Screenshot 2021-09-23 at 15.39.07
Screenshot 2021-09-23 at 15.39.07

Intermittent Fasting – All your questions answered

Intermittent fasting is not a diet, but a way of eating… and it doesn’t change what you eat, but is specifically focused on WHEN you eat. 

Historically, fasting was and is still done for religious or cultural reasons, but has now become a popular go-to diet for weight loss. As an added bonus, there are a number of different methods that can be used when it comes to fasting, from skipping breakfast to only fasting on certain days of the week. 

A number of celebrities and health professionals swear and live by intermittent fasting, but is it actually good for us? And what exactly is it good for

It’s still early days with the fasting trend and many of the studies have been done on rats or in early clinical trials, but there is a substantial volume of evidence to suggest it may be beneficial for more than just losing a few pounds post-summer…


There are a number of approaches when it comes to intermittent fasting:

  • Daily time-restricted eating: eating within a certain time window each day


  • Alternate day fasting: eating a normal diet one day and doing a complete fast or having a smaller meal the next day


  • 5:2 fasting: Fasting for 2 days a week and eating normally for 5


  • Eat Stop Eat: This involves a 24 hour fast once or twice a week. Fasting from dinner one night to dinner the next day adds up to a full 24hr fast. 


  • 48 hour fast: Individuals who chose this option tend to eat dinner on day 1, skip all meals on day 2 and eat their breakfast at midday on day 3, which is a total of 42 hours


  • Fasting mimicking diet: This is a more recent fasting trend that restricts calories for a set period of time. All meals and snacks over the 5 days are from natural, whole foods, being lower in carbs and protein but higher in fat. The meal plan causes the body to generate energy from non-carb sources after glycogen stores are depleted.


  • Water fasts: A fast where the individual does not eat or drink anything other than water. There is no set time that water fasting must last for, but medical advice suggests from 24 hours to 3 days as the maximum to go without food.  


Time-restricted eating is definitely one of the most popular types of fasting, thanks to its ability to fit into the lives of clients who live busy and working lifestyles, they tend to find this method convenient as it often involves pushing breakfast back a couple of hours or going straight into lunch. More to be explained below.


Eating in a time-restricted window is a type of intermittent fasting that limits food intake to a certain number of hours each day. There are two popular methods when it comes to time-restricted eating which include:

  • 16:8- Eating within an 8 hour period and fasting for 16 hours overnight 
  • 14:10- Eating within a 10 hour period and fasting for 14 hours overnight  

There are many reasons why individuals decide to jump on the time-restricted eating wagon, from a strategy for weight loss to a way to boost metabolic health by giving your system a break for longer periods than usual.

Ancestrally it suits us better. Previously, people did not have access to food when and if they needed it 24 hours a day, whereas today the majority of people have the privilege to eat what and when they want around the clock. Some people believe that this excessive eating around the clock without giving our system a break can lead to disruption of the circadian rhythm, potentially leading to metabolic diseases and obesity over time.


CELL AUTOPHAGY- This is the self-cleaning process that recycles and breaks down damaged cells. This gives our body time to become less reliant on glucose as the main energy source, as the body is forced to dip into other energy stores. By stimulating autophagy, we clear out old junky proteins and cellular parts, whilst also stimulating our growth hormone, which tells our body to start producing new useful parts for the body. Current evidence suggests that anywhere between 18 hours to 4 days will trigger autophagy, however, only a small volume of studies measuring this exist in humans. (1, 2)

WEIGHT LOSS: Many people start intermittent fasting with the intention of weight loss. And part of why it works for some is common sense. The fasting process can result in you eating fewer meals, and therefore consuming fewer calories.

However, fasting also enhances hormone function which may work to promote weight loss. Fasting may help to lower insulin levels and increase amounts of norepinephrine, both of which may inhibit the breakdown of body fat and facilitate its use for energy. Therefore, fasting may increase your metabolic rate, helping to burn even more calories. (3)

REDUCE INSULIN RESISTANCE: Fasting may help improve insulin resistance, leading to a positive reduction in blood sugar levels. One human study showed that blood sugar had reduced by 3-6% over the course of 8-12 weeks in individuals with prediabetes who embarked on a diet that included fasting. (4) The idea is that the fasting period gives the cells a chance to rest, helping to increase their sensitivity to sugars over time- which may potentially help reduce the onset or severity of type 2 and pre-diabetes. (5)

GUT HEALTH: Fasting can help to avoid overloading the digestive system. When we’re constantly eating, the gut doesn’t have time to rest and digest, which can have an impact on how our gut health functions. Read more below.

MINDFUL EATING: For those who are constantly grazing around the clock without any structure to their meal times, intermittent fasting may help individuals be more aware of their eating patterns, becoming more in tune with their natural hunger cues. This increased sensitivity to our hunger and fullness can help us to become more mindful of the food we are consuming. 


SHUTTING OFF– If a woman’s body goes into “starvation mode” too often, this can be one of the first biological responses to shut down reproduction. Instead, the body prioritises our energy for necessary biological processes such as digestion and breathing. A by-product of this prioritisation is that the reproductive system takes a hit, resulting in potential changes that include menstrual cycle changes or imbalanced sex hormones.

DISORDERED EATING– Intermittent fasting is a restrictive diet, and those with a history of disordered eating should avoid embarking on fad/restrictive diets until fully recovered. 

