How intermittent fasting works and what you need to know before trying it

Have you heard about fasting and wondered if it’s just another health fad? Far from it – it’s a time-tested practice that taps into your body’s natural ability to heal, reset, and thrive. Beyond weight loss, fasting offers profound metabolic and cellular benefits backed by science. To help you navigate your fasting journey safely and effectively, we spoke to Sandra Cohen, a nutritional therapist, who breaks down everything you need to know about fasting – its benefits, risks, and best practices.

What is intermittent fasting (IF)?

Intermittent fasting (IF) is an eating pattern that alternates between periods of eating and calorie restriction. Unlike focusing on what you eat, IF focuses on when you eat. There are various methods of IF, including fasting for a set number of hours each day or reducing caloric intake on certain days of the week. A popular approach is the 16:8 or 18:6, where eating is confined to an 8-hour or 6-hour window, followed by fasting for the remaining hours.

One type of IF, known as time-restricted eating (TRE) or prolonged nightly fasting, aligns eating with your body’s natural circadian rhythms. This method involves limiting food intake to a 4- to 12-hour window each day, extending the overnight fasting period to at least 10 hours.

Sandra explains, “Aligning your eating schedule with your natural circadian rhythms is one of the most effective ways to enhance metabolism and improve digestion. It helps your body burn fat more efficiently and supports your energy levels throughout the day.”

What happens to your body when you fast?

When you fast, your body switches from using carbohydrates as its primary fuel to burning stored fat. This metabolic shift, known as ketosis, not only enhances fat-burning but optimises mitochondrial function, boosting your energy at a cellular level. Additionally, fasting activates autophagy, a self-cleaning process that removes damaged cells and regenerates healthier ones.

Sandra elaborates, “Fasting promotes cellular repair by activating autophagy, which clears out dead and damaged cells. This process rejuvenates your body, potentially slowing down the ageing process. Fasting also fine-tunes hormonal regulation, increasing insulin sensitivity and amplifying human growth hormone (HGH), which helps with fat metabolism and muscle preservation.”

The science-backed benefits of fasting

As your body enters ketosis and activates autophagy, fasting continues to optimise your health in remarkable ways. Here’s a closer look at some of the key benefits:

• Gut health: Giving your digestive system a break reduces bloating and promotes a healthier gut, which can improve digestion and even help with conditions like IBS.
• Metabolic health: Fasting enhances insulin sensitivity, balances blood sugar levels, and reduces the risk of type 2 diabetes. Sandra highlights, “Fasting also gives your digestive system a much-needed rest, improving the balance of your gut microbiota and enhancing digestion.”
• Brain health: Fasting increases brain-derived neurotrophic factor (BDNF), a protein that supports cognitive function and may help stave off neurodegenerative diseases like Alzheimer’s.
• Fat loss & inflammation reduction: Fasting accelerates fat oxidation and combats chronic inflammation, making it a powerful tool in the fight against obesity and metabolic diseases.
• Cellular rejuvenation & longevity: Autophagy not only removes damaged cells but also promotes longevity by supporting cellular repair and regeneration.

The risks of fasting: is it right for you?

While fasting holds transformative potential, it’s not a one-size-fits-all solution. There are a few important risks to be aware of when considering fasting as part of your health regimen:

• Blood sugar imbalances: Fasting can cause hypoglycemia, particularly in diabetics.
• Hormonal disruptions: Extended fasting may impact menstrual cycles, thyroid function, or reproductive health.
• Muscle depletion: If you fast for extended periods without adequate protein intake, it could lead to muscle loss.
• Electrolyte imbalances: Prolonged fasting can lead to dizziness, fatigue, and irregular heart rhythms if sodium, potassium, and magnesium levels drop too low.

Sandra advises, “If you have any pre-existing health conditions, it’s important to consult with a healthcare professional before starting a fasting regimen. Fasting may not be suitable for everyone,” especially for individuals who are:

• Diabetics on medication (risk of hypoglycaemia)
• Those with adrenal or thyroid dysfunction
• Those under chronic stress or experiencing sleep disturbances (as it may elevate cortisol)
• Pregnant or breastfeeding women (increased caloric needs)
• Those with a history of eating disorders
• Elderly individuals (risk of muscle loss or frailty)
• Children and adolescents (as they require continuous nutrients for growth)
• People with chronic illnesses or those recovering from surgery

How long should you fast?

The ideal fasting duration varies depending on your health goals and how your body responds to fasting. Here’s a breakdown of common fasting durations:

• Intermittent Fasting (12-18 hours): Popular approaches like 16:8 (16 hours fasting, 8-hour eating window) or 18:6 (18 hours fasting, 6-hour eating window) regulate insulin sensitivity and promote fat-burning. It’s a flexible and sustainable option for most people.
• 24-Hour Fasting (OMAD – One Meal A Day): Fasting for a full day and eating one meal within a 1-2 hour window. OMAD boosts autophagy and cellular repair but should be used occasionally, with caution, as extended fasting without proper protein intake can cause muscle loss.
• Prolonged Fasting (48+ hours): Extended fasting triggers deep cellular repair and reduces oxidative stress, but should be done under medical supervision to ensure safety and proper nutrient intake.

