response in healthy adults or demonstrate long-term benefit. People with diabetes need a supervised plan.

When does weight loss begin?

Some studies discussed earlier report approximately half a kilogram per week over short periods with 16–20-hour fasting schedules. Individual weight changes vary, and early losses may include water.

Faster loss is not automatically better. Focus on a safe, sustainable plan rather than a promised weekly result.

Recipes for an intermittent fasting plan

These dishes can be adapted to suitable eating schedules; the portions are examples rather than universal prescriptions.

Couscous with chicken breast, Brussels sprouts, mushrooms and carrot

Ingredients

Couscous 50 g
Brussels sprouts 40 g
Mushrooms 40 g
Carrot 40 g
Olive oil 10 g
Table salt 2 g
Chicken breast 80 g

Method

Put the couscous in a bowl and boil an equal volume of water with a pinch of salt, or use the packet’s specified ratio.
Pour the boiling water over the couscous, stir and cover to hydrate.
Meanwhile, heat a little olive oil and sauté halved or quartered Brussels sprouts, sliced or diced carrot, quartered mushrooms and a pinch of salt.
Once tender, add the diced chicken and cook thoroughly, stirring until browned.
Stir in the couscous and adjust the seasoning.

Figures per serving

Calories: 409
Protein: 25% | Carbohydrates: 46% | Fat: 29%

Quinoa, chickpeas, chicken, sweet potato crisps, kale and soya sprouts