For some, fasting may actually be a trigger for disordered relationships with food. There is a strong biological push to overindulge and overeat following periods of fasting. Appetite hormones can go into overdrive when the body and brain are deprived of food. This can be a vicious cycle, leaving individuals feeling guilty.

HIGH DROP OUT RATES– Past trials of Intermittent Fasting have shown high dropout rates amongst individuals assigned to the fasting regimen. Will you really say no to that next glass of wine because it’s 5 minutes past your fast start time? Fasting doesn’t always fit into your lifestyle, and any diet that gets in the way of socialising or enjoyment should be questioned. 

OVEREATING– One of the potential downfalls of fasting is binge eating/over-eating. Some individuals who fast may feel ravenous by the time they break the fast, which can lead to overeating. In addition, only eating within a short window is not a free ticket to eat everything in sight for eight hours before your fast restarts! This would defeat the purpose of fasting. This can be a bigger struggle for people who are used to eating regularly and may not be in total tune with their body’s hunger cues. 

LONG TERM FASTS – The ultimate risk of fasting, is death by starvation, which is very unlikely to happen after a 48 hour fast. However, there are other drawbacks of fasting for longer lengths of time. Of course, most people fast safely, but it is also worth noting that fasting is not a risk-free experiment. Less serious drawbacks can include mood swings, low energy and irritability, with fasting also lowering blood pressure levels. Many will undergo these longer fasts to “detox”, which may be detrimental to our health. Strict water fasting is also a risk for heart failure due to during a fast, the body’s intracellular stores of minerals that are vital for cardiac function, are depleted. The results of this cardiac muscle loss and mineral deprivation can be tragic. If you want to try fasting, it is best to try one of the easier approaches such as TRE or work with a professional to guide you appropriately.

STRESS LEVELS– Fasting may increase levels of the stress hormones norepinephrine and cortisol, due to depriving the body of food for a period of time. Although fasting does bring some positive health benefits, these could be outweighed you have pre-existing high-stress levels, are at risk of burn out or have a condition that is impacted by stress. High cortisol levels have also been associated with fat storage, which is not ideal if you’ve incorporated fasting as part of your routine for weight loss. If you are already a highly stressed individual, maybe fasting will not be right for you. 


Not only does the food we eat influence our gut microbes that live in the gut, but also WHEN and HOW we eat can have an impact on our gut microbiome.

Digesting food uses up a great deal of energy, with fasting giving your overworked gut a hard-earned break from energy-intensive tasks such as breaking down and digesting the food you eat. Our MMC (migrating motor complex) is our guts cleansing system, that helps to push food through the intestines and reduce bacterial infections and overgrowth. When we fast, this can allow the MMC to do its job. Whilst intermittent fasting can give the body this opportunity, the MMC actually only needs about 120 minutes to do its job. To support your gut health, keep a 12 hour fast overnight and leave 2-3 hour gaps between meals. 

When it comes to gut bacteria, it has been suggested that fasting may improve gut-friendly flora such as Akkermansia. (7)

However, a note that fasting may actually trigger symptoms for some individuals. Long gaps between meals may actually exacerbate symptoms for those with reflux, heartburn or low stomach acid.

So as great as fasting may seem, it will not be the answer for everyone’s gut problems. If you suffer from specific problems that have been ongoing it is so important to work with a nutritionist to get to the root cause of your issues. 


ATHLETES – Intermittent fasting may not be suitable for certain individuals such as athletes who are training intensely and have very high energy requirements. They will most likely not be able to consume enough food to support their intense training in an 8-10 hour window. If you have an intense exercise regime and wish to implement fasting, it is best to work with a professional to provide targeted support.

PREGNANCY/BREASTFEEDING – In addition, pregnant or breastfeeding women need to ensure they are having enough good-quality nutrition to support their growing baby, so fasting will not be suitable for them. 

HEALTH CONDITIONS – some individuals may find that fasting triggers or exacerbates their symptoms. This may be the case for those with fertility struggles, thyroid hormone imbalance and burnout. This is why fasting should be done in a controlled manner, and if you are dealing with a health concern, it is best to work with someone. 

For individuals with diabetes or on any medications, intermittent fasting should always be done under the supervision of a doctor. Some medications require food to work effectively, therefore it is always important and advised to check.


Like any dietary pattern, it’s important to find out what works for you.

My advice on introducing fasting to your routine. Start with a 12 hour fast which can be beneficial for many of us, and gradually increase your fasting hours until you get to a point you feel comfortable. If you notice at 14 hours you feel shaky, dizzy or you overeat after fasting, then 14 hours might be too much for you. For some, you may feel you can stretch past 16 hours.

If you would like to consider intermittent fasting to help your gut or health concerns, send my team an email to book in for a free 15-minute discovery call to find out if it’s right for you 


REFERENCES (1) (2) (3) (4) (5) (6) (7)

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Hi there

London Nutritionist Clarissa Lenherr

I’m Clarissa, a registered nutritionist (mBANT) and workplace wellness expert. In my practice, I have helped hundreds of clients reach optimal health through creating sustainable, effective habits and dietary adjustments. My aim is to empower people with the skills, tools and knowledge to take their health into their own hands and feel the happiest, healthiest versions of themselves. Featured in The Daily Mail, Women’s Health, The Telegraph, and more.




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