Staying hydrated is non-negotiable. Water, herbal teas, black coffee, and bone broth not only support hydration but also maintain electrolyte balance, ensuring that fasting enhances rather than depletes your health.

Sandra Cohen, Nutritional Therapist

Note: Medical supervision is recommended for prolonged fasts or low calories fasts to ensure nutritional adequacy and to prevent excessive metabolic stress. Ultimately, fasting should be a supportive tool, not a stressor. Listening to your body and adjusting fasting frequency, duration, and dietary intake accordingly will ensure that it enhances health rather than disrupting it.

5:2, juice cleanse, or 28-day challenge: what are they and are they worth exploring?

There are other fasting methods worth considering, but they should be approached with caution. Here’s a look at a few:

• 5:2 Fasting: This involves eating normally for five days and restricting calories (500-600) on two non-consecutive days. While effective for weight loss and improving metabolic flexibility, it may cause hunger on fasting days.
• Juice cleanse: Consuming only fruit and vegetable juices for a set period. While it offers vitamins and antioxidants, it lacks protein and fats, which can lead to muscle loss and blood sugar imbalances.
• 28-Day Fasting Challenge: A structured fasting protocol that helps individuals gradually adapt to intermittent fasting over four weeks, progressively increasing fasting windows while maintaining nutrient-dense eating periods.

Breaking your fast like a pro

How you break your fast is just as important as how you fast. To avoid overwhelming your digestive system, Sandra recommends starting with nutrient-dense, easily digestible foods:

• Healthy fats: avocado, bone broth, olive oil
• Lean proteins: eggs, fish, chicken
• Non-starchy vegetables: steamed spinach, courgette, cucumbers
• Fermented foods: sauerkraut, kimchi for gut health

“Start with small portions,” Sandra advises. “Avoid sugar-laden or processed foods, as they can spike your blood sugar and cause digestive discomfort, negating many of the benefits you’ve gained from fasting.”

Can you exercise while fasting? Here’s what to know

Exercising during a fast is possible, but the intensity of your workout matters. Low-intensity exercises, such as walking or yoga, are generally well-tolerated and can be done without overwhelming the body. However, strength training or high-intensity exercise may be better suited for after fasting, when energy levels are restored.

While fasted workouts can increase fat oxidation, they should be approached with caution, particularly for individuals prone to fatigue or dizziness. Listen to your body and adjust your exercise routine to match your energy levels during fasting.

How women can fast without disrupting their hormones

Women’s hormonal systems are highly responsive to caloric restriction and fasting duration, making it crucial to approach fasting with care. During perimenopause and menopause, fluctuating levels of oestrogen and progesterone can make women more vulnerable to the stress response triggered by fasting. Excessive fasting can spike cortisol – the stress hormone – which may increase insulin resistance, disrupt thyroid function, and throw reproductive hormones further out of balance.

For women already dealing with high stress levels, adrenal dysfunction, or HPA axis dysregulation, prolonged fasting can deplete energy reserves, worsening symptoms like fatigue, mood swings, weight gain, and sleep disturbances. To prevent these imbalances, Sandra Cohen recommends:

• Shorter fasting windows (12-14 hours instead of 16+ hours) to avoid overwhelming the body with stress.
• Protein-rich meals during eating windows to support muscle maintenance and hormone health.
• Avoiding fasting during high-stress periods or times of increased physical or mental demand.
• Cyclical fasting, where you fast on some days but not every day, to prevent prolonged caloric restriction.

By adopting a more flexible and balanced approach to fasting, women can enjoy its health benefits without risking their hormonal balance or overall well-being.

Intermittent fasting for weight loss: discover the science behind it

Intermittent fasting has become a powerful tool for weight loss. The science behind intermittent fasting reveals that fasting for periods of time allows your body to shift from burning glucose to burning stored fat for energy. This metabolic switch is what drives weight loss and improves metabolic health.

For weight loss, many experts, including Sandra Cohen, recommend a fasting window of at least 12 hours. This allows the body to enter fat-burning mode while still supporting overall health. Some people may benefit from extending the fasting period to 14-16 hours, which can further enhance fat oxidation and improve insulin sensitivity.

Genetics also play a role in how well an individual tolerates fasting. Certain genetic variations can affect the body’s ability to efficiently switch between fuel sources, which can impact how effective fasting is for weight loss. For example, variations in the PPARGC1A gene, involved in energy metabolism, may affect how well the body adapts to longer fasting periods.

The key to success with intermittent fasting is personalisation. Each individual’s body responds differently, and factors like blood sugar regulation and adrenal function must be taken into account. By monitoring energy levels, hunger cues, and metabolic markers, you can find the ideal fasting window that works best for your weight loss goals.

Fasting for religious purposes: a time for reflection and nourishment

Fasting is a deeply spiritual practice in many religious traditions, offering a unique opportunity for reflection, self-discipline, and empathy for others. While the purpose and methods of fasting may differ across faiths, the emphasis on mindful eating and connecting with one’s faith is universal. Here, Sandra Cohen, a nutritional therapist, shares valuable advice for those observing fasting during Ramadan and Lent, focusing on how to nourish the body during these sacred times.

Fasting during Ramadan

For Muslims observing Ramadan, the fast from sunrise to sunset is a time of spiritual reflection, self-control, and increased empathy for those less fortunate. Fasting during Ramadan includes abstaining from food, drink, smoking, gossip, and sexual relations during daylight hours. This practice is designed to bring individuals closer to God and to heighten their awareness of the needs of others.

Sandra offers insightful advice on how to nourish the body during Ramadan, especially when it comes to the pre-sunrise meal (suhoor) and the post-sunset meal (iftar):

• Suhoor (pre-sunrise meal): Focus on nutrient-dense meals that provide sustained energy throughout the day. Sandra recommends incorporating complex carbohydrates, lean protein, fiber, and healthy fats to prevent energy dips during fasting hours. Good options include whole grains, eggs, avocado, and oats.
• Iftar (post-sunset meal): After a long day of fasting, it’s important to rehydrate and replenish lost nutrients. Sandra advises breaking the fast with hydrating foods such as soups, salads, or smoothies. A balanced meal that includes lean protein, healthy fats, and plenty of vegetables will help restore energy levels. For example, grilled chicken or fish with roasted vegetables or a chickpea salad can provide the necessary nutrients to support the body’s recovery after fasting.

Hydration is key: taying hydrated is essential, especially given that water is restricted during daylight hours. During suhoor and iftar, drink plenty of water, and consider including hydrating foods like cucumbers, watermelon, and soups to support hydration throughout the day.

It’s also wise to avoid excessive sugars and refined carbohydrates during these meals, as they can lead to energy crashes and disrupt the body’s natural rhythms.

Fasting during Lent

Lent, observed by many Catholics, is a period of 40 days (from Ash Wednesday to Easter) where individuals often abstain from certain foods or practices as a form of penance and spiritual growth. During Lent, Catholics are called to refrain from eating meat on Ash Wednesday, Good Friday, and all Fridays of Lent. Additionally, many choose to give up specific foods or fast for extended periods.

When it comes to fasting during Lent, Sandra advises that it’s crucial to find high-quality protein sources, particularly because meat is restricted:

• Fish & seafood: Rich in omega-3 fatty acids, options like salmon, sardines, and mackerel provide both protein and essential fats that support overall health.
• Eggs: A complete protein source, eggs support muscle maintenance and satiety during fasting periods.
• Dairy (if permitted): Greek yogurt, cottage cheese, and hard cheeses are excellent sources of protein and calcium, which are vital during periods of dietary restriction.
• Plant-based proteins: Lentils, chickpeas, quinoa, tofu, tempeh, and nuts (like almonds and cashews) are rich in amino acids, ensuring that your body gets the necessary building blocks for muscle repair and energy.
• Protein combinations: Sandra suggests combining plant-based proteins, such as pairing rice with beans or hummus with whole-grain pita, to create a complete protein profile for optimal nutrition.

Reintroducing foods after Lent

After 40 days of restricting certain foods, it’s important to reintroduce them slowly to prevent digestive discomfort and fluctuations in blood sugar. Sandra recommends a gradual approach:

• Start with small portions: If you’ve given up meat, dairy, or processed foods, begin by reintroducing smaller portions to gauge how your body reacts. For example, if you’ve abstained from meat, start with a smaller portion of chicken or fish.
• Prioritise whole foods first: Rather than jumping straight into fried or highly processed foods, start with nutrient-dense, whole foods like lean proteins, steamed vegetables, and healthy fats. This ensures that your body receives nourishment without overwhelming the digestive system.
• Monitor Your reactions: As you reintroduce certain foods, pay attention to how your body reacts. Symptoms like bloating, gas, fatigue, or skin reactions may indicate food sensitivities.
• Reintroduce one food group at a time: If you’ve eliminated dairy, for instance, start with yogurt before moving on to harder cheeses to see how your body responds.

By reintroducing foods thoughtfully and gradually, you’ll allow your digestive system to adjust and help avoid any negative reactions, while still honouring the spiritual aspects of Lent.

About Sandra Cohen